Minimize Hubble

HST (Hubble Space Telescope) Mission

Sensor Complement   HST Imagery    Hubble Servicing Missions    Ground Segment    References

The HST (Hubble Space Telescope) of NASA is named in honor of the American astronomer Edwin Hubble (1889-1953), Dr. Hubble confirmed an "expanding" universe, which provided the foundation for the big-bang theory. Hubble, the observatory, is the first major optical telescope to be placed in space, the ultimate mountaintop. Above the distortion of the atmosphere, far far above rain clouds and light pollution, Hubble has an unobstructed view of the universe. Scientists have used Hubble to observe the most distant stars and galaxies as well as the planets in our solar system. 1)

The planning for HST started in the early 1970s. The HST was launched into LEO (Low Earth Orbit) on April 24, 1990 on STS-31 (12:33:51 UTC, on Shuttle Discovery). Hubble is operational as of 2019, in its 30th year on orbit, and is one of NASA's Great Observatories. Hubble's launch and deployment in April 1990 marked the most significant advance in astronomy since Galileo's telescope. Thanks to five servicing missions and more than 25 years of operation, our view of the universe and our place within it has never been the same.

Mission:

• Deployment of Hubble: April 25, 1990

• First Image: May 20, 1990: Star cluster NGC 3532

• Servicing Mission 1 (STS-61): December 1993

• Servicing Mission 2 (STS-82): February 1997

• Servicing Mission 3A (STS-103): December 1999

• Servicing Mission 3B (STS-109): February 2002

• Servicing Mission 4 (STS-125): May 2009

Spacecraft: The spacecraft has a length of 13.2 m, a mass at launch of 10,886 kg, post SM (Servicing Mission) 4 of 12,247 kg, and a maximum diameter of 4.2 m.

Orbit: LEO with an altitude of 547 km an inclination of 28.5º, and a period of 95 minutes.

The HST (Hubble Space Telescope) of NASA features a ULE TM(Ultra-Low Expansion) primary mirror of 2.4 m diameter (f/24 Ritchey-Chretien) and a 0.3 m Zerodur secondary mirror. The HST primary mirror was a lightweighted monolithic design (824 kg) by Perkin-Elmer (now Goodrich Inc.), Danbury, CN, using a lightweight, thick egg crate core sandwiched between two plates and fused together.

The HST is the most precisely pointed instrument in spaceborne astronomy. The pointing requirements call for a continuous 24 hour target lock maintenance of 0.007 arcseconds (2 millionth degree).

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Figure 1: IMAX Cargo Bay Camera view of the Hubble Space Telescope at the moment of release, mission STS-31 in April 1990 (image credit: NASA)

Some background:

The telescope's original equipment package included the Wide Field/Planetary Camera (WF/PC), Goddard High Resolution Spectograph (GHRS), Faint Object Camera (FOC), Faint Object Spectograph (FOS), and High Speed Photometer (HSP). 2) 3)

After a few weeks of operation, scientists noticed that images being sent back from Hubble were slightly blurred. While this distortion still allowed scientists to study the cosmos and make significant discoveries, it resulted in less spectacular images, and some of the original mission could not be fulfilled. An investigation finally revealed a spherical aberration in the primary mirror, due to a miscalibrated measuring instrument that caused the edges of the mirror to be ground slightly too flat. Engineers rushed to come up with a fix to the problem in time for Hubble's first scheduled servicing mission in 1993. The system designed to correct the error was designated COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR was a set of optics that compensated for the aberration and would allow all of Hubble's instruments to function normally.

In December, 1993, the crew of STS-61 embarked on a service mission to replace a number of Hubble's parts. Following intensive training on the use of new tools never used before in space, two teams of astronauts completed repairs during a record five back-to-back spacewalks. During the EVAs, COSTAR was installed and the Wide Field/Planetary Camera was replaced with the Wide Field/Planetary Camera 2, which was designed to compensate for the mirror problem. The team also performed basic maintenance on the craft, installed new solar arrays, and replaced four of Hubble's gyroscopes.

Shortly after the crew returned to Earth and the Hubble Space Telescope began returning sharp and spectacular images, NASA deemed the servicing mission a success. Astronomers could now take advantage of a fully functional space telescope, and the public was treated to breathtaking photos of stars, galaxies, nebulae, and other deep-space objects. Subsequent servicing missions improved Hubble's capabilities and performed routine repairs.

In February, 1997, the crew of STS-82 installed the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and the Space Telescope Imaging Spectograph (STIS) to detect infrared light from deep-space objects and take detailed photos of celestial objects. Servicing mission 3A in December, 1999 replaced all six of the telescope's aging gyroscopes, which accurately point the telescope at its target. STS-103 astronauts also replaced one of the telescope's three fine guidance sensors and installed a new computer, all in time to redeploy Hubble into orbit on Christmas Day. The most recent servicing mission to the spacecraft, servicing mission 3B, came aboard STS-109 in March, 2002. Columbia crewmembers installed the new Advanced Camera for Surveys (ACS), which had sharper vision, a wider field of view, and quicker data gathering than the Wide Field/Planetary Camera 2. Astronauts also replaced Hubble's solar panels with a more efficient array and conducted repairs on the NICMOS.

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Figure 2: This photograph of NASA’s Hubble Space Telescope was taken on the fifth servicing mission to the observatory in May 2009 (image credit: NASA)

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Figure 3: Artist's view of the HST in space along with the designation of the key element locations (image credit: NASA)

The Hubble Space Telescope is an international collaboration among NASA and ESA (European Space Agency). NASA has overall responsibility for the Hubble mission and operations. ESA provided the original FOC (Faint Object Camera) and solar panels, and provides science operations support.

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Figure 4: Photo of the Hubble mission operations team at NASA's Goddard Space Flight Center in Greenbelt, Maryland, as of Hubble’s 25th anniversary of flight in April 2015. Since Hubble’s official start in 1977, thousand of people from the United States and Europe have supported the mission through building and testing hardware and software, operating the vehicle, and performing science operations. More than 30 astronauts have flown to Hubble to deploy, upgrade and repair the observatory with the support of a human spaceflight and space shuttle staff. Thousands of astronomers from dozens of countries have used Hubble and analyzed its data to produce more than 15,000 peer reviewed papers to date (image credit: NASA/GSFC, Bill Hrybyk) 4)


Note: At this stage of the mission (2018), no attempt is being made to recover all facets of Hubble regarding the spacecraft, instrumentation and the past history (it would have required a constant accompaniment of the mission with all updates over its lifetime). Instead, some fairly recent images of the mission and the operational status of the mission are presented.

The Hubble Servicing Missions are shortly described in a separate chapter of this file.




HST sensor complement: (ACS, WFC3, STIS, COS, FGS, NICMOS)

The Hubble Space Telescope has three types of instruments that analyze light from the universe: cameras, spectrographs and interferometers. 5)

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Figure 5: Hubble’s scientific instruments analyze different types of light ranging from ultraviolet (UV) to infrared (IR). This graphic shows which wavelengths each instrument studies (image credit: NASA)


Cameras:

Hubble has two primary camera systems to capture images of the cosmos. Called the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3), these two systems work together to provide superb wide-field imaging over a broad range of wavelengths.

ACS (Advanced Camera for Surveys)

Installed on Hubble in 2002, ACS was designed primarily for wide-field imagery in visible wavelengths, although it can also detect ultraviolet and near-infrared light. ACS has three cameras, called channels, that capture different types of images. An electronics failure in January 2007 rendered the two most-used science channels inoperable. In 2009, astronauts were able to repair one of the channels and restored ACS’s capacity to capture high-resolution, wide-field views.

WFC3 (Wide Field Camera 3)

Installed in 2009, WFC3 provides wide-field imagery in ultraviolet, visible and infrared light. WFC3 was designed to complement ACS and expand the imaging capabilities of Hubble in general. While ACS is primarily used for visible-light imaging, WFC3 probes deeper into infrared and ultraviolet wavelengths, providing a more complete view of the cosmos.

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Figure 6: Astronaut Andrew Feustel prepares to install WFC3 (Wide Field Camera 3) on Hubble during Servicing Mission 4 in 2009 (image credit: NASA)


Spectrographs

Spectrographs practice spectroscopy, the science of breaking light down to its component parts, similar to how a prism splits white light into a rainbow. Any object that absorbs or emits light can be studied with a spectrograph to determine characteristics such as temperature, density, chemical composition and velocity.

Hubble currently utilizes two spectrographs: COS (Cosmic Origins Spectrograph) and the STIS (Space Telescope Imaging Spectrograph). COS and STIS are complementary instruments that provide scientists with detailed spectral data for a variety of celestial objects. While STIS is a versatile, “all purpose” spectrograph that handles bright objects well, COS measures exceedingly faint levels of ultraviolet light emanating from distant cosmic sources, such as quasars in remote galaxies. Working together, the two spectrographs provide a full set of spectroscopic tools for astrophysical research.

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Figure 7: Hubble's STIS captured a spectrum (right) of material ejected by a pair of massive stars called Eta Carinae, while the Wide Field and Planetary Camera 2 took an image of the billowing clouds of gas enveloping the stellar pair (left). The spectrum reveals that one of the lobes contains the elements helium (He), argon (Ar), iron (Fe) and nickel (Ni), image credit: NASA, ESA and the Hubble SM4 ERO Team


Interferometers

Hubble’s interferometers serve a dual purpose — they help the telescope maintain a steady aim and also serve as a scientific instrument. The three interferometers aboard Hubble are called the FGS (Fine Guidance Sensors). The Fine Guidance Sensors measure the relative positions and brightnesses of stars.

When Hubble is pointing at a target, two of the three Fine Guidance Sensors are used to lock the telescope onto the target. For certain observations, the third Fine Guidance Sensor can be used to gather scientific information about a target, such as a celestial object’s angular diameter or star positions that are ten times more accurate than those obtained by ground-based telescopes.

The Fine Guidance Sensors are very sensitive instruments. They seek out stable point sources of light (known as “guide stars”) and then lock onto them to keep the telescope pointing steadily. When a light in the sky is not a point source, the Fine Guidance Sensor cannot lock on and so it rejects the guide star. Often, a rejected guide star is actually a faraway galaxy or a double-star system. Since Hubble was launched in 1990, the Fine Guidance Sensors have detected hundreds of double-star systems that were previously thought to be single stars.


Past Instruments

Only one of the instruments remaining on Hubble — the third Fine Guidance Sensor — was launched with the observatory in 1990. The rest of the instruments were installed during Hubble’s five servicing missions. In addition to installing new instruments, astronauts also repaired two instruments (ACS and STIS) while visiting Hubble on Servicing Mission 4 in 2009. The NICMOS (Near-Infrared Camera and Multi-Object Spectrometer) on Hubble is in hibernation following a cryocooler anomaly, but most of its infrared duties have since been taken over by WFC3.

Hubble’s past instruments include:

• High Speed Photometer

• Faint Object Camera

• Faint Object Spectrograph

• Goddard High Resolution Spectrograph

• Wide Field and Planetary Camera

• Wide Field and Planetary Camera 2

• Fine Guidance Sensors (three).


Current Instruments

ACS (Advanced Camera for Surveys) - ACS is a third-generation imaging camera. This camera is optimized to perform surveys or broad imaging campaigns. ACS replaced Hubble's Faint Object Camera (FOC) during Servicing Mission 3B. Its wavelength range extends from the ultraviolet, through the visible and out to the near-infrared (115-1050 nm). ACS has increased Hubble's potential for new discoveries by a factor of ten.

COS (Cosmic Origins Spectrograph) - COS focuses exclusively on ultraviolet (UV) light and is the most sensitive ultraviolet spectrograph ever, increasing the sensitivity at least 10 times in the UV spectrum and up to 70 times when looking at extremely faint objects. It is best at observing points of light, like stars and quasars. COS was installed during during Servicing Mission 4 in May 2009.

STIS (Space Telescope Imaging Spectrograph) - STIS is a second-generation imager/spectrograph. STIS is used to obtain high resolution spectra of resolved objects. STIS has the special ability to simultaneously obtain spectra from many different points along a target. The STIS instrument has a mass of 318 kg and a wavelength range of 115-1000 nm.

STIS spreads out the light gathered by a telescope so that it can be analyzed to determine such properties of celestial objects as chemical composition and abundances, temperature, radial velocity, rotational velocity, and magnetic fields. Its spectrograph can be switched between two different modes of usage:

C So-called "long slit spectroscopy" where spectra of many different points across an object are obtained simultaneously.

1) So-called "echelle spectroscopy" where the spectrum of one object is spread over the detector giving better wavelength resolution in a single exposure.

STIS also has a so-called coronagraph which can block light from bright objects, and in this way enables investigations of nearby fainter objects.

WFC3 (Wide Field Camera 3) - Wide Field Camera 3 is the main imager on the telescope. It has a camera that records visible and ultraviolet (UVIS, 200-1000 nm) wavelengths of light and is 35 times more sensitive in the UV wavelengths than its predecessor. A second camera that is built to view infrared (NIR, 850-1700 nm) light increases Hubble's IR resolution from 65,000 to 1 million pixels. Its combination of field-of-view, sensitivity, and low detector noise results in a 15-20 time improvement over Hubble’s previous IR camera. WFC3 was jointly developed at GSFC, STScI (Space Telescope Science Institute) in Baltimore and Ball Aerospace & Technologies Corporation in Boulder, CO. 6)

FGS (Fine Guidance Sensor) – The FGS provides pointing information for the spacecraft by locking onto guide stars. The FGS can also function as a scientific instrument by precisely measuring the relative positions of stars, detecting rapid changes in a star’s brightness, and resolving double-star systems that appear as point sources even to Hubble’s cameras. Hubble has three FGSs onboard the observatory.

NICMOS (Near Infrared Camera and Multi-Object Spectrometer) – NICMOS has the ability to obtain images and spectroscopic observations of astronomical targets at near-infrared wavelengths. Although NICMOS is currently inactive, most of its functionality is replaced by Hubble’s other science instruments.

Minimize Status and some observation imagery

HST (Hubble Space Telescope) - Status and some observation imagery

• 16 August 2019: When stars like the Sun grow advanced in age, they expand and glow red. These so-called red giants then begin to lose their outer layers of material into space. More than half of such a star's mass can be shed in this manner, forming a shell of surrounding gas. At the same time, the star's core shrinks and grows hotter, emitting ultraviolet light that causes the expelled gases to glow. 7)

- This type of object is called, somewhat confusingly, a planetary nebula, though it has nothing to do with planets. The name derives from the rounded, planet-like appearance of these objects in early telescopes.

- NGC 2022 is located in the constellation of Orion (The Hunter).

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Figure 8: Although it looks more like an entity seen through a microscope than a telescope, this rounded object, named NGC 2022, is certainly no alga or tiny, blobby jellyfish. Instead, it is a vast orb of gas in space, cast off by an ageing star. The star is visible in the orb's center, shining through the gases it formerly held onto for most of its stellar life (image credit: ESA/Hubble & NASA, R. Wade; CC BY 4.0)

• 08 August 2019: Hubble Showcases New Portrait of Jupiter. Among the most striking features in the image are the rich colors of the clouds moving toward the Great Red Spot. This huge anticyclonic storm is roughly the diameter of Earth and is rolling counterclockwise between two bands of clouds that are moving in opposite directions toward it. 8)

- As with previous images of Jupiter taken by Hubble, and other observations from telescopes on the ground, the new image confirms that the huge storm which has raged on Jupiter's surface for at least 150 years continues to shrink. The reason for this is still unknown so Hubble will continue to observe Jupiter in the hope that scientists will be able to solve this stormy riddle. Much smaller storms appear on Jupiter as white or brown ovals that can last as little as a few hours or stretch on for centuries.

- The worm-shaped feature located south of the Great Red Spot is a cyclone, a vortex spinning in the opposite direction to that in which the Great Red Spot spins. Researchers have observed cyclones with a wide variety of different appearances across the planet. The two white oval features are anticyclones, similar to small versions of the Great Red Spot.

- The Hubble image also highlights Jupiter's distinct parallel cloud bands. These bands consist of air flowing in opposite directions at various latitudes. They are created by differences in the thickness and height of the ammonia ice clouds; the lighter bands rise higher and have thicker clouds than the darker bands. The different concentrations are kept separate by fast winds which can reach speeds of up to 650 km/hour.

- These observations of Jupiter form part of the Outer Planet Atmospheres Legacy (OPAL) program, which began in 2014. This initiative allows Hubble to dedicate time each year to observing the outer planets and provides scientists with access to a collection of maps, which helps them to understand not only the atmospheres of the giant planets in the Solar System, but also the atmosphere of our own planet and of the planets in other planetary systems.

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Figure 9: The NASA/ESA Hubble Space Telescope reveals the intricate, detailed beauty of Jupiter's clouds in this new image taken on 27 June 2019 by Hubble's Wide Field Camera 3, when the planet was 644 million kilometers from Earth — its closest distance this year. The image features the planet's trademark Great Red Spot and a more intense color palette in the clouds swirling in the planet's turbulent atmosphere than seen in previous years [image credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley)]

• 02 August 2019: Believe it or not, this long, luminous streak, speckled with bright blisters and pockets of material, is a spiral galaxy like our Milky Way. But how could that be? 9)

- The galaxy is located in the constellation of Leo Minor (The Lesser Lion). Other telescopes that have had NGC 3432 in their sights include those of the Sloan Digital Sky Survey, the Galaxy Evolution Explorer (GALEX), and the Infrared Astronomical Satellite (IRAS).

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Figure 10: It turns out that we see this galaxy, named NGC 3432, orientated directly edge-on to us from our vantage point here on Earth. The galaxy's spiral arms and bright core are hidden, and we instead see the thin strip of its very outer reaches. Dark bands of cosmic dust, patches of varying brightness, and pink regions of star formation help with making out the true shape of NGC 3432 — but it's still somewhat of a challenge! Because observatories such as the NASA/ESA Hubble Space Telescope have seen spiral galaxies at every kind of orientation, astronomers can tell when we happen to have caught one from the side (image credit: ESA/Hubble & NASA, A. Filippenko, R. Jansen; CC BY 4.0)

• 01 August 2019: The scorching hot exoplanet WASP-121b represents a new twist on the phrase "heavy metal." — There are no loud electric guitar riffs, characteristic of heavy metal music, streaming into space. What is escaping the planet is iron and magnesium gas, dubbed heavy metals, because they are heavier than lightweight hydrogen and helium. The observations by the Hubble Space Telescope represent the first time heavy metal gas has been detected floating away from an exoplanet. 10)

- Observations by NASA's Hubble Space Telescope reveal magnesium and iron gas streaming from the strange world outside our solar system known as WASP-121b. The observations represent the first time that so-called "heavy metals"—elements heavier than hydrogen and helium—have been spotted escaping from a hot Jupiter, a large, gaseous exoplanet very close to its star.

- Normally, hot Jupiter-sized planets are still cool enough inside to condense heavier elements such as magnesium and iron into clouds.

- But that's not the case with WASP-121b, which is orbiting so dangerously close to its star that its upper atmosphere reaches a blazing 4,600 degrees Fahrenheit. The WASP-121 system resides about 900 light-years from Earth.

- "Heavy metals have been seen in other hot Jupiters before, but only in the lower atmosphere," explained lead researcher David Sing of the Johns Hopkins University in Baltimore, Maryland. "So you don't know if they are escaping or not. With WASP-121b, we see magnesium and iron gas so far away from the planet that they're not gravitationally bound."

- Ultraviolet light from the host star, which is brighter and hotter than the Sun, heats the upper atmosphere and helps lead to its escape. In addition, the escaping magnesium and iron gas may contribute to the temperature spike, Sing said. "These metals will make the atmosphere more opaque in the ultraviolet, which could be contributing to the heating of the upper atmosphere," he explained.

- The sizzling planet is so close to its star that it is on the cusp of being ripped apart by the star's gravity. This hugging distance means that the planet is football shaped due to gravitational tidal forces.

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Figure 11: This artist's illustration shows an alien world that is losing magnesium and iron gas from its atmosphere. The observations represent the first time that so-called "heavy metals"—elements more massive than hydrogen and helium—have been detected escaping from a hot Jupiter, a large gaseous exoplanet orbiting very close to its star. (image credit: NASA, ESA, and J. Olmsted (STScI))

- "We picked this planet because it is so extreme," Sing said. "We thought we had a chance of seeing heavier elements escaping. It's so hot and so favorable to observe, it's the best shot at finding the presence of heavy metals. We were mainly looking for magnesium, but there have been hints of iron in the atmospheres of other exoplanets. It was a surprise, though, to see it so clearly in the data and at such great altitudes so far away from the planet. The heavy metals are escaping partly because the planet is so big and puffy that its gravity is relatively weak. This is a planet being actively stripped of its atmosphere."

- The researchers used the observatory's STIS (Space Telescope Imaging Spectrograph) to search in ultraviolet light for the spectral signatures of magnesium and iron imprinted on starlight filtering through WASP-121b's atmosphere as the planet passed in front of, or transited, the face of its home star.

- This exoplanet is also a perfect target for NASA's upcoming James Webb Space Telescope to search in infrared light for water and carbon dioxide, which can be detected at longer, redder wavelengths. The combination of Hubble and Webb observations would give astronomers a more complete inventory of the chemical elements that make up the planet's atmosphere.

- The WASP-121b study is part of the Panchromatic Comparative Exoplanet Treasury (PanCET) survey, a Hubble program to look at 20 exoplanets, ranging in size from super-Earths (several times Earth's mass) to Jupiters (which are over 100 times Earth's mass), in the first large-scale ultraviolet, visible, and infrared comparative study of distant worlds.

- The observations of WASP-121b add to the developing story of how planets lose their primordial atmospheres. When planets form, they gather an atmosphere containing gas from the disk in which the planet and star formed. These atmospheres consist mostly of the primordial, lighter-weight gases hydrogen and helium, the most plentiful elements in the universe. This atmosphere dissipates as a planet moves closer to its star.

- "The hot Jupiters are mostly made of hydrogen, and Hubble is very sensitive to hydrogen, so we know these planets can lose the gas relatively easily," Sing said. "But in the case of WASP-121b, the hydrogen and helium gas is outflowing, almost like a river, and is dragging these metals with them. It's a very efficient mechanism for mass loss."

- The results will appear online today in The Astronomical Journal. 11)

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• 26 July 2019: Every now and then, the NASA/ESA Hubble Space Telescope glimpses a common object — say, a spiral galaxy — in an interesting or unusual way. A sharply angled perspective, such as the one shown in this Hubble image, can make it seem as if we, the viewers, are craning our necks to see over a barrier into the galaxy's bright center. 12) 13)

- NGC 3169 is located about 70 million light-years away in the constellation of Sextans (The Sextant). It is part of the Leo I Group of galaxies, which, like the Local Group that houses our home galaxy, the Milky Way, is part of a larger galactic congregation known as the Virgo Supercluster.

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Figure 12: In the case of NGC 3169, this barrier is the thick dust embedded within the galaxy's spiral arms. Cosmic dust comprises a potpourri of particles, including water ice, hydrocarbons, silicates, and other solid material. It has many origins and sources, from the leftovers of star and planet formation to molecules modified over millions of years by interactions with starlight (image credit: ESA/Hubble & NASA, L. Ho; CC BY 4.0)

• 16 July 2019: Astronomers have made a new measurement of how fast the universe is expanding, using an entirely different kind of star than previous endeavors. The revised measurement, which comes from NASA's Hubble Space Telescope, falls in the center of a hotly debated question in astrophysics that may lead to a new interpretation of the universe's fundamental properties. 14)

- Scientists have known for almost a century that the universe is expanding, meaning the distance between galaxies across the universe is becoming ever more vast every second. But exactly how fast space is stretching, a value known as the Hubble constant, has remained stubbornly elusive.

- Now, University of Chicago professor Wendy Freedman and colleagues have a new measurement for the rate of expansion in the modern universe, suggesting the space between galaxies is stretching faster than scientists would expect. Freedman's is one of several recent studies that point to a nagging discrepancy between modern expansion measurements and predictions based on the universe as it was more than 13 billion years ago, as measured by the European Space Agency's Planck satellite.

- As more research points to a discrepancy between predictions and observations, scientists are considering whether they may need to come up with a new model for the underlying physics of the universe in order to explain it.

- "The Hubble constant is the cosmological parameter that sets the absolute scale, size and age of the universe; it is one of the most direct ways we have of quantifying how the universe evolves," said Freedman. "The discrepancy that we saw before has not gone away, but this new evidence suggests that the jury is still out on whether there is an immediate and compelling reason to believe that there is something fundamentally flawed in our current model of the universe."

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Figure 13: These galaxies are selected from a Hubble Space Telescope program to measure the expansion rate of the universe, called the Hubble constant. The value is calculated by comparing the galaxies' distances to the apparent rate of recession away from Earth (due to the relativistic effects of expanding space). - By comparing the apparent brightnesses of the galaxies' red giant stars with nearby red giants, whose distances were measured with other methods, astronomers are able to determine how far away each of the host galaxies are. This is possible because red giants are reliable milepost markers because they all reach the same peak brightness in their late evolution. And, this can be used as a "standard candle" to calculate distance. Hubble's exquisite sharpness and sensitivity allowed for red giants to be found in the stellar halos of the host galaxies. - The red giants were searched for in the halos of the galaxies. The center row shows Hubble's full field of view. The bottom row zooms even tighter into the Hubble fields. The red giants are identified by yellow circles (image credit: NASA/ESA, W. Freedman (University of Chicago), ESO, and the Digitized Sky Survey)

- In a new paper accepted for publication in The Astrophysical Journal, Freedman and her team announced a new measurement of the Hubble constant using a kind of star known as a red giant. Their new observations, made using Hubble, indicate that the expansion rate for the nearby universe is just under 70 km/sec/Mpc (Megaparsec). One parsec is equivalent to 3.26 light-years distance. 15)

- This measurement is slightly smaller than the value of 74 km/sec/Mpc recently reported by the Hubble SH0ES (Supernovae H0 for the Equation of State) team using Cepheid variables, which are stars that pulse at regular intervals that correspond to their peak brightness. This team, led by Adam Riess of the Johns Hopkins University and Space Telescope Science Institute, Baltimore, Maryland, recently reported refining their observations to the highest precision to date for their Cepheid distance measurement technique.

How to Measure Expansion

- A central challenge in measuring the universe's expansion rate is that it is very difficult to accurately calculate distances to distant objects.

- In 2001, Freedman led a team that used distant stars to make a landmark measurement of the Hubble constant. The Hubble Space Telescope Key Project team measured the value using Cepheid variables as distance markers. Their program concluded that the value of the Hubble constant for our universe was 72 km/sec/Mpc.

- But more recently, scientists took a very different approach: building a model based on the rippling structure of light left over from the big bang, which is called the Cosmic Microwave Background. The Planck measurements allow scientists to predict how the early universe would likely have evolved into the expansion rate astronomers can measure today. Scientists calculated a value of 67.4 km/sec/Mpc, in significant disagreement with the rate of 74.0 km/sec/Mpc measured with Cepheid stars.

- Astronomers have looked for anything that might be causing the mismatch. "Naturally, questions arise as to whether the discrepancy is coming from some aspect that astronomers don't yet understand about the stars we're measuring, or whether our cosmological model of the universe is still incomplete," Freedman said. "Or maybe both need to be improved upon."

- Freedman's team sought to check their results by establishing a new and entirely independent path to the Hubble constant using an entirely different kind of star.

- Certain stars end their lives as a very luminous kind of star called a red giant, a stage of evolution that our own Sun will experience billions of years from now. At a certain point, the star undergoes a catastrophic event called a helium flash, in which the temperature rises to about 100 million degrees and the structure of the star is rearranged, which ultimately dramatically decreases its luminosity. Astronomers can measure the apparent brightness of the red giant stars at this stage in different galaxies, and they can use this as a way to tell their distance.

- The Hubble constant is calculated by comparing distance values to the apparent recessional velocity of the target galaxies — that is, how fast galaxies seem to be moving away. The team's calculations give a Hubble constant of 69.8 km/sec/Mpc — straddling the values derived by the Planck and Riess teams.

- "Our initial thought was that if there's a problem to be resolved between the Cepheids and the Cosmic Microwave Background, then the red giant method can be the tie-breaker," said Freedman.

- But the results do not appear to strongly favor one answer over the other say the researchers, although they align more closely with the Planck results.

- NASA's upcoming mission, the Wide Field Infrared Survey Telescope (WFIRST), scheduled to launch in the mid-2020s, will enable astronomers to better explore the value of the Hubble constant across cosmic time. WFIRST, with its Hubble-like resolution and 100 times greater view of the sky, will provide a wealth of new Type Ia supernovae, Cepheid variables, and red giant stars to fundamentally improve distance measurements to galaxies near and far.

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

In 1924, American astronomer Edwin Hubble announced that he discovered galaxies outside of our Milky Way by using the powerful new Hooker telescope perched above Los Angeles. By measuring the distances to these galaxies, he realized the farther away a galaxy is, the faster it appears to be receding from us. This was incontrovertible evidence the universe is uniformly expanding in all directions. This was a big surprise, even to Albert Einstein, who predicted a well-balanced, static universe. The expansion rate is the basis of the Hubble constant. It is a sought-after value because it yields clues to the origin, age, evolution, and future fate of our universe.

For nearly the past century astronomers have worked meticulously to precisely measure the Hubble constant. Before the Hubble Space Telescope was launched in 1990, the universe's age was thought to lie between 10 and 20 billion years, based on different estimates of the Hubble constant. Improving this value was one of the biggest justifications for building the Hubble telescope. This paid off in the early 1990s when a team led by Wendy Freedman of the University of Chicago greatly refined the Hubble constant value to a precision of 10%. This was possible because the Hubble telescope is so sharp at finding and measuring Cepheid variable stars as milepost markers — just as Edwin Hubble did 70 years earlier.

But astronomers strive for ever greater precision, and this requires further refining yardsticks for measuring vast intergalactic distances of billions of light-years. Freedman's latest research looks at aging red giant stars in nearby galaxies. They are also milepost markers because they all reach the same peak brightness at a critical stage of their late evolution. This can be used to calculate distances.

Freedman's research is one of several recent studies that point to a nagging discrepancy between the universe's modern expansion rate and predictions based on the universe as it was more than 13 billion years ago, as measured by the European Space Agency's Planck satellite. This latest measurement offers new evidence suggesting that there may be something fundamentally flawed in the current model of the universe.

Table 1: Red giant stars used as milepost markers

• 11 July 2019: Astronomers using the NASA/ESA Hubble Space Telescope have observed an unexpected thin disc of material encircling a supermassive black hole at the heart of the spiral galaxy NGC 3147, located 130 million light-years away. 16) 17) 18)

- The presence of the black hole disc in such a low-luminosity active galaxy has astronomers surprised. Black holes in certain types of galaxies such as NGC 3147 are considered to be starving as there is insufficient gravitationally captured material to feed them regularly. It is therefore puzzling that there is a thin disc encircling a starving black hole that mimics the much larger discs found in extremely active galaxies.

- Of particular interest, this disc of material circling the black hole offers a unique opportunity to test Albert Einstein's theories of relativity. The disc is so deeply embedded in the black hole's intense gravitational field that the light from the gas disc is altered, according to these theories, giving astronomers a unique peek at the dynamic processes close to a black hole.

- "We've never seen the effects of both general and special relativity in visible light with this much clarity," said team member Marco Chiaberge of AURA (Association of Universities for Research in Astronomy) for ESA, STScI and Johns Hopkins University.

- The disc's material was measured by Hubble to be whirling around the black hole at more than 10% of the speed of light. At such extreme velocities, the gas appears to brighten as it travels toward Earth on one side, and dims as it speeds away from our planet on the other. This effect is known as relativistic beaming. Hubble's observations also show that the gas is embedded so deep in a gravitational well that light is struggling to escape, and therefore appears stretched to redder wavelengths. The black hole's mass is around 250 million times that of the Sun.

- "This is an intriguing peek at a disc very close to a black hole, so close that the velocities and the intensity of the gravitational pull are affecting how we see the photons of light," explained the study's first author, Stefano Bianchi, of Università degli Studi Roma Tre in Italy.

- In order to study the matter swirling deep inside this disc, the researchers used the Hubble Space Telescope Imaging Spectrograph (STIS) instrument. This diagnostic tool divides the light from an object into its many individual wavelengths to determine the object's speed, temperature, and other characteristics at very high precision. STIS was integral to effectively observing the low-luminosity region around the black hole, blocking out the galaxy's brilliant light.

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Figure 14: Artist's impression of the peculiar thin disc of material circling a supermassive black hole at the heart of the spiral galaxy NGC 3147, located 130 million light-years away (image credit: ESA/Hubble, M. Kornmesser)

- The astronomers initially selected this galaxy to validate accepted models about lower-luminosity active galaxies: those with malnourished black holes. These models predict that discs of material should form when ample amounts of gas are trapped by a black hole's strong gravitational pull, subsequently emitting lots of light and producing a brilliant beacon called a quasar.

- "The type of disc we see is a scaled-down quasar that we did not expect to exist," Bianchi explained. "It's the same type of disc we see in objects that are 1000 or even 100 000 times more luminous. The predictions of current models for very faint active galaxies clearly failed."

- The team hopes to use Hubble to hunt for other very compact discs around low-luminosity black holes in similar active galaxies.

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Figure 15: A Hubble Space Telescope image of the spiral galaxy NGC 3147 appears next to an artist's illustration of the supermassive black hole residing at the galaxy's core. The Hubble image shows off the galaxy's sweeping spiral arms, full of young blue stars, pinkish nebulas, and dust in silhouette. However, at the brilliant core of NGC 3147 lurks a monster black hole, weighing about 250 million times the mass of our Sun. Hubble observations of the black hole demonstrate two of Einstein's theories of relativity. The reddish-yellow features swirling around the center are the glow of light from gas trapped by the hefty black hole's powerful gravity. The black hole is embedded deep within its gravitational field, shown by the green grid that illustrates warped space. The gravitational field is so strong that light is struggling to climb out, a principal described in Einstein's theory of general relativity. Material also is whipping so fast around the black hole that it brightens as it approaches Earth on one side of the disk and gets fainter as it moves away. This effect, called relativistic beaming, was predicted by Einstein's theory of special relativity. NGC 3147 is located 130 million light-years away in the northern circumpolar constellation Draco the Dragon [image credit: Hubble Image: NASA, ESA, S. Bianchi (Università degli Studi Roma Tre University), A. Laor (Technion-Israel Institute of Technology), & M. Chiaberge (ESA, STScI, and JHU); illustration: NASA, ESA, and A. Feild and L. Hustak (STScI)]

• 08 July 2019: NGC 1156 is located in the constellation of Aries (The Ram). It is classified as a dwarf irregular galaxy, meaning that it lacks a clear spiral or rounded shape, as other galaxies have, and is on the smaller side, albeit with a relatively large central region that is more densely packed with stars. 19)

- Some pockets of gas within NGC 1156 rotate in the opposite direction to the rest of the galaxy, suggesting that there has been a close encounter with another galaxy in NGC 1156's past. The gravity of this other galaxy — and the turbulent chaos of such an interaction — could have scrambled the likely more orderly rotation of material within NGC 1156, producing the odd behavior we see today.

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Figure 16: The galaxy NGC 1156 resembles a delicate cherry blossom tree flowering in springtime in this Hubble Picture of the Week. The many bright "blooms" within the galaxy are in fact stellar nurseries — regions where new stars are springing to life. Energetic light emitted by newborn stars in these regions streams outwards and encounters nearby pockets of hydrogen gas, causing it to glow with a characteristic pink hue (image credit: ESA/Hubble, NASA, R. Jansen; CC BY 4.0)

• 02 July 2019: Two NASA space telescopes have teamed up to identify, for the first time, the detailed chemical "fingerprint" of a planet between the sizes of Earth and Neptune. No planets like this can be found in our own solar system, but they are common around other stars. 20)

- The planet, Gliese 3470 b (also known as GJ 3470 b), may be a cross between Earth and Neptune, with a large rocky core buried under a deep, crushing hydrogen-and-helium atmosphere. Weighing in at 12.6 Earth masses, the planet is more massive than Earth but less massive than Neptune (which is more than 17 Earth masses).

- Many similar worlds have been discovered by NASA's Kepler space observatory, whose mission ended in 2018. In fact, 80% of the planets in our galaxy may fall into this mass range. However, astronomers have never been able to understand the chemical nature of such a planet until now, researchers say.

- By inventorying the contents of GJ 3470 b's atmosphere, astronomers are able to uncover clues about the planet's nature and origin.

- "This is a big discovery from the planet-formation perspective. The planet orbits very close to the star and is far less massive than Jupiter - 318 times Earth's mass - but has managed to accrete the primordial hydrogen/helium atmosphere that is largely 'unpolluted' by heavier elements," said Björn Benneke of the University of Montreal in Canada. "We don't have anything like this in the solar system, and that's what makes it striking."

- Astronomers enlisted the combined multi-wavelength capabilities NASA's Hubble and Spitzer space telescopes to do a first-of-a-kind study of GJ 3470 b's atmosphere.

- This was accomplished by measuring the absorption of starlight as the planet passed in front of its star (transit) and the loss of reflected light from the planet as it passed behind the star (eclipse). All told, the space telescopes observed 12 transits and 20 eclipses. The science of analyzing chemical fingerprints based on light is called "spectroscopy."

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Figure 17: This artist's illustration shows the theoretical internal structure of the exoplanet GJ 3470 b. It is unlike any planet found in the Solar System. Weighing in at 12.6 Earth masses the planet is more massive than Earth but less massive than Neptune. Unlike Neptune, which is 3 billion miles from the Sun, GJ 3470 b may have formed very close to its red dwarf star as a dry, rocky object. It then gravitationally pulled in hydrogen and helium gas from a circumstellar disk to build up a thick atmosphere. The disk dissipated many billions of years ago, and the planet stopped growing. The bottom illustration shows the disk as the system may have looked long ago. Observation by NASA's Hubble and Spitzer space telescopes have chemically analyzed the composition of GJ 3470 b's very clear and deep atmosphere, yielding clues to the planet's origin. Many planets of this mass exist in our galaxy [image credit: NASA, ESA, and L. Hustak (STScI)]

- "For the first time we have a spectroscopic signature of such a world," said Benneke. But he is at a loss for classification: Should it be called a "super-Earth" or "sub-Neptune?" Or perhaps something else?

- Fortuitously, the atmosphere of GJ 3470 b turned out to be mostly clear, with only thin hazes, enabling the scientists to probe deep into the atmosphere.

- "We expected an atmosphere strongly enriched in heavier elements like oxygen and carbon which are forming abundant water vapor and methane gas, similar to what we see on Neptune," said Benneke. "Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen/helium-rich composition of the Sun."

- Other exoplanets, called "hot Jupiters," are thought to form far from their stars and over time migrate much closer. But this planet seems to have formed just where it is today, said Benneke.

- The most plausible explanation, according to Benneke, is that GJ 3470 b was born precariously close to its red dwarf star, which is about half the mass of our Sun. He hypothesizes that essentially it started out as a dry rock and rapidly accreted hydrogen from a primordial disk of gas when its star was very young. The disk is called a "protoplanetary disk."

- "We're seeing an object that was able to accrete hydrogen from the protoplanetary disk but didn't run away to become a hot Jupiter," said Benneke. "This is an intriguing regime."

- One explanation is that the disk dissipated before the planet could bulk up further. "The planet got stuck being a sub-Neptune," said Benneke.

- NASA's upcoming James Webb Space Telescope will be able to probe even deeper into GJ 3470 b's atmosphere, thanks to Webb's unprecedented sensitivity in the infrared. The new results have already spawned great interest from American and Canadian teams developing the instruments on Webb. They will observe the transits and eclipses of GJ 3470 b at light wavelengths where the atmospheric hazes become increasingly transparent.

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

- The Jet Propulsion Laboratory in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena. Space operations are based at Lockheed Martin Space Systems in Littleton, Colorado. Data are archived at the Infrared Science Archive housed at IPAC at Caltech. Caltech manages JPL for NASA.

• 01 July 2019: Imagine slow-motion fireworks that started exploding nearly two centuries ago and haven't stopped since then. This is how you might describe this double star system located 7500 light-years away in the constellation Carina (The Ship's Keel). In 1838 Eta Carinae underwent a cataclysmic outburst called the Great Eruption, quickly escalating to become in 1844 the second brightest star in the sky by April of that year. The star has since faded, but this new view from the NASA/ESA Hubble Space Telescope shows that the spectacular display is still ongoing, and reveals details that have never been seen before. 21)

- Violent mass ejections are not uncommon in Eta Carinae's history; the system has been blighted by chaotic eruptions, often blasting parts of itself into space But the Great Eruption was particularly dramatic. The larger of the two stars is a massive, unstable star nearing the end of its life, and what astronomers witnessed over a century and a half ago was, in fact, a stellar near-death experience.

- The resulting surge of light was outshone only by Sirius, which is almost one thousand times closer to Earth, and for a time made Eta Carinae an important navigation star for mariners in the southern seas. This close call stopped just short of destroying Eta Carinae, and the light intensity gradually subsided. Researchers studying the star today can still see the signature of the Great Eruption on its surroundings; the huge dumbbell shape is formed of the dust and gas and other filaments that were hurled into space in the expulsion. These hot glowing clouds are known as the Homunculus Nebula, and have been a target of Hubble since its launch in 1990.

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Figure 18: Hubble offers a special view of the double star system Eta Carinae's expanding gases glowing in red, white, and blue. This is the highest resolution image of Eta Carinae taken by the NASA/ESA Hubble Space Telescope [image credit: NASA/ESA, N. Smith (University of Arizona, Tucson), and J. Morse (BoldlyGo Institute, New York)]

- In fact, the volatile star has been imaged by almost every instrument on Hubble over more than 25 years. Astronomers have observed the cosmic drama play out in ever higher resolution. This latest image was created using Hubble's WFC3 (Wide Field Camera 3) to map warm magnesium gas glowing in ultraviolet light (shown in blue).

- Scientists have long known that the outer material thrown off in the 1840s eruption has been heated by shock waves generated when it crashed into material previously ejected from the star . The team who captured this new image were expecting to find light from magnesium coming from the complicated array of filaments seen in the light from glowing nitrogen (shown in red). Instead, a whole new luminous magnesium structure was found in the space between the dusty bipolar bubbles and the outer shock-heated nitrogen-rich filaments.

