Minimize Hubble Space Telescope

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.




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

• 11 September 2019: With data from the NASA/ESA Hubble Space Telescope, water vapor has been detected in the atmosphere of a super-Earth within the habitable zone by UCL (University College London) researchers in a world first. K2-18b, which is eight times the mass of Earth, is now the only planet orbiting a star outside the Solar System, or exoplanet, known to have both water and temperatures that could support life. 7) 8)

- The UCL team used archive data from 2016 and 2017 captured by the NASA/ESA Hubble Space Telescope and developed open-source algorithms to analyze the starlight filtered through K2-18b's atmosphere. 9) The results revealed the molecular signature of water vapor, also indicating the presence of hydrogen and helium in the planet's atmosphere.

- The discovery, published today in Nature Astronomy, is the first successful atmospheric detection of an exoplanet orbiting in its star's habitable zone, at a distance where water can exist in liquid form. 10)

- The authors believe that other molecules, including nitrogen and methane, may be present but they remain undetectable with current observations. Further studies are required to estimate cloud coverage and the percentage of atmospheric water present.

- The planet orbits the cool dwarf star K2-18, which is 110 light years from Earth in the constellation of Leo. Given the high level of activity of its red dwarf star, K2-18b may be more hostile than Earth and is likely to be exposed to more radiation.

- K2-18b was discovered in 2015 and is one of hundreds of super-Earths – planets with masses between those of Earth and Neptune – found by NASA's Kepler spacecraft. NASA's TESS mission is expected to detect hundreds more super-Earths in the coming years.

- Co-author Ingo Waldmann (UCL CSED), said: "With so many new super-Earths expected to be found over the next couple of decades, it is likely that this is the first discovery of many potentially habitable planets. This is not only because super-Earths like K2-18b are the most common planets in our Milky Way, but also because red dwarfs – stars smaller than our Sun – are the most common stars."

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Figure 8: This artist's impression shows the planet K2-18b, it's host star and an accompanying planet in this system. K2-18b is now the only super-Earth exoplanet known to host both water and temperatures that could support life (image credit: ESA/Hubble, M. Kornmesser, CC BY 4.0)

- The next generation of space telescopes, including the NASA/ESA/CSA James Webb Space Telescope and ESA's ARIEL mission, will be able to characterize atmospheres in more detail as they will carry more advanced instruments. ARIEL is expected to launch in 2028 and will observe 1000 planets in detail to get a truly representative picture of what they are like.

- Professor Giovanna Tinetti (UCL CSED), co-author and Principal Investigator for ARIEL, said: "Our discovery makes K2-18b one of the most interesting targets for future study. Over 4000 exoplanets have been detected but we don't know much about their composition and nature. By observing a large sample of planets, we hope to reveal secrets about their chemistry, formation and evolution."

- "This study contributes to our understanding of habitable worlds beyond our Solar System and marks a new era in exoplanet research, crucial to ultimately placing the Earth, our only home, into the greater picture of the Cosmos," said Angelos Tsiaras.

• 09 September 2019: Previous research on the formation and evolution of star clusters has suggested that these systems tend to be compact and dense when they form, before expanding with time to become clusters of both small and large sizes. New Hubble observations in the Large Magellanic Cloud (LMC) galaxy have increased our understanding of how the size of star clusters in the LMC changes with time[1]. 11)

Note [1]: The observations were achieved from a set of long exposures acquired with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) for five old star clusters in the Large Magellanic Cloud galaxy, secured under proposal 14164 (PI: Sarajedini).

- Star clusters are aggregates of many (up to one million) stars. They are active systems in which the mutual gravitational interactions among the stars change their structure over time (known to astronomers as "dynamical evolution"). Because of such interactions, heavy stars tend to progressively sink towards the central region of a star cluster, while low-mass stars can escape from the system. This causes a progressive contraction of the cluster core over different timescales and means that star clusters with the same chronological age can vary greatly in appearance and shape because of their different “dynamical ages”.

- Located nearly 160,000 light-years from Earth, the LMC is a satellite galaxy of the Milky Way which hosts star clusters covering a wide range of ages. This differs from our own Milky Way galaxy which primarily contains older star clusters. The distribution of sizes as a function of age observed for star clusters in the LMC is very puzzling, as the young clusters are all compact, while the oldest systems have both small and large sizes.

- All star clusters, including those in the LMC, have been found to host a special type of re-invigorated stars called blue stragglers [2]. Under certain circumstances, stars receive extra fuel that bulks them up and substantially brightens them. This can happen if one star pulls matter off a neighbor, or if they collide.

Note [2]: Blue stragglers are so called because of their blue color, and the fact that their evolution lags behind that of their neighbors.

- As a result of dynamical aging, heavier stars sink towards the center of a cluster as the cluster ages, in a process similar to sedimentation, called “central segregation”. Blue stragglers are bright, making them relatively easy to observe, and they have high masses, which means that they are affected by central segregation and can be used to estimate the dynamical age of a star cluster [3].

Note [3]: Blue stragglers combine being relatively bright and having high mass by the standards of globular cluster stars, but they are not the only stars within these clusters that are either bright or massive.

