Hubble Space Telescope
HST (Hubble Space Telescope) Mission
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 2018, in its 29th 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.
• 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).
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)
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.
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)
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.
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)
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)
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.
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 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.
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
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.
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).
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
- 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.
Figure 8: 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. 8)
- 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.
Figure 9: 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. 9) 10)
- 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.
Figure 10: 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. 11)
Figure 11: 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. 12)
- 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 12). 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. 13)
- 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
Figure 12: 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. 14)
- 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.
Figure 13: 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. 15)
- 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.
Figure 14: 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. 16)
Figure 15: 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. 17)
- 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."
Figure 16: 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 17: 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.
Figure 18: 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! 18)
- 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).
Figure 19: 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. 19)
- 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 20: 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) 20)
Figure 21: Large Magellanic Cloud [DSS (Digitized Sky Survey)View] with Star Cluster Overlay (Hubble), image credit: NASA, ESA, and A. Riess (STScI/JHU)
Figure 22: Three Steps to the Hubble Constant [image credit: NASA, ESA, and A. Riess (STScI/JHU)]
Figure 23: 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. 21)
- 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.
Figure 24: 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. 22)
- 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.
Figure 25: 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. 23)
- 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.
Figure 26: 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. 24)
- 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 1 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.
• 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. 25)
- “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.
Figure 27: 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. 26)
- 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.
Figure 28: 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. 27)
- 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.
Figure 29: 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. 28)
- 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).
Figure 30: 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. 29) 30)
Figure 31: 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
- "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
- 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. 31)
Figure 32: 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. 32)
- 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. 33)
- 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). 34)
- 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?
Figure 33: 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. 35) 36)
- 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."
Figure 34: 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. 37)
- 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.
Figure 35: 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. 38)
Figure 36: 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. 39) 40)
- The new Hubble view of Neptune shows the dark storm, seen at top center (Figure 37). 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.
Figure 37: 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. 41)
- 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.
Figure 38: 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. 42)
- 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 . 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.
- 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.
Figure 39: 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. 43)
Figure 40: 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. 44) 45)
- "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."
Figure 41: 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 41: 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. 46)
Figure 42: 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. 47)
Figure 43: 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. 48)
• 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. 49) 50)
- 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. 51)
- 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.
Figure 44: 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. 52)
- 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. 53)
- 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. 54)
- 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. 55)
- 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.
Figure 45: Planet OGLE-2012-BLG-0950Lb was detected through gravitational microlensing, a phenomenon that acts as nature's magnifying glass (image credit: LCO, D. Bennett)
- 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.
- 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. 56)
- 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. 57)
- 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.
Figure 46: 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.
Figure 47: 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. 58)
- 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 . 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 .
- 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.
Figure 48: 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 49), 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. 59)
- 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).
Figure 49: 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. 60)
- 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.
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.
- "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." 61)
- 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.
Figure 50: 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. 62)
- The findings, published in the journal of Astronomy and Astrophysics, advance astronomers' knowledge about how planets evolve. 63)
- 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.
Figure 51: 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. 64)
- 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."
Figure 52: 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.
- 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. 65)
- 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. 66)
- 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.)
Figure 53: 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. 67)
- 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.
Figure 54: Tangled — cosmic edition (image credit: ESA/Hubble, NASA)
• 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. 68) 69)
- 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."
Figure 55: 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.
Figure 56: 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.
Table 2: Extended life for ESA's science missions 70)
• 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. 71)
- 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.
Figure 57: 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