- We've discovered a large amount of warm gas that was ejected in the Great Eruption but hasn't yet collided with the other material surrounding Eta Carinae," explained Nathan Smith of Steward Observatory at the University of Arizona, lead investigator of the Hubble program. "Most of the emission is located where we expected to find an empty cavity. This extra material is fast, and it ‘ups the ante' in terms of the total energy of an already powerful stellar blast."

- This newly revealed data is important for understanding how the eruption began, because it represents the fast and energetic ejection of material that may have been expelled by the star shortly before the expulsion of the rest of the nebula. Astronomers need more observations to measure exactly how fast the material is moving and when it was ejected.

- Another striking feature of the image is the streaks visible in the blue region outside the lower-left bubble. These streaks appear where the star's light rays poke through the dust clumps scattered along the bubble's surface. Wherever the ultraviolet light strikes the dense dust, it leaves a long thin shadow that extends beyond the lobe into the surrounding gas. "The pattern of light and shadow is reminiscent of sunbeams that we see in our atmosphere when sunlight streams past the edge of a cloud, though the physical mechanism creating Eta Carinae's light is different," noted team member Jon Morse of BoldlyGo Institute in New York.

- This technique of searching in ultraviolet light for warm gas could be used to study other stars and gaseous nebulae, the researchers say.

- "We had used Hubble for decades to study Eta Carinae in visible and infrared light, and we thought we had a pretty full account of its ejected debris. But this new ultraviolet-light image looks astonishingly different, revealing gas we did not see in either visible-light or infrared images," Smith said. "We're excited by the prospect that this type of ultraviolet magnesium emission may also expose previously hidden gas in other types of objects that eject material, such as protostars or other dying stars; and only Hubble can take these kinds of pictures".

- The causes of Eta Carinae's Great Eruption remain the subject of speculation and debate. A recent theory suggests that Eta Carinae, which may once have weighed as much as 150 Suns, started out as a triple system, and the 1840s mass ejection was triggered when the primary star devoured one of its companions, rocketing more than ten times the mass of our Sun into space. While the exact circumstances of that show-stopping burst of light remain a mystery for now, astronomers are more certain of how this cosmic light show will conclude. Eta Carinae's fireworks display is fated to reach its finale when it explodes as a supernova, greatly surpassing even its last powerful outburst. This may already have happened, but the tsunami of light from such a blinding blast would take 7500 years to reach Earth.

• 24 June 2019: Have you ever been looking for one thing – at home or while browsing the web for example – and accidentally stumbled upon something else, but that is just as interesting? Something similar happened to the NASA/ESA Hubble Space Telescope a couple of years ago. While observing distant galaxies lying billions of light-years away, the telescope serendipitously spotted several asteroids, small Solar System objects that reside ‘only' a few tens to hundreds of millions of kilometers from Earth. 22)

- Asteroids are mainly found in an area called the ‘main belt', between the orbits of Mars and Jupiter. More than 700 000 asteroids have been identified to date, and predictions indicate that many more might be out there, each left over from the early days when planets were taking shape around the Sun.

- The curved or S-shaped streaks in this image are trails created by asteroids as they move along their orbits. Rather than leaving one long trail, the asteroids appear in multiple Hubble exposures that have been combined into one image. The image shows a total of twenty asteroid trails, belonging to seven unique objects; five of these were new discoveries – too faint to be seen previously.

- This week, a team of astronomers, planetary scientists and software engineers based at ESA and other research institutes has launched a new citizen science project: the Hubble Asteroid Hunter. The project was developed as part of the Zooniverse – the world's largest and most popular platform for people-powered research.

- The new project features a collection of archival Hubble images where calculations indicate that an asteroid might have been crossing the field of view at the time of the observation. Everyone can participate! By identifying the asteroids potentially present in these images and marking the exact position of their trails, you too can help the team improve the asteroid orbit determination and better characterize these objects. Precise knowledge of the orbit is particularly important for so-called near-Earth asteroids, those potentially flying close to our planet.

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Figure 19: This image was taken as part of the Frontier Fields program, a Hubble initiative to push the telescope's limits, observing six massive galaxy clusters – huge cosmic objects comprising hundreds of galaxies along with hot gas and dark matter – and exploiting their effect as a gravitational ‘lens' on background sources to capture light from extremely distant galaxies (image credit: NASA, ESA, and B. Sunnquist and J. Mack (STScI); CC BY 4.0; Acknowledgment: NASA, ESA, and J. Lotz (STScI) and the HFF Team)

- While observing each cluster with one of the cameras on Hubble, the team also used a different camera, pointing in a slightly different direction, to photograph six so-called ‘parallel fields'. This maximized Hubble's observational efficiency in doing deep space exposures, imaging a myriad of far away galaxies.

- The picture of Figure 19, first published in 2017, shows the parallel field for the galaxy cluster Abell 370. It was assembled from images taken in visible and infrared light and contains thousands of galaxies, including massive yellowish ellipticals and majestic blue spirals. Much smaller, fragmentary blue galaxies are sprinkled throughout the field. The reddest objects are most likely the farthest galaxies, whose light has been stretched into the red part of the spectrum by the expansion of space.

- The position of this field on the sky is near the ecliptic, the plane of our Solar System. This is the region in which most asteroids orbit the Sun, which is why Hubble astronomers saw so many crossings. Hubble deep-sky observations taken along a line-of-sight near the plane of our Solar System commonly record asteroid trails.

- Each year on 30 June, the worldwide UN-sanctioned Asteroid Day takes place to raise awareness about asteroids and what can be done to protect Earth from possible impact. The day falls on the anniversary of the Tunguska event that took place on 30 June 1908, the most harmful known asteroid related event in recent history. Follow the 48-hour Asteroid Day livecast from https://asteroidday.org/ this weekend, and join the conversation online via #AsteroidDay2019.

• 21 June 2019: This image shows an irregular galaxy named IC 10, a member of the Local Group — a collectiongrouping of over 50 galaxies within our cosmic neighborhood that includes the Milky Way. 23)

- IC 10 is a remarkable object. It is the closest-known starburst galaxy to us, meaning that it is undergoing a furious bout of star formation fueled by ample supplies of cool hydrogen gas. This gas condensescongeals into vast molecular clouds, which then formcondense into dense knots where pressures and temperatures reach a point sufficient to ignite nuclear fusion, thus giving rise to new generations of stars.

- A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Nikolaus Sulzenauer, and went on to win tenth prize.

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Figure 20: Spiral, elliptical, irregular. As an irregular galaxy, IC 10 lacks the majestic shape of spiral galaxies such as the Milky Way, or the rounded, ethereal appearance of elliptical galaxies. It is a faint object, despite its relative proximity to us — just of 2.2 million light-years. In fact, IC 10 only became known to humankind in 1887, when American astronomer Lewis Swift spotted it during an observing campaign. The small galaxy remains difficult to study even today, because it is located along a line -of -sight which is chock-full of cosmic dust and stars (image credit: NASA, ESA and F. Bauer; CC BY 4.0)

• 13 June 2019: Located about 30 million light-years away in the constellation of Pyxis (The Compass), ESO 495-21 is a dwarf starburst galaxy — this means that it is small in size, but ablaze with rapid bursts of star formation. Starburst galaxies form stars at exceptionally high rates, creating stellar newborns of up to 1000 times faster than the Milky Way. 24)

- Hubble has studied the bursts of activity within ESO 495-21 several times. Notably, the space telescope has explored the galaxy's multiple super star clusters, very dense regions only a few million years old and packed with massive stars. These spectacular areas can have a huge impact on their host galaxies. Studying them allows astronomers to investigate the earliest stages of their evolution, in a bid to understand how massive stars form and change throughout the Universe.

- As well as hosting the cosmic fireworks that are super star clusters, ESO 495-21 also may harbor a supermassive black hole at its core. Astronomers know that almost every large galaxy hosts such an object at its center, and, in general, the bigger the galaxy, the more massive the black hole. Our home galaxy, the Milky Way, houses a supermassive black hole, Sagittarius A*, which is over four million times as massive as the Sun. ESO 495-21, also known as Henize 2-10) is a dwarf galaxy, only three percent the size of the Milky Way, and yet there are indications that the black hole at its core is over a million times as massive as the Sun — an extremely unusual scenario.

- This black hole may offer clues as to how black holes and galaxies evolved in the early Universe. The origin of the central supermassive black holes in galaxies is still a matter of debate — do the galaxies form first and then crush material at their centers into black holes, or do pre-existing black holes gather galaxies around them? Do they evolve together — or could the answer be something else entirely?

- With its small size, indistinct shape, and rapid starburst activity, astronomers think ESO 495-21 may be an analogue for some of the first galaxies to have formed in the cosmos. Finding a black hole at the galaxy's heart is therefore a strong indication that black holes may have formed first, with galaxies later developing and evolving around them.

- The data comprising this image were gathered by two of the instruments aboard the NASA/ESA Hubble Space Telescope: the Advanced Camera for Surveys and already decommissioned Wide Field Planetary Camera 2.

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Figure 21: Nestled within this field of bright foreground stars lies ESO 495-21, a tiny galaxy with a big heart. ESO 495-21 may be just 3000 light-years across, but that is not stopping the galaxy from furiously forming huge numbers of stars. It may also host a supermassive black hole; this is unusual for a galaxy of its size, and may provide intriguing hints as to how galaxies form and evolve (image credit: NASA, ESA, W. Vacca)

• 07 June 2019: This striking image was taken by the NASA/ESA Hubble Space Telescope's WFC3 (Wide Field Camera 3), a powerful instrument installed on the telescope in 2009. WFC3 is responsible for many of Hubble's most breathtaking and iconic photographs, including Pictures of the Week. 25) 26)

- By studying galactic specimens such as NGC 7773 throughout the Universe, researchers hope to learn more about the processes that have shaped — and continue to shape — our cosmic home.

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Figure 22: Our galaxy, the Milky Way, is thought to be a barred spiral like NGC 7773. Shown here, NGC 7773 is a beautiful example of a barred spiral galaxy in the constellation Pegasus. A luminous bar-shaped structure cuts prominently through the galaxy's bright core, extending to the inner boundary of NGC 7773's sweeping, pinwheel-like spiral arms. Astronomers think that these bar structures emerge later in the lifetime of a galaxy, as star-forming material makes its way towards the galactic center — younger spirals do not feature barred structures as often as older spirals do, suggesting that bars are a sign of galactic maturity. They are also thought to act as stellar nurseries, as they gleam brightly with copious numbers of youthful stars (image credit: ESA/Hubble & NASA, J. Walsh)

• 03 June 2019: Astronomers have directly imaged two exoplanets that are gravitationally carving out a wide gap within a planet-forming disk surrounding a young star. While over a dozen exoplanets have been directly imaged, this is only the second multi-planet system to be photographed. (The first was a four-planet system orbiting the star HR 8799.) Unlike HR 8799, though, the planets in this system are still growing by accreting material from the disk. 27)

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Figure 23: This artist's illustration shows two gas giant exoplanets orbiting the young star PDS 70. These planets are still growing by accreting material from a surrounding disk. In the process, they have gravitationally carved out a large gap in the disk. The gap extends from distances equivalent to the orbits of Uranus and Neptune in our solar system (image credit: STScI, J. Olmsted)

- "This is the first unambiguous detection of a two-planet system carving a disk gap," said Julien Girard of the Space Telescope Science Institute in Baltimore, Maryland.

- The host star, known as PDS 70, is located about 370 light-years from Earth. The young 6-million-year-old star is slightly smaller and less massive than our Sun, and is still accreting gas. It is surrounded by a disk of gas and dust that has a large gap extending from about 1.9 to 3.8 billion miles.

- PDS 70 b, the innermost known planet, is located within the disk gap at a distance of about 2 billion miles from its star, similar to the orbit of Uranus in our solar system. The team estimates that it weighs anywhere from 4 to 17 times as much as Jupiter. It was first detected in 2018.

- PDS 70 c, the newly discovered planet, is located near the outer edge of the disk gap at about 3.3 billion miles from the star, similar to Neptune's distance from our Sun. It is less massive than planet b, weighing between 1 and 10 times as much as Jupiter. The two planetary orbits are near a 2-to-1 resonance, meaning that the inner planet circles the star twice in the time it takes the outer planet to go around once.

- The discovery of these two worlds is significant because it provides direct evidence that forming planets can sweep enough material out of a protoplanetary disk to create an observable gap.

- "With facilities like ALMA, Hubble, or large ground-based optical telescopes with adaptive optics we see disks with rings and gaps all over. The open question has been, are there planets there? In this case, the answer is yes," explained Girard.

- The team detected PDS 70 c from the ground, using the MUSE spectrograph on the European Southern Observatory's Very Large Telescope (VLT). Their new technique relied on the combination of the high spatial resolution provided by the 8-meter telescope equipped with four lasers and the instrument's medium spectral resolution that allows it to "lock onto" light emitted by hydrogen, which is a sign of gas accretion.

- "This new observing mode was developed to study galaxies and star clusters at higher spatial resolution. But this new mode also makes it suitable for exoplanet imaging, which was not the original science driver for the MUSE instrument," said Sebastiaan Haffert of Leiden Observatory, lead author on the paper. "We were very surprised when we found the second planet," Haffert added. 28)

- In the future, NASA's James Webb Space Telescope may be able to study this system and other planet nurseries using a similar spectral technique to narrow in on various wavelengths of light from hydrogen. This would allow scientists to measure the temperature and density of gas within the disk, which would help our understanding of the growth of gas giant planets. The system might also be targeted by the WFIRST mission, which will carry a high-performance coronagraph technology demonstration that can block out the star's light to reveal fainter light from the surrounding disk and companion planets.

• 31 May 2019: This luminous orb is the galaxy NGC 4621, better known as Messier 59 (Figure 24). As this latter moniker indicates, the galaxy is listed in the famous catalog of deep-sky objects compiled by French comet-hunter Charles Messier in the 18th century. However, German astronomer Johann Gottfried Koehler is credited with discovering the galaxy just days before Messier added it to his collection in 1779. 29)

- Located in the 2,000-strong Virgo cluster of galaxies within the constellation of Virgo (the Virgin), Messier 59 lies approximately 50 million light-years away from us. This image was taken by the NASA/ESA Hubble Space Telescope's Advanced Camera for Surveys.

- Messier 59 is featured in Hubble's Messier catalog, which includes some of the most fascinating objects that can be observed from Earth's Northern Hemisphere. See the NASA-processed image and other Messier objects at: https://www.nasa.gov/content/goddard/hubble-s-messier-catalog

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Figure 24: Modern observations show that Messier 59 is an elliptical galaxy, one of the three main kinds of galaxies along with spirals and irregulars. Ellipticals tend to be the most evolved of the trio, full of old, red stars and exhibiting little or no new star formation. Messier 59, however, bucks this trend somewhat; the galaxy does show signs of star formation, with some newborn stars residing within a disk near the core (image credit: ESA/Hubble & NASA, P. Cote)

• 16 May 2019: The irregular galaxy NGC 4485 has been involved in a dramatic gravitational interplay with its larger galactic neighbor NGC 4490 — out of frame to the bottom right in this image. Found about 30 million light-years away in the constellation of Canes Venatici (the Hunting Dogs), the strange result of these interacting galaxies has resulted in an entry in the Atlas of Peculiar galaxies: Arp 269. 30)

- Having already made their closest approach, NGC 4485 and NGC 4490 are now moving away from each other, vastly altered from their original states. Still engaged in a destructive yet creative dance, the gravitational force between them continues to warp each of them out of all recognition, while at the same time creating the conditions for huge regions of intense star formation.

- This galactic tug-of-war has created a stream of material about 25,000 light-years long which connects the two galaxies. The stream is made up of bright knots and huge pockets of gassy regions, as well as enormous regions of star formation in which young, massive, blue stars are born. Short-lived, however, these stars quickly run out of fuel and end their lives in dramatic explosions. While such an event seems to be purely destructive, it also enriches the cosmic environment with heavier elements and delivers new material to form a new generation of stars.

- Two very different regions are now apparent in NGC 4485; on the left are hints of the galaxy's previous spiral structure, which was at one time undergoing "normal" galactic evolution. The right of the image reveals a portion of the galaxy ripped towards its larger neighbor, bursting with hot, blue stars and streams of dust and gas.

- This image, captured by the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope, adds light through two new filters compared with an image released in 2014. The new data provide further insights into the complex and mysterious field of galaxy evolution.

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Figure 25: The NASA/ESA Hubble Space Telescope has taken a new look at the spectacular irregular galaxy NGC 4485, which has been warped and wound by its larger galactic neighbor. The gravity of the second galaxy has disrupted the ordered collection of stars, gas and dust, giving rise to an erratic region of newborn, hot, blue stars and chaotic clumps and streams of dust and gas (image credit: ESA, NASA)

• 10 May 2019: Dotted across the sky in the constellation of Pictor (The Painter's Easel) is the galaxy cluster highlighted here by the NASA/ESA Hubble Space Telescope: SPT-CL J0615-5746, or SPT0615 for short. First discovered by the South Pole Telescope less than a decade ago, SPT0615 is exceptional among the myriad clusters so far catalogued in our map of the Universe — it is the highest-redshift cluster for which a full, strong lens model is published. 31)

- SPT0615 is a massive cluster of galaxies, one of the farthest observed to cause gravitational lensing. Gravitational lensing occurs when light from a background object is deflected around mass between the object and the observer. Among the identified background objects, there is SPT0615-JD, a galaxy that is thought to have emerged just 500 million years after the Big Bang. This puts it among the very earliest structures to form in the Universe. It is also the farthest galaxy ever imaged by means of gravitational lensing.

- Just as ancient paintings can tell us about the period of history in which they were painted, so too can ancient galaxies tell us about the era of the Universe in which they existed. To learn about cosmological history, astronomers explore the most distant reaches of the Universe, probing ever further out into the cosmos. The light from distant objects travels to us from so far away that it takes an immensely long time to reach us, meaning that it carries information from the past — information about the time at which it was emitted.

- By studying such distant objects, astronomers are continuing to fill the gaps in our picture of what the very early Universe looked like, and uncover more about how it evolved into its current state.

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Figure 26: Hubble Image of the Week: Distant and Ancient (image credit: ESA/Hubble & NASA, I. Karachentsev et al., F. High et al.CC BY 4.0)

• 03 May 2019: Few of the universe's residents are as iconic as the spiral galaxy. These limelight-hogging celestial objects combine whirling, pinwheeling arms with scatterings of sparkling stars, glowing bursts of gas, and dark, weaving lanes of cosmic dust, creating truly awesome scenes — especially when viewed through a telescope such as the NASA/ESA Hubble Space Telescope. In fact, this image from Hubble frames a perfect spiral specimen: the stunning NGC 2903. 32)

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Figure 27: NGC 2903 is located about 30 million light-years away in the constellation of Leo (the Lion), and was studied as part of a Hubble survey of the central regions of roughly 145 nearby disk galaxies. This study aimed to help astronomers better understand the relationship between the black holes that lurk at the cores of galaxies like these, and the rugby-ball-shaped bulge of stars, gas and dust at the galaxy's center — such as that seen in this image (image credit: ESA/Hubble & NASA, L. Ho et al.)

02 May 2019: Astronomers have put together the largest and most comprehensive "history book" of galaxies into one single image, using 16 years' worth of observations from NASA's Hubble Space Telescope. 33)

- The deep-sky mosaic, created from nearly 7,500 individual exposures, provides a wide portrait of the distant universe, containing 265,000 galaxies that stretch back through 13.3 billion years of time to just 500 million years after the big bang. The faintest and farthest galaxies are just one ten-billionth the brightness of what the human eye can see. The universe's evolutionary history is also chronicled in this one sweeping view. The portrait shows how galaxies change over time, building themselves up to become the giant galaxies seen in the nearby universe.

- This ambitious endeavor, called the Hubble Legacy Field, also combines observations taken by several Hubble deep-field surveys, including the eXtreme Deep Field (XDF), the deepest view of the universe. The wavelength range stretches from ultraviolet to near-infrared light, capturing the key features of galaxy assembly over time.

- "Now that we have gone wider than in previous surveys, we are harvesting many more distant galaxies in the largest such dataset ever produced by Hubble," said Garth Illingworth of the University of California, Santa Cruz, leader of the team that assembled the image. "This one image contains the full history of the growth of galaxies in the universe, from their time as 'infants' to when they grew into fully fledged 'adults.'"

- No image will surpass this one until future space telescopes are launched. "We've put together this mosaic as a tool to be used by us and by other astronomers," Illingworth added. "The expectation is that this survey will lead to an even more coherent, in-depth and greater understanding of the universe's evolution in the coming years."

- The image yields a huge catalog of distant galaxies. "Such exquisite high-resolution measurements of the numerous galaxies in this catalog enable a wide swath of extragalactic study," said catalog lead researcher Katherine Whitaker of the University of Connecticut, in Storrs. "Often, these kinds of surveys have yielded unanticipated discoveries which have had the greatest impact on our understanding of galaxy evolution."

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Figure 28: This Hubble Space Telescope image represents a portion of the Hubble Legacy Field, one of the widest views of the universe ever made. The image, a combination of thousands of snapshots, represents 16 years' worth of observations. The Hubble Legacy Field includes observations taken by several Hubble deep-field surveys, including the eXtreme Deep Field (XDF), the deepest view of the universe. The wavelength range stretches from ultraviolet to near-infrared light, capturing all the features of galaxy assembly over time. This cropped image mosaic presents a wide portrait of the distant universe and contains roughly 200,000 galaxies. They stretch back through 13.3 billion years of time to just 500 million years after the universe's birth in the big bang (image credit: NASA, ESA, G. Illingworth and D. Magee (University of California, Santa Cruz), K. Whitaker (University of Connecticut), R. Bouwens (Leiden University), P. Oesch (University of Geneva) and the Hubble Legacy Field team)

Figure 29: The video begins with a view of the thousands of galaxies in the Hubble Ultra Deep Field and slowly zooms out to reveal the larger Hubble Legacy Field, containing 265,000 galaxies [video credit: NASA, ESA, G. Illingworth (University of California, Santa Cruz) and G. Bacon (STScI)]

- Galaxies are the "markers of space," as astronomer Edwin Hubble once described them a century ago. Galaxies allow astronomers to trace the expansion of the universe, offer clues to the underlying physics of the cosmos, show when the chemical elements originated, and enable the conditions that eventually led to the appearance of our solar system and life.

- This wider view contains about 30 times as many galaxies as in the previous deep fields. The new portrait, a mosaic of multiple snapshots, covers almost the width of the full Moon. The XDF, which penetrated deeper into space than this wider view, lies in this region, but it covers less than one-tenth of the full Moon's diameter. The Legacy Field also uncovers a zoo of unusual objects. Many of them are the remnants of galactic "train wrecks," a time in the early universe when small, young galaxies collided and merged with other galaxies.

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Figure 30: This graphic compares the dimensions of the Hubble Legacy Field on the sky with the angular size of the Moon. The Hubble Legacy Field is one of the widest views ever taken of the universe with Hubble. The new portrait, a mosaic of nearly 7,500 exposures, covers almost the width of the full Moon. The Moon and the Legacy Field each subtend about an angle of one-half a degree on the sky (or half the width of your forefinger held at arm's length), image credit: Hubble Legacy Field Image: NASA, ESA, and G. Illingworth and D. Magee (University of California, Santa Cruz); Moon Image: NASA, Goddard Space Flight Center and Arizona State University

- Assembling all of the observations was an immense task. The image comprises the collective work of 31 Hubble programs by different teams of astronomers. Hubble has spent more time on this tiny area than on any other region of the sky, totaling more than 250 days, representing nearly three-quarters of a year.

- "Our goal was to assemble all 16 years of exposures into a legacy image," explained Dan Magee, of the University of California, Santa Cruz, the team's data processing lead. "Previously, most of these exposures had not been put together in a consistent way that can be used by any researcher. Astronomers can select the data in the Legacy Field they want and work with it immediately, as opposed to having to perform a huge amount of data reduction before conducting scientific analysis."

- The image, along with the individual exposures that make up the new view, is available to the worldwide astronomical community through the Mikulski Archive for Space Telescopes (MAST). MAST, an online database of astronomical data from Hubble and other NASA missions, is located at the STScI (Space Telescope Science Institute) in Baltimore, Maryland.

- The Hubble Space Telescope has come a long way in taking ever deeper "core samples" of the distant universe. After Hubble's launch in 1990, astronomers debated if it was worth spending a chunk of the telescope's time to go on a "fishing expedition" to take a very long exposure of a small, seemingly blank piece of sky. The resulting Hubble Deep Field image in 1995 captured several thousand unseen galaxies in one pointing. The bold effort was a landmark demonstration and a defining proof-of-concept that set the stage for future deep field images. In 2002, Hubble's Advanced Camera for Surveys went even deeper to uncover 10,000 galaxies in a single snapshot. Astronomers used exposures taken by Hubble's Wide Field Camera 3 (WFC3), installed in 2009, to assemble the eXtreme Deep Field snapshot in 2012. Unlike previous Hubble cameras, the telescope's WFC3 covers a broader wavelength range, from ultraviolet to near-infrared.

- This new image mosaic is the first in a series of Hubble Legacy Field images. The team is working on a second set of images, totaling more than 5,200 Hubble exposures, in another area of the sky. In the future, astronomers hope to broaden the multiwavelength range in the legacy images to include longer-wavelength infrared data and high-energy X-ray observations from two other NASA Great Observatories, the Spitzer Space Telescope and Chandra X-ray Observatory.

- The vast number of galaxies in the Legacy Field image are also prime targets for future telescopes. "This will really set the stage for NASA's planned Wide Field Infrared Survey Telescope (WFIRST)," Illingworth said. "The Legacy Field is a pathfinder for WFIRST, which will capture an image that is 100 times larger than a typical Hubble photo. In just three weeks' worth of observations by WFIRST, astronomers will be able to assemble a field that is much deeper and more than twice as large as the Hubble Legacy Field."

- In addition, NASA's upcoming James Webb Space Telescope will allow astronomers to push much deeper into the legacy field to reveal how the infant galaxies actually grew. Webb's infrared coverage will go beyond the limits of Hubble and Spitzer to help astronomers identify the first galaxies in the universe.

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• 26 April 2019: Messier 75 lies in the constellation of Sagittarius (The Archer), around 67,000 light-years away from Earth. The majority of the cluster's stars, about 400,000 intotal, are found in its core; it is one of the most densely populated clusters ever found, with a phenomenal luminosity of some 180,000 times that of the Sun. No wonder it photographs so well! 34)

- Discovered in 1780 by Pierre Méchain, Messier 75 was also observed by Charles Messier and added to his catalog later that year. This image of Messier 75 was captured by the NASA/ESA Hubble Space Telescope's AWS (Advanced Camera for Surveys).

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Figure 31: This sparkling burst of stars is Messier 75. It is a globular cluster: a spherical collection of stars bound together by gravity. Clusters like this orbit around galaxies and typically reside in their outer and less-crowded areas, gathering to form dense communities in the galactic suburbs (image credit: ESA/Hubble & NASA, F. Ferraro et al.; CC BY 4.0)

• 25 April 2019: Astronomers using NASA's Hubble Space Telescope say they have crossed an important threshold in revealing a discrepancy between the two key techniques for measuring the universe's expansion rate. The recent study strengthens the case that new theories may be needed to explain the forces that have shaped the cosmos. 35)

- A brief recap: The universe is getting bigger every second. The space between galaxies is stretching, like dough rising in the oven. But how fast is the universe expanding? As Hubble and other telescopes seek to answer this question, they have run into an intriguing difference between what scientists predict and what they observe.

- Hubble Space Telescope measurements suggest a faster expansion rate in the modern universe than expected, based on how the universe appeared more than 13 billion years ago. These measurements of the early universe come from the European Space Agency's Planck satellite. This discrepancy has been identified in scientific papers over the last several years, but it has been unclear whether differences in measurement techniques are to blame, or whether the difference could result from unlucky measurements.

- The latest Hubble data lower the possibility that the discrepancy is only a fluke to 1 in 100,000. This is a significant gain from an earlier estimate, less than a year ago, of a chance of 1 in 3,000.

- These most precise Hubble measurements to date bolster the idea that new physics may be needed to explain the mismatch.

- "The Hubble tension between the early and late universe may be the most exciting development in cosmology in decades," said lead researcher and Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, in Baltimore, Maryland. "This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This disparity could not plausibly occur just by chance."

Figure 32: Measurements of today's expansion rate do not match the rate that was expected based on how the Universe appeared shortly after the Big Bang over 13 billion years ago. Using new data from the NASA/ESA Hubble Space Telescope, astronomers have significantly lowered the possibility that this discrepancy is a fluke (video credit: ESA/Hubble video, Hubblecast, ID: heic1908a, Released: 25 April 2019) 36)

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Figure 33: Large Magellanic Cloud [DSS (Digitized Sky Survey)View] with Star Cluster Overlay (Hubble), image credit: NASA, ESA, and A. Riess (STScI/JHU)

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Figure 34: Three Steps to the Hubble Constant [image credit: NASA, ESA, and A. Riess (STScI/JHU)]

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Figure 35: Compass Image of Large Magellanic Cloud [image credit: NASA, ESA, and A. Riess (STScI/JHU)]

Tightening the bolts on the 'cosmic distance ladder'

- Scientists use a "cosmic distance ladder" to determine how far away things are in the universe. This method depends on making accurate measurements of distances to nearby galaxies and then moving to galaxies farther and farther away, using their stars as milepost markers. Astronomers use these values, along with other measurements of the galaxies' light that reddens as it passes through a stretching universe, to calculate how fast the cosmos expands with time, a value known as the Hubble constant. Riess and his SH0ES (Supernovae H0 for the Equation of State) team have been on a quest since 2005 to refine those distance measurements with Hubble and fine-tune the Hubble constant.

- In this new study, astronomers used Hubble to observe 70 pulsating stars called Cepheid variables in the Large Magellanic Cloud. The observations helped the astronomers "rebuild" the distance ladder by improving the comparison between those Cepheids and their more distant cousins in the galactic hosts of supernovas. Riess's team reduced the uncertainty in their Hubble constant value to 1.9% from an earlier estimate of 2.2%.

- As the team's measurements have become more precise, their calculation of the Hubble constant has remained at odds with the expected value derived from observations of the early universe's expansion. Those measurements were made by Planck, which maps the cosmic microwave background, a relic afterglow from 380,000 years after the big bang.

- The measurements have been thoroughly vetted, so astronomers cannot currently dismiss the gap between the two results as due to an error in any single measurement or method. Both values have been tested multiple ways.

- "This is not just two experiments disagreeing," Riess explained. "We are measuring something fundamentally different. One is a measurement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding. If these values don't agree, there becomes a very strong likelihood that we're missing something in the cosmological model that connects the two eras."

How the new study was done

- Astronomers have been using Cepheid variables as cosmic yardsticks to gauge nearby intergalactic distances for more than a century. But trying to harvest a bunch of these stars was so time-consuming as to be nearly unachievable. So, the team employed a clever new method, called DASH (Drift And Shift), using Hubble as a "point-and-shoot" camera to snap quick images of the extremely bright pulsating stars, which eliminates the time-consuming need for precise pointing.

- "When Hubble uses precise pointing by locking onto guide stars, it can only observe one Cepheid per each 90-minute Hubble orbit around Earth. So, it would be very costly for the telescope to observe each Cepheid," explained team member Stefano Casertano, also of STScI and Johns Hopkins. "Instead, we searched for groups of Cepheids close enough to each other that we could move between them without recalibrating the telescope pointing. These Cepheids are so bright, we only need to observe them for two seconds. This technique is allowing us to observe a dozen Cepheids for the duration of one orbit. So, we stay on gyroscope control and keep 'DASHing' around very fast."

- The Hubble astronomers then combined their result with another set of observations, made by the Araucaria Project, a collaboration between astronomers from institutions in Chile, the U.S., and Europe. This group made distance measurements to the Large Magellanic Cloud by observing the dimming of light as one star passes in front of its partner in eclipsing binary-star systems.

- The combined measurements helped the SH0ES Team refine the Cepheids' true brightness. With this more accurate result, the team could then "tighten the bolts" of the rest of the distance ladder that extends deeper into space.

- The new estimate of the Hubble constant is 74 km (46 miles) per second per megaparsec. This means that for every 3.3 million light-years farther away a galaxy is from us, it appears to be moving 74 km (46 miles) per second faster, as a result of the expansion of the universe. The number indicates that the universe is expanding at a 9% faster rate than the prediction of 67 km (41.6 miles) per second per megaparsec, which comes from Planck's observations of the early universe, coupled with our present understanding of the universe.

So, what could explain this discrepancy?

- One explanation for the mismatch involves an unexpected appearance of dark energy in the young universe, which is thought to now comprise 70% of the universe's contents. Proposed by astronomers at Johns Hopkins, the theory is dubbed "early dark energy," and suggests that the universe evolved like a three-act play.

- Astronomers have already hypothesized that dark energy existed during the first seconds after the big bang and pushed matter throughout space, starting the initial expansion. Dark energy may also be the reason for the universe's accelerated expansion today. The new theory suggests that there was a third dark-energy episode not long after the big bang, which expanded the universe faster than astronomers had predicted. The existence of this "early dark energy" could account for the tension between the two Hubble constant values, Riess said.

- Another idea is that the universe contains a new subatomic particle that travels close to the speed of light. Such speedy particles are collectively called "dark radiation" and include previously known particles like neutrinos, which are created in nuclear reactions and radioactive decays.

- Yet another attractive possibility is that dark matter (an invisible form of matter not made up of protons, neutrons, and electrons) interacts more strongly with normal matter or radiation than previously assumed.

- But the true explanation is still a mystery.

- Riess doesn't have an answer to this vexing problem, but his team will continue to use Hubble to reduce the uncertainties in the Hubble constant. Their goal is to decrease the uncertainty to 1%, which should help astronomers identify the cause of the discrepancy.

- The team's results have been accepted for publication in The Astrophysical Journal.

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• On 24 April 1990, the NASA/ESA Hubble Space Telescope was launched on the space shuttle Discovery. It has since revolutionized how astronomers and the general public see the Universe. The images it provides are spectacular from both a scientific and a purely aesthetic point of view. 37)

- Each year the telescope dedicates a small portion of its precious observing time to take a special anniversary image, focused on capturing particularly beautiful and meaningful objects. This year's image is the Southern Crab Nebula, and it is no exception. Note: The Southern Crab Nebula is so named to distinguish it from the better-known Crab Nebula, a supernova remnant visible in the constellation of Taurus.

- This peculiar nebula, which exhibits nested hourglass-shaped structures, has been created by the interaction between a pair of stars at its center. The unequal pair consists of a red giant and a white dwarf. The red giant is shedding its outer layers in the last phase of its life before it too lives out its final years as a white dwarf. Some of the red giant's ejected material is attracted by the gravity of its companion.

- When enough of this cast-off material is pulled onto the white dwarf, it too ejects the material outwards in an eruption, creating the structures we see in the nebula. Eventually, the red giant will finish throwing off its outer layers, and stop feeding its white dwarf companion. Prior to this, there may also be more eruptions, creating even more intricate structures.

- Astronomers did not always know this, however. The object was first written about in 1967, but was assumed to be an ordinary star until 1989, when it was observed using telescopes at the European Southern Observatory's La Silla Observatory. The resulting image showed a roughly crab-shaped extended nebula, formed by symmetrical bubbles of gas and dust.

- These observations only showed the outer hourglass emanating from a bright central region that could not be resolved. It was not until Hubble observed the Southern Crab in 1999 that the entire structure came into view. This image revealed the inner nested structures, suggesting that the phenomenon that created the outer bubbles had occurred twice in the (astronomically) recent past.

- It is fitting that Hubble has returned to this object twenty years after its first observation. This new image adds to the story of an active and evolving object and contributes to the story of Hubble's role in our evolving understanding of the Universe.

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Figure 36: This incredible image of the hourglass-shaped Southern Crab Nebula was taken to mark the NASA/ESA Hubble Space Telescope's 29th anniversary in space. The nebula, created by a binary star system, is one of the many objects that Hubble has demystified throughout its productive life. This new image adds to our understanding of the nebula and demonstrates the telescope's continued capabilities (image credit: NASA, ESA, and STScI, CC BY 4.0)

• 15 April 2019: A stellar Easter egg. The Egg Nebula is a preplanetary nebula, created by a dying star in the process of becoming a planetary nebula. Planetary nebulas have nothing to do with planets – the name arose when 18th century astronomers spotted them in their telescopes and thought they looked like planets. Instead, they are the remnants of material expelled by Sun-like stars in the later stages of their lives. 38)

- The preplanetary nebula phase is extremely short-lived in astronomical terms – only a few thousand years. This makes them rare objects and, combined with the fact that they are quite faint, rather difficult to spot. The Egg Nebula, located around 3000 light years from us, was the first of its kind to be discovered in the 1970s.

- During the preplanetary nebula phase, the central star periodically sheds its outer layers, which are then illuminated by the dying star at the center. Eventually the star stops shedding material and the core remnant heats up, exciting the expelled gas so that it glows brightly and becomes a planetary nebula.

- The dark band, sweeping beams, and crisscrossing arcs in this image can reveal a lot about the complex environment of a dying star. The central band is a cocoon of dust hiding the star from view.

- Beams of light emanate from the obscured star, and it is thought that they are due to starlight escaping from the ring-shaped holes in the dusty cocoon that surrounds the star. The holes are possibly carved by a high-speed stream of matter, although the cause of these jets are unknown. The spoke-like features are shadows cast by blobs of material within the region of the holes in the cocoon.

- Numerous bright arcs intersect the beams: these are shells of matter ejected by the star. The arcs are like tree rings, and can tell us something about the object's age as they reveal that the rate of mass ejection has varied between 100 and 500 years throughout its 10,000 year history. The gas is expanding at a rate of 20 km/s and matter has been detected out to a radius of 0.6 light years, providing an estimate of the amount of matter in the nebula.

- This image was previously published on NASA's and ESA's Hubble websites.

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Figure 37: The Egg Nebula is a 'preplanetary nebula'. These objects occur as a dying star's hot remains briefly illuminates material it has expelled, lighting up the gas and dust that surrounds it. This image is based on observations performed in the mid 1990s in red light with the Wide Field and Planetary Camera 2 (WFPC2) on the NASA/ESA Hubble Space Telescope (image credit: Raghvendra Sahai and John Trauger (JPL), the WFPC2 science team, and NASA/ESA)

• 05 April 2019: Star clusters are commonly featured in cosmic photoshoots, and are also well-loved by the keen eye of the NASA/ESA Hubble Space Telescope. These large gatherings of celestial gems are striking sights — and Messier 2 is certainly no exception. 39)

- Messier 2 is located in the constellation of Aquarius (the Water Bearer), about 55,000 light-years away. It is a globular cluster, a spherical group of stars all tightly bound together by gravity. With a diameter of roughly 175 light-years, a population of 150,000 stars, and an age of 13 billion years, Messier 2 is one of the largest clusters of its kind and one of the oldest associated with the Milky Way.

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Figure 38: This Hubble image of Messier 2's core was created using visible and infrared light. Most of the cluster's mass is concentrated at its center, with shimmering streams of stars extending outward into space. It is bright enough that it can even be seen with the naked eye when observing conditions are extremely good (image credit: ESA/Hubble & NASA, G. Piotto et al.)

• 04 April 2019: NASA has selected 24 new fellows for its prestigious NASA Hubble Fellowship Program (NHFP). The program enables outstanding postdoctoral scientists to pursue independent research across NASA astrophysics, using theory, observation, experimentation or instrument development. Each fellowship provides the awardee up to three years of support. 40)

- The NHFP preserves the legacy of NASA's previous postdoctoral fellowship programs — the Hubble, Einstein and Sagan fellowships. Once selected, fellows are named to one of three sub-categories corresponding to three broad scientific questions NASA has sought to answer about the universe:

a) How does the universe work? – Einstein Fellows

b) How did we get here? – Hubble Fellows

c) Are we alone? – Sagan Fellows

- The NHFP is one of the highlights of NASA's pursuit of excellence in space science.

- "I am excited that this outstanding group of young scientists have accepted NASA Hubble Fellowships," said Paul Hertz, Astrophysics Division director at NASA Headquarters in Washington. "I am confident that the research they do will address the most compelling questions in astrophysics and have an impact on our field far beyond the three-year duration of their fellowships."

- The newly selected NHFP Fellows will begin their programs in the fall of 2019 at a host university or research center of their choosing in the United States. Table 2 provides the names of the 2019 awardees, their host institutions, and their proposed research topics.

- An important part of the NHFP are the symposia, which allow fellows the opportunity to present results of their research, and to meet each other and the scientific and administrative staff who manage the program.

- The Space Telescope Science Institute in Baltimore, Maryland, administers the NHFP on behalf of NASA, in collaboration with the Chandra X-ray Center at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, and the NASA Exoplanet Science Institute at Caltech/IPAC in Pasadena, California.

How does the universe work? – Einstein Fellows:

- Dillon Brout, University of Pennsylvania, Improving Cosmological Constraints with the Dark Energy Supernova Program and a Decade of Type Ia Supernovae

- Andrew Chael, Princeton Center for Theoretical Science, Simulating and Imaging Flaring Black Holes on Horizon Scales

- Jose Maria Ezquiaga, University of Chicago, Precision Cosmology with Present and Next Generation Gravitational Wave Detectors

- Ting Li, Carnegie Observatories, Constraining Dark Matter with Stellar Streams and Dwarf Galaxies

- Renee Ludlam, California Institute of Technology, A New Light on Neutron Stars

- Yao-Yuan Mao, Rutgers University, The Galaxy-Halo Connection: Probing the Dark Universe with Galaxies

- Shuo Zhang, Boston University, Supermassive Black Holes and Exotic Physics in the Galactic Nuclei of Local Galaxies

How did we get here? – Hubble Fellows:

- Emma Beasor, National Optical Astronomy Observatory, The Evolution of Massive Stars to Supernovae

- John Chisholm, University of California, Santa Cruz, What Reionized the Universe?