- Francesco Ferraro of the University of Bologna in Italy and his team used the Hubble Space Telescope to observe blue stragglers in five (coeval) old LMC star clusters with different sizes and succeeded in ranking them in terms of their dynamical age. 12)

- “We demonstrated that different structures of star clusters are due to different levels of dynamical ageing: they are in different physical shape despite the fact that they were born at the same cosmic time. This is the first time that the effect of dynamical ageing has been measured in the LMC clusters” says Ferraro.

- “These findings present intriguing areas for further research, since they reveal a novel and valuable way of reading the observed patterns of LMC star clusters, providing new hints about the cluster formation history in the LMC galaxy,” adds co-author Barbara Lanzoni.

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Figure 9: Just as people of the same age can vary greatly in appearance and shape, so do collections of stars or stellar aggregates. New observations from the NASA/ESA Hubble Space Telescope suggest that chronological age alone does not tell the complete story when it comes to the evolution of star clusters. — This image from the NASA/ESA Hubble Space Telescope reveals an ancient, glimmering ball of stars called NGC 1466. It is a globular cluster — a gathering of stars all held together by gravity — that is slowly moving through space on the outskirts of the Large Magellanic Cloud, one of our closest galactic neighbors. NGC 1466 certainly is one for extremes. It has a mass equivalent to roughly 140,000 Suns and an age of around 13.1 billion years, making it almost as old as the Universe itself. This fossil-like relic from the early Universe lies some 160,000 light-years away from us. NGC 1466 is one of the 5 clusters in the LMC in which the level of dynamical evolution (or "dynamical age") was measured (image credit: ESA/Hubble & NASA)

• 06 September 2019: This Picture of the Week shows a dwarf galaxy named UGC 685. Such galaxies are small and contain just a tiny fraction of the number of stars in a galaxy like the Milky Way. Dwarf galaxies often show a hazy structure, an ill-defined shape, and an appearance somewhat akin to a swarm or cloud of stars — and UGC 685 is no exception to this. Classified as an SAm galaxy — a type of unbarred spiral galaxy — it is located about 15 million light-years from Earth. 13)

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Figure 10: These data were gathered under the NASA/ESA Hubble Space Telescope’s LEGUS (Legacy ExtraGalactic UV Survey) Program, the sharpest and most comprehensive ultraviolet survey of star-forming galaxies in the nearby Universe (image credit: ESA/Hubble & NASA; the LEGUS team, B. Tully, D. Calzetti Acknowledgement(s): Judy Schmidt (Geckzilla); CC BY 4.0)

- LEGUS is imaging 50 spiral and dwarf galaxies in our cosmic neighborhood in multiple colors using Hubble’s Wide Field Camera 3. The survey is picking apart the structures of these galaxies and resolving their constituent stars, clusters, groups, and other stellar associations. Star formation plays a huge role in shaping its host galaxy; by exploring these targets in detail via both new observations and archival Hubble data, LEGUS will shed light on how stars form and cluster together, how these clusters evolve, how a star’s formation affects its surroundings, and how stars explode at the end of their lives.

• 23 August 2019: This atmospheric Picture of the Week, taken with the NASA/ESA Hubble Space Telescope, shows a dark, gloomy scene in the constellation of Gemini (The Twins). The subject of this image confused astronomers when it was first studied — rather than being classified as a single object, it was instead recorded as two objects, owing to its symmetrical lobed structure (known as NGC 2371 and NGC 2372, though sometimes referred to together as NGC 2371/2). 14)

- The structure of this region is complex. It is filled with dense knots of gas, fast-moving jets that appear to be changing direction over time, and expanding clouds of material streaming outwards on diametrically opposite sides of the remnant star. Patches of this scene glow brightly as the remnant star emits energetic radiation that excites the gas within these regions, causing it to light up. This scene will continue to change over the next few thousand years; eventually the knotty lobes will dissipate completely, and the remnant star will cool and dim to form a white dwarf.

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Figure 11: Two lobes are visible to the upper right and lower left of the frame, and together form something known as a planetary nebula. Despite the name, such nebulae have nothing to do with planets; NGC 2371/2 formed when a Sun-like star reached the end of its life and blasted off its outer layers, shedding the constituent material and pushing it out into space to leave just a superheated stellar remnant behind. This remnant is visible as the orange-tinted star at the center of the frame, sitting neatly between the two lobes (image credit: ESA/Hubble & NASA, R. Wade et al.; CC BY 4.0)

• 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. 15)

- 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 12: 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. 16)

- 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 13: 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? 17)

- 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 14: 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. 18)

- 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 15: 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. 19)

- 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. 20) 21)

- 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 16: 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. 22)

- 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 17: 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. 23)

- 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. 24) 25) 26)

- 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 18: 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 19: 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. 27)

- 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 20: 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. 28)

- 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 21: 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. 29)

- 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 22: 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. 30)

- 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 23: 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 23, 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. 31)

- 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 24: 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. 32)

- 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 25: 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. 33) 34)

- 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 26: 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. 35)

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Figure 27: 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. 36)

- 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 28). 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. 37)

- 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 28: 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)