Hubble Space Telescope
Hubble status and imagery in the period 2020
• December 18, 2020: The narrow galaxy elegantly curving around its spherical companion in this image is a fantastic example of a truly strange and very rare phenomenon. 1)
- First theorized to exist by Einstein in his general theory of relativity, this object’s unusual shape can be explained by a process called gravitational lensing, which causes light shining from far away to be bent and pulled by the gravity of an object between its source and the observer. In this case, the light from the background galaxy has been distorted into the curve we see by the gravity of the galaxy cluster sitting in front of it. The near exact alignment of the background galaxy with the central elliptical galaxy of the cluster, seen in the middle of this image, has warped and magnified the image of the background galaxy around itself into an almost perfect ring. The gravity from other galaxies in the cluster is soon to cause additional distortions.
- Objects like these are the ideal laboratory in which to research galaxies too faint and distant to otherwise see.
Figure 1: This image, taken with the NASA/ESA Hubble Space Telescope, depicts GAL-CLUS-022058s, located in the southern hemisphere constellation of Fornax (The Furnace). GAL-CLUS-022058s is the largest and one of the most complete Einstein rings ever discovered in our Universe. The object has been nicknamed by the Principal Investigator and his team who are studying this Einstein ring as the "Molten Ring", which alludes to its appearance and host constellation (image credit: ESA/Hubble & NASA, S. Jha; CC BY 4.0 Acknowledgement: L. Shatz)
• December 15, 2020: When NASA's Voyager 2 spacecraft flew by Neptune in 1989 after a nearly 3-billion-mile odyssey, astronomers expected to get a close-up look at a blue-green planet that seemed as featureless as a marble. Instead, they were shocked and intrigued to see a dynamic and turbulent world of whirling storms, including a giant feature dubbed the Great Dark Spot, looming in Neptune's far southern hemisphere. 2)
Figure 2: Hubble uncovers a pair of Dark Vortices on Neptune. This Hubble Space Telescope snapshot of the dynamic blue-green planet Neptune reveals a monstrous dark storm [top center] and the emergence of a smaller dark spot nearby [top right]. The giant vortex, which is wider than the Atlantic Ocean, was traveling south toward certain doom by atmospheric forces at the equator when it suddenly made a U-turn and began drifting back northward. Hubble's WFC3 (Wide Field Camera 3) captured this visible-light image on Jan. 7, 2020, the same time a slightly smaller dark spot mysteriously appeared nearby. That spot then vanished a few months later. The smaller feature may have been a piece of the giant storm that broke off as the larger vortex approached the equator. - Hubble uncovered the giant storm in September 2018 in Neptune's northern hemisphere. The feature is roughly 4,600 miles across. The estimated width of the smaller spot is 3,900 miles. - The large storm is the fourth transient dark spot Hubble has observed since 1993. NASA's Voyager 2 spacecraft first imaged two dark features in Neptune's southern hemisphere in 1989 as Voyager flew by the distant planet. Those storms had disappeared by the time Hubble looked at Neptune in 1994. However, Hubble detected two new dark spots in the planet's northern hemisphere in 1994 and 1996. - It's unclear how these storms form. Their clouds may be rising to higher altitudes, compared to surrounding regions in the gas giant's atmosphere. - Neptune's predominant blue color is due to the absorption of red light by the distant planet's methane-rich atmosphere [image credits: NASA, ESA, STScI, M. H. Wong (University of California, Berkeley) and L. A. Sromovsky and P. M. Fry (University of Wisconsin-Madison)]
- The vortex was reminiscent of Jupiter's legendary Great Red Spot, a monstrous storm that has been raging for hundreds of years. Had this Great Dark Spot been brewing for the same amount of time? Or, was it a more ephemeral tempest?
- Scientists had to wait until 1994, when the Hubble Space Telescope and its crisp vision peered at distant Neptune. The mysterious spot had vanished! This game of planetary peek-a-boo continued when Hubble spotted another dark storm appearing in Neptune's northern hemisphere in 1995. Over the past three decades, Hubble has continued to observe the planet, watching several more dark spots come and go.