- Eric Coughlin, Princeton University, The Appearance of Disappearing Stars: Mass Ejection, Fallback Accretion, and Jets from Weak and Failed Supernovae

- Anna-Christina Eilers, Massachusetts Institute of Technology, The Formation and Growth of Supermassive Black Holes at Early Cosmic Epochs

- Hui Li, Massachusetts Institute of Technology, Bridging the Gap Between Galaxy and Star Formation with Star Clusters

- Anna Faye McLeod, University of California, Berkeley, Stellar Feedback in the Era of Integral Field Spectroscopy

- Georgia Virginia Panopoulou, California Institute of Technology, A Unique Approach to the Determination of the Galactic Magnetic Field Using Starlight and Synchrotron Polarization Observations

- Vadim Semenov, Harvard University, Modeling the Turbulent Evolution of Galaxies over Cosmic Time

- Justin Spilker, University of Texas, Austin, Taking a Census of Galactic Winds with JWST, ALMA, and SOFIA

- Feige Wang, University of Arizona, Probing Cosmic Reionization and the Growth of the Earliest Super-Massive Black Holes

- Coral Wheeler, Carnegie Observatories, Ultra-High Resolution Simulations of the Milky Way and its Satellites.

Are we alone? – Sagan Fellows:

- Jaehan Bae, Carnegie Department of Terrestrial Magnetism, Constraining Initial Phases of Planet Formation

- Jennifer Bergner, University of Chicago, Connecting Interstellar and Planetary Chemistry

- Sebastiaan Haffert, University of Arizona, Seeing the Formation of Planets with High-Contrast Spectroscopy at MagAO-X

- Joshua Krissansen-Totton, University of California, Santa Cruz, Inverse Modeling of the Atmospheric Evolution of Lifeless Worlds to Understand Exoplanet Biosignatures

- Antonija Oklopcic, Harvard University, Spectral Signatures of Atmospheric Escape in Exoplanets

- Christopher Theissen, University of California, San Diego, Planetary Collisions around Low-Mass Stars: Constraining the Timescale for Collisions and Testing the Origin of the Kepler Dichotomy

Table 2: The 2019 NASA Hubble Fellowship Program

• 02 April 2019: NASA's Space Telescope Science Institute (STScI) recently awarded SwRI (Southwest Research Institute) the largest Hubble Space Telescope (HST) solar system program ever, with 206 of Hubble's orbits around the Earth allocated to the project. The SSOLS (Solar System Origins Legacy Survey) will focus on Kuiper Belt objects (KBOs), particularly binary populations. 41)

- "The Kuiper Belt is a unique remnant of the solar system's primordial planetesimal disk," said Dr. Alex Parker, the SwRI planetary scientist leading the survey. "This cold, calm region has preserved an extraordinarily large population of binary objects, particularly those where the two objects have similar mass."

- Hubble orbits at an altitude of about 350 miles (564 km), circling the Earth every 97 minutes. Most HST time is dedicated to studying interstellar space phenomena. The Kuiper Belt is a distant reservoir of ancient material that lies at the edge of our solar system, beyond all the terrestrial and giant planets. At the present time, the properties of the Kuiper Belt's unique population of binary systems can only be accurately measured with Hubble. SwRI leads this large HST project focused on characterizing the binary and color properties of over 200 unique KBOs. Team members are spread across the USA, Canada and Northern Ireland.

- "These binary systems are powerful tracers of the processes that built the planets," Parker said. "We will use Hubble to test the theory that many planetesimals formed as binary systems from the get-go, and that today's Kuiper Belt binaries did not come from mergers of initially solitary objects. Binary objects orbit around each other as they collectively circle the Sun. Recent models of small body formation suggest that binaries are leftovers of the very earliest times of our solar system, when pairs of bodies could form directly from collapsing swarms of small-scale "pebbles."

- Competing theories of planetesimal formation predict different size and color distributions for binary and solitary KBOs. If objects first formed through an accretion process and were merged into binaries later, scientists expect the objects in binary systems to have dissimilar colors and to have a different size distribution than solitary objects. However, if planetesimals formed through a rapid collapse process that produced some solitary objects and some binary systems from the start, scientists would expect objects in binary systems to have a similar surface color and a size distribution similar to that of solitary objects.

- The SSOLS program builds upon the legacy of the OSSOS (Outer Solar System Origins Survey) and the CFEPS (Canada-France Ecliptic Plane Survey), the two largest well-characterized Kuiper Belt surveys ever conducted. By drawing targets from these well-characterized surveys, SSOLS will provide a coherent framework to test leading theories of planetesimal formation and the origin and evolution of the outer solar system's architecture.

- The SSOLS team will be posting updates, images, and results on its website, https://www.ssols.space/

- SSOLS is a Cycle 26 HST treasury program administered by STScI, which is operated by the AURA (Association of Universities for Research in Astronomy) and based in Baltimore, Maryland. STScI is charged with helping humanity explore the universe with advanced space telescopes and ever-growing data archives.

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Figure 39: The SwRI-led SSOLS (Solar System Origins Legacy Survey) will search for Kuiper Belt objects such as those shown in this artist's illustration of a widely separated binary (image credit: Courtesy of Southwest Research Institute and Alex H. Parker)

• 29 March 2019: This star-studded image shows us a portion of Messier 11, an open star cluster in the southern constellation of Scutum (The Shield). Messier 11 is also known as the Wild Duck Cluster, as its brightest stars form a "V" shape that somewhat resembles a flock of ducks in flight. 42)

- Messier 11 is one of the richest and most compact open clusters currently known. By investigating the brightest, hottest main sequence stars in the cluster astronomers estimate that it formed roughly 220 million years ago. Open clusters tend to contain fewer and younger stars than their more compact globular cousins, and Messier 11 is no exception: at its center lie many blue stars, the hottest and youngest of the cluster's few thousand stellar residents.

- The lifespans of open clusters are also relatively short compared to those of globular ones; stars in open clusters are spread further apart and are thus not as strongly bound to each other by gravity, causing them to be more easily and quickly drawn away by stronger gravitational forces. As a result Messier 11 is likely to disperse in a few million years as its members are ejected one by one, pulled away by other celestial objects in the vicinity.

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Figure 40: Wild cosmic ducks (image credit: ESA/Hubble & NASA, P. Dobbie et al.; CC BY 4.0)

• 28 March 2019: A small asteroid has been caught in the process of spinning so fast it's throwing off material, according to new data from NASA's Hubble Space Telescope and other observatories. 43)

- Images from Hubble show two narrow, comet-like tails of dusty debris streaming from the asteroid (6478) Gault. Each tail represents an episode in which the asteroid gently shed its material — key evidence that Gault is beginning to come apart.

- Discovered in 1988, the 2.5-mile-wide (4-kilometer-wide) asteroid has been observed repeatedly, but the debris tails are the first evidence of disintegration. Gault is located 214 million miles (344 million kilometers) from the Sun. Of the roughly 800,000 known asteroids between Mars and Jupiter, astronomers estimate that this type of event in the asteroid belt is rare, occurring roughly once a year.

- Watching an asteroid become unglued gives astronomers the opportunity to study the makeup of these space rocks without sending a spacecraft to sample them.

- "We didn't have to go to Gault," explained Olivier Hainaut of ESO (European Southern Observatory) in Garching, Germany, a member of the Gault observing team. "We just had to look at the image of the streamers, and we can see all of the dust grains well-sorted by size. All the large grains (about the size of sand particles) are close to the object and the smallest grains (about the size of flour grains) are the farthest away because they are being pushed fastest by pressure from sunlight."

- Gault is only the second asteroid whose disintegration has been strongly linked to a process known as a YORP effect. (YORP stands for "Yarkovsky–O'Keefe–Radzievskii–Paddack," the names of four scientists who contributed to the concept.) When sunlight heats an asteroid, infrared radiation escaping from its warmed surface carries off angular momentum as well as heat. This process creates a tiny torque that can cause the asteroid to continually spin faster. When the resulting centrifugal force starts to overcome gravity, the asteroid's surface becomes unstable, and landslides may send dust and rubble drifting into space at a couple miles per hour, or the speed of a strolling human. The researchers estimate that Gault could have been slowly spinning up for more than 100 million years.

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Figure 41: This Hubble Space Telescope image reveals the gradual self-destruction of an asteroid, whose ejected dusty material has formed two long, thin, comet-like tails. The longer tail stretches more than 500,000 miles (800,000 km) and is roughly 3,000 miles (4,800 km) wide. The shorter tail is about a quarter as long. The streamers will eventually disperse into space [image credit: NASA, ESA, K. Meech and J. Kleyna (University of Hawaii), and O. Hainaut (European Southern Observatory)]

- Piecing together Gault's recent activity is an astronomical forensics investigation involving telescopes and astronomers around the world. All-sky surveys, ground-based telescopes, and space-based facilities like the Hubble Space Telescope pooled their efforts to make this discovery possible.

- The initial clue was the fortuitous detection of the first debris tail, observed on Jan. 5, 2019, by the NASA-funded ATLAS (Asteroid Terrestrial-Impact Last Alert System) telescope in Hawaii. The tail also turned up in archival data from December 2018 from ATLAS and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) telescopes in Hawaii. In mid-January, a second shorter tail was spied by the Canada–France–Hawaii Telescope in Hawaii and the Isaac Newton Telescope in Spain, as well as by other observers. An analysis of both tails suggests the two dust events occurred around Oct. 28 and Dec. 30, 2018.

- Follow-up observations with the William Herschel Telescope and ESA's (European Space Agency) Optical Ground Station in La Palma and Tenerife, Spain, and the Himalayan Chandra Telescope in India measured a two-hour rotation period for the object, close to the critical speed at which a loose "rubble-pile" asteroid begins to break up.

- "Gault is the best ‘smoking gun' example of a fast rotator right at the two-hour limit," said team member Jan Kleyna of the University of Hawaii in Honolulu.

- An analysis of the asteroid's surrounding environment by Hubble revealed no signs of more widely distributed debris, which rules out the possibility of a collision with another asteroid causing the outbursts.

- The asteroid's narrow streamers suggest that the dust was released in short bursts, lasting anywhere from a few hours to a few days. These sudden events puffed away enough debris to make a "dirt ball" approximately 150 m across if compacted together. The tails will begin fading away in a few months as the dust disperses into interplanetary space.

- Based on observations by the Canada–France–Hawaii Telescope, the astronomers estimate that the longer tail stretches over half a million miles (800,000 km) and is roughly 3,000 miles (4,800 km) wide. The shorter tail is about a quarter as long.

- Only a couple of dozen active asteroids have been found so far. Astronomers may now have the capability to detect many more of them because of the enhanced survey capabilities of observatories such as Pan-STARRS and ATLAS, which scan the entire sky. "Asteroids such as Gault cannot escape detection anymore," Hainaut said. "That means that all these asteroids that start misbehaving get caught."

- The researchers hope to monitor Gault for more dust events. The team's results have been accepted for publication by The Astrophysical Journal Letters.

• 18 March 2019: This fuzzy orb of light is a giant elliptical galaxy filled with an incredible 200 billion stars. Unlike spiral galaxies, which have a well-defined structure and boast picturesque spiral arms, elliptical galaxies appear fairly smooth and featureless. This is likely why this galaxy, named Messier 49, was discovered by French astronomer Charles Messier in 1771. At a distance of 56 million light-years, and measuring 157,000 light-years across, M49 was the first member of the Virgo Cluster of galaxies to be discovered, and it is more luminous than any other galaxy at its distance or nearer. 44)

- Elliptical galaxies tend to contain a larger portion of older stars than spiral galaxies and also lack young blue stars. Messier 49 itself is very yellow, which indicates that the stars within it are mostly older and redder than the Sun. In fact, the last major episode of star formation was about six billion years ago — before the Sun was even born!

- Messier 49 is also rich in globular clusters; it hosts about 6000, a number that dwarfs the 150 found in and around the Milky Way. On average, these clusters are 10 billion years old. Messier 49 is also known to host a supermassive black hole at its centre with the mass of more than 500 million Suns, identifiable by the X-rays pouring out from the heart of the galaxy (as this Hubble image comprises infrared observations, these X-rays are not visible here).

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Figure 42: Invisible X-rays of Messier 49 (image credit: ESA/Hubble & NASA, J. Blakenslee, P Cote et al.)

• 07 March 2019: In a striking example of multi-mission astronomy, measurements from the NASA/ESA Hubble Space Telescope and the ESA Gaia mission have been combined to improve the estimate of the mass of our home galaxy the Milky Way: 1.5 trillion (1.5 x 1012) solar masses. 45) 46)

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Figure 43: This artist's impression shows a computer generated model of the Milky Way and the accurate positions of the globular clusters used in this study surrounding it. Scientists used the measured velocities of these 44 globular clusters to determine the total mass of the Milky Way, our cosmic home. Satellite: Hubble Space Telescope (image credit: ESA/Hubble, NASA, L. Calçada)

- The mass of the Milky Way is one of the most fundamental measurements astronomers can make about our galactic home. However, despite decades of intense effort, even the best available estimates of the Milky Way's mass disagree wildly. Now, by combining new data from the European Space Agency (ESA) Gaia mission with observations made with the NASA/ESA Hubble Space Telescope, astronomers have found that the Milky Way weighs in at about 1.5 trillion solar masses within a radius of 129,000 light-years from the galactic center.

- Previous estimates of the mass of the Milky way ranged from 500 billion (500 x 109) to 3 trillion (3 x 1012) times the mass of the Sun. This huge uncertainty arose primarily from the different methods used for measuring the distribution of dark matter – which makes up about 90% of the mass of the galaxy.

- "We just can't detect dark matter directly," explains Laura Watkins (European Southern Observatory, Germany), who led the team performing the analysis. "That's what leads to the present uncertainty in the Milky Way's mass – you can't measure accurately what you can't see!"

- Given the elusive nature of the dark matter, the team had to use a clever method to weigh the Milky Way, which relied on measuring the velocities of globular clusters – dense star clusters that orbit the spiral disc of the galaxy at great distances.
Note: Globular clusters formed prior to the construction of the Milky Way's spiral disk, where our Sun and the Solar System later formed. Because of their great distances, globular star clusters allow astronomers to trace the mass of the vast envelope of dark matter surrounding our galaxy far beyond the spiral disk.

- "The more massive a galaxy, the faster its clusters move under the pull of its gravity" explains N. Wyn Evans (University of Cambridge, UK). "Most previous measurements have found the speed at which a cluster is approaching or receding from Earth, that is the velocity along our line of sight. However, we were able to also measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated."
Note: The total velocity of an object is made up of three motions – a radial motion plus two defining the sideway motions. However, in astronomy most often only line-of-sight velocities are available. With only one component of the velocity available, the estimated masses depend very strongly on the assumptions for the sideway motions. Therefore measuring the sideway motions directly significantly reduces the size of the error bars for the mass.

- The group used Gaia's second data release as a basis for their study. Gaia was designed to create a precise three-dimensional map of astronomical objects throughout the Milky Way and to track their motions. Its second data release includes measurements of globular clusters as far as 65,000 light-years from Earth.

- "Global clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our galaxy" said Tony Sohn of STScI (Space Telescope Science Institute), Baltimore, MD, USA, who led the Hubble measurements.

- The team combined these data with Hubble's unparalleled sensitivity and observational legacy. Observations from Hubble allowed faint and distant globular clusters, as far as 130,000 light-years from Earth, to be added to the study. As Hubble has been observing some of these objects for a decade, it was possible to accurately track the velocities of these clusters as well.

- "We were lucky to have such a great combination of data," explained Roeland P. van der Marel of STScI. "By combining Gaia's measurements of 34 globular clusters with measurements of 12 more distant clusters from Hubble, we could pin down the Milky Way's mass in a way that would be impossible without these two space telescopes."

- Until now, not knowing the precise mass of the Milky Way has presented a problem for attempts to answer a lot of cosmological questions. The dark matter content of a galaxy and its distribution are intrinsically linked to the formation and growth of structures in the Universe. Accurately determining the mass for the Milky Way gives us a clearer understanding of where our galaxy sits in a cosmological context. 47)

Figure 44: Hubblecast 117 Light: Hubble & Gaia weigh the Milky Way. Measurements from the NASA/ESA Hubble Space Telescope and the ESA Gaia mission have been combined to improve the estimate of the mass of our home galaxy the Milky Way: 1.5 trillion solar masses (video credit: Hubble ESA, NASA)

• 06 March 2019: NASA has recovered the Hubble Space Telescope's ACS (Advanced Camera for Surveys) instrument, which suspended operations on Thursday, Feb. 28, 2019. The final tests were conducted and the instrument was brought back to its operational mode on March 6. 48)

- At 8:31 p.m. EST on 28 February, the ACS aboard NASA's Hubble Space Telescope suspended operations after an error was detected as the instrument was performing a routine boot procedure. The error indicated that software inside the camera had not loaded correctly in a small section of computer memory. The Hubble operations team ran repeated tests to reload the memory and check the entire process. No errors have been detected since the initial incident, and it appears that all circuits, computer memory and processors that are part of that boot process are now operating normally. The instrument has now been brought back to its standard operating mode for normal operations.

- The ACS was installed in 2002 and repaired during the last servicing mission to Hubble back in 2009 after a power supply failure. More than 5,500 peer-reviewed scientific papers have been published from its data, and it is credited with some of Hubble's most iconic images, including the Hubble Ultra Deep Field, the furthest look into the universe at that time.

- Hubble itself is in its 29th year of operations, well surpassing its original 15-year lifetime. With its primary and backup systems, it is expected that Hubble will operate simultaneously with the upcoming JWST (James Webb Space Telescope) to obtain multiwavelength observations of astronomical objects. Scheduled to launch in 2021, the JWST is designed to see near- and mid-infrared light while Hubble is optimized for ultraviolet and visible light.

• 01 March 2019: At 8:31 p.m. EST on 28 February 2019, the Advanced Camera for Surveys (ACS) aboard NASA's Hubble Space Telescope suspended operations after an error was detected as the instrument was performing a routine boot procedure. The error indicated that software inside the camera had not loaded correctly. A team of instrument system engineers, flight software experts, and flight operations personnel quickly organized to download and analyze instrument diagnostic information. This team is currently working to identify the root cause and then to construct a recovery plan. 49)

- The telescope continues to operate normally, executing observations with the other three science instruments — the Wide Field Camera 3 (WFC3), the Cosmic Origins Spectrograph (COS), and the Space Telescope Imaging Spectrograph (STIS) — that are all performing nominally. There are no critical observations using the ACS scheduled for the remainder of this week or next week, and the observations that were planned over the next two weeks can be easily rescheduled.

- Originally required to last 15 years, Hubble has now been operating for more than 28 years. The final servicing mission in 2009, expected to extend Hubble's lifetime an additional five years, has now produced more than nine years of science observations. During that servicing mission, astronauts repaired the ACS, installed in 2002, after its power supply failed in 2007.

• 01 March 2019: Globular clusters like NGC 2419, visible in this image taken with the NASA/ESA Hubble Space Telescope, are not only beautiful, but also fascinating. They are spherical groups of stars which orbit the center of a galaxy; in the case of NGC 2419, that galaxy is the Milky Way. NGC 2419 can be found around 300,000 light-years from the Solar System, in the constellation Lynx (the Lynx). 50)

- The stars populating globular clusters are very similar to one another, with similar properties such as metallicity. The similarity of these stellar doppelgängers is due to their formation early in the history of the galaxy. As the stars in a globular cluster all formed at around the same time, they tend to display reasonably homogeneous properties. It was believed that this similarity also extended to the stellar helium content; that is, it was thought that all stars in a globular cluster would contain comparable amounts of helium.

- However, Hubble's observations of NGC 2419 have shown that this is not always the case. This surprising globular cluster turns out to be made up of two separate populations of red giant stars, one of which is unusually helium-rich. Other elements within the different stars in NGC 2419 vary too — nitrogen in particular. On top of this, these helium-rich stars were found to be predominantly in the center of the globular cluster, and to be rotating. These observations have raised questions about the formation of globular clusters; did these two drastically different groups of stars form together? Or did this globular cluster come into being by a different route entirely?

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Figure 45: The two mysterious populations of NGC 2419 (image credit: ESA/Hubble & NASA, S. Larsen et al.; CC BY 4.0)

• 20 February 2019: Astronomers call it "the moon that shouldn't be there." — After several years of analysis, a team of planetary scientists using NASA's Hubble Space Telescope has at last come up with an explanation for a mysterious moon around Neptune that they discovered with Hubble in 2013. 51) 52)

- The tiny moon, named Hippocamp, is unusually close to a much larger Neptunian moon called Proteus. Normally, a moon like Proteus should have gravitationally swept aside or swallowed the smaller moon while clearing out its orbital path.

- So why does the tiny moon exist? Hippocamp is likely a chipped-off piece of the larger moon that resulted from a collision with a comet billions of years ago. The diminutive moon, only 20 miles (about 34 km) across, is 1/1000th the mass of Proteus (which is 260 miles, ~418 km across).

- "The first thing we realized was that you wouldn't expect to find such a tiny moon right next to Neptune's biggest inner moon," said Mark Showalter of the SETI Institute in Mountain View, California. "In the distant past, given the slow migration outward of the larger moon, Proteus was once where Hippocamp is now."

- This scenario is supported by Voyager 2 images from 1989 that show a large impact crater on Proteus, almost large enough to have shattered the moon. "In 1989, we thought the crater was the end of the story," said Showalter. "With Hubble, now we know that a little piece of Proteus got left behind and we see it today as Hippocamp." The orbits of the two moons are now 7,500 miles (about 12,070 km) apart.

- Neptune's satellite system has a violent and tortured history. Many billions of years ago, Neptune captured the large moon Triton from the Kuiper Belt, a large region of icy and rocky objects beyond the orbit of Neptune. Triton's gravity would have torn up Neptune's original satellite system. Triton settled into a circular orbit and the debris from shattered Neptunian moons re-coalesced into a second generation of natural satellites. However, comet bombardment continued to tear things up, leading to the birth of Hippocamp, which might be considered a third-generation satellite.

- "Based on estimates of comet populations, we know that other moons in the outer solar system have been hit by comets, smashed apart, and re-accreted multiple times," noted Jack Lissauer of NASA's Ames Research Center in California's Silicon Valley, a coauthor on the new research. "This pair of satellites provides a dramatic illustration that moons are sometimes broken apart by comets."

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Figure 46: Artist's concept of Neptune's Moon Hippocamp [image credit: NASA, ESA, and J. Olmsted (STScI)]

- Hippocamp is a half-horse half-fish from Greek mythology. The scientific name for the seahorse is Hippocampus, also the name of an important part of the human brain. The rules of the International Astronomical Union require that the moons of Neptune are named after Greek and Roman mythology of the undersea world.

- The team of astronomers in this study consists of M. Showalter (SETI Institute, Mountain View, California), I. de Pater (University of California, Berkeley, California), J. Lissauer (NASA Ames Research Center, Silicon Valley, California), and R. French (SETI Institute, Mountain View, California).

- The paper will appear in the February 21 issue of the science journal Nature. 53)

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

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Figure 47: Neptune's inner Moons and their diameters. This diagram shows the orbital positions of Neptune's inner moons, which range in size from 20 to 260 miles across. The outer moon Triton was captured from the Kuiper belt many billions of years ago. This would have torn up Neptune's original satellite system. Triton settled into a circular orbit and the debris from shattered moons re-coalesced into a second generation of inner satellites seen today. However, comet bombardment continued to tear things up, leading to the birth of Hippocamp, which is a broken-off piece of Proteus. Therefore, it is a third-generation satellite. Not shown is Neptune's outermost known satellite, Nereid, which is in a highly eccentric orbit, and may be a survivor from the era of that Triton capture [image credit: NASA, ESA, and A. Feild (STScI)]

• 15 February 2019: Stars are born in dark clouds of gas and dust like this. But star formation is an energetic process, and newly-formed stars can send out a brilliant display of lights called Herbig-Haro objects. These objects form as jets of hot gas spewed by the newborn star collide with the surrounding matter at high speeds. 54)

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Figure 48: In this image, the NASA/ESA Hubble Space Telescope has captured the smoking gun of a newborn star, the Herbig–Haro objects numbered 7 to 11 (HH 7–11). These five objects, visible in blue in the top center of the image, lie within NGC 1333, a reflection nebula full of gas and dust found about a thousand light-years away from Earth (image credit: ESA/Hubble & NASA, K. Stapelfeldt)

- Bright patches of nebulosity near newborn stars, Herbig-Haro objects like HH 7–11 are transient phenomena. Traveling away from the star that created them at a speed of up to about 150,000 miles per hour, they disappear into nothingness within a few tens of thousands of years. The young star that is the source of HH 7–11 is called SVS 13, and all five objects are moving away from SVS 13 toward the upper left. The current distance between HH 7 and SVS 13 is about 20,000 times the distance between Earth and the Sun.

- Herbig–Haro objects are formed when jets of ionized gas ejected by a young star collide with nearby clouds of gas and dust at high speeds. The Herbig-Haro objects visible in this image are no exception to this and were formed when the jets from the newborn star SVS 13 collided with the surrounding clouds. These collisions created the five brilliant clumps of light within the reflection nebula.

• 11 February 2019: Like Earth, Uranus and Neptune have seasons, which likely drive some of the features in their atmospheres. But their seasons are much longer than on Earth, spanning decades rather than months. 55) 56)

- The new Hubble view of Neptune shows the dark storm, seen at top center (Figure 49). Appearing during the planet's southern summer, the feature is the fourth and latest mysterious dark vortex captured by Hubble since 1993. Two other dark storms were discovered by the Voyager 2 spacecraft in 1989 as it flew by the remote planet. Since then, only Hubble has had the sensitivity in blue light to track these elusive features, which have appeared and faded quickly. A study led by University of California, Berkeley, undergraduate student Andrew Hsu estimated that the dark spots appear every four to six years at different latitudes and disappear after about two years.

- Hubble uncovered the latest storm in September 2018 in Neptune's northern hemisphere. The feature is roughly 6,800 miles across. To the right of the dark feature are bright white "companion clouds." Hubble has observed similar clouds accompanying previous vortices. The bright clouds form when the flow of ambient air is perturbed and diverted upward over the dark vortex, causing gases to freeze into methane ice crystals. These clouds are similar to clouds that appear as pancake-shaped features when air is pushed over mountains on Earth (though Neptune has no solid surface). The long, thin cloud to the left of the dark spot is a transient feature that is not part of the storm system.

- It's unclear how these storms form. But like Jupiter's Great Red Spot, the dark vortices swirl in an anti-cyclonic direction and seem to dredge up material from deeper levels in the ice giant's atmosphere.

- The Hubble observations show that as early as 2016, increased cloud activity in the region preceded the vortex's appearance. The images indicate that the vortices probably develop deeper in Neptune's atmosphere, becoming visible only when the top of the storm reaches higher altitudes.

- The snapshot of Uranus, like the image of Neptune, reveals a dominant feature: a vast bright stormy cloud cap across the north pole.

- Scientists believe this new feature is a result of Uranus' unique rotation. Unlike every other planet in the solar system, Uranus is tipped over almost onto its side. Because of this extreme tilt, during the planet's summer the Sun shines almost directly onto the north pole and never sets. Uranus is now approaching the middle of its summer season, and the polar-cap region is becoming more prominent. This polar hood may have formed by seasonal changes in atmospheric flow.

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Figure 49: During its routine yearly monitoring of the weather on our solar system's outer planets, NASA's Hubble Space Telescope has uncovered a new mysterious dark storm on Neptune (right, image taken with WFC3 in September and November 2018) and provided a fresh look at a long-lived storm circling around the north polar region on Uranus (left). The Uranus image was taken with the WFC3 of Hubble in November 2018. [image credit: NASA, ESA, A. Simon (NASA Goddard Space Flight Center), and M. H. Wong and A. Hsu (University of California, Berkeley)]

- Near the edge of the polar storm is a large, compact methane-ice cloud, which is sometimes bright enough to be photographed by amateur astronomers. A narrow cloud band encircles the planet north of the equator. It is a mystery how bands like these are confined to such narrow widths, because Uranus and Neptune have very broad westward-blowing wind jets.

- Both planets are classified as ice giant planets. They have no solid surface but rather mantles of hydrogen and helium surrounding a water-rich interior, itself perhaps wrapped around a rocky core. Atmospheric methane absorbs red light but allows blue-green light to be scattered back into space, giving each planet a cyan hue.

- The new Neptune and Uranus images are from the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project, led by Amy Simon of NASA's Goddard Space Flight Center in Greenbelt, Maryland, that annually captures global maps of our solar system's outer planets when they are closest to Earth in their orbits. OPAL's key goals are to study long-term seasonal changes, as well as capture comparatively transitory events, such as the appearance of Neptune's dark spot. These dark storms may be so fleeting that in the past some of them may have appeared and faded during multi-year gaps in Hubble's observations of Neptune. The OPAL program ensures that astronomers won't miss another one.

- These images are part of a scrapbook of Hubble snapshots of Neptune and Uranus that track the weather patterns over time on these distant, cold planets. Just as meteorologists cannot predict the weather on Earth by studying a few snapshots, astronomers cannot track atmospheric trends on solar system planets without regularly repeated observations. Astronomers hope that Hubble's long-term monitoring of the outer planets will help them unravel the mysteries that still persist about these faraway worlds.

- Analyzing the weather on these worlds also will help scientists better understand the diversity and similarities of the atmospheres of solar-system planets, including Earth.

• 04 February 2019: This atmospheric image shows a galaxy named Messier 85, captured in all its delicate, hazy glory by the NASA/ESA Hubble Space Telescope. Messier 85 slants through the constellation of Coma Berenices (Berenice's Hair), and lies around 50 million light-years from Earth. It was first discovered by Charles Messier's colleague Pierre Méchain in 1781, and is included in the Messier catalogue of celestial objects. 57)

- Messier 85 is intriguing — its properties lie somewhere between those of a lenticular and an elliptical galaxy, and it appears to be interacting with two of its neighbors: the beautiful spiral NGC 4394, located out of frame to the upper left, and the small elliptical MCG 3-32-38, located out of frame to the center bottom.

- The galaxy contains some 400 billion stars, most of which are very old. However, the central region hosts a population of relatively young stars of just a few billion years in age; these stars are thought to have formed in a late burst of star formation, likely triggered as Messier 85 merged with another galaxy over four billion years ago. Messier 85 has a further potentially strange quality. Almost every galaxy is thought to have a supermassive black hole at its center, but from measurements of the velocities of stars in this galaxy, it is unclear whether Messier 85 contains such a black hole.

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Figure 50: This image combines infrared, visible and ultraviolet observations from Hubble's Wide Field Camera 3 (image credit: ESA/Hubble & NASA, R. O'Connell)

• 31 January 2019: An international team of astronomers recently used the NASA/ESA Hubble Space Telescope to study white dwarf stars within the globular cluster NGC 6752. The aim of their observations was to use these stars to measure the age of the globular cluster, but in the process they made an unexpected discovery. 58)

- In the outer fringes of the area observed with Hubble's Advanced Camera for Surveys a compact collection of stars was visible. After a careful analysis of their brightnesses and temperatures, the astronomers concluded that these stars did not belong to the cluster – which is part of the Milky Way – but rather they are millions of light-years more distant.

- Our newly discovered cosmic neighbor, nicknamed Bedin I by the astronomers, is a modestly sized, elongated galaxy. It measures only around 3000 light-years at its greatest extent – a fraction of the size of the Milky Way. Not only is it tiny, but it is also incredibly faint. These properties led astronomers to classify it as a dwarf spheroidal galaxy.

- Dwarf spheroidal galaxies are defined by their small size, low-luminosity, lack of dust and old stellar populations [1]. 36 galaxies of this type are already known to exist in the Local Group of Galaxies, 22 of which are satellite galaxies of the Milky Way.
[1] While similar to dwarf elliptical galaxies in appearance and properties, dwarf spheroidal galaxies are in general approximately spherical in shape and have a lower luminosity.

- While dwarf spheroidal galaxies are not uncommon, Bedin I has some notable features. Not only is it one of just a few dwarf spheroidals that have a well established distance but it is also extremely isolated. It lies about 30 million light-years from the Milky Way and 2 million light-years from the nearest plausible large galaxy host, NGC 6744. This makes it possibly the most isolated small dwarf galaxy discovered to date.

- From the properties of its stars, astronomers were able to infer that the galaxy is around 13 billion years old – nearly as old as the Universe itself. Because of its isolation – which resulted in hardly any interaction with other galaxies – and its age, Bedin I is the astronomical equivalent of a living fossil from the early Universe.

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Figure 51: The accidentally discovered galaxy Bedin I (image credit: ESA/Hubble, NASA, Bedin et al., CC BY 4.0)

- The discovery of Bedin I was a truly serendipitous find. Very few Hubble images allow such faint objects to be seen, and they cover only a small area of the sky. Future telescopes with a large field of view, such as the WFIRST telescope, will have cameras covering a much larger area of the sky and may find many more of these galactic neighbors. 59)

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Figure 52: This composite image shows the location of the accidentally discovered dwarf galaxy Bedin I behind the globular cluster NGC 6752. The lower image, depicting the complete cluster, is a ground-based observation from the Digitized Sky Survey 2. The upper right image shows the full field of view of the NASA/ESA Hubble Space Telescope. The upper left one highlights the part containing the galaxy Bedin I (image credit: ESA/Hubble, NASA, Bedin et al., Digitized Sky Survey 2, CC BY 4.0)

• 24 January 2019: The rough-and-tumble environment near the center of the massive Coma galaxy cluster is no match for a wayward spiral galaxy. New images from NASA's Hubble Space Telescope show a spiral galaxy being stripped of its gas as it plunges toward the cluster's center. A long, thin streamer of gas and dust stretches like taffy from the galaxy's core and on into space. Eventually, the galaxy, named D100, will lose all of its gas and become a dead relic, deprived of the material to create new stars and shining only by the feeble glow of old, red stars. 60) 61)

- "This galaxy stands out as a particularly extreme example of processes common in massive clusters, where a galaxy goes from being a healthy spiral full of star formation to a 'red and dead galaxy,'" said William Cramer of Yale University in New Haven, Connecticut, leader of the team using the Hubble observations. "The spiral arms disappear, and the galaxy is left with no gas and only old stars. This phenomenon has been known about for several decades, but Hubble provides the best imagery of galaxies undergoing this process."

- Called "ram pressure stripping," the process occurs when a galaxy, due to the pull of gravity, falls toward the dense center of a massive cluster of thousands of galaxies, which swarm around like a hive of bees. During its plunge, the galaxy plows through intergalactic material, like a boat moving through water. The material pushes gas and dust from the galaxy. Once the galaxy loses all of its hydrogen gas — fuel for starbirth — it meets an untimely death because it can no longer create new stars. The gas-stripping process in D100 began roughly 300 million years ago.

- In the massive Coma cluster this violent gas-loss process occurs in many galaxies. But D100 is unique in several ways. Its long, thin tail is its most unusual feature. The tail, a mixture of dust and hydrogen gas, extends nearly 200,000 light-years, about the width of two Milky Way galaxies. But the pencil-like structure is comparatively narrow, only 7,000 light-years wide.

- "The tail is remarkably well-defined, straight and smooth, and has clear edges," explained team member Jeffrey Kenney, also of Yale University. "This is a surprise because a tail like this is not seen in most computer simulations. Most galaxies undergoing this process are more of a mess. The clean edges and filamentary structures of the tail suggest that magnetic fields play a prominent role in shaping it. Computer simulations show that magnetic fields form filaments in the tail's gas. With no magnetic fields, the tail is more clumpy than filamentary."

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Figure 53: The spiral galaxy D100, on the far right of this Hubble Space Telescope image, is being stripped of its gas as it plunges toward the center of the giant Coma galaxy cluster. The dark brown streaks near D100's central region are silhouettes of dust escaping from the galaxy. The dust is part of a long, thin tail, also composed of hydrogen gas, that stretches like taffy from the galaxy's core. Hubble, however, sees only the dust. The telescope's sharp vision also uncovered the blue glow of clumps of young stars in the tail. The brightest clump in the middle of the tail (the blue feature) contains at least 200,000 stars, fueled by the ongoing loss of hydrogen gas from D100 [image credit: NASA, ESA, M. Sun (University of Alabama), and W. Cramer and J. Kenney (Yale University)]

Legend to Figure 53: The gas-loss process occurs when D100, due to the pull of gravity, begins falling toward the dense center of the massive Coma cluster, consisting of thousands of galaxies. During its plunge, D100 plows through intergalactic material like a boat plowing through water. This material pushes gas and dust out of the galaxy. Once D100 loses all of its hydrogen gas, its star-making fuel, it can no longer create new stars. The gas-stripping process in the beleaguered galaxy began roughly 300 million years ago.

The reddish galaxies in the image contain older stars between the ages of 500 million to 13 billion years old. One of those galaxies is D99, just below and to the left of D100. It was stripped of its gas by the same process as the one that is siphoning gas from D100. The blue galaxies contain a mixture of young and old stars. Some of the stars are less than 500 million years old. The Coma cluster is located 330 million light-years from Earth. — The Hubble image is a blend of several exposures taken in visible light between May 10 and July 10, 2016, and November 2017 to January 2018, by the Advanced Camera for Surveys.

- The researchers' main goal was to study star formation along the tail. Hubble's sharp vision uncovered the blue glow of clumps of young stars. The brightest clump in the middle of the tail contains at least 200,000 stars, triggered by the ongoing gas loss from the galaxy. However, based on the amount of glowing hydrogen gas contained in the tail, the team had expected Hubble to uncover three times more stars than it detected.

- The Subaru Telescope in Hawaii observed the glowing tail in 2007 during a survey of the Coma cluster's galaxies. But the astronomers needed Hubble observations to confirm that the hot hydrogen gas contained in the tail was a signature of star formation.

- "Without the depth and resolution of Hubble, it's hard to say if the glowing hydrogen-gas emission is coming from stars in the tail or if it's just from the gas being heated," Cramer said. "These Hubble visible-light observations are the first and best follow-up of the Subaru survey."

- The Hubble data show that the gas-stripping process began on the outskirts of the galaxy and is moving in towards the center, which is typical in this type of mass loss. Based on the Hubble images, the gas has been cleared out all the way down to the central 6,400 light-years.

- Within that central region, there is still a lot of gas, as seen in a burst of star formation. "This region is the only place in the galaxy where gas exists and star formation is taking place," Cramer said. "But now that gas is being stripped out of the center, forming the long tail."

- Adding to this compelling narrative is another galaxy in the image that foreshadows D100's fate. The object, named D99, began as a spiral galaxy similar in mass to D100. It underwent the same violent gas-loss process as D100 is now undergoing, and is now a dead relic. All of the gas was siphoned from D99 between 500 million and 1 billion years ago. Its spiral structure has mostly faded away, and its stellar inhabitants consist of old, red stars. "D100 will look like D99 in a few hundred million years," Kenney said. — The Coma cluster is located 330 million light-years from Earth.

• 24 January, 2019: The Whirlpool Galaxy is a magnificent spiral galaxy that has been studied across the spectrum by NASA's Great Observatories. This remarkable video uses two dimensional images and three dimensional visualizations to contrast and compare the different views of infrared (Spitzer Space Telescope), visible (Hubble Space Telescope), and X-ray (Chandra X-ray Observatory) observations. Within these spectral bands, each wavelength region illustrates a different component of the stars, gas, and dust that comprise the galaxy. By both separating and combining seven multiwavelength views, astronomers gain a broader and richer look into the detailed structure of a spiral galaxy. 62)

Figure 54: A multiwavelength examination of the majestic Whirlpool Galaxy [video credit: NASA's Universe of Learning, Visualization: Frank Summers, Joseph DePasquale, Dani Player (STScI), Kim Arcand (SAO/CXC), Robert Hurt (Caltech/IPAC), Music: "Cylinder Five", Chris Zabriskie, CC BY 4.0]

• 21 January 2019: Gravitational lensing can help astronomers study objects that would otherwise be too faint or appear too small for us to view. When a massive object — such as a massive cluster of galaxies, as seen here — distorts space with its immense gravitational field, it causes light from more distant galaxies to travel along altered and warped paths. It also amplifies the light, making it possible for us to observe and study its source. 63)

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Figure 55: This picture showcases a gravitational lensing system called SDSS J0928+2031. Quite a few images of this type of lensing have been featured as Pictures of the Week in past months, as NASA/ESA Hubble Space Telescope data is currently being used to research how stars form and evolve in distant galaxies (image credit: ESA/Hubble & NASA, M. Gladders et al.)

- In this image, we see two dominant elliptical galaxies near the center of the image. The gravity from the galaxy cluster that is the home of these galaxies is acting as the aforementioned gravitational lens, allowing us to view the more distant galaxies sitting behind them. We see the effects of this lensing as narrow, curved streaks of light surrounding both of the large galaxies.

- This image was observed by Hubble as part of the Sloan Giant Arcs Survey program.

• 17 January 2019: The Hubble Space Telescope's Wide Field Camera 3 was brought back to full operational status and completed its first science observations just after noon EST today, 17 January. The instrument autonomously shut down on 8 January after internal data erroneously indicated invalid voltage levels. 64)

• 15 January 2019: NASA has moved closer to conducting science operations again with the Hubble Space Telescope's WFC3 (Wide Field Camera 3) instrument, which suspended operations on 8 January 2019. As of 15 January, the instrument was brought back to its operations mode. 65) 66)

- Shortly after noon EST (Eastern Standard Time) on 8 January, software installed on the Wide Field Camera 3 detected that some voltage levels within the instrument were out of the predefined range. The instrument autonomously suspended its operations as a safety precaution. Upon further investigation, the voltage levels appeared to be within normal range, yet the engineering data within the telemetry circuits for those voltage levels were not accurate. In addition, all other telemetry within those circuits also contained erroneous values indicating that this was a telemetry issue and not a power supply issue.

- After resetting the telemetry circuits and associated boards, additional engineering data were collected and the instrument was brought back to operations. All values were normal. Additional calibration and tests will be run over the next 48 to 72 hours to ensure that the instrument is operating properly. Further investigation using both the new and the previously collected engineering data will be conducted to determine why those data values were originally incorrect.

- Assuming that all tests work as planned, it is expected that the Wide Field Camera 3 will start to collect science images again by the end of the week.

- The Wide Field Camera 3 was installed during the last servicing mission to Hubble back in 2009. Over 2,000 peer-reviewed published papers have been produced from its data. Hubble itself is in its 29th year of operations, well surpassing its original 15-year lifetime.

- Hubble operations, like other satellite operations, are excepted activities as defined in the NASA furlough/shutdown plan. The current partial government shutdown does not affect its flight operations.

• 14 January 2019: Messier 89 is slightly smaller than the Milky Way, but has a few interesting features that stretch far out into the surrounding space. One structure of gas and dust extends up to 150 000 light-years out from the galaxy's center, which is known to house a supermassive black hole. Jets of heated particles reach out to 100 000 light-years from the galaxy, suggesting that Messier 89 may have once been far more active — perhaps an active quasar or radio galaxy — than it is now. It is also surrounded by an extensive system of shells and plumes, which may have been caused by past mergers with smaller galaxies — and implies that Messier 89 as we know it may have formed in the relatively recent past. 67)

- Messier 89 was discovered by astronomer Charles Messier in 1781, when Messier had been cataloguing astronomical objects for 23 years — ever since he mistook a faint object in the sky for Halley's Comet. Upon closer inspection, he realized the object was actually the Crab Nebula. To prevent other astronomers from making the same error, he decided to catalogue all the bright, deep-sky objects that could potentially be mistaken for comets. His methodical observations of the night sky led to the first comprehensive catalogue of astronomical objects: the Messier catalogue! Messier 89 holds the record for being the last ever giant elliptical to be found by Messier, and the most perfectly spherical galaxy in the entire catalogue of 110 objects.

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Figure 56: This huge ball of stars — around 100 billion in total — is an elliptical galaxy located some 55 million light-years away from us. Known as Messier 89, this galaxy appears to be perfectly spherical; this is unusual for elliptical galaxies, which tend to be elongated ellipsoids. The apparently spherical nature of Messier 89 could, however, be a trick of perspective, and be caused by its orientation relative to the Earth (image credit: ESA/Hubble & NASA, S. Faber, et al.)

• 9 January 2019: At 17:23 UTC on 8 January, the WFC3 (Wide Field Camera 3) on the Hubble Space Telescope suspended operations due to a hardware problem. Hubble will continue to perform science observations with its other three active instruments, while the Wide Field Camera 3 anomaly is investigated. WFC3, installed during Servicing Mission 4 in 2009, is equipped with redundant electronics should they be needed to recover the instrument. 68)

- There are concerns, however, that "engineers are unlikely to be able to fix the aging telescope until the ongoing U.S. government shutdown ends — whenever that might be," according to the science journal Nature. Engineers are unlikely to be able to fix the ageing telescope until the ongoing US government shutdown ends — whenever that might be. 69)

- Hubble's mission operations are based at NASA's Goddard Space Flight Center in Greenbelt, Maryland, where most employees are on involuntary leave during the shutdown. A few people who operate spacecraft that are actively flying, including Hubble, have been allowed to keep working.

- But fixing the telescope, which is almost 30 years old, will almost certainly require additional government employees who are forbidden to work during the shutdown. NASA has formed an investigative team, composed primarily of contractors and experts from its industry partners, to examine the technical troubles.

- Federal law allows agencies to keep some personnel working during a shutdown if they are deemed necessary for protecting life and property. It is not clear whether NASA will request an emergency exception to allow repairs to Hubble before the shutdown — now on its nineteenth day — ends.

- An e-mail to a NASA press officer seeking comment prompted this automatic reply: "I am in furlough status and unable to respond to your message at this time."

- Last October, Hubble stopped working entirely for three weeks after the failure of one of the gyroscopes that it uses to orient itself in space. Engineers fixed the problem, but the rescue effort required input from experts from across NASA, including many who are currently furloughed.

- The STScI (Space Telescope Science Institute) in Baltimore, Maryland, which runs Hubble's science operations, remains open for now, using money it received from NASA before the shutdown started. But many of Hubble's technical experts are based at Goddard, which is closed.

- The risk of not being able to fix Hubble if something broke is one of the impacts scientists were worried about as the government shutdown began on 22 December.

- The shutdown, which affects roughly 75% of the government, is now in its third week with no end in sight. If it persists until 12 January, it will break the record for longest shutdown, which was set by a 21-day event that began on 16 December 1995.

• 8 January 2019: Astronomers have found a new exoplanet that could alter the standing theory of planet formation. With a mass that's between that of Neptune and Saturn, and its location beyond the "snow line" of its host star, an alien world of this scale was supposed to be rare. 70)

- Aparna Bhattacharya, a postdoctoral researcher from the University of Maryland and NASA's Goddard Space Flight Center (GSFC), led the team that made the discovery, which was announced today during a press conference at the 233rd Meeting of the American Astronomical Society in Seattle.

- Using the Near-Infrared Camera, second generation (NIRC2) instrument on the 10-meter Keck II telescope of the W. M. Keck Observatory on Maunakea, Hawaii and the WFC3 (Wide Field Camera 3) instrument on the Hubble Space Telescope, the researchers took simultaneous high-resolution images of the exoplanet, named OGLE-2012-BLG-0950Lb, allowing them to determine its mass.

- "We were surprised to see the mass come out right in the middle of the predicted intermediate giant planet mass gap," said Bhattacharya. "It's like finding an oasis in the middle of the exoplanet desert!"

- "I was very pleased with how quickly Aparna completed the analysis," said co-author David Bennett, a senior research scientist at the University of Maryland and GSFC. "She had to develop some new methods to analyze this data — a type of analysis that had never been done before."

- In an uncanny timing of events, another team of astronomers (which included Bhattacharya and Bennett) published a statistical analysis at almost the same time showing that such sub-Saturn mass planets are not rare after all.

- "We were just finishing up the analysis when the mass measurements of OGLE-2012- BLG-0950Lb came in," said lead author Daisuke Suzuki of Japan's ISAS (Institute of Space and Astronautical Science). "This planet confirmed our interpretation of the statistical study."

- The teams' results on OGLE-2012-BLG-0950Lb are published in the December issue of The Astronomical Journal and the statistical study was published in the December 20th issue of the Astrophysical Journal Letters. 71)

- OGLE-2012-BLG-0950Lb was among the sub-Saturn planets in the statistical study; all were detected through microlensing, the only method currently sensitive enough to detect planets with less than Saturn's mass in Jupiter-like orbits.

- Microlensing leverages a consequence of Einstein's theory of general relativity: the bending and magnification of light near a massive object like a star, producing a natural lens on the sky. In the case of OGLE-2012-BLG-0950Lb, the light from a distant background star was magnified by OGLE-2012-BLG-0950L (the exoplanet's host star) over the course of two months as it passed close to perfect alignment in the sky with the background star.

- By carefully analyzing the light during the alignment, an unexpected dimming with a duration of about a day was observed, revealing the presence of OGLE-2012-BLG-0950Lb via its own influence on the lensing.

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Figure 57: Planet OGLE-2012-BLG-0950Lb was detected through gravitational microlensing, a phenomenon that acts as nature's magnifying glass (image credit: LCO, D. Bennett)

Methodology

- OGLE-2012-BLG-0950Lb was first detected by the microlensing survey telescopes of the Optical Gravitational Lensing Experiment (OGLE) and the Microlensing Observations in Astrophysics (MOA) collaborations.

- Bhattacharya's team then conducted follow-up observations using Keck Observatory's powerful adaptive optics system in combination with NIRC2.

- "The Keck observations allowed us to determine that the sub-Saturn or super-Neptune size planet has a mass of 39 times that of the Earth, and that its host star is 0.58 times the mass of the Sun," said Bennett. "They measured the separation of the foreground planetary system from the background star. This allowed us to work out the complete geometry of the microlensing event. Without this data, we only knew the star-planet mass ratio, not the individual masses."

- For the statistical study, Suzuki's team and MOA analyzed the properties of 30 sub-Saturn planets found by microlensing and compared them to predictions from the core accretion theory.

Challenging the Theory

- What is unique about the microlensing method is its sensitivity to sub-Saturn planets like OGLE-2012-BLG-0950Lb that orbit beyond the "snow line" of their host stars.

- The snow line, or frost line, is the distance in a young solar system, (a.k.a. a protoplanetary disk) at which it is cold enough for water to condense into ice. At and beyond the snow line there is a dramatic increase in the amount of solid material needed for planet formation. According to the core accretion theory, the solids are thought to build up into planetary cores first through chemical and then gravitational processes.

- "A key process of the core accretion theory is called "runaway gas accretion," said Bennett. "Giant planets are thought to start their formation process by collecting a core mass of about 10 times the Earth mass in rock and ice. At this stage, a slow accretion of hydrogen and helium gas begins until the mass has doubled. Then, the accretion of hydrogen and helium is expected to speed up exponentially in this runaway gas accretion process. This process stops when the supply is exhausted. If the supply of gas is stopped before runaway accretion stops, we get "failed Jupiter" planets with masses of 10-20 Earth-masses (like Neptune)."

- The runaway gas accretion scenario of the core accretion theory predicts that planets like OGLE-2012- BLG-0950Lb are expected to be rare. At 39 times the mass of the Earth, planets this size are thought to be continuing through a stage of rapid growth, ending in a much more massive planet. This new result suggests that the runaway growth scenario may need revision.

- Suzuki's team compared the distribution of planet-star mass ratios found by microlensing to distributions predicted by the core accretion theory. They found that the core accretion theory's runaway gas accretion process predicts about 10 times fewer intermediate mass giant planets like OGLE-2012- BLG-0950Lb than are seen in the microlensing results.

- This discrepancy implies that gas giant formation may involve processes that have been overlooked by existing core accretion models, or that the planet forming environment varies considerably as a function of host star mass.

Next Steps

- This discovery has not only called into question an established theory, it was made using a new technique that will be a key part of NASA's next big planet finding mission, the Wide Field Infra-Red Survey Telescope (WFIRST), which is scheduled to launch into orbit in the mid-2020s.

- "This is exactly the method that WFIRST will use to measure the masses of the planets that it discovers with its exoplanet microlensing survey. Until WFIRST comes online, we need to develop this method with observations from our Keck Key Strategic Mission Support (KSMS) program as well as observations from Hubble," said Bennett.

- "It's very exciting to see Keck and Hubble combine forces to provide this surprising new result," said Keck Observatory Chief Scientist John O'Meara. "And it's equally exciting to know that we can make these kind of advances today to help facilitate the best science from WFIRST and Keck's partnership in the future."

- The NASA Keck KSMS program will continue to make follow-up observations of microlensing events detected by telescopes on the ground and in space.

• 8 January 2019: Rocky planets orbiting red dwarf stars may be bone dry and lifeless, according to a new study using NASA's Hubble Space Telescope. Water and organic compounds, essential for life as we know it, may get blown away before they can reach the surface of young planets. 72)

- This hypothesis is based on surprising observations of a rapidly eroding dust-and-gas disk encircling the young, nearby red dwarf star AU Microscopii (AU Mic) by Hubble and the European Southern Observatory's VLT (Very Large Telescope) in Chile. Planets are born in disks like this one. - Red dwarfs, which are smaller and fainter than our Sun, are the most abundant and longest-lived stars in the galaxy.

- Fast-moving blobs of material appear to be ejecting particles from the AU Mic disk. If the disk continues to dissipate at this rapid pace, it will be gone in about 1.5 million years. In that short time, icy material from comets and asteroids could be cleared out of the disk. Comets and asteroids are important because they are believed to have seeded rocky planets such as Earth with water and organic compounds, the chemical building blocks for life. If this same transport system is needed for planets in the AU Mic system, then they may end up "dry" and dusty—inhospitable for life as we know it.

- "The Earth, we know, formed 'dry,' with a hot, molten surface, and accreted atmospheric water and other volatiles for hundreds of millions of years, being enriched by icy material from comets and asteroids transported from the outer solar system," said co-investigator Glenn Schneider of Steward Observatory in Tucson, Arizona. The observations are led by John Wisniewski of the University of Oklahoma in Norman, whose team is composed of 14 astronomers from the U.S. and Europe. 73)

- If the activity around AU Mic is typical of the planet-birthing process among red dwarfs, it could further reduce prospects of habitable worlds across our galaxy. Previous observations suggest that a torrent of ultraviolet light from young red dwarf stars quickly strips away the atmosphere of any orbiting planets. This particular star is only 23 million years old.

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Figure 58: These two NASA Hubble Space Telescope images, taken six years apart, show fast-moving blobs of material sweeping outwardly through a debris disk around the young, nearby red dwarf star AU Microscopii (AU Mic). Red dwarfs are the most abundant and longest-lived stars in our Milky Way galaxy. AU Mic is approximately 23 million years old. The top image was taken in 2011; the bottom in 2017. Hubble's Space Telescope Imaging Spectrograph (STIS) took the images in visible light. This comparison of the two images shows the six-year movement of one of the known blobs (marked by an arrow). Researchers estimate that the blob, which is zipping along at nearly 15,000 miles an hour, traveled more that 820 million miles between 2011 and 2017. That is about the distance from Earth to Saturn. Astronomers do not know how the blobs are launched through the system. Eventually, the blob highlighted in the image will sweep through the disk, escape the star's gravitational grip, and race out into space. Astronomers expect the string of blobs to clear out the disk within 1.5 million years. Their estimated ejection speeds are between 9,000 miles per hour and 27,000 miles per hour, fast enough to escape the star's gravitational clutches. They currently range in distance from roughly 930 million miles to more than 5.5 billion miles from the star. The disk, seen edge-on, is illuminated by scattered light from the star. The glare of the star, located at the center of the disk, has been blocked out by the STIS coronagraph so that astronomers can see more structure in the disk. The bright dot above the left side of the disk in the 2017 image is a background star. The system resides 32 light-years away in the southern constellation Microscopium [image credit: NASA, ESA, J. Wisniewski (University of Oklahoma), C. Grady (Eureka Scientific), and G. Schneider (Steward Observatory)]

- Surveys have shown that terrestrial planets are common around red dwarfs. In fact, they should contain the bulk of our galaxy's planet population, which could number tens of billions of worlds. Planets have been found within the habitable zone of several nearby red dwarfs, but their physical characteristics are largely unknown.

Blown Out by Blobs

- Observations by Hubble's Space Telescope Imaging Spectrograph (STIS) and the VLT show that the AU Mic circumstellar disk is being excavated by fast-moving blobs of circumstellar material, which are acting like a snowplow by pushing small particles—possibly containing water and other volatiles—out of the system. Researchers don't yet know how the blobs were launched. One theory is that powerful mass ejections from the turbulent star expelled them. Such energetic activity is common among young red dwarfs.

- "These observations suggest that water-bearing planets might be rare around red dwarfs because all the smaller bodies transporting water and organics are blown out as the disk is excavated," explained Carol Grady of Eureka Scientific in Oakland, California, co-investigator on the Hubble observations.

- Conventional theory holds that billions of years ago Earth formed as a comparatively dry planet. Gravitationally perturbed asteroids and comets, rich in water from the cooler outer solar system, bombarded Earth and seeded the surface with ice and organic compounds. "However, this process may not work in all planetary systems," Grady said.

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Figure 59: The Hubble Space Telescope image on the left is an edge-on view of a portion of a vast debris disk around the young, nearby red dwarf star AU Microscopii (AU Mic). Though planets may have already formed in the disk, Hubble is tracking the movement of several huge blobs of material that could be "snowplowing" remaining debris out of the system, including comets and asteroids. The box in the image at left highlights one blob of material extending above and below the disk. Hubble's Space Telescope Imaging Spectrograph (STIS) took the picture in 2018, in visible light. The glare of the star, located at the center of the disk, has been blocked out by the STIS coronagraph so that astronomers can see more structure in the disk. The STIS close-up image at right reveals, for the first time, details in the blobby material, including a loop-like structure and a mushroom-shaped cap. Astronomers expect the train of blobs to clear out the disk within only 1.5 million years. The consequences are that any rocky planets could be left bone-dry and lifeless, because comets and asteroids will no longer be available to glaze the planets with water or organic compounds. AU Mic is approximately 23 million years old. The system resides 32 light-years away in the southern constellation Microscopium. Credit: NASA, ESA, J. Wisniewski (University of Oklahoma), [image credit: C. Grady (Eureka Scientific), and G. Schneider (Steward Observatory)]

- The team determined the disk's lifespan by using an estimated mass of the disk from an independent study, as well as calculating the mass of the escaping blobs in their STIS visible-light data. The mass of each blob is about four ten-millionths the mass of Earth. The disk's mass—about 1.7 times more massive than Earth—is based on data taken by the ALMA (Atacama Large Millimeter/submillimeter Array ).

- Although the mass of the wayward blobs seems tiny, the diameter of each blob could stretch at least from the Sun to Jupiter. At present, the team has spotted six outbound blobs, but it is possible that there is a continuous stream of them. Groups of blobs careening through the disk could sweep out material fairly quickly.

- "The fast dissipation of the disk is not something I would have expected," Grady said. "Based on the observations of disks around more luminous stars, we had expected disks around fainter red dwarf stars to have a longer time span. In this system, the disk will be gone before the star is 25 million years old." She added that AU Mic likely started out with an outer rim of small icy bodies, like the Kuiper belt found within our own solar system. If the disk weren't being eroded, it would have provided ices to any dry inner planets.

Probing the Blob Mystery

- Hubble astronomers spotted the blobs in STIS visible-light images taken in 2010-2011. As a follow-up to the Hubble study, the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) instrument mounted on the European Southern Observatory's Very Large Telescope in Chile, made near-infrared observations. Features in the disk were hinted at in observations taken in 2004 by ground-based telescopes and Hubble's Advanced Camera for Surveys.

- So far, the team has uncovered blobs on the disk's southeast side, with estimated ejection speeds between 9,000 miles per hour and 27,000 miles per hour, fast enough to escape the star's gravitational clutches. They currently range in distance from roughly 930 million miles to more than 5.5 billion miles from the star.

- Hubble is also showing that these blobs may not just be giant balls of dusty debris. The telescope has resolved substructure in one of the blobs, including a mushroom-shaped cap above the plane of the disk itself and a complex "loop-like" structure below the disk. "These structures could yield clues to the mechanisms that drive these blobs," Schneider said. - The system resides 32 light-years away in the southern constellation Microscopium.

- "AU Mic is ideally placed," Schneider said. "But it is only one of about three or four red-dwarf systems with known starlight-scattering disks of circumstellar debris. The other known systems are typically about six times farther away, so it's challenging to conduct a detailed study of the types of features in those disks that we see in AU Mic."

- However, astronomers are beginning to identify some possibly similar activity in these other systems. "It shows that AU Mic is not unique," Grady said. "In fact, you could argue that because it is one of the nearest systems of this type, it would be unlikely that it would be unique."

- The AU Mic observations show the importance of a star's disk environment on planet formation and evolution. "What we have learned is that disks seem to be a normal part of the history of planetary systems," Grady said. "If you don't understand a star's disk, you don't have a good understanding of the resulting planetary system."

• 7 January 2019: The NASA/ESA Hubble Space Telescope has captured the most detailed image yet of a close neighbor of the Milky Way – the Triangulum Galaxy, a spiral galaxy located at a distance of only three million light-years. This panoramic survey of the third-largest galaxy in our Local Group of galaxies provides a mesmerizing view of the 40 billion stars that make up one of the most distant objects visible to the naked eye. 74)

- This new image of the Triangulum Galaxy – also known as Messier 33 or NGC 598 – has a staggering 665 million pixels and showcases the central region of the galaxy and its inner spiral arms. To stitch together this gigantic mosaic, Hubble's ACS (Advanced Camera for Surveys) needed to create 54 separate images.

- Under excellent dark-sky conditions, the Triangulum Galaxy can be seen with the naked eye as a faint, blurry object in the constellation of Triangulum (the Triangle), where its ethereal glow is an exciting target for amateur astronomers.

- At only three million light-years from Earth, the Triangulum Galaxy is a notable member of the Local Group – it is the group's third-largest galaxy, but also the smallest spiral galaxy in the group [1]. It measures only about 60 000 light-years across, compared to the 200 000 light-years of the Andromeda Galaxy; the Milky Way lies between these extremes at about 100 000 light-years in diameter [2].
Note 1: Our galaxy, the Milky Way, is part of the Local Group, an assembly of more than 50 galaxies bound together by gravity. Its largest member is the Andromeda Galaxy – also known as Messier 31 – followed by the Milky Way and the Triangulum Galaxy. The remaining members of the Local Group are dwarf galaxies, each orbiting one of the three larger ones.
Note 2: The much bigger Andromeda Galaxy was mapped by Hubble in 2015, creating the sharpest and largest image of this galaxy and the largest Hubble image ever (heic1502).

- The Triangulum Galaxy is not only surpassed in size by the other two spirals, but by the multitude of stars they contain. The Triangulum Galaxy has at least an order of magnitude less stars than the Milky Way and two orders of magnitude less than Andromeda. These numbers are hard to grasp when already in this image 10 to 15 million individual stars are visible.

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Figure 60: This gigantic image of the Triangulum Galaxy – also known as Messier 33 – is a composite of about 54 different pointings with Hubble's Advanced Camera for Surveys. With a staggering size of 34,372 times 19,345 pixels, it is the second-largest image ever released by Hubble. The mosaic of the Triangulum Galaxy showcases the central region of the galaxy and its inner spiral arms. Millions of stars, hundreds of star clusters and bright nebulae are visible. This image is too large to be easily displayed at full resolution and is best appreciated using the zoom tool [image credit: NASA, ESA, and M. Durbin, J. Dalcanton, and B. F. Williams (University of Washington)]

- In contrast to the two larger spirals, the Triangulum Galaxy doesn't have a bright bulge at its center and it also lacks a bar connecting its spiral arms to the center. It does, however, contain a huge amount of gas and dust, giving rise to rapid star formation. New stars form at a rate of approximately one solar mass every two years.

- The abundance of gas clouds in the Triangulum Galaxy is precisely what drew astronomers to conduct this detailed survey. When stars are born, they use up material in these clouds of gas and dust, leaving less fuel for new stars to emerge. Hubble's image shows two of the four brightest of these regions in the galaxy: NGC 595 and NGC 604. The latter is the second most luminous region of ionized hydrogen within the Local Group and it is also among the largest known star formation regions in the Local Group.

- These detailed observations of the Triangulum Galaxy have tremendous legacy value – combined with those of the Milky Way, the Andromeda Galaxy and the irregular Magellanic Cloud galaxies, they will help astronomers to better understand star formation and stellar evolution.

• 21 December 2018: The bright southern hemisphere star RS Puppis, at the center of the image (Figure 61), is swaddled in a gossamer cocoon of reflective dust illuminated by the glittering star. The super star is ten times more massive than the Sun and 200 times larger. 75)

- RS Puppis rhythmically brightens and dims over a six-week cycle. It is one of the most luminous in the class of so-called Cepheid variable stars. Its average intrinsic brightness is 15,000 times greater than the Sun's luminosity.

- The nebula flickers in brightness as pulses of light from the Cepheid propagate outwards. Hubble took a series of photos of light flashes rippling across the nebula in a phenomenon known as a "light echo." Even though light travels through space fast enough to span the gap between Earth and the Moon in a little over a second, the nebula is so large that reflected light can actually be photographed traversing the nebula.

- By observing the fluctuation of light in RS Puppis itself, as well as recording the faint reflections of light pulses moving across the nebula, astronomers are able to measure these light echoes and pin down a very accurate distance. The distance to RS Puppis has been narrowed down to 6,500 light-years (with a margin of error of only one percent).

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Figure 61: This festive NASA Hubble Space Telescope image resembles a holiday wreath made of sparkling lights (image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA) – Hubble/Europe Collaboration; Acknowledgement: H. Bond (STScI and Pennsylvania State University)

• 20 December 2018: Astronomers using data from the NASA/ESA Hubble Space Telescope have employed a revolutionary method to detect dark matter in galaxy clusters. The method allows astronomers to "see" the distribution of dark matter more accurately than any other method used to date and it could possibly be used to explore the ultimate nature of dark matter. The results were published in the journal Monthly Notices of the Royal Astronomical Society. 76)

- In recent decades astronomers have tried to understand the true nature of the mysterious substance that makes up most of the matter in the Universe – dark matter – and to map its distribution in the Universe.
Note 1: Dark matter makes up about 85% of the matter in the Universe, and about a quarter of its total energy density. Dark matter does not emit any kind of electromagnetic radiation – its presence can only be determined via gravitational effects.

Now two astronomers from Australia and Spain have used data from the Frontier Fields program of the NASA/ESA Hubble Space Telescope to accurately study the distribution of dark matter.
Note 2: The Hubble Frontier Fields program was a deep imaging initiative designed to utilize the strong gravitational lensing effects in galaxy clusters to see extremely distant galaxies and thereby gain insight into the early Universe and the evolution of galaxies since that time. The program observed six galaxy clusters over 630 hours of Hubble's time. To receive the new results presented here the data was used in a different way, without using gravitational lensing.

- "We have found a way to 'see' dark matter," explains Mireia Montes (University of New South Wales, Australia), lead author of the study. "We have found that very faint light in galaxy clusters, the intracluster light, maps how dark matter is distributed." 77)

- Intracluster light is a byproduct of interactions between galaxies. In the course of these interactions, individual stars are stripped from their galaxies and float freely within the cluster. Once free from their galaxies, they end up where the majority of the mass of the cluster, mostly dark matter, resides.

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Figure 62: Abell S1063, a galaxy cluster, was observed by the NASA/ESA Hubble Space Telescope as part of the Frontier Fields program. The huge mass of the cluster – containing both baryonic matter and dark matter – acts as cosmic magnification glass and deforms objects behind it. In the past astronomers used this gravitational lensing effect to calculate the distribution of dark matter in galaxy clusters (image credit: NASA, ESA, and M. Montes (University of New South Wales, Sydney, Australia)

- "These stars have an identical distribution to the dark matter, as far as our current technology allows us to study," explained Montes. Both the dark matter and these isolated stars – which form the intracluster light – act as collisionless components. These follow the gravitational potential of the cluster itself. The study showed that the intracluster light is aligned with the dark matter, tracing its distribution more accurately than any other method relying on luminous tracers used so far.

- This method is also more efficient than the more complex method of using gravitational lensing. While the latter requires both accurate lensing reconstruction and time-consuming spectroscopic campaigns, the method presented by Montes utilizes only deep imaging. This means more clusters can be studied with the new method in the same amount of observation time.

- The results of the study introduce the possibility of exploring the ultimate nature of dark matter. "If dark matter is self-interacting we could detect this as tiny departures in the dark matter distribution compared to this very faint stellar glow," highlights Ignacio Trujillo (Instituto de Astrofísica de Canarias, Spain), co-author of the study. Currently, all that is known about dark matter is that it appears to interact with regular matter gravitationally, but not in any other way. To find that it self-interacts would place significant constraints on its identity.

- For now, Montes and Trujillo plan to survey more of the original six clusters to see if their method remains accurate. Another important test of their method will be the observation and analysis of additional galaxy clusters by other research teams, to add to the data set and confirm their findings.

- The team can also look forward to the application of the same techniques using future space-based telescopes like the NASA/ESA/CSA James Webb Space Telescope, which will have even more sensitive instruments able to resolve faint intracluster light in the distant Universe.

- "There are exciting possibilities that we should be able to probe in the upcoming years by studying hundreds of galaxy clusters," concludes Ignacio Trujillo.

• 14 December 2018: The speed and distance at which planets orbit their respective blazing stars can determine each planet's fate - whether the planet remains a longstanding part of its solar system or evaporates into the universe's dark graveyard more quickly. - In their quest to learn more about far-away planets beyond our own solar system, astronomers discovered that a medium-sized planet roughly the size of Neptune, GJ 3470b, is evaporating at a rate 100 times faster than a previously discovered planet of similar size, GJ 436b. 78)

- The findings, published in the journal of Astronomy and Astrophysics, advance astronomers' knowledge about how planets evolve. 79)

- The study is part of the Panchromatic Comparative Exoplanet Treasury (PanCET) program, led by Sing, which aims to measure the atmospheres of 20 exoplanets in ultraviolet, optical and infrared light, as they orbit their stars. PanCET is the largest exoplanet observation program to be run with NASA's Hubble Space Telescope.

- One particular issue of interest to astronomers is how planets lose their mass through evaporation. Planets such as "super" Earths and "hot" Jupiters orbit more closely to their stars and are therefore hotter, causing the outermost layer of their atmospheres to be blown away by evaporation.

- While these larger Jupiter-sized and smaller Earth-sized exoplanets are plentiful, medium Neptune-sized exoplanets (roughly four times larger than Earth) are rare. Researchers hypothesize that these Neptunes get stripped of their atmospheres and ultimately become smaller planets.

- It's difficult, however, to actively witness them doing so because they can only be studied in UV light, which limits researchers to examining nearby stars no greater than 150 light-years away from earth, not obscured by interstellar material. GJ 3470b is 96 light-years away and circles a red dwarf star in the general direction of the constellation Cancer.

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Figure 63: This graphic plots exoplanets based on their size and distance from their star. Each dot represents an exoplanet. Planets the size of Jupiter (located at the top of the graphic) and planets the size of Earth and so-called super-Earths (at the bottom) are found both close and far from their star. But planets the size of Neptune (in the middle of the plot) are scarce close to their star. This so-called desert of hot Neptunes shows that such alien worlds are rare, or, they were plentiful at one time, but have since disappeared. The detection that GJ 3470b, a warm Neptune at the border of the desert, is fast losing its atmosphere suggests that hotter Neptunes may have eroded down to smaller, rocky super-Earths (image credit: International Team, STScI)

- In this study, Hubble found that exoplanet GJ 3470b had lost significantly more mass and had a noticeably smaller exosphere than the first Neptune-sized exoplanet studied, GJ 436b, due to its lower density and receipt of a stronger radiation blast from its host star.

- GJ 3470b's lower density makes it unable to gravitationally hang on to the heated atmosphere, and while the star hosting GJ 436b was between 4 billion and 8 billion years old, the star hosting GJ 3470b is only 2 billion years old; a younger star is more active and powerful, and, therefore, has more radiation to heat the planet's atmosphere.

- Sing's team estimates that GJ 3470b may have already lost up to 35 percent of its total mass and, in a few billion years, all of its gas may be stripped off, leaving behind only a rocky core.

- "We're starting to better understand how planets are shaped and what properties influence their overall makeup," Sing said. "Our goal with this study and the overarching PanCET program is to take a broad look at these planets' atmospheres to determine how each planet is affected by its own environment. By comparing different planets, we can start piecing together the larger picture in how they evolve."

- Looking forward, Sing and the team hope to study more exoplanets by searching for helium in infrared light, which will allow a greater search range than searching for hydrogen in UV light.

- Currently, planets, which are made largely of hydrogen and helium, can only be studied through tracing hydrogen in UV light. Using Hubble, the upcoming NASA James Webb Space Telescope (which will have a greater sensitivity to helium), and a new instrument called Carmenes that Sing recently found can precisely track the trajectory of helium atoms, astronomers will be able to broaden their pursuit of distant planets.

• 4 December 2018: Twenty-five years ago this week, NASA held its collective breath as seven astronauts on space shuttle Endeavour caught up with the Hubble Space Telescope 353 miles (568 kilometers) above Earth. Their mission: to fix a devastating flaw in the telescope's primary mirror. 80)

- Hubble Space Telescope has a primary mirror of 2.4 m in aperture. The largest optical telescope launched into space, where it could observe the universe free from the distorting effects of Earth's atmosphere, Hubble had a lot riding on it. But after the first images were obtained and carefully analyzed following the telescope's deployment on April 25, 1990, it was clear that something was wrong: The images were blurry.

- Astronomers and engineers rallied to study a variety of solutions to the problem, and NASA convened an independent committee to find the source. They all came to the same conclusion: Hubble's primary mirror, which looks like a very shallow bowl, had been polished into the wrong shape. The error was smaller than the width of a human hair, but the effect was significant. If the error went uncorrected, Hubble would never reach its full potential.

- During the week of 6 December 1993, the astronaut crew installed two pieces of hardware intended to fix the error. The Corrective Optics Space Telescope Axial Replacement (COSTAR) was designed and built by a team at NASA's Goddard Spaceflight Center in Greenbelt, Maryland, and would correct for the mirror error in three of the five instruments on Hubble.

- The second instrument was the Wide Field and Planetary Camera 2 (WFPC2), designed and built at NASA's Jet Propulsion Laboratory in Pasadena, California. WFPC2, which actually contains four cameras, would go on to produce many of Hubble's breathtaking images, helping transform our view of the cosmos.

- The size of baby grand piano, the instrument imaged objects and events that occurred in our own solar system - such as comet Shoemaker-Levy 9's crash into Jupiter - to the most distant cosmological images that had ever been taken in visible light. It generated breathtaking snapshots of galaxies, exploded stars and nebulae where new stars are born. During the instrument's tenure, Hubble managers pointed the telescope at a single, black patch of sky for more than a week and found thousands of previously unseen galaxies.

- But WFPC2's success was far from guaranteed. The instrument was built on an incredibly tight timeline, and designing it to correct the flaw was something JPL's John Trauger, principal investigator for WFPC2, would later describe as being akin to "trying to play baseball on the side of a hill."

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Figure 64: Replacing the Wide Field and Planetary Camera. Astronaut Jeffrey Hoffman removes Wide Field and Planetary Camera 1 (WFPC 1) during change-out operations (image credit: NASA)

- "There's a lot of pressure when you're building a space instrument even under normal circumstances," said Dave Gallagher, JPL's associate director for strategic integration, who served as integration and test manager for WFPC2. "But when you're fixing something that will essentially make or break the reputation of the entire agency, the pressure goes through the roof."

A Mirror Image

- In June 1990, NASA announced that the Hubble telescope was not working as expected. WFPC2 team members say they remember that the reaction from the public and the media was often pessimistic or even incredulous. Trauger watched network news anchor Tom Brokaw begin his program that evening by saying, "The Hubble Telescope you've heard so much about - it's broken."

- "The promise of the Hubble program, the application of our best technology to push back the frontiers of astronomy, had been instantly transformed in the public eye to an icon of technical failure," Trauger wrote in an essay in 2007.

- Trauger brought his team together to work the problem. The telescope's primary and secondary mirrors collected light and fed it to the five onboard science instruments. The primary mirror could not be replaced and could not be returned to Earth for repairs. A solution would have to be found for each of Hubble's instruments. The COSTAR device provided corrective optics for three of them, eliminating the need to fully replace those instruments. But the same approach wouldn't work for the telescope's Wide Field and Planetary Camera (WFPC), the predecessor of WFPC2.

- Trauger and his team came up with a potential solution. The primary mirror error caused light striking different parts of the mirror to come into focus at different locations, so the team had to figure out how to redirect it to the appropriate focal point. Their solution was to reverse-engineer the problem: They would place four identical nickel-sized mirrors inside the instrument - one for each of the four cameras inside WFPC2 - with the same error as the flawed primary mirror, but where the primary mirror was too flat, the new mirrors would be curved too deeply. Together, these two errors would cancel each other, producing the equivalent of a single mirror with the correct shape.

- NASA accepted JPL's proposal to build a WFPC replacement. The agency had planned to carry out Hubble repair missions every three years and decided to maintain this schedule. The first repair mission was set for the fall of 1993. JPL would need to deliver the replacement by the winter of 1992 - just over 2 years away. The race to repair Hubble was on.

Under Pressure

- Two years was nowhere near enough time to build a new camera instrument from scratch. Thankfully, WFPC2 was already under construction at JPL; NASA had intended to eventually use it as an upgrade for WFPC or a replacement if the instrument ever failed.

- Even with work on WFPC2 already under way, the deadline required an accelerated schedule. Dave Rodgers and Larry Simmons, the WFPC2 project managers, held daily meetings with the leaders of each of WFPC2's several components to help stay on target.

- "The daily meetings kept the pressure on all of us, all the time," said Simmons, who retired from JPL in 2005. "We knew we only had a few years, and we had to get it done."

- While the corrective mirrors were small, they affected nearly every step of the building process and created "an endless string of novel problems," according to Trauger.

- To minimize the chance for error during WFPC2's installation in low-Earth orbit, the seven astronauts who were scheduled to execute the repair mission traveled to JPL to learn about the instrument and be trained on how to install it. They would be inserting WFPC2 into a cavity in the telescope's body, as if sliding it in a drawer. And although they would need to make sure that the electrical connections at the back of the instrument were secure, they had no way of reaching those connections; they could control only how they inserted the instrument.

- Complicating matters further was the weight of WFPC2: At more than 600 pounds (272 kilograms), it was unwieldy even in the microgravity of low-Earth orbit. One of the instrument's mirrors, called the pickoff mirror, was mounted on a short arm located outside the protective casing. Merely bumping the mirror would misalign the system and essentially ruin the entire instrument. During WFPC2's construction, Trauger and colleagues showed a model of the instrument to an astronaut, who bumped the pickoff mirror. Trauger couldn't help but wonder, "Is this an omen?"

Time to Fly

- The leaders of the WFPC2 team traveled to NASA's Kennedy Space Center in Florida for the early morning launch on Dec. 2, 1993. After departing Kennedy and seeking out an early breakfast, Gallagher remembers looking up at the predawn sky to see the space shuttle passing overhead and nearing Hubble; the objects appeared as two faint points of light in the sky as they orbited Earth.

- On the sixth day of the mission, astronauts Jeffrey Hoffman and Story Musgrave conducted a spacewalk to remove WFPC from Hubble and install WFPC2. Everything seemed to go as planned, but the real test was yet to come.

- The astronauts returned to Earth on Dec. 13, and the first raw data from WFPC2 came back on Dec. 18. The team put the data through the image-processing software and watched anxiously as the pictures began to ratchet across the screen. There was instant relief.

- "They were sharp," Trauger said of the images. "And it wasn't just that we had pictures that looked amazing, it was that we were making new discoveries right away. There were things in the images that we'd never seen before."

- NASA released those first images to the public on Jan. 13, 1994. The next day, the WFPC2 team presented the results to an overflow audience at the winter meeting of the American Astronomical Society.

- "When we showed the first images, the room erupted; we got a standing ovation," Trauger said. "You don't usually see that at an astronomy meeting!"

- The WFPC2 instrument operated on Hubble for over 15 years and took more than 135,000 observations of the universe. More than 3,500 science papers were written based on that data before the instrument was retired in 2009, and over 2,000 more have been published since.

- "WFPC2 didn't succeed by magic or luck; it succeeded because we had a competent and hardworking group of people who understood what was at stake and stepped up to the challenge," Gallagher said. "And just like with every project, I wish I could have transported that team with me to the next mission."

- In May of 2009, astronauts removed WFPC2 from Hubble and replaced it with the Wide Field Camera 3 (WFC3), which continues to operate today - 28 years after Hubble first switched on. WFPC2 was later placed on public display at the Smithsonian Air and Space Museum in Washington, D.C.

- The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.

• 29 November 2018: Gazing across 300 million light-years into a monstrous city of galaxies, astronomers have used NASA's Hubble Space Telescope to do a comprehensive census of some of its most diminutive members: a whopping 22,426 globular star clusters found to date. 81)

- The survey, published in the November 9, 2018, issue of The Astrophysical Journal, will allow for astronomers to use the globular cluster field to map the distribution of matter and dark matter in the Coma galaxy cluster, which holds over 1,000 galaxies that are packed together. 82)

- Because globular clusters are much smaller than entire galaxies — and much more abundant — they are a much better tracer of how the fabric of space is distorted by the Coma cluster's gravity. In fact, the Coma cluster is one of the first places where observed gravitational anomalies were considered to be indicative of a lot of unseen mass in the universe — later to be called "dark matter."

- Among the earliest homesteaders of the universe, globular star clusters are snow-globe-shaped islands of several hundred thousand ancient stars. They are integral to the birth and growth of a galaxy. About 150 globular clusters zip around our Milky Way galaxy, and, because they contain the oldest known stars in the universe, were present in the early formative years of our galaxy.

- Some of the Milky Way's globular clusters are visible to the naked eye as fuzzy-looking "stars." But at the distance of the Coma cluster, its globulars appear as dots of light even to Hubble's super-sharp vision. The survey found the globular clusters scattered in the space between the galaxies. They have been orphaned from their home galaxy due to galaxy near-collisions inside the traffic-jammed cluster. Hubble revealed that some globular clusters line up along bridge-like patterns. This is telltale evidence for interactions between galaxies where they gravitationally tug on each other like pulling taffy.

- Astronomer Juan Madrid of the Australian Telescope National Facility in Sydney, Australia, first thought about the distribution of globular clusters in Coma when he was examining Hubble images that show the globular clusters extending all the way to the edge of any given photograph of galaxies in the Coma cluster.

- He was looking forward to more data from one of the legacy surveys of Hubble that was designed to obtain data of the entire Coma cluster, called the Coma Cluster Treasury Survey. However, halfway through the program, in 2006, Hubble's powerful Advanced Camera for Surveys (ACS) had an electronics failure. (The ACS was later repaired by astronauts during a 2009 Hubble servicing mission.)

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Figure 65: This is a Hubble Space Telescope mosaic of a portion of the immense Coma cluster of over 1,000 galaxies, located 300 million light-years from Earth. Hubble's incredible sharpness was used to do a comprehensive census of the cluster's most diminutive members: a whopping 22,426 globular star clusters. Among the earliest homesteaders of the universe, globular star clusters are snow-globe-shaped islands of several hundred thousand ancient stars. The survey found the globular clusters scattered in the space between the galaxies. They have been orphaned from their home galaxies through galaxy tidal interactions within the bustling cluster. Astronomers will use the globular cluster field for mapping the distribution of matter and dark matter in the Coma galaxy cluster [image credit: NASA, ESA, J. Mack (STScI) and J. Madrid (Australian Telescope National Facility)]

- To fill in the survey gaps, Madrid and his team painstakingly pulled numerous Hubble images of the galaxy cluster taken from different Hubble observing programs. These are stored in the Space Telescope Science Institute's Mikulski Archive for Space Telescopes in Baltimore, Maryland. He assembled a mosaic of the central region of the cluster, working with students from the National Science Foundation's Research Experience for Undergraduates program. "This program gives an opportunity to students enrolled in universities with little or no astronomy to gain experience in the field," Madrid said.

- The team developed algorithms to sift through the Coma mosaic images that contain at least 100,000 potential sources. The program used globular clusters' color (dominated by the glow of aging red stars) and spherical shape to eliminate extraneous objects — mostly background galaxies unassociated with the Coma cluster.

- Though Hubble has superb detectors with unmatched sensitivity and resolution, their main drawback is that they have tiny fields of view. "One of the cool aspects of our research is that it showcases the amazing science that will be possible with NASA's planned Wide Field Infrared Survey Telescope (WFIRST) that will have a much larger field of view than Hubble," said Madrid. "We will be able to image entire galaxy clusters at once."

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

• 26 November 2018: This dark, tangled web is an object named SNR 0454-67.2. It formed in a very violent fashion — it is a supernova remnant, created after a massive star ended its life in a cataclysmic explosion and threw its constituent material out into surrounding space. This created the messy formation we see in this NASA/ESA Hubble Space Telescope image, with threads of red snaking amidst dark, turbulent clouds. 83)

- SNR 0454-67.2 is situated in the Large Magellanic Cloud, a dwarf spiral galaxy that lies close to the Milky Way. The remnant is likely the result of a Type Ia supernova explosion; this category of supernovae is formed from the death of a white dwarf star, which grows and grows by siphoning material from a stellar companion until it reaches a critical mass and then explodes.

- As they always form via a specific mechanism — when the white dwarf hits a particular mass — these explosions always have a well-known luminosity, and are thus used as markers (standard candles) for scientists to obtain and measure distances throughout the Universe.

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Figure 66: Tangled — cosmic edition (image credit: ESA/Hubble, NASA)

Minimize Hubble continued

• 15 November 2018: Astronomers may have finally uncovered the long-sought progenitor to a specific type of exploding star by sifting through NASA Hubble Space Telescope archival data. The supernova, called a Type Ic, is thought to detonate after its massive star has shed or been stripped of its outer layers of hydrogen and helium. 83) 84)

- These stars could be among the most massive known — at least 30 times heftier than our Sun. Even after shedding some of their material late in life, they are expected to be big and bright. So it was a mystery why astronomers had not been able to nab one of these stars in pre-explosion images.

- Finally, in 2017, astronomers got lucky. A nearby star ended its life as a Type Ic supernova. Two teams of astronomers pored through the archive of Hubble images to uncover the putative precursor star in pre-explosion photos taken in 2007. The supernova, catalogued as SN 2017ein, appeared near the center of the nearby spiral galaxy NGC 3938, located roughly 65 million light-years away.

- This potential discovery could yield insight into stellar evolution, including how the masses of stars are distributed when they are born in batches.

- "Finding a bona fide progenitor of a supernova Ic is a big prize of progenitor searching," said Schuyler Van Dyk of the California Institute of Technology (Caltech) in Pasadena, lead researcher of one of the teams. "We now have for the first time a clearly detected candidate object." His team's paper was published in June in The Astrophysical Journal.

- A paper by a second team, which appeared in the Oct. 21, 2018, issue of the Monthly Notices of the Royal Astronomical Society, is consistent with the earlier team's conclusions.

- "We were fortunate that the supernova was nearby and very bright, about 5 to 10 times brighter than other Type Ic supernovas, which may have made the progenitor easier to find," said Charles Kilpatrick of the University of California, Santa Cruz, leader of the second team. "Astronomers have observed many Type Ic supernovas, but they are all too far away for Hubble to resolve. You need one of these massive, bright stars in a nearby galaxy to go off. It looks like most Type Ic supernovas are less massive and therefore less bright, and that's the reason we haven't been able to find them."

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Figure 66: This is an artist's concept of a blue supergiant star that once existed inside a cluster of young stars in the spiral galaxy NGC 3938, located 65 million light-years away. It exploded as a supernova in 2017, and Hubble Space Telescope archival photos were used to locate the doomed progenitor star, as it looked in 2007. The star may have been as massive as 50 suns and burned at a furious rate, making it hotter and bluer than our Sun. It was so hot, it had lost its outer layers of hydrogen and helium. When it exploded in 2017, astronomers categorized it as a Type Ic supernova because of the lack of hydrogen and helium in the supernova's spectrum. In an alternative scenario (not shown here) a binary companion to the massive star may have stripped off its hydrogen and helium layers [image credits: NASA, ESA, and J. Olmsted (STScI)]

- An analysis of the object's colors shows that it is blue and extremely hot. Based on that assessment, both teams suggest two possibilities for the source's identity. The progenitor could be a single hefty star between 45 and 55 times more massive than our Sun. Another idea is that it could have been a massive binary-star system in which one of the stars weighs between 60 and 80 solar masses and the other roughly 48 suns. In this latter scenario, the stars are orbiting closely and interact with each other. The more massive star is stripped of its hydrogen and helium layers by the close companion, and eventually explodes as a supernova.

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Figure 67: This NASA Hubble Space Telescope image of the nearby spiral galaxy NGC 3938 shows the location of supernova 2017ein, in a spiral arm near the bright core. The exploded star is a Type Ic supernova, thought to detonate after its massive star has shed or been stripped of its outer layers of hydrogen and helium. Progenitor stars to Type Ic supernovas have been hard to find. But astronomers sifting through Hubble archival images may have uncovered the star that detonated as supernova 2017ein. The location of the candidate progenitor star is shown in the left pullout box at the bottom, taken in 2007. The bright object in the box at bottom right is a close-up image of the supernova, taken by Hubble in 2017, shortly after the stellar blast. NGC 3938 resides 65 million light-years away in the constellation Ursa Major. The Hubble image of NGC 3938 was taken in 2007 [image credits: NASA, ESA, S. Van Dyk (Caltech), and W. Li (University of California)]

- The possibility of a massive double-star system is a surprise. "This is not what we would expect from current models, which call for lower-mass interacting binary progenitor systems," Van Dyk said.

- Expectations on the identity of the progenitors of Type Ic supernovas have been a puzzle. Astronomers have known that the supernovas were deficient in hydrogen and helium, and initially proposed that some hefty stars shed this material in a strong wind (a stream of charged particles) before they exploded. When they didn't find the progenitors stars, which should have been extremely massive and bright, they suggested a second method to produce the exploding stars that involves a pair of close-orbiting, lower-mass binary stars. In this scenario, the heftier star is stripped of its hydrogen and helium by its companion. But the "stripped" star is still massive enough to eventually explode as a Type Ic supernova.

- "Disentangling these two scenarios for producing Type Ic supernovas impacts our understanding of stellar evolution and star formation, including how the masses of stars are distributed when they are born, and how many stars form in interacting binary systems," explained Ori Fox of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, a member of Van Dyk's team. "And those are questions that not just astronomers studying supernovas want to know, but all astronomers are after."

- Type Ic supernovas are just one class of exploding star. They account for about 20 percent of massive stars that explode from the collapse of their cores.

- The teams caution that they won't be able to confirm the source's identity until the supernova fades in about two years. The astronomers hope to use either Hubble or the upcoming NASA James Webb Space Telescope to see whether the candidate progenitor star has disappeared or has significantly dimmed. They also will be able to separate the supernova's light from that of stars in its environment to calculate a more accurate measurement of the object's brightness and mass.

- SN 2017ein was discovered in May 2017 by Tenagra Observatories in Arizona. But it took the sharp resolution of Hubble to pinpoint the exact location of the possible source. Van Dyk's team imaged the young supernova in June 2017 with Hubble's Wide Field Camera 3. The astronomers used that image to pinpoint the candidate progenitor star nestled in one of the host galaxy's spiral arms in archival Hubble photos taken in December 2007 by the Wide Field Planetary Camera 2.

- Kilpatrick's group also observed the supernova in June 2017 in infrared images from one of the 10-meter telescopes at the W. M. Keck Observatory in Hawaii. The team then analyzed the same archival Hubble photos as Van Dyk's team to uncover the possible source.

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

14 November 2018: The SPC (Science Program Committee) of ESA has confirmed the continued operations of ten scientific missions in the Agency's fleet up to 2022. After a comprehensive review of their scientific merits and technical status, the SPC has decided to extend the operation of the five missions led by ESA's Science Program: Cluster, Gaia, INTEGRAL, Mars Express, and XMM-Newton. The SPC also confirmed the Agency's contributions to the extended operations of Hinode, Hubble, IRIS, SOHO, and ExoMars TGO. 85)

- This includes the confirmation of operations for the 2019–2020 cycle for missions that had been given indicative extensions as part of the previous extension process, and indicative extensions for an additional two years, up to 2022.
Note: Every two years, all missions whose approved operations end within the following four years are subject to review by the advisory structure of the Science Directorate. Extensions are granted to missions that satisfy the established criteria for operational status and science return, subject to the level of financial resources available in the science program. These extensions are valid for the following four years, subject to a mid-term review and confirmation after two years.

- The decision was taken during the SPC meeting at ESA/ESAC (European Space Astronomy Center) near Madrid, Spain, on 14 November.

- ESA's science missions have unique capabilities and are prolific in their scientific output. Cluster, for example, is the only mission that, by varying the separation between its four spacecraft, allows multipoint measurements of the magnetosphere in different regions and at different scales, while Gaia is performing the most precise astrometric survey ever realized, enabling unprecedented studies of the distribution and motions of stars in the Milky Way and beyond.

- Many of the science missions are proving to be of great value to pursue investigations that were not foreseen at the time of their launch. Examples include the role of INTEGRAL and XMM-Newton in the follow-up of recent gravitational wave detections, paving the way for the future of multi-messenger astronomy, and the many discoveries of diverse exoplanets by Hubble.

- Collaboration between missions, including those led by partner agencies, is also of great importance. The interplay between solar missions like Hinode, IRIS and SOHO provides an extensive suite of complementary instruments to study our Sun; meanwhile, Mars Express and ExoMars TGO are at the forefront of the international fleet investigating the Red Planet.

- Another compelling factor to support the extension is the introduction of new modes of operation to accommodate the evolving needs of the scientific community, as well as new opportunities for scientists to get involved with the missions.

Table 3: Extended life for ESA's science missions 85)

• 08 November 2018: Blue compact dwarf galaxies take their name from the intensely blue star-forming regions that are often found within their cores. One such region can be seen embedded in ESO 338-4, which is populated with bright young stars voraciously consuming hydrogen. These massive stars are doomed to a short existence, as despite their vast supplies of hydrogen fuel. The nuclear reactions in the cores of these stars will burn through these supplies in only millions of years — a mere blink of an eye in astronomical terms. 86)

- The young blue stars nestled within a cloud of dust and gas in the center of this image are the result of a recent galaxy merger between a wandering galaxy and ESO 388-4. This galactic interaction disrupted the clouds of gas and dust surrounding ESO 338-4 and led to the rapid formation of a new population of stars.

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Figure 68: This captivating image from the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 shows a lonely dwarf galaxy, a staggering 100 million light-years away from Earth. This image depicts the blue compact dwarf galaxy ESO 338-4, which can be found in the constellation of Corona Australis (the Southern Crown), image credit: ESA/Hubble & NASA

• 05 November 2018: On the first day of the 15th annual European Space Weather Week, this image from the NASA/ESA Hubble Space Telescope fittingly shows a striking occurrence of celestial weather in the outer reaches of the Solar System: an aurora on Uranus. 87)

- Auroras, also known as polar lights, are a relatively familiar type of space weather to Earth-based stargazers, but have also been spied on many other planets in the Solar System.

- Views of the Earth’s Northern and Southern Lights show glowing sheets and rippling waves of bright light painting the sky in striking shades of green and even red, blue, and purple; these breath-taking scenes are created as streams of energetic charged particles hit the upper layers of Earth’s atmosphere at altitudes of up to a few hundreds of kilometers, and interact with resident atoms and molecules of mostly oxygen and nitrogen. These emit photons at specific visible wavelengths or colors – green and red for oxygen, blue and purple for nitrogen – and fill the sky with an eerie auroral glow.

- Hubble has observed auroras on Uranus on various occasions: in 2011, when the telescope became the first to image the phenomenon from the vicinity of Earth, then again in 2012 and 2014, taking extra data beyond visible light.

- By pointing Hubble’s ultraviolet eye on Uranus twice during the same month, from 1 to 5 and 22 to 24 November 2014, scientists were able to determine that the planet’s glimmering auroras rotate along with the planet. The observations also helped to locate Uranus’ magnetic poles, and allowed scientists to track two so-called interplanetary shocks that propagated through the Solar System. These shocks were triggered by two powerful bursts of material flung out by the Sun via the solar wind, an ongoing flow of charged particles constantly emanating from our star, and caused the most intense auroras ever seen on Uranus.

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Figure 69: This image, originally published in 2017, shows the auroras as wispy patches of white against the planet’s azure blue disc, and combines optical and ultraviolet observations from Hubble with archive data from NASA’s Voyager 2 probe. Voyager 2 was the first and only craft to visit the outermost planets in the Solar System; it flew past Uranus in January 1986, and past Neptune in August 1989. These icy planets have not been visited since. NASA and ESA have been studying a possible joint mission that would target the two ice giant planets in order to explore their intriguing role in our planetary system (image credit: ESA/Hubble & NASA, L. Lamy / Observatoire de Paris)

• 31 October 2018: The NASA/ESA Hubble Space Telescope has captured part of the wondrous Serpens Nebula, lit up by the star HBC 672. This young star casts a striking shadow – nicknamed the Bat Shadow – on the nebula behind it, revealing telltale signs of its otherwise invisible protoplanetary disc. 88)

- The Serpens Nebula, located in the tail of the Serpent (Serpens Cauda) about 1300 light-years away, is a reflection nebula that owes most of its sheen to the light emitted by stars like HBC 672 –  a young star nestled in its dusty folds. In this image the NASA/ESA Hubble Space Telescope has exposed two vast cone-like shadows emanating from HBC 672.

- These colossal shadows on the Serpens Nebula are cast by the protoplanetary disc surrounding HBC 672. By clinging tightly to the star the disc creates an imposing shadow, much larger than the disc – approximately 200 times the diameter of our own Solar System. The disc's shadow is similar to that produced by a cylindrical lamp shade. Light escapes from the top and bottom of the shade, but along its circumference, dark cones of shadow form.

- The disc itself is so small and far away from Earth that not even Hubble can detect it encircling its host star. However, the shadow feature – nicknamed the Bat Shadow – reveals details of the disc's shape and nature. The presence of a shadow implies that the disc is being viewed nearly edge-on.

- Whilst most of the shadow is completely opaque, scientists can look for color differences along its edges, where some light gets through. Using the shape and color of the shadow, they can determine the size and composition of dust grains in the disc.

- The whole Serpens Nebula, of which this image shows only a tiny part, could host more of these shadow projections. The nebula envelops hundreds of young stars, many of which could also be in the process of forming planets in a protoplanetary disc.

- Although shadow-casting discs are common around young stars, the combination of an edge-on viewing angle and the surrounding nebula is rare. However, in an unlikely coincidence, a similar looking shadow phenomenon can be seen emanating from another young star, in the upper left of the image.

- These precious insights into protoplanetary discs around young stars allow astronomers to study our own past. The planetary system we live in once emerged from a similar protoplanetary disc when the Sun was only a few million years old. By studying these distant discs we get to uncover the formation and evolution of our own cosmic home.

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Figure 70: This image, taken with the NASA/ESA Hubble Space Telescope shows the Serpens Nebula, a stellar nursery about 1300 light-years away. Within the nebula, in the upper right of the image, a shadow is created by the protoplanetary disc surrounding the star HBC 672. While the disc of debris is too tiny to be seen even by Hubble, its shadow is projected upon the cloud in which it was born. In this view, the feature – nicknamed the Bat Shadow – spans approximately 200 times the diameter of our own Solar System. - A similar looking shadow phenomenon can be seen emanating from another young star, in the upper left of the image (image credit: NASA, ESA, and STScI, CC BY 4.0)

NASA’s Hubble Space Telescope returned to normal operations late Friday, Oct. 26, and completed its first science observations on Saturday, 27 October 2018 at 2:10 AM EDT. The observations were of the distant, star-forming galaxy DSF2237B-1-IR and were taken in infrared wavelengths with the WFC3 (Wide Field Camera 3) instrument. The return to conducting science comes after successfully recovering a backup gyroscope, that had replaced a failed gyro three weeks earlier. 89)

- One of Hubble’s gyros failed on 5 October, and the spacecraft’s operations team activated a backup gyro the next day. However, the backup incorrectly returned rotation rates that were far in excess of the actual rates.

- Last week the operations team commanded Hubble to perform numerous maneuvers, or turns, and switched the gyro between different operational modes, which successfully cleared what was believed to be blockage between components inside the gyro that produced the excessively high rate values. Next, the team monitored and tested the gyro with additional maneuvers to make sure that the gyro was stable. The team then installed additional safeguards on the spacecraft in case the excessive rate values return, although this is not anticipated.

- On 26 October, the team began the process to restore the scientific instruments to standard operating status. Hubble successfully completed maneuvers to get on target for the first science observations, and the telescope collected its first science data since 5 October.

- Hubble is now back in its normal science operations mode with three fully functional gyros. Originally required to last 15 years, Hubble has now been at the forefront of scientific discovery for more than 28 years. The team expects the telescope will continue to yield amazing discoveries well into the next decade, enabling it to work alongside the James Webb Space Telescope.

• 26 October 2018: The constellation of Cassiopeia, named after a vain queen in Greek mythology, forms the easily recognizable "W" shape in the night sky. The central point of the W is marked by a dramatic star named Gamma Cassiopeiae. 90) 91) 92)

- The remarkable Gamma Cassiopeiae is a blue-white subgiant variable star that is surrounded by a gaseous disc. This star is 19 times more massive and 65,000 times brighter than our Sun. It also rotates at the incredible speed of 1.6 million km/hour – more than 200 times faster than our parent star. This frenzied rotation gives it a squashed appearance. The fast rotation causes eruptions of mass from the star into a surrounding disk. This mass loss is related to the observed brightness variations.

- The radiation of Gamma Cassiopeiae is so powerful that it even affects IC 63, sometimes nicknamed the Ghost Nebula, that lies several light years away from the star. IC 63 is visible in this image taken by the NASA/ESA Hubble Space Telescope.

- The colors in the eerie nebula showcase how the nebula is affected by the powerful radiation from the distant star. The hydrogen within IC 63 is being bombarded with ultraviolet radiation from Gamma Cassiopeiae, causing its electrons to gain energy which they later release as hydrogen-alpha radiation – visible in red in this image.

- This hydrogen-alpha radiation makes IC 63 an emission nebula, but we also see blue light in this image. This is light from Gamma Cassiopeiae that has been reflected by dust particles in the nebula, meaning that IC 63 is also a reflection nebula.

- This colorful and ghostly nebula is slowly dissipating under the influence of ultraviolet radiation from Gamma Cassiopeiae. However, IC 63 is not the only object under the influence of the mighty star. It is part of a much larger nebulous region surrounding Gamma Cassiopeiae that measures approximately two degrees on the sky – roughly four times as wide as the full Moon.

- The region is best seen from the Northern Hemisphere during autumn and winter. Though it is high in the sky and visible all year round from Europe, it is very dim, so observing it requires a fairly large telescope and dark skies.

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Figure 71: IC 63, the Ghost Nebula. From above Earth's atmosphere, Hubble gives us a view that we cannot hope to see with our eyes. This photo is possibly the most detailed image that has ever been taken of IC 63, and it beautifully showcases Hubble's capabilities (image credit: ESA/Hubble, NASA, CC BY 4.0)

• 23 October 2018: An international team of astronomers have discovered two stars in a binary pair that complete an orbit around each other in a little over three hours, residing in the planetary nebula M3-1. Remarkably, the stars could drive a nova explosion, an entirely unexpected event based on our current understanding of binary star evolution. The team, led by David Jones of the IAC (Instituto Astrofisica de Canarias) and the Universidad de La Laguna, report their findings in Monthly Notices of the Royal Astronomical Society: Letters. 93) 94)

- Planetary nebulae are the glowing shells of gas and dust formed from the outer layers of stars like our own Sun, which they throw off during the final stages of their evolution. In many cases, interaction with a nearby companion star plays an important role in the ejection of this material and the formation of the elaborate structures seen in the resulting planetary nebulae.

- The planetary nebula M3-1 is located in the constellation of Canis Major, at a distance of roughly 14,000 light years. M3-1 was a firm candidate to host a binary central star, as its structure with prominent jets and filaments is typical of these binary star interactions.

- Using the telescopes of the European Southern Observatory (ESO) in Chile, Jones's team looked at M3-1 over a period of several years. In the process they discovered and studied the binary stars in the center of the nebula.

- "We knew M3-1 had to host a binary star, so we set about acquiring the observations required to prove this and to relate the properties of the nebula with the evolution of the star or stars that formed it," says Brent Miszalski, researcher at the Southern African Large Telescope, and co-author of the study.

- The two stars are so close together that they cannot be resolved from the ground, so instead the presence of the second star is inferred from the variation of their observed combined brightness — most obviously by periodic eclipses of one star by the other which produce marked drops in the brightness.

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Figure 72: An image obtained with the Hubble Space Telescope of the planetary nebula M3-1, the central star of which is actually a binary system with one of the shortest orbital periods known (image credit: David Jones - IAC)

- "When we began the observations, it was immediately clear that the system was a binary" explains Henri Boffin, researcher at the European Southern Observatory in Germany. "We saw that the apparently single star at the center of the nebula was rapidly changing in brightness, and we knew that this must be due to the presence of a companion star."

- The team discovered that the central star of the planetary nebula M3-1 has one of the shortest orbital period binary central stars known to date, at just over three hours. The ESO observations also show that the two stars — most likely a white dwarf with a low-mass main sequence companion — are almost touching.

- As a result, the pair are likely to undergo a so-called nova eruption, the result of the transfer of material from one star to the other. When this reaches a critical mass, a violent thermonuclear explosion takes place and the system temporarily increases in brightness by up to a million times.

- "After the various observing campaigns in Chile, we had enough data to begin to understand the properties of the two stars — their masses, temperatures and radii" says Paulina Sowicka, a PhD student at the Nicolas Copernicus Astronomical Center in Poland. "It was a real surprise that the two stars were so close together and so large that they were almost touching one another. A nova explosion could take place in just a few thousand years from now."

- Theory suggests that binary stars should be well separated after the formation of a planetary nebula. It should then take a long time before they begin to interact again and events such as novae become possible.

- In 2007, astronomers observed a different nova explosion, known as Nova Vul 2007, inside another planetary nebula. Jones comments: "The 2007 event was particularly difficult to explain. By the time the two stars are close enough for a nova, the material in the planetary nebula should have expanded and dissipated so much that it's no longer visible."

- The new event adds to the conundrum, adds Jones: "In the central stars of M3-1, we've found another candidate for a similar nova eruption in the relatively near future."

- The team now hope to carry out further study of the nebula and others like it, helping to shed light on the physical processes and origins of novae and supernovae, some of the most spectacular and violent phenomena in the Universe.

• 22 October 2018: NASA took great strides last week to press into service a Hubble Space Telescope backup gyroscope (gyro) that was incorrectly returning extremely high rotation rates. The backup gyro was turned on after the spacecraft entered safe mode due to a failed gyro on Friday, 5 October. The rotation rates produced by the backup gyro have since reduced and are now within an expected range. Additional tests will be performed to ensure Hubble can return to science operations with this gyro. 95)

- A gyro is a device that measures the speed at which the spacecraft is turning, and is needed to help Hubble turn and lock on to new targets.

- A wheel inside the gyro spins at a constant rate of 19,200 revolutions per minute. This wheel is mounted in a sealed cylinder, called a float, which is suspended in a thick fluid. Electricity is carried to the motor by thin wires, approximately the size of a human hair, that are immersed in the fluid. Electronics within the gyro detect very small movements of the axis of the wheel and communicate this information to Hubble’s central computer. These gyros have two modes — high and low. High mode is a coarse mode used to measure large rotation rates when the spacecraft turns across the sky from one target to the next. Low mode is a precision mode used to measure finer rotations when the spacecraft locks onto a target and needs to stay very still.

- In an attempt to correct the erroneously high rates produced by the backup gyro, the Hubble operations team executed a running restart of the gyro on 16 October. This procedure turned the gyro off for one second, and then restarted it before the wheel spun down. The intention was to clear any faults that may have occurred during startup on 6 October, after the gyro had been off for more than 7.5 years. However, the resulting data showed no improvement in the gyro’s performance.

- On 18 October, the Hubble operations team commanded a series of spacecraft maneuvers, or turns, in opposite directions to attempt to clear any blockage that may have caused the float to be off-center and produce the exceedingly high rates. During each maneuver, the gyro was switched from high mode to low mode to dislodge any blockage that may have accumulated around the float.

- Following the 18 October maneuvers, the team noticed a significant reduction in the high rates, allowing rates to be measured in low mode for brief periods of time. On 19 October, the operations team commanded Hubble to perform additional maneuvers and gyro mode switches, which appear to have cleared the issue. Gyro rates now look normal in both high and low mode.

- Hubble then executed additional maneuvers to make sure that the gyro remained stable within operational limits as the spacecraft moved. The team saw no problems and continued to observe the gyro through the weekend to ensure that it remained stable.

- The Hubble operations team plans to execute a series of tests to evaluate the performance of the gyro under conditions similar to those encountered during routine science observations, including moving to targets, locking on to a target, and performing precision pointing. After these engineering tests have been completed, Hubble is expected to soon return to normal science operations.

• 19 October 2018: Discovered in November 1834 by British astronomer John Herschel, NGC 1898 has been scrutinized numerous times by the NASA/ESA Hubble Space Telescope. Today we know that globular clusters are some of the oldest known objects in the universe and that they are relics of the first epochs of galaxy formation. While we already have a pretty good picture on the globular clusters of the Milky Way — still with many unanswered questions — our studies on globular clusters in nearby dwarf galaxies just started. The observations of NGC 1898 will help to determine whether their properties are similar to the ones found in the Milky Way, or if they have different features, due to being in a different cosmic environment. 96)

- The image of Figure 73 was taken by Hubble’s Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3).

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Figure 73: This glittering ball of stars is the globular cluster NGC 1898, which lies toward the center of the Large Magellanic Cloud — one of our closest cosmic neighbors. The Large Magellanic Cloud is a dwarf galaxy that hosts an extremely rich population of star clusters, making it an ideal laboratory for investigating star formation (image credit: ESA/Hubble & NASA)

• 18 October 2018: New observations by two Arizona State University astronomers using the Hubble Space Telescope have caught a red dwarf star in a violent outburst, or superflare. The blast of radiation was more powerful than any such outburst ever detected from the Sun, and would likely affect the habitability of any planets orbiting it. 97)

- Moreover, the astronomers say, such superflares appear more common in younger red dwarfs, which erupt 100 to 1000 times more powerfully than they will when they age.

- The superflare was detected as part of a Hubble Space Telescope observing program dubbed HAZMAT (HAbitable Zones and M dwarf Activity across Time). The program surveys red dwarfs (also known as M dwarfs) at three different ages—young, intermediate, and old—and observes them in ultraviolet light, where they show the most activity.

- "Red dwarf stars are the smallest, most common, and longest-lived stars in the galaxy," says Evgenya Shkolnik, an assistant professor in ASU's School of Earth and Space Exploration and the HAZMAT program's principal investigator. "In addition, we think that most red dwarf stars have systems of planets orbiting them."

- The Hubble telescope's orbit above Earth's atmosphere gives it clear, unhindered views at ultraviolet wavelengths. The flares are believed to be powered by intense magnetic fields that get tangled by the roiling motions of the stellar atmosphere. When the tangling gets too intense, the fields break and reconnect, unleashing tremendous amounts of energy.

- ASU postdoctoral researcher Parke Loyd is the first author on the paper (to be published in the Astrophysical Journal) that reports on the stellar outbursts. 98)

- He says, "When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, 'Whoa.'"

- Loyd notes, "Gathering data on young red dwarfs has been especially important because we suspected these stars would be quite unruly in their youth, which is the first hundred million years or so after they form." - He adds, "Most of the potentially-habitable planets in our galaxy have had to withstand intense flares like the ones we observed at some point in their life. That's a sobering thought."

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Figure 74: Violent outbursts of seething gas from young red dwarfs may make conditions uninhabitable on fledgling planets. In this artist's rendering, an active, young red dwarf (right) is stripping the atmosphere from an orbiting planet (left). ASU astronomers have found that flares from the youngest red dwarfs they surveyed — approximately 40 million years old — are 100 to 1000 times more energetic than when the stars are older. They also detected one of the most intense stellar flares ever observed in ultraviolet light — more energetic than the most powerful flare ever recorded from our Sun [image credit: NASA, ESA, and D. Player (STScI)]

Rough environment for planets

- About three-quarters of the stars in our Milky Way galaxy are red dwarfs. Most of the galaxy's "habitable-zone" planets—planets orbiting their stars at a distance where temperatures are moderate enough for liquid water to exist on their surface—orbit red dwarfs. In fact, the nearest star to our Sun, a red dwarf named Proxima Centauri, has an Earth-size planet in its habitable zone.

- However, red dwarfs—especially young red dwarfs—are active stars, producing flares that could blast out so much energy that it disrupts and possibly strips off the atmospheres of these fledgling planets.

- "The goal of the HAZMAT program is to understand the habitability of planets around low-mass stars," explains Shkolnik. "These low-mass stars are critically important in understanding planetary atmospheres." Ultraviolet radiation can modify the chemistry in a planet's atmosphere, or potentially remove that atmosphere.

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Figure 75: Observations with the Hubble Space Telescope discovered a superflare (red line) that caused a red dwarf star's brightness in the far ultraviolet to abruptly increase by a factor of nearly 200 (image credit: P. Loyd/ASU)

- The observations reported in the Astrophysical Journal examined the flare frequency of 12 young (40 million year old) red dwarfs and represent just the first part of the HAZMAT program. These stars show that young low-mass stars flare much more frequently and more energetically than old stars and middle-age stars like our Sun—as evidenced by the superflare.

- "With the Sun, we have a hundred years of good observations," says Loyd. "And in that time, we've seen one, maybe two, flares that have an energy approaching that of the superflare."

- However, he says, "In a little less than a day's worth of Hubble observations of these young stars, we caught the superflare. This means that we're looking at superflares happening every day or even a few times a day."

- Could superflares of such frequency and intensity bathe young planets in so much ultraviolet radiation that they forever rule out any chance of habitability?

- According to Loyd, "Flares like we observed have the capacity to strip away the atmosphere from a planet. But that doesn't necessarily mean doom and gloom for life on the planet. It just might be different life than we imagine. Or there might be other processes that could replenish the atmosphere of the planet. It's certainly a harsh environment, but I would hesitate to call it a sterile environment."

- The next part of the HAZMAT study will be to study intermediate-age red dwarfs that are 650 million years old. Then the oldest red dwarfs will be analyzed and compared with the young and intermediate stars to understand the evolution of the high-energy-radiation environment for planets around these low-mass stars.

- Red dwarfs, which are estimated to burn as long as a trillion years, have a vast stretch of time available to eventually host evolving, habitable planets.

- "They just have many more opportunities for life to evolve, given their longevity," says Shkolnik. "I don't think we know for sure one way or another about whether planets orbiting red dwarfs are habitable just yet, but I think time will tell."

- She says, "It's great that we're living in a time when we have the technology to actually answer these kinds of questions, rather than just philosophize about them."

• 12 October 2018: This image from the NASA/ESA Hubble Space Telescope (Figure 76) reveals a spiral galaxy named Messier 95 (also known as M95 or NGC 3351). Located about 35 million light-years away in the constellation of Leo (The Lion), this swirling spiral was discovered by astronomer Pierre Méchain in 1781, and cataloged by French astronomer Charles Messier just four days later. Messier was primarily a comet hunter, and was often left frustrated by objects in the sky that resembled comets but turned out not to be. To help other astronomers avoid confusing these objects in the future, he created his famous catalog of Messier objects. 99)

- Most definitely not a comet, Messier 95 is actually a barred spiral galaxy. The galaxy has a bar cutting through its center, surrounded by an inner ring currently forming new stars. Also our own Milky Way is a barred spiral.

- As well as hosting this stellar nursery, Messier 95 is a known host of the dramatic and explosive final stages in the lives of massive stars: supernovae. In March 2016 a spectacular supernova named SN 2012aw was observed in the outer regions of one of Messier 95’s spiral arms. Once the light from the supernova had faded, astronomers were able to compare observations of the region before and after the explosion to find out which star had “disappeared” — the progenitor star. In this case, the star was an especially huge red supergiant up to 26 times more massive than the Sun.

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Figure 76: This Hubble image reveals a detailed view of part of the spiral galaxy Messier 95 (image credit: ESA/Hubble & NASA; CC BY 4.0)

• 08 October 2018: NASA is working to resume science operations of the Hubble Space Telescope after the spacecraft entered safe mode on Friday, 5 October , shortly after 6:00 p.m. EDT. Hubble’s instruments still are fully operational and are expected to produce excellent science for years to come. 100)

- Hubble entered safe mode after one of the three gyroscopes, actively being used to point and steady the telescope, failed. Safe mode puts the telescope into a stable configuration until ground control can correct the issue and return the mission to normal operation.

- Built with multiple redundancies, Hubble had six new gyros installed during Servicing Mission-4 in 2009. Hubble usually uses three gyros at a time for maximum efficiency, but can continue to make scientific observations with just one.

- The gyro that failed had been exhibiting end-of-life behavior for approximately a year, and its failure was not unexpected; two other gyros of the same type had already failed. The remaining three gyros available for use are technically enhanced and therefore expected to have significantly longer operational lives.

- Two of those enhanced gyros are currently running. Upon powering on the third enhanced gyro that had been held in reserve, analysis of spacecraft telemetry indicated that it was not performing at the level required for operations. As a result, Hubble remains in safe mode. Staff at NASA’s Goddard Space Flight Center and the Space Telescope Science Institute are currently performing analyses and tests to determine what options are available to recover the gyro to operational performance.

- Science operations with Hubble have been suspended while NASA investigates the anomaly. An Anomaly Review Board, including experts from the Hubble team and industry familiar with the design and performance of this type of gyro, is being formed to investigate this issue and develop the recovery plan. If the outcome of this investigation results in recovery of the malfunctioning gyro, Hubble will resume science operations in its standard three-gyro configuration.

- If the outcome indicates that the gyro is not usable, Hubble will resume science operations in an already defined “reduced-gyro” mode that uses only one gyro. While reduced-gyro mode offers less sky coverage at any particular time, there is relatively limited impact on the overall scientific capabilities.

• 03 October 2018: A pair of Columbia University astronomers using NASA's Hubble Space Telescope and the Kepler Space Telescope have assembled compelling evidence for the existence of a moon orbiting a gas-giant planet 8,000 light-years away. 101) 102)

- In a paper published 03 October in the journal Science Advances, Alex Teachey and David Kipping report that the detection of a candidate exomoon—that is, moons orbiting planets in other star systems—is unusual because of its large size, comparable to the diameter of Neptune. Such gargantuan moons do not exist in our own solar system, where nearly 200 natural satellites have been cataloged. 103)

- "This would be the first case of detecting a moon outside our solar system," said Kipping, an assistant professor of astronomy at Columbia. "If confirmed by follow-up Hubble observations, the finding could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets."

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Figure 77: Artist's impression of the exoplanet Kepler-1625b, transiting the star, with the candidate exomoon in tow (image credit: Dan Durda)

- In looking for exomoons, the researchers analyzed data from 284 Kepler-discovered planets that were in comparatively wide orbits, with periods greater than 30 days, around their host star. The observations measured the momentary dimming of starlight as a planet passed in front of its star, called a transit. The researchers found one instance, in Kepler 1625b, that had intriguing anomalies.

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Figure 78: Artist’s impression of the exoplanet Kepler-1625b transiting the star with the candidate exomoon in tow (image credit: Dan Durda)

- "We saw little deviations and wobbles in the light curve that caught our attention," Kipping said.

- The Kepler results were enough for the team to get 40 hours of time with Hubble to intensively study the planet, obtaining data four times more precise than that of Kepler. The researchers monitored the planet before and during its 19-hour-long transit across the face of the star. After it ended, Hubble detected a second and much smaller decrease in the star's brightness 3.5 hours later, consistent with "a moon trailing the planet like a dog following its owner on a leash," Kipping said. "Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured."

- In addition to this dip in light, Hubble provided supporting evidence for the moon hypothesis by measuring that the planet began its transit 1.25 hours earlier than predicted. This is consistent with the planet and moon orbiting a common center of gravity (barycenter) that would cause the planet to wobble from its predicted location.

- "An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit," Kipping said.

- The researchers note that in principle this anomaly could be caused by the gravitational pull of a hypothetical second planet in the system, although Kepler found no evidence for additional planets around the star during its four-year mission.

- "A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," said lead author Teachey, NSF Graduate Fellow in astronomy at Columbia. "It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation."

- The moon is estimated to be only 1.5 percent the mass of its companion planet, which itself estimated to be several times the mass of Jupiter. This value is close to the mass-ratio between the Earth and its moon. But in the case of the Earth-Moon system and the Pluto-Charon system—the largest of the five known natural satellites of the dwarf planet Pluto—an early collision with a larger body is hypothesized to have blasted off material that later coalesced into a moon. Kepler 1625b and its satellite, however, are gaseous, not rocky, and, therefore, such a collision may not lead to the condensation of a satellite.

- Exomoons are difficult to find because they are smaller than their companion planet and so their transit signal is weak; they also shift position with each transit because the moon is orbiting the planet. In addition, the ideal candidate planets hosting moons are in large orbits, with long and infrequent transit times. In this search, the Neptune-sized moon would have been among the easiest to first detect because of its large size.

- The host planet and its moon lie within the solar mass star's (Kepler 1625) habitable zone, where moderate temperatures allow for the existence of liquid water on any solid planetary surface. "Both bodies, however, are considered to be gaseous and therefore unsuitable for life as we know it," Kipping said.

- Future searches will target Jupiter-sized planets that are farther from their star than Earth is from the Sun. There are just a handful of these in the Kepler database. NASA's upcoming James Webb Space Telescope could really "clean-up" in the satellite search, Kipping said. "We can expect to see really tiny moons."

• 21 September 2018: In the northern constellation of Coma Berenices (Berenice's Hair) lies the impressive Coma Cluster — a structure of over a thousand galaxies bound together by gravity. Many of these galaxies are elliptical types, as is the brighter of the two galaxies dominating this image: NGC 4860 (center of Figure 79). However, the outskirts of the cluster also host younger spiral galaxies that proudly display their swirling arms. Again, this image shows a wonderful example of such a galaxy in the shape of the beautiful NGC 4858, which can be seen to the left of its bright neighbor and which stands out on account of its unusual, tangled, fiery appearance. 104)

- NGC 4858 is special. Rather than being a simple spiral, it is something called a “galaxy aggregate”, which is, just as the name suggests, a central galaxy surrounded by a handful of luminous knots of material that seem to stem from it, extending and tearing away and adding to or altering its overall structure. It is also experiencing an extremely high rate of star formation, possibly triggered by an earlier interaction with another galaxy. As we see it, NGC 4858 is forming stars so frantically that it will use up all of its gas long before it reaches the end of its life. The color of its bright knots indicates that they are formed of hydrogen, which glows in various shades of bright red as it is energized by the many young, hot stars lurking within.

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Figure 79: This scene of the impressive Coma Cluster was captured by the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3), a powerful camera designed to explore the evolution of stars and galaxies in the early Universe (image credit: ESA/Hubble & NASA, CC BY 4.0)

• 14 September 2018: Gravity is so much a part of our daily lives that it is all too easy to forget its awesome power — but on a galactic scale, its power becomes both strikingly clear and visually stunning. 105)

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Figure 80: This image was taken with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3) and shows an object named SDSS J1138+2754. It acts as a gravitational lens illustrating the true strength of gravity: A large mass — a galaxy cluster in this case — is creating such a strong gravitational field that it is bending the very fabric of its surroundings. This causes the billion-year-old light from galaxies sitting behind it to travel along distorted, curved paths, transforming the familiar shapes of spirals and ellipticals (visible in other parts of the image) into long, smudged arcs and scattered dashes (image credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt; CC BY 4.0)

Legend to Figure 80: Some distant galaxies even appear multiple times in this image. Since galaxies are wide objects, light from one side of the galaxy passes through the gravitational lens differently than light from the other side. When the galaxies’ light reaches Earth it can appear reflected, as seen with the galaxy on the lower left part of the lens, or distorted, as seen with the galaxy to the upper right. — These data were taken as part of a research project on star formation in the distant Universe, building on Hubble’s extensive legacy of deep-field images. Hubble observed 73 gravitationally-lensed galaxies for this project.

• 10 September 2018: Figure 81 is a composite image taken by Hubble on 6 June 2018 showing a fully-illuminated Saturn and its rings, along with six of its 62 known moons. The visible moons are (from left to right) Dione, Enceladus, Tethys, Janus, Epimetheus and Mimas (click here for an annotated version). Dione is the largest moon in the picture, with a diameter of 1123 km, compared to the smallest, oddly-shaped Epimetheus with a diameter around 116 km. 106)

- During Cassini’s mission, Enceladus was identified as one of the most intriguing moons, with the discovery of water vapor jets spewing from the surface implying the existence of a subsurface ocean. Icy moons with subsurface oceans could potentially offer the conditions to harbor life, and understanding their origins and properties are essential for furthering our knowledge of the Solar System. ESA's JUpiter ICy moons Explorer (Juice), due to launch in 2022, aims to continue this theme by studying Jupiter's ocean-bearing moons: Ganymede, Europa, and Callisto.

- The Hubble image of Figure 82 was taken shortly before Saturn's opposition on 27 June, when the Sun, Earth and Saturn were aligned so that the Sun fully illuminated Saturn as seen from Earth. Saturn's closest approach to Earth occurs around the same time as opposition, which makes it appear brighter and larger and allows the planet to be imaged in greater detail.

- In this image the planet’s rings are seen near their maximum tilt towards Earth. Towards the end of Cassini’s mission, the spacecraft made multiple dives through the gap between Saturn and its rings, gathering spectacular data in this previously unchartered territory.

- The image also shows a hexagonal atmospheric feature around the north pole, with the remnants of a storm, seen as a string of bright clouds. The hexagon-shaped cloud phenomenon is a stable and persistent feature first seen by the Voyager 1 space probe when it flew past Saturn 1981. In a study published just last week, scientists using Cassini data collected between 2013 and 2017, as the planet approached northern summer, identified a hexagonal vortex above the cloud structure, showing there is still much to learn about the dynamics of Saturn’s atmosphere.

- The Hubble observations making up this image were performed as part of the Outer Planet Atmospheres Legacy (OPAL) project, which uses Hubble to observe the outer planets to understand the dynamics and evolution of their complex atmospheres. This was the first time that Saturn was imaged as part of OPAL. This image was first published on 26 July.

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Figure 81: A composite image taken by Hubble on 6 June 2018 showing a fully-illuminated Saturn and its rings, along with six of its 62 known moons. The visible moons are (from left to right) Dione, Enceladus, Tethys, Janus, Epimetheus and Mimas (image credit: NASA, ESA, A. Simon (GSFC) and the OPAL Team, and J. DePasquale (STScI); CC BY 4.0)

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Figure 82: This composite image, taken by the NASA/ESA Hubble Space Telescope on 6 June 2018, shows the ringed planet Saturn with six of its 62 known moons. With a diameter of 1123 km, Dione is the fourth-largest of Saturn’s moons and the largest of the siblings in this family portrait. The smallest satellite in this picture is the irregularly shaped Epimetheus, with a size of 143 x 108 x 98 km. The image is a composite because the moons move during the Saturn exposures, and individual frames must be realigned to make a color portrait [image credit: NASA, ESA, A. Simon (GSFC) and the OPAL Team, and J. DePasquale (STScI)]

• 30 August 2018: Astronomers using the NASA/ESA Hubble Space Telescope have taken a series of spectacular images featuring the fluttering auroras at the north pole of Saturn. The observations were taken in ultraviolet light and the resulting images provide astronomers with the most comprehensive picture so far of Saturn's northern aurora. 107)

- In 2017, over a period of seven months, the NASA/ESA Hubble Space Telescope took images of auroras above Saturn's north pole region using the Space Telescope Imaging Spectrograph. The observations were taken before and after the Saturnian northern summer solstice. These conditions provided the best achievable viewing of the northern auroral region for Hubble.

- On Earth, auroras are mainly created by particles originally emitted by the Sun in the form of solar wind. When this stream of electrically charged particles gets close to our planet, it interacts with the magnetic field, which acts as a gigantic shield. While it protects Earth's environment from solar wind particles, it can also trap a small fraction of them. Particles trapped within the magnetosphere — the region of space surrounding Earth in which charged particles are affected by its magnetic field — can be energized and then follow the magnetic field lines down to the magnetic poles. There, they interact with oxygen and nitrogen atoms in the upper layers of the atmosphere, creating the flickering, colorful lights visible in the polar regions here on Earth.
Note: The auroras here on Earth have different names depending on which pole they occur at. Aurora Borealis, or the northern lights, is the name given to auroras around the north pole and Aurora Australis, or the southern lights, is the name given for auroras around the south pole.

- However, these auroras are not unique to Earth. Other planets in our Solar System have been found to have similar auroras. Among them are the four gas giants Jupiter, Saturn, Uranus and Neptune. Because the atmosphere of each of the four outer planets in the Solar System is — unlike the Earth — dominated by hydrogen, Saturn's auroras can only be seen in ultraviolet wavelengths; a part of the electromagnetic spectrum which can only be studied from space.

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Figure 83: Saturn and its northern auroras (composite image), image credit: ESA/Hubble, NASA, A. Simon (GSFC) and the OPAL Team, J. DePasquale (STScI), L. Lamy (Observatoire de Paris)

- Hubble allowed researchers to monitor the behavior of the auroras at Saturn's north pole over an extended period of time. The Hubble observations were coordinated with the "Grand Finale" of the Cassini spacecraft, when the spacecraft simultaneously probed the auroral regions of Saturn. The Hubble data allowed astronomers to learn more about Saturn’s magnetosphere, which is the largest of any planet in the Solar System other than Jupiter.
Note: Cassini was a collaboration between NASA, ESA and the Italian Space Agency. It spent 13 years orbiting Saturn, gathering information and giving astronomers a great insight into the inner workings of Saturn. Cassini took more risks at the end of its mission, travelling through the gap between Saturn and its rings. No spacecraft had previously done this, and Cassini gathered spectacular images of Saturn as well as new data for scientists to work with. On 15 September 2017 Cassini was sent on a controlled crash into Saturn.

- The images show a rich variety of emissions with highly variable localized features. The variability of the auroras is influenced by both the solar wind and the rapid rotation of Saturn, which lasts only about 11 hours. On top of this, the northern aurora displays two distinct peaks in brightness — at dawn and just before midnight. The latter peak, unreported before, seems specific to the interaction of the solar wind with the magnetosphere at Saturn’s solstice.

- The main image presented here is a composite of observations made of Saturn in early 2018 in the optical and of the auroras on Saturn’s north pole region, made in 2017, demonstrating the size of the auroras along with the beautiful colors of Saturn.

- Hubble has studied Saturn's auroras in the past. In 2004, it studied the southern auroras shortly after the southern solstice (heic0504) and in 2009 it took advantage of a rare opportunity to record Saturn when its rings were edge-on (heic1003). This allowed Hubble to observe both poles and their auroras simultaneously.

• 24 August 2018: This dramatic image from the NASA/ESA Hubble Space Telescope shows the planetary nebula NGC 3918, a brilliant cloud of colorful gas in the constellation of Centaurus, around 4,900 light-years from Earth. 108)

- In the center of the cloud of gas, and completely dwarfed by the nebula, are the dying remnants of a red giant. During the final convulsive phase in the evolution of these stars, huge clouds of gas are ejected from the surface of the star before it emerges from its cocoon as a white dwarf. The intense ultraviolet radiation from the tiny remnant star then causes the surrounding gas to glow like a fluorescent sign. These extraordinary and colorful planetary nebulas are among the most dramatic sights in the night sky, and often have strange and irregular shapes, which are not yet fully explained.

- NGC 3918’s distinctive eye-like shape, with a bright inner shell of gas and a more diffuse outer shell that extends far from the nebula, looks as if it could be the result of two separate ejections of gas. But this is in fact not the case: studies of the object suggest that they were formed at the same time, but are being blown from the star at different speeds. The powerful jets of gas emerging from the ends of the large structure are estimated to be shooting away from the star at speeds of up to 350,000 km/hr.

- By the standards of astronomical phenomena, planetary nebulas like NGC 3918 are very short-lived, with a lifespan of just a few tens of thousands of years.

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Figure 84: This Hubble image shows the planetary nebula NGC 3918, a brilliant cloud of colorful gas in the constellation of Centaurus. The image is a composite of visible and near-infrared snapshots taken with Hubble’s Wide Field and Planetary Camera 2 (image credit: ESA/Hubble and NASA)

• 16 August 2018: Astronomers using the ultraviolet vision of NASA’s Hubble Space Telescope have captured one of the largest panoramic views of the fire and fury of star birth in the distant universe. The field features approximately 15,000 galaxies, about 12,000 of which are forming stars. Hubble’s ultraviolet vision opens a new window on the evolving universe, tracking the birth of stars over the last 11 billion years back to the cosmos’ busiest star-forming period, which happened about 3 billion years after the big bang. 109)

- Ultraviolet light has been the missing piece to the cosmic puzzle. Now, combined with infrared and visible-light data from Hubble and other space and ground-based telescopes, astronomers have assembled one of the most comprehensive portraits yet of the universe’s evolutionary history.

- The image straddles the gap between the very distant galaxies, which can only be viewed in infrared light, and closer galaxies, which can be seen across a broad spectrum. The light from distant star-forming regions in remote galaxies started out as ultraviolet. However, the expansion of the universe has shifted the light into infrared wavelengths. By comparing images of star formation in the distant and nearby universe, astronomers glean a better understanding of how nearby galaxies grew from small clumps of hot, young stars long ago.

- Because Earth’s atmosphere filters most ultraviolet light, Hubble can provide some of the most sensitive space-based ultraviolet observations possible.

- The program, called the Hubble Deep UV (HDUV) Legacy Survey, extends and builds on the previous Hubble multi-wavelength data in the CANDELS-Deep (Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey) fields within the central part of the GOODS (Great Observatories Origins Deep Survey) fields. This mosaic is 14 times the area of the Hubble Ultra Violet Ultra Deep Field released in 2014.

- The image of Figure 85 is a portion of the GOODS-North field, which is located in the northern constellation Ursa Major.

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Figure 85: Astronomers have just assembled one of the most comprehensive portraits yet of the universe’s evolutionary history, based on a broad spectrum of observations by the Hubble Space Telescope and other space and ground-based telescopes. In particular, Hubble’s ultraviolet vision opens a new window on the evolving universe, tracking the birth of stars over the last 11 billion years back to the cosmos’ busiest star-forming period, about 3 billion years after the big bang. This photo encompasses a sea of approximately 15,000 galaxies — 12,000 of which are star-forming — widely distributed in time and space. This mosaic is 14 times the area of the Hubble Ultra Violet Ultra Deep Field released in 2014 [image credit: NASA, ESA, P. Oesch (University of Geneva), and M. Montes (University of New South Wales)]

- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• 10 August 2018: This Picture of the Week shows the colorful globular cluster NGC 2108. The cluster is nestled within the Large Magellanic Cloud, in the constellation of the Swordfish (Dorado). It was discovered in 1835 by the astronomer, mathematician, chemist and inventor John Herschel, son of the famous William Herschel. 110)

- The most striking feature of this globular cluster is the gleaming ruby-red spot at the center left of the image (Figure 86). What looks like the cluster’s watchful eye is actually a carbon star. Carbon stars are almost always cool red giants, with atmospheres containing more carbon than oxygen — the opposite to our Sun. Carbon monoxide forms in the outer layer of the star through a combination of these elements, until there is no more oxygen available. Carbon atoms are then free to form a variety of other carbon compounds, such as C2, CH, CN, C3 and SiC2, which scatter blue light within the star, allowing red light to pass through undisturbed.

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Figure 86: This image was captured by the NASA/ESA Hubble Space Telescope’s Advanced Camera for Surveys (ACS), using three different filters (image credit: ESA/Hubble & NASA, CC BY 4.0)

• 03 August 2018: Gravitational lenses — such as this galaxy cluster SDSS J1152+3313 — possess immense masses that wrap their surroundings and bend the light from faraway objects into rings, arcs, streaks, blurs, and other odd shapes. This lens, however, is not only wrapping the appearance of a distant galaxy — it is also amplifying its light, making it appear much brighter than it would be without the lens. Combined with the high image quality obtainable with Hubble, this gives valuable clues into how stars formed in the early Universe. 111)

- Star formation is a key process in astronomy. Everything that emits light is somehow connected to stars, so understanding how stars form is key to understanding countless objects lying across the cosmos. Astronomers can probe these early star-forming regions to learn about the sizes, luminosities, formation rates, and generations of different types of stars.

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Figure 87: Obtained for a research program on star formation in old and distant galaxies, this NASA/ESA Hubble Space Telescope image obtained with its Wide Field Camera 3 (WFC3) demonstrates the immense effects of gravity; more specifically, it shows the effects of gravitational lensing caused by an object called SDSS J1152+3313 (image credit: ESA/Hubble & NASA: Acknowledgement: Judy Schmidt (Geckzilla), CC BY 4.0)

• 26 July 2018: In the summer of 2018 the planets Mars and Saturn are, one after the other, in opposition to Earth. During this event the planets are relatively close to Earth, allowing astronomers to observe them in greater detail. Hubble took advantage of this preferred configuration and imaged both planets to continue its long-standing observation of the outer planets in the Solar System. 112) 113)

- Since the NASA/ESA Hubble Space Telescope was launched, its goal has always been to study not only distant astronomical objects, but also the planets within our Solar System. Hubble’s high-resolution images of our planetary neighbors can only be surpassed by pictures taken from spacecraft that actually visit these bodies. However, Hubble has one advantage over space probes: it can look at these objects periodically and observe them over much longer periods than any passing probe could.

- In the last months the planets Mars and Saturn have each been in opposition to Earth — Saturn on 27 June and Mars on 27 July. An opposition occurs when the Sun, Earth and an outer planet are lined up, with Earth sitting in between the Sun and the outer planet. During an opposition, a planet is fully lit by the Sun as seen from Earth, and it also marks the time when the planet is closest to Earth, allowing astronomers to see features on the planet’s surface in greater detail.
Note: The dates of opposition and closest approach differ slightly. This difference is caused by the elliptical orbit of the planets and the fact that the orbits are not in exactly the same plane.

- A month before Saturn's opposition — on 6 June — Hubble was used to observe the ringed planet . At this time Saturn was approximately 1.4 billion kilometers from Earth. The taken images show Saturn’s magnificent ring system near its maximum tilt toward Earth, allowing a spectacular view of the rings and the gaps between them. Though all of the gas giants boast rings, Saturn’s are the largest and most spectacular, stretching out to eight times the radius of the planet.
Note: The observations of Saturn were made as part of the Outer Planet Atmospheres Legacy (OPAL) project. OPAL is helping astronomers understand the atmospheric dynamics and evolution of the gas giant planets in our Solar System. Jupiter, Uranus and Neptune have already been observed several times as part of this project, but this is the first time Saturn was observed as part of OPAL.

- Alongside a beautiful view of the ring system, Hubble's new image reveals a hexagonal pattern around the north pole — a stable and persistent wind feature discovered during the flyby of the Voyager 1 space probe in 1981. To the south of this feature a string of bright clouds is visible: remnants of a disintegrating storm.

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Figure 88: This image shows the recent observations of the planets Mars (right) and Saturn (left) made with the NASA/ESA Hubble Space Telescope. The observations of both objects were made in June and July 2018 and show the planets close to their opposition (image credit: Saturn: NASA, ESA, A. Simon (GSFC) and the OPAL Team, and J. DePasquale (STScI); Mars: NASA, ESA, and STScI)

- While observing the planet, Hubble also managed to capture images of six of Saturn's 62 currently known moons: Dione, Enceladus, Tethys, Janus, Epimetheus, and Mimas. Scientists hypothesize that a small, wayward moon like one of these disintegrated 200 million years ago to form Saturn’s ring system.

- Hubble shot the second portrait, of the planet Mars, on 18 July, just 13 days before Mars reached its closest approach to Earth. This year Mars will get as close as 57.6 million km from Earth. This makes it the closest approach since 2003, when the red planet made its way closer to us than at any other time in almost 60 000 years (opo0322).

- While previous images showed detailed surface features of the planet, this new image is dominated by a gigantic sandstorm enshrouding the entire planet. Still visible are the white polar caps, Terra Meridiani, the Schiaparelli Crater, and Hellas Basin — but all of these features are slightly blurred by the dust in the atmosphere.

- Comparing these new images of Mars and Saturn with older data gathered by Hubble, other telescopes and even space probes allows astronomers to study how cloud patterns and large-scale structures on other planets in our Solar System change over time.

• 13 July 2018: In November 2008, 14-year-old Caroline Moore from New York discovered a supernova in UGC 12682. This made her the youngest person at the time to have discovered a supernova. Follow-up observations by professional astronomers of the so-called SN 2008ha showed that it was peculiarly interesting in many different ways: its host galaxy UGC 12862 rarely produces supernovae. It is one of the faintest supernovae ever observed and after the explosion it expanded very slowly, suggesting that the explosion did not release copious amounts of energy as usually expected. 114)

- Astronomers have now classified SN 2008ha as a subclass of a Type Ia supernova, which is the explosion of a white dwarf that hungrily accretes matter from a companion star. SN 2008ha may have been the result of a partially failed supernova, explaining why the explosion failed to decimate the whole star.

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Figure 89: Glowing warmly against the dark backdrop of the Universe, this image from the NASA/ESA Hubble Space Telescope shows an irregular galaxy called UGC 12682. Located approximately 70 million light-years away in the constellation of Pegasus (The Winged Horse), UGC 12682 is distorted and oddly-structured, with bright pockets of star formation (image credit: ESA/Hubble & NASA, CC BY 4.0)

• 3 July 2018: Like a July 4 fireworks display, a young, glittering collection of stars resembles an aerial burst. The cluster is surrounded by clouds of interstellar gas and dust - the raw material for new star formation. The nebula, located 20,000 light-years away in the constellation Carina, contains a central cluster of huge, hot stars, called NGC 3603. 115)

- Appearing colorful and serene, this environment is anything but. Ultraviolet radiation and violent stellar winds have blown out an enormous cavity in the gas and dust enveloping the cluster. Most of the stars in the cluster were born around the same time but differ in size, mass, temperature and color. The course of a star's life is determined by its mass, so a cluster of a given age will contain stars in various stages of their lives, giving an opportunity for detailed analyses of stellar life cycles. NGC 3603 also contains some of the most massive stars known. These huge stars live fast and die young, burning through their hydrogen fuel quickly and ultimately ending their lives in supernova explosions.

- Star clusters like NGC 3603 provide important clues to understanding the origin of massive star formation in the early, distant universe. Astronomers also use massive clusters to study distant starbursts that occur when galaxies collide, igniting a flurry of star formation. The proximity of NGC 3603 makes it an excellent lab for studying such distant and momentous events.

- This Hubble Space Telescope image was captured in August 2009 and December 2009 with the Wide Field Camera 3 in both visible and infrared light, which trace the glow of sulfur, hydrogen, and iron.

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Figure 90: This Hubble Space Telescope image was captured in August 2009 and December 2009 with the Wide Field Camera 3 in both visible and infrared light, which trace the glow of sulfur, hydrogen, and iron [image credit: NASA, ESA, R. O'Connell (University of Virginia), F. Paresce (National Institute for Astrophysics, Bologna, Italy), E. Young (Universities Space Research Association/Ames Research Center), the WFC3 Science Oversight Committee, and the Hubble Heritage Team (STScI/AURA)]

• 25 June 2018: For years the Hubble Space Telescope has captured crisp spectral images of exoplanets transiting their host stars. Because those images include light filtered through the exoplanets’ atmospheres, they contain clues about atmospheric composition. Absorption features in such spectra have produced evidence of water, carbon dioxide, methane, and even clouds in the atmospheres of extrasolar planets. 116)

- But Hubble’s workhorse detector for exoplanet atmosphere observations, the Wide Field Camera 3, collects light in only 13 wavelength bins. The James Webb Space Telescope, scheduled for a 2020 launch, will be able to resolve spectra into hundreds of bins. The abundance of data could yield far more detailed portraits of extrasolar atmospheres, but it also creates a challenge: how to decipher all that information.

- Enter Kevin Heng and his coworkers at the University of Bern in Switzerland. The researchers have now demonstrated that machine learning can be used to extract atmospheric properties from even the most complicated transmission spectra. Heng and his colleagues trained their machine on tens of thousands of model spectra that were calculated analytically for atmospheres of varying temperature, cloudiness, and composition. The machine learning algorithm plots the spectra in N-dimensional space, where N is the number of wavelength bins in each spectrum, and then identifies clusters in that multidimensional space. Model atmospheres belonging to the same cluster tend to share similar physical attributes, so when the trained machine is given a real-life spectrum to analyze, it plots the spectrum and assigns to it the physical attributes of the nearest cluster.

- Reassuringly, a test-run analysis of the gas-giant planet WASP-12b yielded results similar to those of more conventional techniques. The test was implemented in 13-dimensional space, to match Hubble’s 13 spectral bins, but future implementations using more detailed spectra could include thousands of dimensions. 117)

• 25 June 2018: As if this Hubble Space Telescope picture isn't cluttered enough with myriad galaxies, nearby asteroids photobomb the image, their trails sometimes mimicking background astronomical phenomena. 118)

- The stunningly beautiful galaxy cluster Abell 370 (Figure 91) contains an astounding assortment of several hundred galaxies tied together by the mutual pull of gravity. Located approximately four billion light years away in the constellation Cetus, the Sea Monster, this immense cluster is a rich mix of a variety of galaxy shapes.

- Entangled among the galaxies are thin, white trails that look like curved or S-shaped streaks. These are trails from asteroids that reside, on average, only about 260 million kilometers from Earth – right around the corner in astronomical terms. The trails appear in multiple Hubble exposures that have been combined into one image. Of the 22 total asteroid sightings for this field, five are unique objects. These asteroids are so faint that they were not previously identified.

- The asteroid trails look curved due to an observational effect called parallax. As Hubble orbits around Earth, an asteroid will appear to move along an arc with respect to the vastly more distant background stars and galaxies. The motion of Earth around the Sun, and the motion of the asteroids along their orbits, are other contributing factors to the apparent skewing of asteroid paths.

- All the asteroids were found manually, the majority by “blinking” consecutive exposures to capture apparent asteroid motion. Astronomers found a unique asteroid for every 10 to 20 hours of exposure time.

- These asteroid trails should not be confused with the mysterious-looking arcs of blue light that are actually distorted images of distant galaxies behind the cluster. Many of these far-flung galaxies are too faint for Hubble to see directly. Instead, in a dramatic example of “gravitational lensing,” the cluster functions as a natural telescope, warping space and affecting light traveling through the cluster toward Earth.

- The study was part of the Frontier Fields program and the image, assembled from several exposures taken in visible and infrared light, was first published on 6 November 2017.

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Figure 91: This image was assembled from several exposures taken in visible and infrared light. The field's position on the sky is near the ecliptic, the plane of our Solar System. This is the zone in which most asteroids reside, which is why Hubble astronomers saw so many crossings. Hubble deep-sky observations taken along a line-of-sight near the plane of our Solar System commonly record asteroid trails (image credit: NASA, ESA, and B. Sunnquist and J. Mack (STScI) Acknowledgment: NASA, ESA, and J. Lotz (STScI) and the HFF Team)

- Every year on 30 June, the global “Asteroid Day” event takes place to raise awareness about asteroids and what can be done to protect Earth from possible impact. The day falls on the anniversary of the Tunguska event that took place on 30 June 1908, the most harmful known asteroid related event in recent history. This year, ESA is co-hosting a live webcast with the European Southern Observatory packed with expert interviews, news on some of the most recent asteroid science results, and the truth about the dinosaurs. Watch 30 June at 13:00 CEST via http://www.esa.int/Our_Activities/Space_Engineering_Technology/Asteroid_day

• 21 June 2018: An international team of astronomers using the NASA/ESA Hubble Space Telescope and the European Southern Observatory's VLT (Very Large Telescope) has made the most precise test of general relativity yet outside our Milky Way. The nearby galaxy ESO 325-G004 acts as a strong gravitational lens, distorting light from a distant galaxy behind it to create an Einstein ring around its center. By comparing the mass of ESO 325-G004 with the curvature of space around it, the astronomers found that gravity on these astronomical length-scales behaves as predicted by general relativity. This rules out some alternative theories of gravity. 119) 120)

- Using the NASA/ESA Hubble Space Telescope and European Southern Observatory's VLT, a team led by Thomas Collett (University of Portsmouth, UK), was able to perform the most precise test of general relativity outside the Milky Way to date.

- The theory of general relativity predicts that objects deform spacetime, causing any light that passes by to be deflected and resulting in a phenomenon known as gravitational lensing. This effect is only noticeable for very massive objects. A few hundred strong gravitational lenses are known, but most are too distant to precisely measure their mass. However, the elliptical galaxy ESO 325-G004 is amongst the closest lenses at just 450 million light-years from Earth.

- Using the MUSE (Multi Unit Spectroscopic Explorer) instrument on the VLT the team calculated the mass of ESO 325-G004 by measuring the movement of stars within it. Using Hubble the scientists were able to observe an Einstein ring resulting from light from a distant galaxy being distorted by the intervening ESO 325-G004. Studying the ring allowed the astronomers to measure how light, and therefore spacetime, is being distorted by the huge mass of ESO 325-G004.

- Collett comments: "We know the mass of the foreground galaxy from MUSE and we measured the amount of gravitational lensing we see from Hubble. We then compared these two ways to measure the strength of gravity – and the result was just what general relativity predicts, with an uncertainty of only nine percent. This is the most precise test of general relativity outside the Milky Way to date. And this using just one galaxy!"

- General relativity has been tested with exquisite accuracy on Solar System scales, and the motions of stars around the black hole at the center of the Milky Way are under detailed study, but previously there had been no precise tests on larger astronomical scales. Testing the long range properties of gravity is vital to validate our current cosmological model.

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Figure 92: An image of the nearby galaxy ESO 325-G004, created using data collected by the NASA/ESA Hubble Space Telescope and the MUSE instrument on the ESO's Very Large Telescope. MUSE measured the velocity of stars in ESO 325-G004 to produce the velocity dispersion map that is overlaid on top of the Hubble Space Telescope image. Knowledge of the velocities of the stars allowed the astronomers to infer the mass of ESO 325-G004. The inset shows the Einstein ring resulting from the distortion of light from a more distant source by intervening lens ESO 325-004, which becomes visible after subtraction of the foreground lens light (image credit: ESO, ESA/Hubble, NASA)

- These findings may have important implications for models of gravity alternative to general relativity. These alternative theories predict that the effects of gravity on the curvature of spacetime are "scale dependent". This means that gravity should behave differently across astronomical length-scales from the way it behaves on the smaller scales of the Solar System. Collett and his team found that this is unlikely to be true unless these differences only occur on length scales larger than 6000 light-years.

- "The Universe is an amazing place providing such lenses which we can use as our laboratories," adds team member Bob Nichol (University of Portsmouth). "It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was."

• 31 May 2018: Though it resembles a peaceful rose swirling in the darkness of the cosmos, NGC 3256 is actually the site of a violent clash. This distorted galaxy is the relic of a collision between two spiral galaxies, estimated to have occurred 500 million years ago. Today it is still reeling in the aftermath of this event. 121)

- Located about 100 million light-years away in the constellation of Vela (The Sails), NGC 3256 is approximately the same size as our Milky Way and belongs to the Hydra-Centaurus Supercluster. It still bears the marks of its tumultuous past in the extended luminous tails that sprawl out around the galaxy, thought to have formed 500 million years ago during the initial encounter between the two galaxies, which today form NGC 3256. These tails are studded with young blue stars, which were born in the frantic but fertile collision of gas and dust.

- When two galaxies merge, individual stars rarely collide because they are separated by such enormous distances, but the gas and dust of the galaxies do interact – with spectacular results. The brightness blooming in the center of NGC 3256 gives away its status as a powerful starburst galaxy, host to vast amounts of infant stars born into groups and clusters. These stars shine most brightly in the far infrared, making NGC 3256 exceedingly luminous in this wavelength domain. Because of this radiation, it is classified as a Luminous Infrared Galaxy.

- NGC 3256 has been the subject of much study due to its luminosity, its proximity, and its orientation: astronomers observe its face-on orientation, that shows the disc in all its splendor. NGC 3256 provides an ideal target to investigate starbursts that have been triggered by galaxy mergers. It holds particular promise to further our understanding of the properties of young star clusters in tidal tails.

- As well as being lit up by over 1000 bright star clusters, the central region of NGC 3256 is also home to crisscrossing threads of dark dust and a large disc of molecular gas spinning around two distinct nuclei – the relics of the two original galaxies. One nucleus is largely obscured, only unveiled in infrared, radio and X-ray wavelengths.

- These two initial galaxies were gas-rich and had similar masses, as they seem to be exerting roughly equal influence on each other. Their spiral disks are no longer distinct, and in a few hundred million years' time, their nuclei will also merge and the two galaxies will likely become united as a large elliptical galaxy.

- NGC 3256 was previously imaged through fewer filters by the NASA/ESA Hubble Space Telescope as part of a large collection of 59 images of merging galaxies, released for Hubble's 18th anniversary on 24 April 2008.

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Figure 93: This image, taken with the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS), both installed on the NASA/ESA Hubble Space Telescope, shows the peculiar galaxy NGC 3256. The galaxy is about 100 million light-years from Earth and is the result of a past galactic merger, which created its distorted appearance. As such, NGC 3256 provides an ideal target to investigate starbursts that have been triggered by galaxy mergers (image credit: ESA/Hubble, NASA, CC BY 4.0)

• 28 May 2018: This NASA/ESA Hubble Space Telescope image shows a cluster of hundreds of galaxies located about 7.5 billion light-years from Earth (Figure 94). The brightest galaxy within this cluster named SDSS J1156+1911 and known as the Brightest Cluster Galaxy (BCG), is visible in the lower middle of the frame. It was discovered by the Sloan Giant Arcs Survey which studied data maps covering huge parts of the sky from the Sloan Digital Sky Survey. The survey found more than 70 galaxies that look to be significantly affected by a cosmic phenomenon known as gravitational lensing. 122)

- Gravitational lensing is one of the predictions of Albert Einstein's General Theory of Relativity. The mass contained within a galaxy is so immense that it can actually warp and bend the very fabric of its surroundings (known as space-time), forcing light to travel along curved paths. As a result, the image of a more distant galaxy appears distorted and amplified to an observer, as the light from it has been bent around the intervening galaxy. This effect can be very useful in astronomy, allowing astronomers to see galaxies that are either obscured or too distant to be otherwise detected by our current instruments.

- Galaxy clusters are giant structures containing hundreds to thousands of galaxies, some with masses over one million billion times the mass of the Sun! SDSS J1156+1911 is only roughly 600 billion times the mass of the Sun, making it less massive than the average galaxy. However, it is massive enough to produce the fuzzy, greenish streak seen just below the brightest galaxy — the lensed image of a more distant galaxy.

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Figure 94: This Hubble image shows a cluster of hundreds of galaxies located about 7.5 billion light-years from Earth (image credit: ESA/Hubble & NASA; Acknowledgment: Judy Schmidt (Geckzilla))

• 17 May 2018: Ultraviolet light is a major tracer of the youngest and hottest stars. These stars are short-lived and intensely bright. Astronomers have now finished a survey called LEGUS (Legacy ExtraGalactic UV Survey) that captured the details of 50 local galaxies within 60 million light-years of Earth in both visible and ultraviolet light. 123) 124)

- The LEGUS team carefully selected its targets from among 500 candidate galaxies compiled from ground-based surveys. They chose the galaxies based on their mass, star-formation rate, and their abundances of elements heavier than hydrogen and helium. Because of the proximity of the selected galaxies, Hubble was able to resolve them into their main components: stars and star clusters. With the LEGUS data, the team created a catalog with about 8000 young clusters and it also created a star catalog comprising about 39 million stars that are at least five times more massive than our Sun.

- The data, gathered with Hubble’s WFC3 (Wide Field Camera 3) and ACS (Advanced Camera for Surveys), provide detailed information on young, massive stars and star clusters, and how their environment affects their development. As such, the catalogue offers an extensive resource for understanding the complexities of star formation and galaxy evolution.

- One of the key questions the survey may help astronomers answer is the connection between star formation and the major structures, such as spiral arms, that make up a galaxy. These structured distributions are particularly visible in the youngest stellar populations.

- By resolving the fine details of the studied galaxies, while also studying the connection to larger galactic structures, the team aims to identify the physical mechanisms behind the observed distribution of stellar populations within galaxies.

- Figuring out the final link between gas and star formation is key to fully understanding galaxy evolution. Astronomers are studying this link by looking at the effects of the environment on star clusters, and how their survival is linked to their surroundings.

- LEGUS will not only allow astronomers to understand the local Universe. It will also help interpret views of distant galaxies, where the ultraviolet light from young stars is stretched to infrared wavelengths due to the expansion of space. The NASA/ESA/CSA James Webb Space Telescope and its ability to observe in the far infrared will complement the LEGUS views.

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Figure 95: The glowing spiral arms of NGC 6744. This image shows the galaxy NGC 6744, about 30 million light-years away. It is one of 50 galaxies observed as part of the Hubble Space Telescope’s Legacy ExtraGalactic UV Survey (LEGUS), the sharpest, most comprehensive ultraviolet-light survey of star-forming galaxies in the nearby Universe, offering an extensive resource for understanding the complexities of star formation and galaxy evolution. The image is a composite using both ultraviolet light and visible light, gathered with Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys (image credit: NASA, ESA, and the LEGUS team)

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Figure 96: Dwarf galaxy UGCA 281. UGCA 281 is a blue compact dwarf galaxy located in the constellation of Canes Venatici. Within it, two giant star clusters appear brilliant white and are swaddled by greenish hydrogen gas clouds. These clusters are responsible for most of the recent star formation in UGCA 281; the rest of the galaxy is comprised of older stars and appears redder in color. The reddish objects in the background are background galaxies that appear through the diffuse dwarf galaxy. The image is a composite using both ultraviolet light and visible light, gathered with Hubble's Wide Field Camera 3 and Advanced Camera for Surveys (image credit: NASA, ESA, and the LEGUS team)

• 16 My 2018: Resembling a wizard’s staff set aglow, NGC 1032 cleaves the quiet darkness of space in two in this image from the NASA/ESA Hubble Space Telescope (Figure 97). 125)

- NGC 1032 is located about a hundred million light years away in the constellation Cetus (The Sea Monster). Although beautiful, this image perhaps does not do justice to the galaxy’s true aesthetic appeal: NGC 1032 is actually a spectacular spiral galaxy, but from Earth, the galaxy’s vast disc of gas, dust and stars is seen nearly edge-on.

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Figure 97: A handful of other galaxies can be seen lurking in the background, scattered around the narrow stripe of NGC 1032. Many are oriented face-on or at tilted angles, showing off their glamorous spiral arms and bright cores. Such orientations provide a wealth of detail about the arms and their nuclei, but fully understanding a galaxy’s three-dimensional structure also requires an edge-on view. This gives astronomers an overall idea of how stars are distributed throughout the galaxy and allows them to measure the “height” of the disc and the bright star-studded core (image credit: ESA/Hubble & NASA, CC BY 4.0)

• 02 May 2018: Astronomers using the NASA/ESA Hubble Space Telescope have detected helium in the atmosphere of the exoplanet WASP-107b. This is the first time this element has been detected in the atmosphere of a planet outside the Solar System. The discovery demonstrates the ability to use infrared spectra to study exoplanet extended atmospheres. 126)

- The international team of astronomers, led by Jessica Spake, a PhD student at the University of Exeter in the UK, used Hubble's Wide Field Camera 3 to discover helium in the atmosphere of the exoplanet WASP-107b. This is the first detection of its kind.

- Spake explains the importance of the discovery: "Helium is the second-most common element in the Universe after hydrogen. It is also one of the main constituents of the planets Jupiter and Saturn in our Solar System. However, up until now helium had not been detected on exoplanets - despite searches for it."

- The team made the detection by analyzing the infrared spectrum of the atmosphere of WASP-107b. Previous detections of extended exoplanet atmospheres have been made by studying the spectrum at ultraviolet and optical wavelengths; this detection therefore demonstrates that exoplanet atmospheres can also be studied at longer wavelengths.
Note: The measurement of an exoplanet's atmosphere is performed when the planet passes in front of its host star. A tiny portion of the star's light passes through the exoplanet's atmosphere, leaving detectable fingerprints in the spectrum of the star. The larger the amount of an element present in the atmosphere, the easier the detection becomes.

- "The strong signal from helium we measured demonstrates a new technique to study upper layers of exoplanet atmospheres in a wider range of planets," says Spake "Current methods, which use ultraviolet light, are limited to the closest exoplanets. We know there is helium in the Earth's upper atmosphere and this new technique may help us to detect atmospheres around Earth-sized exoplanets – which is very difficult with current technology."

- WASP-107b is one of the lowest density planets known: While the planet is about the same size as Jupiter, it has only 12% of Jupiter's mass. The exoplanet is about 200 light-years from Earth and takes less than six days to orbit its host star.

- The amount of helium detected in the atmosphere of WASP-107b is so large that its upper atmosphere must extend tens of thousands of kilometers out into space. This also makes it the first time that an extended atmosphere has been discovered at infrared wavelengths.

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Figure 98: Artist's impression of WASP-107b (image credit: ESA/Hubble, NASA, M. Kornmesser, CC BY 4.0)

- Since its atmosphere is so extended, the planet is losing a significant amount of its atmospheric gases into space – between ~0.1-4% of its atmosphere's total mass every billion years.
Note: Stellar radiation has a significant effect on the rate at which a planet's atmosphere escapes. The star WASP-107 is highly active, supporting the atmospheric loss. As the atmosphere absorbs radiation it heats up, so the gas rapidly expands and escapes more quickly into space.

- As far back as the year 2000, it was predicted that helium would be one of the most readily-detectable gases on giant exoplanets, but until now, searches were unsuccessful.

- David Sing, co-author of the study also from the University of Exeter, concludes: "Our new method, along with future telescopes such as the NASA/ESA/CSA James Webb Space Telescope, will allow us to analyze atmospheres of exoplanets in far greater detail than ever before." 127)

• 19 April 2018: To celebrate its 28th anniversary in space the NASA/ESA Hubble Space Telescope took this amazing and colorful image of the Lagoon Nebula (Figure 99). The whole nebula, about 4000 light-years away, is an incredible 55 light-years wide and 20 light-years tall. This image shows only a small part of this turbulent star-formation region, about four light-years across. 128)

- This stunning nebula was first catalogued in 1654 by the Italian astronomer Giovanni Battista Hodierna, who sought to record nebulous objects in the night sky so they would not be mistaken for comets. Since Hodierna’s observations, the Lagoon Nebula has been photographed and analysed by many telescopes and astronomers all over the world.

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Figure 99: The observations were taken by Hubble’s Wide Field Camera 3 between 12 February and 18 February 2018 (image credit: NASA, ESA, STScI, CC BY 4.0)

• 10 April 2018: This NASA/ESA Hubble Space Telescope image (Figure 100) shows a massive galaxy cluster glowing brightly in the darkness. Despite its beauty, this cluster bears the distinctly unpoetic name of PLCK_G308.3-20.2. 129)

- Galaxy clusters can contain thousands of galaxies all held together by the glue of gravity. At on130)e point in time they were believed to be the largest structures in the Universe — until they were usurped in the 1980s by the discovery of superclusters, which typically contain dozens of galaxy clusters and groups and span hundreds of millions of light-years. However, clusters do have one thing to cling on to; superclusters are not held together by gravity, so galaxy clusters still retain the title of the biggest structures in the Universe bound by gravity.

- One of the most interesting features of galaxy clusters is the stuff that permeates the space between the constituent galaxies: the intracluster medium (ICM). High temperatures are created in these spaces by smaller structures forming within the cluster. This results in the ICM being made up of plasma — ordinary matter in a superheated state. Most luminous matter in the cluster resides in the ICM, which is very luminous X-rays. However, the majority of the mass in a galaxy cluster exists in the form of non-luminous dark matter. Unlike plasma, dark matter is not made from ordinary matter such as protons, neutrons and electrons. It is a hypothesized substance thought to make up 80 % of the Universe’s mass, yet it has never been directly observed.

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Figure 100: This image was taken by Hubble’s Advanced Camera for Surveys and Wide-Field Camera 3 as part of an observing program called RELICS (Reionization Lensing Cluster Survey). RELICS imaged 41 massive galaxy clusters with the aim of finding the brightest distant galaxies for the forthcoming NASA/ESA/CSA James Webb Space Telescope(JWST) to study (image credit: ESA/Hubble & NASA, RELICS)

• 02 April 2018: Astronomers using the NASA/ESA Hubble Space Telescope have found the most distant star ever discovered. The hot blue star existed only 4.4 billion years after the Big Bang. This discovery provides new insight into the formation and evolution of stars in the early Universe, the constituents of galaxy clusters and also on the nature of dark matter. 131)

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Figure 101: Appearance of the most distant star (image credit: NASA & ESA and P. Kelly (University of California, Berkeley))

- The international team, led by Patrick Kelly (University of Minnesota, USA), Jose Diego (Instituto de Física de Cantabria, Spain) and Steven Rodney (University of South Carolina, USA), discovered the distant star in the galaxy cluster MACS J1149-2223 in April 2016. The observations with Hubble were actually performed in order to detect and follow the latest appearance of the gravitationally lensed supernova explosion nicknamed "Refsdal" (heic1525), when an unexpected point source brightened in the same galaxy that hosted the supernova.

- "Like the Refsdal supernova explosion the light of this distant star got magnified, making it visible for Hubble," says Patrick Kelly. "This star is at least 100 times farther away than the next individual star we can study, except for supernova explosions."

- The observed light from the newly discovered star, called Lensed Star 1 (LS1) was emitted when the Universe was only about 30 percent of its current age – about 4.4 billion years after the Big Bang. The detection of the star through Hubble was only possible because the light from the star was magnified 2000 times.

- "The star became bright enough to be visible for Hubble thanks to a process called gravitational lensing," explains Jose Diego. The light from LS1 was magnified not only by the huge total mass of the galaxy cluster, but also by another compact object of about three times the mass of the Sun within the galaxy cluster itself; an effect known as gravitational microlensing.
Note: Gravitational lensing magnifies the light from fainter, background objects, allowing Hubble to see objects it would otherwise not be able to detect. The process was first predicted by Albert Einstein and is now used to find some of the most distant objects in the Universe. Usually the lensing object is a galaxy or a galaxy cluster, but in some cases it can also be a star or even a planet. When it involves these smaller objects the process is called microlensing.

- "The discovery of LS1 allows us to gather new insights into the constituents of the galaxy cluster. We know that the microlensing was caused by either a star, a neutron star, or a stellar-mass black hole," explains Steven Rodney. LS1 therefore allows astronomers to study neutron stars and black holes, which are otherwise invisible and they can estimate how many of these dark objects exist within this galaxy cluster.

- As galaxy clusters are among the largest and most massive structures in the Universe, learning about their constituents also increases our knowledge about the composition of the Universe overall. This includes additional information about the mysterious dark matter.

- "If dark matter is at least partially made up of comparatively low-mass black holes, as it was recently proposed, we should be able to see this in the light curve of LS1. Our observations do not favor the possibility that a high fraction of dark matter is made of these primordial black holes with about 30 times the mass of the Sun", highlights Kelly.

- After the discovery the researchers used Hubble again to measure a spectrum of LS1. Based on their analysis, the astronomers think that LS1 is a B-type supergiant star. These stars are extremely luminous and blue in color, with a surface temperature between 11,000 and 14,000 degrees Celsius; making them more than twice as hot as the Sun.

- But this was not the end of the story. Observations made in October 2016 suddenly showed a second image of the star. "We were actually surprised to not have seen this second image in earlier observations, as also the galaxy the star is located in can be seen twice," comments Diego. "We assume that the light from the second image has been deflected by another moving massive object for a long time – basically hiding the image from us. And only when the massive object moved out of the line of sight the second image of the star became visible." This second image and the blocking object add another piece of the puzzle to reveal the makeup of galaxy clusters.

- With more research and the arrival of new, more powerful telescopes like the NASA/ESA/CSA James Webb Space Telescope, the astronomers suggest that with microlensing, it will be possible to study the evolution of the earliest stars in the Universe in greater detail than ever expected.

•30 March 2018: The image of Figure 102, captured by the ACS (Advanced Camera for Surveys) on the NASA/ESA Hubble Space Telescope, shows the spiral galaxy NGC 5714, about 130 million light-years away in the constellation of Boötes (the Herdsman). NGC 5714 is classified as a Sc spiral galaxy, but its spiral arms — the dominating feature of spiral galaxies — are almost impossible to see, as NGC 1787 presents itself at an almost perfectly edge-on angle. 132) 133)

- Discovered by William Herschel in 1787, NGC 5714 was host to a fascinating and rare event in 2003. A faint supernova appeared about 8000 light-years below the central bulge of NGC 5714. Supernovae are the huge, violent explosions of dying stars, and the one that exploded in NGC 5714 — not visible in this much later image — was classified as a Type Ib/c supernova and named SN 2003dr. It was particularly interesting because its spectrum showed strong signatures of calcium.

- Calcium-rich supernovae are rare and hence of great interest to astronomers. Astronomers still struggle to explain these particular explosions as their existence presents a challenge to both observation and theory. In particular, their appearance outside of galaxies, their lower luminosity compared to other supernovae, and their rapid evolution are still open questions for researchers.

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Figure 102: Image of the spiral galaxy NGC 5714, captured by the NASA/ESA Hubble Space Telescope (image credit: ESA/Hubble & NASA)

• 28 March 2018: An international team of researchers using the NASA/ESA Hubble Space Telescope and several other observatories have, for the first time, uncovered a galaxy in our cosmic neighborhood that is missing most – if not all – of its dark matter. This discovery of the galaxy NGC 1052-DF2 challenges currently-accepted theories of and galaxy formation and provides new insights into the nature of dark matter. The results are published in Nature. 134) 135)

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Figure 103: A ghostly galaxy lacking dark matter (image credit: NASA, ESA, and P. van Dokkum (Yale University))

- Astronomers using Hubble and several ground-based observatories have found a unique astronomical object: a galaxy that appears to contain almost no dark matter. Hubble helped to accurately confirm the distance of NGC 1052-DF2 to be 65 million light-years and determined its size and brightness. Based on these data the team discovered that NGC 1052-DF2 larger than the Milky Way, but contains about 250 times fewer stars, leading it to be classified as an ultra diffuse galaxy.
Note 1: The galaxy was identified with the Dragonfly Telephoto Array (DFA) and also observed by the Sloan Digital Sky Survey (SDSS). As well as the NASA/ESA Hubble Space Telescope, the Gemini Observatory and the Keck Observatory were used to study the object in more detail.

- "I spent an hour just staring at this image," lead researcher Pieter van Dokkum of Yale University says as he recalls first seeing the Hubble image of NGC 1052-DF2. "This thing is astonishing: a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy."

- Further measurements of the dynamical properties of ten globular clusters orbiting the galaxy allowed the team to infer an independent value of the galaxies mass. This mass is comparable to the mass of the stars in the galaxy, leading to the conclusion that NGC 1052-DF2 contains at least 400 times less dark matter than astronomers predict for a galaxy of its mass, and possibly none at all. This discovery is unpredicted by current theories on the distribution of dark matter and its influence on galaxy formation.
Note 2: Since 1884 astronomers have invoked dark matter to explain why galaxies do not fly apart, given the speed at which the stars within galaxies move. From Kepler's Second Law it is expected that the rotation velocities of stars will decrease with distance from the center of a galaxy. This is not observed.

- "Dark matter is conventionally believed to be an integral part of all galaxies – the glue that holds them together and the underlying scaffolding upon which they are built," explains co-author Allison Merritt from Yale University and the Max Planck Institute for Astronomy, Germany. And van Dokkum adds: "This invisible, mysterious substance is by far the most dominant aspect of any galaxy. Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work." - Merritt remarks: "There is no theory that predicts these types of galaxies – how you actually go about forming one of these things is completely unknown."

- Although counterintuitive, the existence of a galaxy without dark matter negates theories that try to explain the Universe without dark matter being a part of it. The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.
Note3: The MOND theory – Modified Newtonian Dynamics – suggests that the phenomena usually attributed to dark matter can be explained by modifying the laws of gravity. The result of this would be that a signature usually attributed to dark matter should always be detected, and is an unavoidable consequence of the presence of ordinary matter.

- Meanwhile, the researchers already have some ideas about how to explain the missing dark matter in NGC 1052-DF2. Did a cataclysmic event such as the birth of a multitude of massive stars sweep out all the gas and dark matter? Or did the growth of the nearby massive elliptical galaxy NGC 1052 billions of years ago play a role in NGC 1052-DF2's dark matter deficiency?

- These ideas, however, still do not explain how this galaxy formed. To find an explanation, the team is already hunting for more dark-matter deficient galaxies as they analyze Hubble images of 23 ultra-diffuse galaxies – three of which appear to be similar to NGC 1052-DF2.


• April 24, 2017: Since its launch on 24 April 1990, Hubble has been nothing short of a revolution in astronomy. The first orbiting facility of its kind, for 27 years the telescope has been exploring the wonders of the cosmos. Astronomers and the public alike have witnessed what no other humans in history have before. In addition to revealing the beauty of the cosmos, Hubble has proved itself to be a treasure chest of scientific data that astronomers can access. 136) 137)

- NASA and ESA celebrate Hubble's birthday each year with a spectacular image. This year's anniversary image features a pair of spiral galaxies known as NGC 4302 – seen edge-on – and NGC 4298, both located 55 million light-years away in the northern constellation of Coma Berenices (Berenice's Hair). The pair, discovered by astronomer William Herschel in 1784, form part of the Virgo Cluster, a gravitationally bound collection of nearly 2000 individual galaxies. Such objects were first simply called "spiral nebulas," because it wasn't known how far away they were. In the early 20th century, Edwin Hubble discovered that galaxies are other island cities of stars far outside our Milky Way.

- At their closest points, the galaxies are separated from each other in projection by only around 7000 light-years. Given this very close arrangement, astronomers are intrigued by the galaxies' apparent lack of any significant gravitational interaction; only a faint bridge of neutral hydrogen gas – not visible in this image – appears to stretch between them. The long tidal tails and deformations in their structure that are typical of galaxies lying so close to each other are missing completely.

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Figure 104: HST images of spiral galaxies NGC 4302 (left) and NGC 4298 (right), both located 55 million light-years away. They were observed by Hubble to celebrate its 27th year in orbit. The image in visible and infrared light brilliantly captures their warm stellar glow and brown, mottled patterns of dust [image credit: NASA, ESA, and M. Mutchler (STScI)]

- The edge-on galaxy is called NGC 4302, and the tilted galaxy is NGC 4298. These galaxies look quite different because we see them angled at different positions on the sky. They are actually very similar in terms of their structure and contents.

- From our view on Earth, researchers report an inclination of 90 degrees for NGC 4302, which is exactly edge on. NGC 4298 is tilted 70 degrees.

- In NGC 4298, the telltale, pinwheel-like structure is visible, but it's not as prominent as in some other spiral galaxies. In the edge-on NGC 4302, dust in the disk is silhouetted against rich lanes of stars. Absorption by dust makes the galaxy appear darker and redder than its companion. A large blue patch appears to be a giant region of recent star formation.

Figure 105: This animation zooms through the Virgo Cluster of nearly 2,000 galaxies into tight Hubble Space Telescope images of spiral galaxies NGC 4302 (left) and NGC 4298 (right) in visible and infrared light. Located approximately 55 million light-years away, the starry pair offers a glimpse of what our Milky Way galaxy would look like to an outside observer [image credit: NASA, ESA, and G. Bacon, J. DePasquale, and Z. Levay (STScI) Acknowledgment: A. Fujii; Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and UKSTU/AAO; B. Franke (Focal Point Observatory); and M. Mutchler (STScI)]

- A typical spiral galaxy has arms of young stars that wind outward from its center. The bright arms are regions of intense star formation. Such galaxies have a central bulge and are surrounded by a faint halo of stars. Many spiral galaxies also have bars that extend from the central bulge to the arms.

- The edge-on NGC 4302 is about 87,000 light-years in diameter, which is about 60 percent the size of the Milky Way. It is about 110 billion solar masses, approximately one-tenth of the Milky Way's mass.

- The tilted NGC 4298 is about 45,000 light-years in diameter, about one third the size of the Milky Way. At 17 billion solar masses, it is less than 2 percent of the Milky Way galaxy's 1 trillion solar masses.

- The Hubble observations were taken between 2 - 22 January, 2017 with the WFC3 (Wide Field Camera 3) instrument in three visible light bands.


Hubble's 25th anniversary on orbit on April 24, 2015

From planets to planetary nebula, and from star formation to supernova explosions, the NASA/ESA Hubble Space Telescope has captured a wealth of astronomical objects in its 25-year career. The montage of Figure 106 presents 25 images that sample the space telescope’s rich contribution to our understanding of the Universe around us. 138) 139) 140)

The NASA/ESA Hubble was launched into orbit by the Space Shuttle on 24 April 1990 (12:33:51 UTC). It was the first space telescope of its kind, and has surpassed all expectations, providing a quarter of a century of discoveries, stunning images and outstanding science.

The anniversary image (Figure 108) is bursting with silver anniversary fireworks, showing off a giant young star cluster known as Westerlund 2, sparkling with the light of about 3000 stars. Hubble’s sharp vision resolves the dense concentration of stars in the central cluster, which measures only about 10 light-years across.

A new anniversary image of Hubble is released every year and shown in Figure 106.

At the center of the collage is Star cluster Westerlund 2, the image released on the occasion of Hubble’s 25th anniversary.

Top row (from left to right): Interacting galaxies , Abell 2218 , Comet ISON , Jupiter , Green filament in the Teacup galaxy , Star formation in 30 Doradus , Interacting galaxies Arp 273

Second row: Saturn , The Ring Nebula , “Mystic Mountain” in the Carina Nebula , Crab Nebula

Third row: The horsehead nebula , Carina Nebula , Planetary nebula NGC 6302 , Star formation in M17

Fourth row: Globular cluster NGC 121 , “Pillars of creation” , Ring galaxy AM 0644-741,

Fifth row: Colliding galaxies Arp 272, Star cluster NGC 602, Hubble Ultra Deep Field, Mars, Variable star RS Puppis , Orion Nebula

Table 4: Links to the Hubble anniversary images in Figure 106 (Ref. 138)

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Figure 106: Collage of 25 images representing Hubble's rich contribution to our understanding of the Universe (image credit: NASA/ESA)

Figure 107: Horse of a Different Color: Hubble's Universe Unfiltered [video credit: Frank Summers, STScI (Space Telescope Science Institute)]

Legend to Figure 107: The Horsehead Nebula is a striking, dark gas cloud just below Orion's belt. It is a favorite of both professional and amateur astronomers. However, as a dark nebula, most of its true structure is hidden from visible light observations. To celebrate the 23rd anniversary of the Hubble Space Telescope, we revealed the considerable detail of that unseen nebular structure via an infrared portrait. The result is even more striking, and something one just doesn't see very often: a veritable astronomical horse of a different color. (Published on Sep 5, 2013).

"Hubble's Universe" is a recurring broadcast from HubbleSite, online home of the Hubble Space Telescope. Astrophysicist Frank Summers takes viewers on an in-depth tour of the latest Hubble discoveries. Find more episodes at: The Future of Space Astronomy: Hubble's Universe Unfiltered

• This glittering tapestry of young stars flaring into life in the star cluster Westerlund 2 has been released to celebrate the NASA/ESA Hubble Space Telescope’s 25th year in orbit and a quarter of a century of discoveries, stunning images and outstanding science. 141)

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Figure 108: NASA unveils Celestial Fireworks as Official Image for Hubble's 25th Anniversary on April 24, 2015. The image was acquired with WFC-3 (Wide Field Camera-3) piercing through the dusty veil shrouding the stellar nursery in near-infrared light, giving astronomers a clear view of the nebula and the dense concentration of stars in the central cluster. (image credit: NASA, ESA, STScI) 142) 143) 144)

Legend to Figure 108: The sparkling centerpiece of Hubble’s anniversary fireworks is a giant cluster of about 3,000 stars called Westerlund 2, named for Swedish astronomer Bengt Westerlund who discovered the grouping in the 1960s. The cluster resides in a raucous stellar breeding ground known as Gum 29, located 20,000 light-years away from Earth in the constellation Carina.

The giant star cluster is only about two million years old, but contains some of the brightest, hottest and most massive stars ever discovered. Some of these are carving deep cavities in the surrounding material through their intense ultraviolet light and the high-speed charged particles contained in their stellar winds.

This image is a testament to Hubble’s observational power and demonstrates that, even with 25 years of operations under its belt, its story is by no means over. Hubble has set the stage for the James Webb Space Telescope – scheduled for launch in 2018 – but will not be immediately replaced by this next-generation observatory, instead working alongside it. Now, 25 years after launch, is the time to celebrate Hubble’s future potential as well as its remarkable history.

• November 2, 2015: Eerie, dramatic new pictures from NASA's Hubble Space Telescope show newborn stars emerging from "eggs" - not the barnyard variety - but rather dense, compact pockets of interstellar gas called evaporating gaseous globules (EGGs). Hubble found the "EGGs," appropriately enough, in the Eagle nebula, a nearby star-forming region 6,500 light- years away in the constellation Serpens (Figure 109). 145)

- "For a long time astronomers have speculated about what processes control the sizes of stars - about why stars are the sizes that they are," said Jeff Hester of Arizona State University, Tempe, AZ. "Now in M16 we seem to be watching at least one such process at work right in front of our eyes."

- Striking pictures taken by Hester and co-investigators with Hubble's Wide Field and Planetary Camera 2 (WFPC2) resolve the EGGs at the tip of finger-like features protruding from monstrous columns of cold gas and dust in the Eagle nebula (also called M16 - 16th object in the Messier catalog). The columns - dubbed "elephant trunks" - protrude from the wall of a vast cloud of molecular hydrogen, like stalagmites rising above the floor of a cavern. Inside the gaseous towers, which are light-years long, the interstellar gas is dense enough to collapse under its own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings.

- Hubble gives a clear look at what happens as a torrent of ultraviolet light from nearby young, hot stars heats the gas along the surface of the pillars, "boiling it away" into interstellar space - a process called "photoevaporation. "The Hubble pictures show photoevaporating gas as ghostly streamers flowing away from the columns. But not all of the gas boils off at the same rate. The EGGs, which are denser than their surroundings, are left behind after the gas around them is gone.

- "It's a bit like a wind storm in the desert," said Hester. "As the wind blows away the lighter sand, heavier rocks buried in the sand are uncovered. But in M16, instead of rocks, the ultraviolet light is uncovering the denser egg-like globules of gas that surround stars that were forming inside the gigantic gas columns."

- Some EGGs appear as nothing but tiny bumps on the surface of the columns. Others have been uncovered more completely, and now resemble "fingers" of gas protruding from the larger cloud. (The fingers are gas that has been protected from photoevaporation by the shadows of the EGGs). Some EGGs have pinched off completely from the larger column from which they emerged, and now look like teardrops in space.

- By stringing together these pictures of EGGs caught at different stages of being uncovered, Hester and his colleagues from the Wide Field and Planetary Camera Investigation Definition Team are getting an unprecedented look at what stars and their surroundings look like before they are truly stars.

- "This is the first time that we have actually seen the process of forming stars being uncovered by photoevaporation," Hester emphasized. "In some ways it seems more like archaeology than astronomy. The ultraviolet light from nearby stars does the digging for us, and we study what is unearthed."

- "In a few cases we can see the stars in the EGGs directly in the WFPC2 images," says Hester. "As soon as the star in an EGG is exposed, the object looks something like an ice cream cone, with a newly uncovered star playing the role of the cherry on top."

- Ultimately, photoevaporation inhibits the further growth of the embyronic stars by dispersing the cloud of gas they were "feeding" from. "We believe that the stars in M16 were continuing to grow as more and more gas fell onto them, right up until the moment that they were cut off from that surrounding material by photoevaporation," said Hester.

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Figure 109: Gas Pillars in the Eagle Nebula (M16): Pillars of Creation in a star-forming region (image credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University))


Super Nova SN 1987A in the Large Magellanic Cloud

Thirty years ago, on 23 February 1987, the light from a stellar explosion marking the death of a massive star arrived at Earth to shine in Southern Hemisphere skies. Located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, SN 1987A was the closest observed supernova to Earth since the invention of the telescope. Studying it for the last 30 years has revolutionized our understanding of the explosive death of massive stars. 146)

- In operation since 1990, the NASA/ESA Hubble Space Telescope has observed the supernova remnant many times, as highlighted in this montage of Figure 110. The images show its evolution between 1994 and 2016, and highlight the main ring that blazes around the exploded star.

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Figure 110: Hubble follows the evolution of an expanding supernova remnant over three decades (image credit: NASA, ESA and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) and P. Challis (Harvard-Smithsonian Center for Astrophysics)

A new wide-field image(Figure 111) was also taken by Hubble in January 2017 to mark the 30 year anniversary. By observing the expanding remnant material over the years, Hubble has helped to show that the material within the ring was likely ejected 20,000 years before the actual explosion took place.

The initial burst of light from the supernova initially illuminated the rings. They slowly faded over the first decade after the explosion, until a fast-moving shell of gas ejected during the supernova slammed into the central ring, sending a powerful shockwave through the gas, heating it to searing temperatures and generating strong X-ray emission.

This caused clumps of denser gas within the ring to light up like a string of pearls, seen as the increasing number of bright spots, which are now fading again. As the shock wave continues to move through the shells ejected by the dying star in its final throes of life, who knows what new details will be revealed?

Since its launch in 1990 Hubble has observed the expanding dust cloud of SN 1987A several times and this way helped astronomers to create a better understanding of these cosmic explosions.

Supernova 1987A is located in the center of the image amidst a backdrop of stars. The bright ring around the central region of the exploded star is composed of material ejected by the star about 20,000 years before the actual explosion took place. The supernova is surrounded by gaseous clouds. The clouds' red color represents the glow of hydrogen gas.

The colors of the foreground and background stars were added from observations taken by Hubble's WFPC2 ( Wide Field Planetary Camera 2).

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Figure 111: This new image of the supernova remnant SN 1987A was taken by the NASA/ESA Hubble Space Telescope in January 2017 using its WFC3 (Wide Field Camera 3), image credit: NASA, ESA, and R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation) and P. Challis (Harvard-Smithsonian Center for Astrophysics)

Left: For comparison, a picture taken with the WFPC-1 camera in wide field mode, on November 27, 1993, just a few days prior to the STS-61 servicing mission. The effects of optical aberration in HST's 2.4-meter primary mirror blur starlight, smear out fine detail, and limit the telescope's ability to see faint structure. Both Hubble images are "raw;" they have not been subject to computer image reconstruction techniques commonly used in aberrated images made before the servicing mission.

Target Information of M100: The galaxy M100 (100th object in the Messier Catalog of non-stellar objects) is one of the brightest members of the Virgo Cluster of galaxies. The galaxy is in the spring constellation Coma Berenices and can be seen through a moderate-sized amateur telescope. M100 is spiral shaped, like our Milky Way, and tilted nearly face-on as seen from earth. The galaxy has two prominent arms of bright stars and several fainter arms. Though the galaxy is estimated to be tens of millions of light-years away, Hubble reveals the sort of detail only seen previously (with ground based telescopes) in neighboring galaxies that are ten times closer. Before HST, astronomers could only see such a level of detail in roughly a dozen galaxies in our Local Group. Now, with Hubble's improved vision, the portion of the universe which can be studied with such clarity has grown a thousand fold. Only the future will tell what revelations await as Hubble's spectacular vision is applied to a host of fascinating and important questions about the universe and our place in it.




Hubble Servicing Missions

HST (Hubble Space Telescope) was launched April 24, 1990 on Shuttle flight STS-31. However, after Hubble's deployment, scientist realized that the telescope's primary mirror had a flaw called “spherical aberration.” The outer edge of the mirror was ground too flat by a depth of 4 μm. This aberration resulted in images that were fuzzy because some of the light from the objects being studied was being scattered. After the amount of aberration was understood, scientists and engineers developed WFPC2 (Wide Field/Planetary Camera 2) and COSTAR (Corrective Optics Space Telescope Axial Replacement), which were installed in Hubble during the first space shuttle servicing mission in 1993. - Without periodic onboard servicing, HST would have been a disaster and would not have produced all of the great science it has. 147) 148) 149)

All Hubble Servicing Missions were conducted by berthing the Hubble Spacecraft inside the Shuttle Payload Bay. The Shuttle resources SRMS/OBSS (Shuttle Remote Manipulator Arm/Orbiter Boom Sensor System), SSRMS (Space Station Remote Manipulator System, Canadarm2 ), communication system, etc. were used to conduct the inspections and repairs of Hubble.

The servicing missions involved intensive coordination between NASA's Kennedy Space Center in Florida, Johnson Space Center in Houston, and Goddard Space Flight Center in Greenbelt, Maryland. Preparation activities included astronaut training at all three centers; simulations of telescope operations during the mission at the Space Telescope Operations Control Center (STOCC) at Goddard; testing and preparing instruments and hardware for flight at Goddard; and preparing the space shuttle for launch, flight and landing at Kennedy.

During the missions, operations took place primarily at Johnson and in Goddard's STOCC. Johnson’s Mission Control Center (MCC) monitored every aspect of the space shuttle and astronauts, including spacewalks, procedures and schedules, crew activities and health, and in-cabin and cargo bay systems and experiments. The STOCC ground crew handled telescope operations, sending commands to Hubble to place the instruments into “safe hold” (hibernation), close the aperture door (which covers the precious optical components), and perform maneuvers to position the telescope for grappling by the shuttle’s robotic arm, operated by astronauts to bring Hubble into the shuttle’s payload bay.

After each new system part or science instrument was installed, STOCC personnel performed tests to make sure each instrument and component had power and operated as it should. During the astronauts' sleep cycles, the STOCC team performed more detailed tests on each newly installed component to determine if additional service from astronauts would be required.

After all the servicing tasks had been performed via a three- to five-day series of spacewalks, STOCC controllers and Johnson Mission Control prepared the telescope for release. Often this also involved using the shuttle’s thrusters to climb into a higher orbit before releasing Hubble.

The astronaut crew then used the shuttle’s robotic arm to slowly raise Hubble from the payload bay and out into space, where controllers at STOCC opened Hubble's aperture door and made sure the telescope was functioning on its own. Returning Hubble to full science observations after a servicing mission usually took a few months.


SM-1 (Servicing Mission-1)

The primary goal of Servicing Mission 1 was to restore Hubble’s vision. Because Hubble’s primary mirror was incorrectly shaped, the telescope could not focus all the light from an object to a single sharp point. Instead, it saw a fuzzy halo around objects it observed. Astronauts on space shuttle Endeavour’s STS‐61 mission spent five days tuning it up. They installed two new devices - WFPC2 and COSTAR.

The first Hubble repair mission was launched Dec. 2, 1993 on STS-61 (Endeavour, landing on 13 Dec. 1993 at KSC). Installation of COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR deployed corrective optics in front of three of Hubble's first-generation instruments – the Faint Object Camera, the Goddard High Resolution Spectrometer, and the Faint Object Spectrograph. 150) 151) 152)
In addition, SM-1 included the installation and replacement of other components including: solar arrays, solar array drive electronics, magnetometers, two rate sensor units, two gyroscope electronic control units, etc.

After SM-1, Hubble became operational transmitting stupefying images of supernovas, gigantic explosions that marked the death of a star and revealed mysterious black holes in the center of virtually all galaxies. Thanks to these observations, delivered with 10 times the clarity of the most powerful telescopes on Earth, astronomers have been able to confirm that the universe is expanding at an accelerating rate and to calculate its age with greater precision as an estimated 13.7 billion years.

The shuttle flight of 1993 was one of most challenging and complex manned missions ever attempted. During a record five back-to-back space walks totaling 35 hours and 28 minutes, two teams of astronauts completed the first servicing of the Hubble Space Telescope (HST). In many instances, tasks were completed sooner than expected and a few contingencies that did arise were handled smoothly. 153)

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Figure 112: STS-61 Crew photo with Commander Richard O. Covey, Pilot Kenneth D. Bowersox, Payload Commander F. Story Musgrave and Mission Specialists Kathryn C. Thornton, Claude Nicollier, Jeffrey A. Hoffman and Tom Akers (image credit: NASA)

WFPC2 significantly improved ultraviolet performance over WFPC1, the original instrument. In addition to having more advanced detectors and more stringent contamination control, it also incorporated built-in corrective optics.

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Figure 113: WFPC2 in the enclosure Image credit: NASA)

• This comparison image of the core of galaxy M100 shows the dramatic improvement in the Hubble telescope's view of the universe of the universe after the first Hubble Servicing Mission in December 1993. The new image (right) was taken with the second generation Wide Field and Planetary Camera (WFPC2), which was installed during the STS-61 Hubble Servicing Mission. 154)

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Figure 114: This comparison image of the core of the galaxy M100 shows the dramatic improvement in Hubble Space Telescope's view of the universe after the first servicing mission in December 1993. The original view, taken a few days before the servicing mission, is on the left (image credit: NASA, Ref. 149)

Legend to Figure 114: This picture beautifully demonstrates that the camera's corrective optics compensate fully for the optical aberration in Hubble's primary mirror. With the new camera, the Hubble explored the universe with unprecedented clarity and sensitivity, and fulfilled its most important scientific objectives for which the telescope was originally built. 155)

Right: The core of the grand design spiral galaxy M100, as imaged by Hubble Space Telescope's Wide Field Planetary Camera 2 in its high resolution channel. The WFPC-2 contains modified optics that correct for Hubble's previously blurry vision, allowing the telescope for the first time to cleanly resolve faint structure as small as 30 light-years across in a galaxy which is tens of millions of light years away. The image was taken on December 31, 1993.

Figure 115: Servicing Mission 1, December 1993: The primary goal of Servicing Mission 1 was to restore Hubble's vision. Because Hubble's primary mirror was incorrectly shaped, the telescope could not focus all the light from an object to a single sharp point. Instead, it saw a fuzzy halo around objects it observed. Astronauts on the space shuttle Endeavor (STS-61) spent five days tuning it up. They installed two new devices — the Wide Field and Planetary Camera 2 and the Corrective Optics Space Telescope Axial Replacement — to compensate for the primary mirror's incorrect shape. Astronauts also installed new solar arrays, to reduce the "jitter" caused by excessive flexing of the solar panels during the telescope's orbital transition from cold darkness into warm daylight, and new gyroscopes to help point and track the telescope, along with fuse plugs and electronic units (video credit: NASA, published on 7 January 2007)

• December 2, 2013. Although the SM-1 mission was a triumph, and it marked the beginning of the Hubble telescope's long and illustrious career, astronaut Jeffrey Hoffman recalled the SM-1 events at a symposium to mark the 20th anniversary of STS-61. "There were a lot of people who doubted that we could accomplish all the things we had set out to, but here we were at the end of the fifth of five [spacewalks], and we had accomplished 13 of the 12 tasks that had been assigned to us — so we were, justifiably, very happy." 156)


SM-2 (Servicing Mission-2)

The second Hubble service flight was on STS-82 (Feb. 11-21, 1997). 157) The installation of new instruments extended Hubble's wavelength range into the near infrared for imaging and spectroscopy, allowing to probe the most distant reaches of the universe. The replacement of the failed or degraded spacecraft components increased efficiency and performance. The newly installed instruments were: STIS (Space Telescope Imaging Spectrograph), and NICMOS (Near Infrared Camera and Multi-Object Spectrometer).

• NICMOS enabled Hubble to observe infrared wavelengths (0.8-2.5 µm), crucial for viewing very distant optical sources that have lost energy traveling across most of the visible universe and now radiate in the infrared band. NICMOS consists of three cameras. It is capable of both infrared imaging and spectroscopic observations of astronomical targets.

• STIS could take detailed pictures of celestial objects and hunt for black holes. Both instruments featured technology that wasn’t available when scientists designed and built the original Hubble instruments in the late 1970s. STIS's two-dimensional detectors have allowed the instrument to gather 30 times more spectral data and 500 times more spatial data than the previous spectrographs on Hubble. These were capable of only looking at one place at a time.

One of the greatest advantages to using STIS is in the study of supermassive black holes. STIS searches for massive black holes by studying the star and gas dynamics around galactic centers. It measures the distribution of matter in the universe by studying quasar absorption lines. It also uses its high sensitivity and spatial resolution to study star formation in distant galaxies and perform spectroscopic mapping of solar system objects.

The astronauts also installed a refurbished FGS (Fine Guidance Sensor), one of three essential instruments used to keep Hubble steady while viewing objects and to calculate celestial distances; a Solid State Recorder to replace one of Hubble’s data recorders; and a refurbished, spare Reaction Wheel Assembly, part of the Pointing Control Subsystem.

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Figure 116: STS-82 Crew photo with Commander Kenneth D. Bowersox, Pilot Scott J. Horowitz, Mission Specialists Mark C. Lee, Steven A.Hawley, Gregory J. Harbaugh, Steven L. Smith and Joseph R. Tanner (image credit: NASA)


SM-3 (Servicing Mission-3)

Scheduled for June 2000, the third mission to the Hubble Space Telescope was originally planned to carry out preventive repairs. However, urgency to address the failure of Hubble’s third gyroscope led NASA managers to split SM3 into two parts (SM3A and SM3B), scheduling an early servicing mission (SM3A) for December 1999. 158)

The unexpected failure of the fourth of Hubble’s six gyroscopes on 13 November 1999, with SM3A already planned, caused NASA to place Hubble into safe mode. Unable to conduct science without at least three working gyros, Hubble went into a sort of protective hibernation until 19 December 1999, when a crew of astronauts aboard the Discovery Space Shuttle flew to its rescue and replaced all the gyroscopes.

Since the second Servicing Mission in February 1997, three of the gyroscopes had failed and caused some concern among NASA officials. Additionally, NASA deemed necessary the replacement of one of Hubble’s three Fine Guidance Sensors (FGS). Both devices are part of Hubble’s advanced pointing control system, and as such, they keep the telescope steady during observations.

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Figure 117: STS-103 Crew photo with Commander Curtis L. Brown, Pilot Scott J. Kelly, Mission Specialists Steven L. Smith, C. Michael Foale, John M. Grunsfeld, Claude Nicollier and Jean-Francois Clervoy (image credit: NASA) 159)

SM-3A (Servicing Mission-3A)

The Hubble service flight on STS-103 took place Dec. 19-27, 1999. Objective: replacement of gyroscopes (after the third of Hubble's six gyroscopes failed), a fine guidance sensor and a S/C computer. Installation of six voltage/temperature kits for the S/C batteries. Installation of a new transmitter, solid-state recorder (12 Gbit), and thermal insulation blankets. 160) 161)

What was originally conceived as a mission of preventive maintenance turned more urgent on November 13, 1999, when the fourth of six gyros failed and Hubble temporarily closed its eyes on the universe. Unable to conduct science without three working gyros, Hubble entered a state of dormancy called safe mode. Essentially, Hubble "went to sleep" while it waited for help.

STS-103 restored the Hubble Space Telescope to working order and upgraded some of its systems, allowing the decade-old observatory to get ready to begin its second scheduled decade of astronomical observations (Ref. 159).

The first few days of the 8-day mission, the crew prepared for the rendezvous and capture of the Hubble Space Telescope and the three maintenance spacewalks to follow. After a 30-orbit chase Commander Brown and Kelly maneuvered the orbiter to a point directly beneath Hubble, then moved upward toward it. Mission Specialist Clervoy grappled Hubble using the orbiter's robotic arm and placed it on the Flight Support System in the rear of Discovery's cargo bay.

EVA No. 1: Mission Specialists Steven Smith and John Greenfield conducted the mission's first spacewalk. The two made numerous repairs, including replacing the telescope's three Rate Sensor Units — each containing two gyroscopes. They also installed six Voltage/Temperature Improvement Kits between Hubble's solar panels and its six 10-year-old batteries. The kits, the size of cell telephones, were designed to prevent any overheating or overcharging of those batteries. A few minor objectives were left undone, such as taking close-up photos of the Voltage/Temperature Improvement Kits. The 8-hour, 15-minute space walk was second to the longest space walk from Endeavour on STS-49 in May 1992. A few minor problems helped account for the length of the space walk. The astronauts had difficulty in removing one of the old RSUs, and opening valves and removing caps on the NICMOS. The tasks were eventually completed.

EVA No. 2: During the mission's second space walk, Mission Specialists Michael Foale and Claude Nicollier installed a new advanced computer — 20 times faster than Hubble's old one — and a new, 250 kg fine guidance sensor. This 8-hour, 10 minute space walk was the third longest in history. With all major activities accomplished, controllers reported that power was reaching both of the new pieces of equipment. "The brains of Hubble have been replaced," said Mission Specialist Grunsfeld. About 30 minutes later, Hubble began thinking with those new brains.

EVA No. 3: Smith and Grunsfeld again teamed up to make the mission's third and final space walk. Like the first two, it also lasted more than 8 hours, making it the fourth longest in history. The team installed a transmitter that sends scientific data from Hubble to the ground. It replaced one that failed in 1998. The astronauts used special tools developed for the task because transmitters, usually very reliable, were not designed to be replaced in orbit. Smith and Grunsfeld also installed a solid state digital recorder, replacing an older mechanical reel-to-reel recorder.

Hubble was released from Discovery's cargo bay on Christmas Day.

The Hubble team has left the telescope far more fit and capable than ever before. The new, improved, and upgraded equipment included six fresh gyroscopes, six battery voltage/temperature improvement kits, a faster, more powerful, main computer, a next-generation solid state data recorder, a new transmitter, an enhanced fine guidance sensor, and new insulation.

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Figure 118: Hubble berthed in the Space Shuttle bay during Servicing Mission 3A. Astronauts Steven L. Smith, and John M. Grunsfeld, appear as small figures in this wide scene photographed during EVA (Extravehicular Activity), image credit: NASA/ESA


SM-3B (Servicing Mission-3B)

A routine servicing mission to HST took place Mar. 1- 11, 2002 on STS-109 (Columbia). Installation of ACS (Advanced Camera for Surveys), built by Ball Aerospace for NASA and consisting of three cameras in the spectral range of 0.12-1.0 μm. The WFC (Wide Field Camera) uses a CCD area array of 16 Mpixel (4096 x 4096). The second is a HRC (High Resolution Camera) using a 1024 x 1024 CCD array and a high sensitivity in the UV. The third camera, the SBC (Solar-Blind Camera), is a far-ultraviolet, pulse-counting array that has a relatively high throughput at 121 nm. SA-3 (Solar Array-3) installation and PCU (Power Control Unit). Installation of a new experimental cryocooler for NICMOS (70 K cooling to revive its IR vision, and extend its life by several years). 162) 163) 164) 165)

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Figure 119: Illustration of the ACS instrument configuration (image credit: NASA)

Solar Array 3 (SA3) Installation: Four large flexible solar array (SA) panels (wings) provide power to the observatory. During SM1, the original arrays were replaced by SA2 and have powered Hubble for over 8 years. Radiation and debris take their toll on sensitive electronics, which will be replaced to ensure uninterrupted service for the remainder of the mission.

The new solar arrays (SA3) are rigid arrays, which do not roll up and therefore are more robust. Hubble gets a brand new look with its latest set of solar wings. Although one-third smaller than the first two pairs, the power increase was between 20 and 30 percent. They are less susceptible to extreme temperatures and their smaller-sized will reduce the effects of atmospheric drag on the spacecraft.

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Figure 120: The Hubble Space Telescope (HST) heads back toward its normal routine, after a week of servicing and upgrading by the STS‐109 astronaut crew in 2002 (image credit: NASA, Ref. 166)


SM-4 (Servicing Mission-4)

The launch of SM-4 or flight STS-125 on Space Shuttle Atlantis, took place on May 11, 2009 (landing on May 24, 2009) with seven astronauts aboard (RMS capture, repair and upgrade of the 11,000 kg HST spacecraft at an orbital altitude of 560 km). Five spacewalks are required to refurbish Hubble with state-of-the-art science instruments designed to improve the telescope's discovery capabilities. The goal of the long overdue service mission is to extend the star-gazer's life by at least five years (the 2003 Columbia disaster that saw the shuttle disintegrate as it re-entered Earth's atmosphere, killing all seven crew members was the main reason for the long delay).

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Figure 121: The STS‐125 crew members take a moment to pose for a crew photo before a training session in the Space Vehicle Mockup Facility at NASA’s Johnson Space Center. From the left are astronauts Mike Massimino, Michael Good, both mission specialists; Gregory C. Johnson, pilot; Scott Altman, commander; Megan McArthur, John Grunsfeld and Andrew Feustel, all mission specialists (image credit: NASA)

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Figure 122: This graphic depicts the location of the STS‐125 payload hardware (image credit: NASA)

The priorities of the servicing mission are: 166)

• Three Rate Sensor Unit (gyroscope) removal and replacement (only two of the six gyros are currently in operation)

WFC3 (Wide Field Camera 3). WFC3 replaces WFPC2 (Wide Field Planetary Camera 2). Use of 4 k x 2 k CCD e2v detector array providing full-frame imaging. — The WFPC2 was originally installed in the first Hubble servicing mission in 1993, and was nicknamed “the camera that saved Hubble” because its special optics were able to overcome the spherical aberration in the telescope’s main mirror.

- The WFC3 is configured as a two‐channel instrument. Its wide‐wavelength coverage with high efficiency is made possible by this dual‐channel design using two detector technologies. The incoming light beam from the Hubble telescope is directed into WFC3 using a pick‐off mirror, and is directed to either the Ultraviolet‐Visible (UVIS) channel or the Near‐Infrared (NIR) channel. The light‐sensing detectors in both channels are solid‐state devices. For the UVIS channel a large format CCD (Charge Coupled Device), similar to those found in digital cameras, is used. In the NIR detector the crystalline photosensitive surface is composed of mercury, cadmium and tellurium (HgCdTe). 167)

- The high sensitivity to light of the 16 megapixel UVIS CCD, combined with a wide field of view (160 x 160 arcsec), yields about a 35‐times improvement in discovery power versus Hubble’s current most sensitive ultraviolet imager, the ACS High Resolution Channel. The NIR channel’s HgCdTe detector is a highly advanced and larger (one megapixel) version of the 65,000 pixel detectors in the current near‐infrared instrument, NICMOS. The combination of field‐of‐view, sensitivity, and low detector noise results in a 15‐20 x enhancement in capability for WFC3 over NICMOS.

- An important design innovation for the WFC3 NIR channel results from tailoring its detector to reject infrared light (effectively “heat”) longer in wavelength than 1700 nm. In this way it becomes unnecessary to use a cryogen (e.g., liquid or solid nitrogen) to keep it cold. Instead the detector is chilled with an electrical device called a Thermo‐Electric Cooler (TEC). This greatly simplifies the design and will give WFC3 a longer operational life.

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Figure 123: The Wide Field Camera 3 is inspected and readied for flight aboard STS‐125 (image credit: NASA)

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Figure 124: Overview of the WFC3 instrument (image credit: NASA) 168)

- Science instrument C&DH (Command & Data Handling) system swap out (replacement of a unit that failed in Sept. 2008)

- COS (Cosmic Origins Spectrograph) installation and replacement of COSTAR of SM-1. COS is a medium resolution spectrograph specifically designed to observe in the near and mid ultraviolet spectral range. COS was designed with one overriding objective in mind: to collect as many ultraviolet photons of light as possible and hence make possible the effective study of the huge, dark reservoir of gas that exists between the galaxies both near and far — t he so-called "cosmic web" of matter which represents the largest-scale structure in the universe.

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Figure 125: Illustration of the COS configuration (image credit: NASA)

- Battery module replacement installation (Bays 2 and 3). This is the first battery replacement in 19 years.

- Fine Guidance Sensor 2 removal and replacement (it is one of three sensors that help point and lock the telescope on targets)

- Repair of ACS (Advanced Camera for Surveys): ACS has been inoperable since January 2007, when its backup power supply system failed. Replacement of the entire electronics box, which will be powered by a separate low‐voltage power supply.

One piece of new technology is an ASIC, 169) 170) that enables an entire circuit board’s worth of electronics to be condensed into a very small package. It will be a part of the new CCD in the CEB (CCD Electronics Box) that will be installed to repair the failed ACS instrument. - The ASIC design is the same as the one already developed and tested for the JWST (James Webb Space Telescope) mission. However, the electronics packaging for Hubble is different because of the different operating conditions such as temperature and electronics environments.

- New Outer Blanket Layer installation (Bays 8, 5 & 7)

- Reboost of the HST spacecraft altitude.

- Reboost of the HST spacecraft altitude.


EVA1: The first spacewalk of the mission, performed by astronauts John Grunsfeld and Drew Feustel lasted a little over 7 1/2 hours. They successfully installed the new Wide Field Camera 3 science instrument and a new Science Instrument Command and Data Handling Unit. Both WFC-3 and the SI C&DH passed their “aliveness” tests, which essentially means the devices powered on correctly. The WFC-3 also passed its functional test, meaning the capabilities of the instrument itself were tested. The SI C&DH unit has also received an initial OK on its functional test, pending final review of data sent down to the ground. 171)

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Figure 126: Andrew Feustel hauls the new WFC3 on the robotic arm, to install the camera on Hubble. (image credit: NASA)

EVA2: The second EVA of the mission provided some challenges to astronauts Michael Good and Mike Massimino. However, they achieved all the objectives for this spacewalk, it just took them awhile — 7 hours and 56 minutes. They installed three Rate Sensor Units (RSUs), with a pair of gyros in each, and the first of two new battery module units.

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Figure 127: Astronaut Michael Good works with the Hubble Space Telescope in the cargo bay of the Earth-orbiting Space Shuttle Atlantis along with Mike Massimino (image credit: NASA)

EVA3: The third EVA of the mission went like clockwork as Grunsfeld and Feustel teamed up again. They removed the COSTAR (Corrective Optics Space Telescope Axial Replacement) and installed in its place the new COS (Cosmic Origins Spectrograph). They also completed an unprecedented repair of the Advanced Camera for Surveys replacing an electronic card and installed a new electronics box and cable.

To do the repairs on ACS, Grunsfeld removed 32 screws from an access panel to replace the camera’s four circuit boards and installed a new power supply. The two astronauts used specially designed tools to do a job that was never intended to be done on orbit. But they did it, and with efficiency.

Engineers at Goddard have already performed “aliveness” tests on both COS and ACS to verify they have electrical power. However while a functional test of the ACS indicated success in reviving the instrument’s heavily used wide-field channel, officials said early Sunday that it appears the repairs failed to resolve power problem with the camera’s stricken high-resolution channel and it appears “down for the count.”

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Figure 128: STS-125 astronauts John Grunsfeld and Andrew Feustel work together on EVA 3 to navigate the exterior of the Hubble Space Telescope on the end of the remote manipulator system arm, controlled from inside Atlantis' crew cabin (image credit: NASA)

On May 18, 2009, the astronauts successfully completed all the EVAs for the Hubble Servicing Mission, accomplishing all the mission goals. 172) 173)

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Figure 129: With his feet firmly anchored on the shuttle’s robotic arm, astronaut Mike Good maneuvers to retrieve the tool caddy required to repair the Space Telescope Imaging Spectrograph during the final Hubble servicing mission in May 2009. Periodic upgrades have kept the telescope equipped with state-of-the-art instruments, which have given astronomers increasingly better views of the cosmos (image credit: NASA)

Figure 130: During the Servicing Mission 4 (SM4), astronauts will make the final trip to the Hubble Telescope. Over the course of five spacewalks, they will install two new instruments, repair two inactive ones, and perform the component replacements that will keep the telescope functioning at least into 2014. The effort-intensive, rigorously researched, exhaustively tested mission also involves diverse groups of people on the ground throughout the country (video credit: NASA, published on 17 January 2009)

• 21 May 2019: Retired NASA astronaut John Grunsfeld hosts this six-part mini-series about the tools used on the Hubble Space Telescope servicing missions. Hubble was uniquely designed to be serviced in space so that components could be repaired and upgraded. Astronauts using custom-designed tools performed challenging spacewalks on five servicing missions from 1993 to 2009 to keep Hubble operating so that it could change our fundamental understanding of the universe. 174)

- Join John, EVA engineer Ed Rezac, and astronaut trainer Christy Hansen in this episode of Hubble Tool Time to learn about creating a Fastener Capture Plate to capture 111 screws in order to repair the Space Telescope Imaging Spectrograph on Servicing Mission 4 in 2009.

- In addition to enabling Hubble's scientific discoveries, the tools developed by teams at NASA's Goddard Space Flight Center and tested in collaboration with the Johnson Space Center furthered NASA's human exploration capabilities. These tools and the knowledge gleaned from the Hubble servicing missions are used today by astronauts on the International Space Station, and will be critical to NASA's future crewed missions to the Moon and Mars.

Figure 131: Hubble Tool Time Episode 6 - Servicing Mission 4 (video credit: NASA's Goddard Space Flight Center/Katrina Jackson)

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Figure 132: This image depicts the release of the Hubble Space Telescope on Flight Day 9 (image credit: NASA)




Ground Segment of the HST (Hubble Space Telescope) Mission

The NASA/GSFC (Goddard Space Flight Center) in Greenbelt Maryland is home to the Hubble Space Telescope Operations Project, the government’s team of technical managers and scientists who oversee all aspects of the Hubble mission. Under its direction, an integrated group of civil servants and contractors at Goddard collectively known as the operations team is responsible for Hubble’s mission operations—those functions of the mission that operate together to assure the health, safety, and performance of the spacecraft. Examples include monitoring and adjusting the spacecraft’s subsystems (e.g., power, thermal, data management, pointing control, etc.), flight software development, sustaining engineering of the control center hardware and software, and systems administration of the network and ground system components. 175)

A separate contractor team at the Space Telescope Science Institute (STScI) in Baltimore is similarly responsible for science operations—the functions necessary to award telescope time, schedule observations, calibrate the received data, and archive the datasets. Working closely together, Goddard and the STScI operate Hubble 24 hours a day, 7 days a week, though most of the commanding to the telescope and receipt of its science data is accomplished by computers via automated operations.

The Space Telescope Operations Control Center (STOCC) is located at Goddard. It consists of a Mission Operations Room (MOR) and an Operations Support Room (OSR). The MOR is Hubble’s primary command and control room; both manual and automated operations are performed from this location.

Prior to the implementation of automated operations in May 2011, a team of console operators staffed the MOR around the clock. They executed the required procedures to acquire communications with the telescope, manage Hubble’s science and engineering data recorders, and load Hubble’s computers with command sequences. They also monitored telemetry from the spacecraft and reported any problems or concerns to the appropriate subsystem engineers.

These functions, which form Hubble’s daily routine, are now performed autonomously, enabling STOCC personnel to focus on special operations and various tests. Since the advent of automated operations, the STOCC is staffed only eight hours a day, five days a week. If an anomaly occurs on the spacecraft or within the ground system when the facility is unoccupied, a high-reliability text messaging system immediately alerts the appropriate members of the operations team.

In the Operations Support Room (OSR), STOCC personnel interface with a high-fidelity spacecraft simulator (or the spacecraft) to conduct a variety of tasks. These include testing any configuration changes planned for Hubble, analyzing engineering telemetry, testing flight or ground software updates, and running any contingency procedures or other special commanding in response to an observatory anomaly. The operations team continuously examines spacecraft subsystem performance, looking for trends that could signal component degradation and identifying ways to improve system performance and extend the mission’s lifetime.

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Figure 133: The Mission Operations Room during observatory commanding (image credit: NASA)

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Figure 134: This graphic illustrates how Hubble observations, converted to data, are transmitted from the telescope to the ground via the Tracking and Data Relay Satellite System (TDRSS), image credit: NASA

Communications with Hubble are accomplished via a network interface from the STOCC to NASA’s White Sands Test Facility complex located in White Sands, New Mexico, near Las Cruces. The large antennas there transmit radio waves to NASA’s Tracking and Data Relay Satellite System (TDRSS), which forwards them to Hubble. Science observations and engineering data that the telescope stores on solid-state recorders are returned to Goddard using the reverse path. Once received and quality checked, the science data is forwarded to the Space Telescope Science Institute via dedicated high-speed network links where it is processed, archived, and distributed.

The Hubble mission and science operations teams strive to set the standard for NASA’s great observatories by continuously improving the telescope’s science productivity. The optical quality of the telescope, the excellent pointing performance of the spacecraft, the diverse capabilities of its instruments, and the skillful dedication of the mission’s staff keep Hubble at the forefront of observational astrophysics. The observatory is operating at peak performance thanks to the astronauts who successfully completed Servicing Mission 4 in May 2009. In that mission, space-shuttle-based astronauts installed two advanced-technology instruments, repaired two others, replaced a Fine Guidance Sensor, six gyros, six batteries, and the instrument computer, and outfitted certain equipment bays with protective thermal blankets. These upgrades position Hubble to continue its mission of discovery into the next decade.


Simulation

The Vehicle Electrical System Test (VEST) facility provides Hubble operations and engineering personnel with a configurable, high-fidelity simulator of the Hubble observatory and its operating environment.

The VEST is used in three primary ways. First, it is employed to troubleshoot problems seen on the spacecraft by simulating the conditions under which the anomaly was observed and to test possible solutions. Second, it is used to verify updates to the software, commands, and engineering parameters before they are transmitted from the control center to the spacecraft. This process includes a thorough test of the ground procedures used for uplinking and installing these items. Finally, the VEST is used to verify operational procedures before they are executed by the flight operations team.

The VEST consists of several distinct elements. The largest is the integrated VEST structure itself. This is a mechanical and electrical engineering model of Hubble’s middle section (known as the Support Systems Module and the Optical Telescope Assembly Equipment Section) that contains the majority of the spacecraft’s hardware subsystems. Mounted within the structure are spaceflight-qualified modules developed as spares for servicing HST on orbit, and engineering duplicates (where no spares exist) of such items as computers, data recorders, interface boxes, power control units, etc.

A second element includes specially fabricated units designed to mimic the functionality of subsystems on Hubble that are too big or require emulation of the space environment. One such area is the spacecraft’s electrical power subsystem, which includes its two solar arrays, drive mechanisms (used to turn the arrays toward the sun) and six large batteries. Another area is the attitude guidance system that controls Hubble’s pointing and the observatory’s motion from target to target. In this area are units that simulate Hubble’s four 100-pound reaction wheels, six sensitive gyroscopes, sun sensors, and other miscellaneous components.

A third element simulates Hubble’s complex science instruments — both cameras and spectrographs. In this case, large computer racks developed for each instrument known as instrument benches simulate the electronics portion of an instrument and its optical parts like filter wheels, calibration lamps and channel-select mechanisms. Additional imbedded components generate realistic science data used to conduct end-to-end dataflow tests of the larger system.

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Figure 135: The VEST facility uses both hardware and software simulators to replicate the operation of the Hubble observatory in orbit. In this view, the main VEST structure is seen on the left, while ancillary equipment, including some of the instrument benches, appear on the right (image credit: NASA)

Prior to each of the five servicing missions to Hubble, the VEST structure was a critical element for ensuring the fit of new spaceflight hardware into the spacecraft. Many of the items taken aloft by the astronauts to Hubble were first installed into the VEST structure to verify mechanical, electrical and software compatibility with the observatory. During that time, the VEST structure was located inside a large clean room whose environment was vital to maintaining the pristine condition of the hardware, as even a speck of dust could have potentially contaminated Hubble’s sensitive instruments and subsystems.

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Figure 136: Astronauts underwent training at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, prior to each of the HST servicing missions. In this image the Servicing Mission 3A (December 1999) astronauts are seen receiving instruction from Hubble project personnel on various spacecraft subsystems. At the time, the VEST main structure was located in Goddard’s large building 29 clean room (image credit: NASA)




Some background of the HST (Hubble Stace Telescope) Program and STScI (Space Telescope Science Institute)

In 1979, NASA issued a request for proposals for a Space Telescope Science Institute (now also known as STScI), including a site where the institute would be located. After reviewing applications from prestigious institutions and universities, NASA selected the Association of Universities for Research in Astronomy (AURA) in 1981, which named the Johns Hopkins University’s Homewood campus in Baltimore, Maryland, as its base of operations. In the same year, Dr. Riccardo Giacconi was selected as STScI’s first director for his scientific leadership, technical knowledge, and his pioneering work in space astronomy and the creation of the field of X-ray astronomy. He initiated the hiring of STScI staff to support several critical science center functions: the definition of technical management and operations concepts, the development of a guide star selection system, and the creation of a science data reduction and analysis system. 176) 177)

NASA announced the Large Space Telescope’s official name in 1983: The Hubble Space Telescope (HST), honoring Dr. Edwin Hubble, who demonstrated in a 1929 publication that the universe is expanding based on observations showing that more distant galaxies are receding faster than nearby galaxies. His work also revealed that there were other galaxies beyond the Milky Way—revolutionizing our view of the cosmos and our place within it.

In January 1986, the loss of the entire crew of the space shuttle Challenger, which exploded after liftoff, led NASA to delay the launch of Hubble. However, STScI’s staff knew there was significant work yet to complete, which led to the 1989 release of the Guide Star Catalog and the software that supports it, revolutionizing how astronomers lock onto star positions to gather data from any ground- or space-based observatory, and ultimately automating Hubble’s observations.

In April 1990, the crew of the space shuttle Discovery successfully launched and deployed the Hubble Space Telescope. In May, its first image was released, showing a more clearly resolved star (HD96755 in the open cluster NGC 3532) compared to ground-based telescope observations. However, as institute staff collaborated with NASA and the scientific community to analyze additional data in the months that followed, they realized that Hubble’s primary mirror had an aberration; the images it returned were not perfectly focused.

Staff at the institute worked to implement software that allowed scientists to accurately revise Hubble’s data, providing improved images to the scientific community and the public. The issue was completely resolved during the first servicing mission in December 1993, which included the installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) and the Wide Field Planetary Camera 2 (WFPC 2), correcting Hubble’s optics and immediately clarifying the data. NASA astronauts completed four additional servicing missions through 2009, replacing and upgrading Hubble’s instruments with improved optics and increasing its resolution, power, and efficiency each time, allowing the telescope to become one of the world’s most productive scientific instruments.

Hubble’s success is due in part to STScI’s data archive, established at the outset of the mission in 1990. The archive was expanded to include data from other ultraviolet and optical space astronomy missions in 1997 and was renamed the Multi-mission Archive at Space Telescope (MAST). STScI provides secure storage and reliable retrieval services for observation data, creates user-friendly and scientifically useful search tools, and offers support services to the astronomy community. Today, MAST has a user community of more than 10,000 astronomers around the world. In addition to Hubble data, MAST houses the data of more than 20 missions in the ultraviolet, optical, and near-infrared wavelength range. In 2012, MAST was renamed in honor of Maryland’s U.S. Senator Barbara A. Mikulski, who was a steadfast supporter of NASA, the Hubble Space Telescope, and the Space Telescope Science Institute.

Even before Hubble was launched, STScI and NASA began organizing workshops to plan a “Next Generation Space Telescope” that would be significantly larger than Hubble and focused on infrared wavelengths of the electromagnetic spectrum. The idea gained momentum throughout the 1990s and was formally recommended as the top space-based priority by the National Academies of Sciences Decadal Survey in 2001. After construction on the new project began, it was renamed in honor of former NASA Administrator James E. Webb in 2002. The observatory is built with new technologies that will allow it to see deeper into the universe, answer fundamental questions about the origins of planets and stars in the Milky Way, and examine the nature of the first light in the universe.

In 2001 STScI was selected to oversee the science and mission operations of the James Webb Space Telescope, planned to launch in 2021. STScI staff are uniquely prepared to support Webb. Scientists and engineers have mined more than 30 years of experience preparing for and operating Hubble to create a framework for operations that will ensure the health of Webb and maximize its scientific productivity.

STScI will also play a key role in the science operations for NASA’s next flagship observatory following Webb, the Wide Field Infrared Survey Telescope (WFIRST), which is planned to launch in the mid-2020s. With a view 100 times wider than the Hubble Space Telescope at the same sensitivity and resolution, WFIRST will build wide-field maps of large regions of the sky in near-infrared light, and has the potential to answer vital questions in exoplanet and dark energy research. Looking forward, the staff at STScI will continue to follow the institute’s mission: to help humanity explore the universe with advanced space telescopes and ever-growing data archives.


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58) "Hubble fortuitously discovers a new galaxy in the cosmic neighborhood [heic1903]," ESA, Hubble Science & Technology, 31 January 2019, URL: http://sci.esa.int/hubble/61075-hubble-fortuitously-discovers
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59) L. R. Bedin, M. Salaris, R. M. Rich, H. Richer, J. Anderson, D. Bettoni, D. Nardiello, A. P. Milone, A. F .Marino, M. Libralato, A. Bellini, A. Dieball, P. Bergeron, A. J. Burgasser, D. Apai, "The HST Large Program on NGC 6752. I. Serendipitous discovery of a dwarf Galaxy in background," Monthly Notices of the Royal Astronomical Society: Letters, Volume 484, Issue 1, 21 March 2019, Pages L54–L58, https://doi.org/10.1093/mnrasl/slz004,

60) "Hubble Sees Plunging Galaxy Losing Its Gas," Hubblesite, 24 January 2019, URL: http://hubblesite.org/news_release/news/2019-05

61) W. J. Cramer, J. D. P. Kenney, M. Sun, H. Crowl, M. Yagi, P. Jáchym, E. Roediger, W. Waldron, "Spectacular Hubble Space Telescope Observations of the Coma Galaxy D100 and Star Formation in Its Ram Pressure–stripped Tail," The Astrophysical Journal, Vol. 870, No 2, Published 8 January 2019, https://doi.org/10.3847/1538-4357/aaefff , URL: https://iopscience.iop.org/article/10.3847/1538-4357/aaefff/pdf

62) "Shedding new light on the Whirlpool Galaxy," Hubblesite, 24 January 2019, URL: http://hubblesite.org/video/1195/category/109-scientific-visualizations

63) "Hubble Image of the Week – Peering Into The Past," Scitech Daily, 21 January 2019, URL: https://scitechdaily.com/hubble-image-of-the-week-peering-into-the-past/

64) "Hubble's Wide Field Camera 3 Recovered, Collecting Science Data," NASA, 17 January 2019, URL: https://www.nasa.gov/feature/goddard/2019/wide-field
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65) "Hubble's Wide Field Camera 3 to Resume Operations," Hubblesite, News Release number: STScI-2019-10, 15 January 2019, URL: http://hubblesite.org/news_release/news/2019-10

66) "Hubble's Wide Field Camera 3 to Resume Operations," NASA, 15 January 2019, URL: https://www.nasa.gov/feature/goddard/2019/wide
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67) "Messier Monday," ESA, 14 January 2019, URL: https://www.spacetelescope.org/images/potw1902a/

68) Rob Garner, "Wide Field Camera 3 Anomaly on Hubble Space Telescope," NASA, 9 January 2019, URL: https://www.nasa.gov/feature/goddard/2019/wide-field-
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69) Alexandra Witze, "Hubble telescope camera is broken — and US government shutdown could delay repairs," Nature News, 9 January 2019, URL: https://www.nature.com/articles/d41586-019-00094-0

70) "Nature's magnifying glass reveals unexpected intermediate mass exoplanets," Science Daily, 8 January 2019, URL: https://www.sciencedaily.com/releases/2019/01/190108141339.htm

71) Daisuke Suzuki, David P. Bennett, Shigeru Ida, Christoph Mordasini, Aparna Bhattacharya, Ian A. Bond, Martin Donachie, Akihiko Fukui, Yuki Hirao, Naoki Koshimoto, Shota Miyazaki, Masayuki Nagakane, Clément Ranc, Nicholas J. Rattenbury, Takahiro Sumi, Yann Alibert, and Douglas N. C. Lin, "Microlensing Results Challenge the Core Accretion Runaway Growth Scenario for Gas Giants," The Astrophysical Journal Letters, Volume 869, Number 2, Published 19 December 2018, L34 DOI: 10.3847/2041-8213/aaf577, URL of abstract: http://iopscience.iop.org/article/10.3847/2041-8213/aaf577/meta

72) "Young planets orbiting red dwarfs may lack ingredients for life," Phys.org, 8 January 2019, URL: https://phys.org/news/2019-01-young-planets-orbiting-red-dwarfs.html

73) A. Boccaletti, E. Sezestre, A.-M. Lagrange, P. Thébault, R. Gratton, M. Langlois, C. Thalmann, M. Janson, P. Delorme, J.-C. Augereau, G. Schneider, J. Milli, C. Grady, J. Debes, Q. Kral, J. Olofsson, J. Carson, A. L. Maire, T. Henning, J. Wisniewski, J. Schlieder, C. Dominik, S. Desidera, C. Ginski, D. Hines, F. Ménard, D. Mouillet, N. Pawellek, A. Vigan, E. Lagadec, H. Avenhaus, J.-L. Beuzit, B. Biller, M. Bonavita, M. Bonnefoy, W. Brandner, F. Cantalloube, G. Chauvin, A. Cheetham, M. Cude, C. Gry, S. Daemgen, M. Feldt, R. Galicher, J. Girard, J. Hagelberg, P. Janin-Potiron, M. Kasper, H. Le Coroller, D. Mesa, S. Peretti, C. Perrot, M. Samland, E. Sissa, F. Wildi, A. Zurlo, S. Rochat, E. Stadler, L. Gluck, A. Origné, M. Llored, P. Baudoz, G. Rousset, P. Martinez and F. Rigal, "Observations of fast-moving features in the debris disk of AU Mic on a three-year timescale: Confirmation and new discoveries," Astronomy & Astrophysics, Volume 614, 15 June 2018, https://doi.org/10.1051/0004-6361/201732462, URL: https://www.aanda.org/articles/aa/pdf/2018/06/aa32462-17.pdf

74) "Hubble takes gigantic image of the Triangulum Galaxy [heic1901]," ESA Science % Technology, 7 January 2019, URL: http://sci.esa.int/hubble/61036-hubble-takes
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75) "Hubble's Cosmic Holiday Wreath," NASA, 21 December 2018, URL: https://www.nasa.gov/image-feature/goddard/2018/hubbles-cosmic-holiday-wreath

76) "Faint starlight in Hubble images reveals distribution of dark matter [heic1820],"

77) Mireia Montes, Ignacio Trujillo, "Intracluster light: a luminous tracer for dark matter in clusters of galaxies," Monthly Notices of the Royal Astronomical Society, Volume 482, Issue 2, 11 January 2019, Pages 2838–2851, https://doi.org/10.1093/mnras/sty2858, Published: 23 October 2018,

78) "Hubble finds faraway planet vanishing at record speed," Space Daily, 14 December 2018, URL: http://www.spacedaily.com/reports/Hubble_finds_
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79) V. Bourrier, A. Lecavelier des Etangs, D. Ehrenreich, J. Sanz-Forcada, R. Allart, G. E. Ballester, L. A. Buchhave, O. Cohen, D. Deming, T. M. Evans, A. García Munoz, G. W. Henry, T. Kataria, P. Lavvas, N. Lewis, M. López-Morales, M. Marley, D. K. Sing, H. R. Wakeford, "Hubble PanCET: an extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470b," Astronomy & Astrophysics, Vol. 620, A147, Published online: 13 December 2018, https://doi.org/10.1051/0004-6361/201833675 , URL: https://www.aanda.org/articles/aa/pdf/2018/12/aa33675-18.pdf

80) "The 'Camera That Saved Hubble' Turns 25," NASA/JPL Feature 2018-279 , 4 December 2018, URL: https://www.jpl.nasa.gov/news/news.php?feature=72
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81) Ray Villard, Juan Madrid, "Hubble Uncovers Thousands of Globular Star Clusters Scattered Among Galaxies," NASA, 29 November 2018, URL: https://www.nasa.gov/feature/goddard/2018/hubble-uncovers-thousands
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82) Juan P. Madrid, Conor R. O'Neill, Alexander T. Gagliano, Joshua R. Marvil, "A Wide-field Map of Intracluster Globular Clusters in Coma," The Astrophysical Journal, Vol. 867 No 2, published 9 November 2018, DOI: 10.3847/1538-4357/aae206, URL of abstract: http://iopscience.iop.org/article/10.3847/1538-4357/aae206/meta

83) "Tangled — cosmic edition," Hubble Space Telescope, 26 November 2018, URL: https://www.spacetelescope.org/images/potw1848a/

84) "Astronomers Find Possible Elusive Star Behind Supernova," NASA, 15 November 2018, URL: https://www.nasa.gov/feature/goddard/2018/astronomers-find-possible-elusive-star-behind-supernova

85) Schuyler D. Van Dyk, WeiKang Zheng, Thomas G. Brink, Alexei V. Filippenko, Dan Milisavljevic, Jennifer E. Andrews, Nathan Smith, Michele Cignoni, Ori D. Fox, Patrick L. Kelly, Angela Adamo, Sameen Yunus, Keto Zhang, and Sahana Kumar, "SN 2017ein and the Possible First Identification of a Type Ic Supernova Progenitor," The Astrophysical Journal, Vol. 860, No 2, URL of abstract: http://iopscience.iop.org/article/10.3847/1538-4357/aac32c/meta

86) "Extended life for ESA's science missions," ESA Science & Technology, 14 November 2018, URL: http://sci.esa.int/director-desk/60943-extended-life-for-esas-science-missions/

87) "Feeling blue," ESA, 8 November 2018, URL: https://m.esa.int/spaceinimages/Images/2018/11/Feeling_blue

88) "Polar lights on Uranus," ESA, Space Science Image of the Week, 05 November 2018, URL: http://m.esa.int/spaceinimages/Images/2018/11/Polar_lights_on_Uranus

89) "Hubble reveals cosmic Bat Shadow in the Serpent's Tail [heic1819]," ESA Science and Technology, 31 October 2018, URL: http://sci.esa.int/hubble/60899-hubble-reveals-
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90) "NASA's Hubble Space Telescope Returns to Science Operations," NASA, 27 October 2018, URL: https://www.nasa.gov/feature/goddard/2018/update
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91) "The Ghost Nebula," ESA, 26, October 2018, URL: http://m.esa.int/spaceinimages/Images/2018/10/The_Ghost_Nebula

92) "The ghost of Cassiopeia [heic1818]," ESA, 25 October 2018, URL: http://sci.esa.int/hubble/60874-the-ghost-of-cassiopeia/

93) "Hubble Captures the Ghost of Cassiopeia," NASA, 25 October 2018, URL: https://www.nasa.gov/image-feature/goddard/2018/hubble-captures-the-ghost-of-cassiopeia

94) "Ultra-close stars discovered inside a planetary nebula," ScienceDaily, 23 October 2018, URL: https://www.sciencedaily.com/releases/2018/10/181023130545.htm

95) David Jones, Henri M J Boffin, Paulina Sowicka, Brent Miszalski, Pablo Rodríguez-Gil, Miguel Santander-García, Romano L M Corradi, "The short orbital period binary star at the heart of the planetary nebula M 3-1," Monthly Notices of the Royal Astronomical Society: Letters, 2018; DOI: 10.1093/mnrasl/sly142, published 02 August 2018, URL: https://academic.oup.com/mnrasl/article/482/1/L75/5064238

96) "Hubble Moving Closer to Normal Science Operations," NASA, 22 October 2018, URL: https://www.nasa.gov/feature/goddard/2018/
update-on-the-hubble-space-telescope-safe-mode

97) "Hubble Spies Glittering Star Cluster in Nearby Galaxy," NASA, 19 October 2018, URL: https://www.nasa.gov/image-feature/
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98) "Astronomers catch red dwarf star in a superflare outburst," Arizona State University, 18 October 2018, URL: https://phys.org/news/2018-10-astronomers-red-dwarf-star-superflare.html

99) R. O. Parke Loyd, Evgenya L. Shkolnik, Adam C. Schneider, Travis S. Barman, Victoria S. Meadows, Isabella Pagano, Sarah Peacock, "HAZMAT. IV. Flares and Superflares on Young M Stars in the Far Ultraviolet," Astrophysical Journal, arXiv:1810.03277 [astro-ph.SR], 8 October 2018, URL: https://arxiv.org/pdf/1810.03277.pdf

100) "Rings upon rings," ESA, 12 October 2018, URL: http://m.esa.int/spaceinimages/Images/2018/10/Rings_upon_rings

101) Felicia Chou, "Hubble in Safe Mode as Gyro Issues are Diagnosed," NASA, 8 October 2018, URL: https://www.nasa.gov/feature/goddard/2018/hubb
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102) "Astronomers find first compelling evidence for a moon outside our solar system," Columbia University, 03 October 2018, URL: https://phys.org/news/2018-10-astronomers-compelling-evidence-moon-solar.html

103) Felicia Chou, Ray Villard, Alison Hawkes,"Astronomers Find First Evidence of Possible Moon Outside Our Solar System," NASA Release 18-081, 03 October 2018, URL: https://www.nasa.gov/press-release/astronomers-find-first-evidence-of-possible-moon-outside-our-solar-system

104) Alex Teachey, David M. Kipping, "Evidence for a large exomoon orbiting Kepler-1625b," Science Advances, 03 October 2018, Vol. 4, No. 10, eaav1784, DOI: 10.1126/sciadv.aav1784

105) "Knots and bursts," Hubble Image of the Week 17 September 2018, ESA, URL: http://m.esa.int/spaceinimages/Images/2018/09/Knots_and_bursts

106) "Awesome gravity," ESA , Our week through the lens, 10-14 September 2018, URL: http://m.esa.int/spaceinimages/Images/2018/09/Awesome_gravity

107) "Saturn and its moons at opposition," ESA, Space Science Image of the Week: It's been a year since Cassini ended its mission at Saturn, but Hubble still checks in on the ringed planet and its moons from time to time, 10 September 2018, URL: http://m.esa.int/spaceinimages/Images/2018/08/Saturn_and_its_moons_at_opposition

108) "Hubble observes energetic lightshow at Saturn's north pole [heic1815]," ESA, 30 August 2018, URL: http://sci.esa.int/hubble/60570-hubble-
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109) Karl Hille, "A Piercing Celestial Eye Stares Back at Hubble," NASA, 24 August 2018, URL: https://www.nasa.gov/image-feature/goddard/2018
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110) Ann Jenkins, Ray Villard, Pascal Oesch,Mireia Montes, "Hubble Paints Picture of the Evolving Universe," NASA, 16 August 2018, URL: https://www.nasa.gov/feature/goddard/2018/hubble-paints-picture-of-the-evolving-universe

111) "A globular cluster's striking red eye," ESA, 10 August 2018, URL: http://m.esa.int/spaceinimages/Images/2018/08/A_globular_cluster_s_striking_red_eye

112) "Probing the distant past," ESA, 03 August 2018, URL: http://m.esa.int/spaceinimages/Images/2018/07/Probing_the_distant_past

113) "New family photos of Mars and Saturn from Hubble," heic1814 — Photo Release, 26 July 2018, URL: https://www.spacetelescope.org/news/heic1814/

114) "New family photos of Mars and Saturn from Hubble [heic1814]," Hubble, 26 July 2018, URL: http://sci.esa.int/hubble/60521-new-family-
photos-of-mars-and-saturn-from-hubble-heic1814/

115) "A failed supernova?,"ESA, 13 July 2018, URL: http://m.esa.int/spaceinimages/Images/2018/07/A_failed_supernova

116) "Burst of Celestial Fireworks," NASA, 3 July 2018, URL: https://www.nasa.gov/image-feature/burst-of-celestial-fireworks

117) Ashley G. Smart, "Machine learning solves an exoplanet atmosphere," Physics Today, 25 June 2018, URL: https://tinyurl.com/ydh8o777

118) Pablo Márquez-Neila, Chloe Fischer, Raphael Sznitman, Kevin Heng, "Supervised machine learning for analysing spectra of exoplanetary atmospheres," Nature Astronomy, 25 July 2018, URL of abstract: https://www.nature.com/articles/s41550-018-0504-2

119) "Dance of the asteroids:Nearby asteroids photobomb distant galaxies," ESA Space Science Image of the Week, 25 June 2018, URL:http://m.esa.int/spaceinimages/Images/2018
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120) "Hubble proves Einstein correct on galactic scales (heic1812)," ESA, 21 June 2018, http://sci.esa.int/hubble/60441-hubble-proves
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121) Thomas E. Collett, Lindsay J. Oldham, Russell J. Smith, Matthew W. Auger, Kyle B. Westfall, David Bacon, Robert C. Nichol, Karen L. Masters, Kazuya Koyama, Remco van den Bosch, "A precise extragalactic test of General Relativity," Science, Vol. 360, Issue 6395, pp. 1342-1346, 22 Jun 2018, DOI: 10.1126/science.aao2469

122) "Cosmic collision lights up the darkness [heic1811]," ESA, 31 May 2018, URL: http://sci.esa.int/hubble/60372-cosmic-collision-lights-up-the-darkness/

123) "Hubble spots a green cosmic arc," NASA, 1 June 2018, URL: https://www.nasa.gov/image-feature/goddard/2018/hubble-spots-a-green-cosmic-arc

124) "Hubble shows the local Universe in ultraviolet," Hubble Space Telescope, HEIC (Hubble European Space Agency Information Center) 1810, 17 May 2018, URL: http://www.spacetelescope.org/news/heic1810/?utm_source=feedbur
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125) "Astronomers Release Most Complete Ultraviolet-Light Survey of Nearby Galaxies," Hubblesite, 17 May 2018, URL: http://hubblesite.org/news_release/news/2018-27

126) "A spiral disguised," ESA, 16 May, 2018, URL: http://m.esa.int/spaceinimages/Images/2018/05/A_spiral_disguised

127) "Hubble detects helium in the atmosphere of an exoplanet for the first time [heic1809]," ESA, 02 May 2018, URL: http://sci.esa.int/hubble/60245-hubble-detects-helium-in
-the-atmosphere-of-an-exoplanet-for-the-first-time-heic1809/

128) J. J. Spake, D. K. Sing, T. M. Evans, A. Oklopčić, V. Bourrier, L. Kreidberg, B. V. Rackham, J. Irwin, D. Ehrenreich, A. Wyttenbach, H. R. Wakeford, Y. Zhou, K. L. Chubb, N. Nikolov, J. M. Goyal, G. W. Henry, M. H. Williamson, S. Blumenthal, D. R. Anderson, C. Hellier, D. Charbonneau, S. Udry, N. Madhusudhan, "Helium in the eroding atmosphere of an exoplanet," Nature, Volume 557, pages68–70, Published online 02 May 2018, doi:10.1038/s41586-018-0067-5

129) "Hubble's 28th birthday picture: The Lagoon Nebula," Hubble Space Telescope, 19 April 2018, URL: https://www.spacetelescope.org/images/heic1808a/

130) "A colossal cluster," ESA, 10 April 2018, URL: http://m.esa.int/spaceinimages/Images/2018/04/A_colossal_cluster

131) "Hubble Catches a Colossal Cluster," NASA, 13 April, 2018, URL: https://www.nasa.gov/image-feature/goddard/2018/hubble-catches-a-colossal-cluster

132) "Hubble uses cosmic lens to discover most distant star ever observed [heic1807]," ESA, 02 April 2018, URL: http://sci.esa.int/hubble/60140-hubble-uses-cosmic-
lens-to-discover-most-distant-star-ever-observed-heic1807/

133) "The curious case of calcium-rich supernovae," ESA, 30 March 2018, URL: http://m.esa.int/spaceinimages/Images/2018/03/The_curious_case_of_calcium-rich_supernovae

134) Steven K. Blau, "A galaxy with surprisingly little dark matter," Physics Today, 2 April 2018, URL: https://tinyurl.com/yargmbuf

135) "Hubble finds first galaxy in the local Universe without dark matter [heic1806]," ESA, 28 March 2018, URL: http://sci.esa.int/hubble/60113-hubble-finds-first-galaxy
-in-the-local-universe-without-dark-matter-heic1806/

136) Pieter van Dokkum, Shany Danieli, Yotam Cohen, Allison Merritt, Aaron J. Romanowsky, Roberto Abraham, Jean Brodie, Charlie Conroy, Deborah Lokhorst, Lamiya Mowla, Ewan O'Sullivan, Jielai Zhang, "A galaxy lacking dark matter," Nature, Volume 555, pages 629–632, Published 28 March 2018, doi:10.1038/nature25767

137) "Hubble celebrates 27 years with two close friends [heic1709]," ESA, April 20, 2017, URL: http://sci.esa.int/hubble/59018-hubble-celebrates-27-years-with-two-close-friends-heic1709/

138) "A New Angle on Two Spiral Galaxies for Hubble's 27th Birthday," NASA, April 20, 2017, URL: https://www.nasa.gov/feature/goddard/2017/a-new-angle
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139) "Hubble 25," ESA, April 23, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/04/Hubble_25_without_title

140) Tony Phillips, "Handprints on Hubble," NASA, June 26, 2015, URL: http://science.nasa.gov/science-news/science-at-nasa/2015/26jun_handprints/

141) Felicia Chou, Donna Weaver, Ray Villard. "NASA Unveils Celestial Fireworks as Official Image for Hubble 25th Anniversary," NASA, Release 15-066, April 23, 2015, URL: https://www.nasa.gov/press-release/nasa-unveils-celestial-fireworks-as-official-image-for-hubble-25th-anniversary

142) "Celebrating Hubble's silver anniversary," ESA, 23 April 2015, URL: http://m.esa.int/spaceinimages/Images/2015/04/Celebrating_Hubble_s_silver_anniversary

143) Felicia Chou, Donna Weaver, Ray Villard. "NASA Unveils Celestial Fireworks as Official Image for Hubble 25th Anniversary," NASA, Release 15-066, April 23, 2015, URL: http://www.nasa.gov/press-release/nasa-unveils-celestial-fireworks-as-official-image-for-hubble-25th-anniversary

144) "Celestial Fireworks celebrate Hubble's 25th Anniversary," ESA, April 23, 2015, URL: http://www.esa.int/Our_Activities/Space_Science/
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145) "Hubble Space Telescope Celebrates 25 Years of Unveiling the Universe," STCcI, April 23, 2015, News release: STScI-2015-12, URL: http://hubblesite.org/newscenter/archive/releases/2015/12/image/a/

146) "Embryonic Stars Emerge from Interstellar "Eggs"," Hubblesite, News Release number: STScI-1995-44, 2 Nov. 2015, URL: http://hubblesite.org/news_release/news/1995-44

147) "The evolution of SN 1987A," ESA, Space Science Image of the Week, February 27, 2017, URL: http://m.esa.int/spaceinimages/Images/2017/02/The_evolution_of_SN_1987A

148) Edwin P. Hubble (Nov. 20, 1889-Sept. 28, 1953) was an American astronomer. He profoundly changed astronomers' understanding of the nature of the universe by demonstrating the existence of other galaxies besides the Milky Way. He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the distance of that galaxy from the Milky Way. This became known as Hubble's law, and would help establish that the universe is expanding.

149) Note: When originally planned in 1979, the Large Space Telescope program called for return to Earth, refurbishment, and re-launch every 5 years, with on-orbit servicing every 2.5 years. Hardware lifetime and reliability requirements were based on that 2.5 year interval between servicing missions. In 1985, contamination and structural loading concerns associated with return to Earth aboard the Shuttle eliminated the concept of ground return from the program. NASA decided that on-orbit servicing might be adequate to maintain HST for its 15 year design life.

150) "Hubble Servicing Missions Overview," NASA Hubble Space Telescope, 3 August 2017, URL: https://www.nasa.gov/mission_pages/hubble/servicing/index.html

151) "Corrective Optics Space Telescope Axial Replacement," NASA Facts, June 1993, URL: https://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/facts/COSTAR.pdf

152) "NASA's Optical Verification Program," NASA Facts, November 1993, URL: https://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/facts/OpticalVerification.pdf

153) "HST Servicing Mission Observatory Verification," NASA Facts, June 1993, URL: https://asd.gsfc.nasa.gov/archive/hubble/a_pdf/
news/facts/HST_SM_Obs_Verification.pdf

154) https://www.nasa.gov/mission_pages/shuttle/
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155) "Picture Perfect: Hubble's New Improved Optics Probe the Core of a Distant Galaxy," Hubblesite, Release 1025 of January 13, 1994, URL: http://hubblesite.org/news_release/news/1994-01

156) "M100 Galactic Nucleus: Pictures of Galaxy M100 with Hubble's Old and New Optics," Hubblesite, News release ID: STScI-1994-01, Release Date: Jan 13, 1994, URL: http://hubblesite.org/image/123/news_release/1994-01

157) Denise Chow, "Saving Hubble: Astronauts Recall 1st Space Telescope Repair Mission 20 Years Ago," Space.com, 2 December 2013, URL: https://www.space.com/23640-hubble-space-telescope-repair-anniversary.html

158) https://asd.gsfc.nasa.gov/archive/hubble/missions/sm2.html

159) "Servicing Mission 3A," ESA, URL: https://www.spacetelescope.org/about/history/servicing_mission_3a/

160) https://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-103.html

161) https://asd.gsfc.nasa.gov/archive/hubble/missions/sm3a.html

162) "Hubble Space Telescope, Servicing Mission 3A, Media Reference Guide," Prepared for NASA by Lockheed Martin, URL: https://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/SM3A-MediaGuide.pdf

163) https://asd.gsfc.nasa.gov/archive/hubble/missions/sm3b.html

164) "Advanced Camera for Surveys (ACS)," Hubble Facts, URL: https://asd.gsfc.nasa.gov/archive/hubble/a
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165) "Servicing Mission 3B - Another refurbishment for Hubble," ESA, Hubble Space Telescope, URL: http://www.spacetelescope.org/about/history/servicing_mission_3b/

166) "Hubble's instruments: ACS - Advanced Camera for Surveys," ESA, Hubble Space telescope, URL: http://www.spacetelescope.org/about/general/instruments/acs/

167) "Space Shuttle Mission STS-125, The Final Visit to Hubble," NASA Press Kit, URL: http://www.nasa.gov/pdf/331922main_STS-125_Shuttle_Press_Kit.pdf

168) "Hubble Space Telescope – Wide Field Camera 3," NASA Facts, FS-2015-3-256-GSFC, URL: https://www.nasa.gov/sites/default/files/atoms/files/hstwfc3.pdf

169) "Shuttle Mission STS-125 Atlantis," NASA, URL: https://asd.gsfc.nasa.gov/archive/hubble/missions/sm4.html

170) "Hubble to Receive High-Tech JWST Technology," May 8, 2009, URL: http://www.spaceref.com/news/viewpr.html?pid=28167

171) Timothy J. Cole, "On-Orbit Repair of Satellites using Fastener Capture Plates to Eliminate Debris," 2011 IEEE Aerospace Conference, Big Sky, MT, USA, March 5-12, 2011

172) Nancy Atkinson, "Hubble Servicing Mission 4 in Pictures, Part 1," Universe Today, May 17, 2009, URL: http://www.universetoday.com/2009/05/17/hubble-
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173) Nancy Atkinson, "Super-Tools Essential to Hubble Mission Success," Universe Today, May 18, 2009, URL: http://www.universetoday.com/2009/05/18/super-tools-essential-to-hubble-mission-success/

174) Nancy Atkinson, "Gallery: Behind the Scenes Images of the Final Hubble Servicing Mission," Universe Today, April 1, 2015, URL: http://www.universetoday.com/119638/gallery-behind-the-
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175) "Hubble Tool Time Episode 6 - Servicing Mission 4," NASA Goddard Media Studios, 21 May 2019, URL: https://svs.gsfc.nasa.gov/13180

176) "Hubble Mission Operations," NASA, URL: https://www.nasa.gov/content/hubble-mission-operations

177) "STScI History," URL: http://www.stsci.edu/who-we-are/history

178) STScI (Space Telescope Science Institute), URL: http://www.stsci.edu/institute/
 


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

Sensor Complement   HST Imagery    Hubble Servicing Missions    Ground Segment    References   
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