- Only Hubble can study these spots because it has the sharp vision to observe them in visible light. Hubble has shown that these storms live for a few years before vanishing or fading away.
- Researchers thought the current giant storm in the northern hemisphere was heading to destruction when it mysteriously halted its southern journey and began drifting northward. At the same time as the spot's stunning reversal, a new, slightly smaller dark feature appeared near its bigger cousin and later disappeared. These surprising events add to the mystery of this dynamic world.
- Astronomers using NASA's Hubble Space Telescope watched a mysterious dark vortex on Neptune abruptly steer away from a likely death on the giant blue planet.
- The storm, which is wider than the Atlantic Ocean, was born in the planet's northern hemisphere and discovered by Hubble in 2018. Observations a year later showed that it began drifting southward toward the equator, where such storms are expected to vanish from sight. To the surprise of observers, Hubble spotted the vortex change direction by August 2020, doubling back to the north. Though Hubble has tracked similar dark spots over the past 30 years, this unpredictable atmospheric behavior is something new to see.
- Equally as puzzling, the storm was not alone. Hubble spotted another smaller dark spot in January this year that temporarily appeared near its larger cousin. It might possibly have been a piece of the giant vortex that broke off, drifted away, and then disappeared in subsequent observations.
- "We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process," said Michael H. Wong of the University of California at Berkeley. "This is a process that's never been observed. We have seen some other dark spots fading away and they're gone, but we've never seen anything disrupt, even though it’s predicted in computer simulations."
- The large storm, which is 4,600 miles across, is the fourth dark spot Hubble has observed on Neptune since 1993. Two other dark storms were discovered by the Voyager 2 spacecraft in 1989 as it flew by the distant planet, but they had disappeared before Hubble could observe them. Since then, only Hubble has had the sharpness and sensitivity in visible light to track these elusive features, which have sequentially appeared and then faded away over a duration of about two years each. Hubble uncovered this latest storm in September 2018.
- Neptune's dark vortices are high-pressure systems that can form at mid-latitudes and may then migrate toward the equator. They start out remaining stable due to Coriolis forces, which cause northern hemisphere storms to rotate clockwise, due to the planet's rotation. (These storms are unlike hurricanes on Earth, which rotate counterclockwise because they are low-pressure systems.) However, as a storm drifts toward the equator, the Coriolis effect weakens and the storm disintegrates. In computer simulations by several different teams, these storms follow a more-or-less straight path to the equator, until there is no Coriolis effect to hold them together. Unlike the simulations, the latest giant storm didn't migrate into the equatorial "kill zone."
- "It was really exciting to see this one act like it's supposed to act and then all of a sudden it just stops and swings back," Wong said. "That was surprising."
Dark Spot Jr.
- The Hubble observations also revealed that the dark vortex’s puzzling path reversal occurred at the same time that a new spot, informally deemed "dark spot jr.," appeared. The newest spot was slightly smaller than its cousin, measuring about 3,900 miles across. It was near the side of the main dark spot that faces the equator—the location that some simulations show a disruption would occur.
- However, the timing of the smaller spot's emergence was unusual. "When I first saw the small spot, I thought the bigger one was being disrupted," Wong said. "I didn't think another vortex was forming because the small one is farther towards the equator. So it's within this unstable region. But we can't prove the two are related. It remains a complete mystery.
- "It was also in January that the dark vortex stopped its motion and started moving northward again," Wong added. "Maybe by shedding that fragment, that was enough to stop it from moving towards the equator."
- The researchers are continuing to analyze more data to determine whether remnants of dark spot jr. persisted through the rest of 2020.
Dark Storms Still Puzzling
- It's still a mystery how these storms form, but this latest giant dark vortex is the best studied so far. The storm's dark appearance may be due to an elevated dark cloud layer and it could be telling astronomers about the storm's vertical structure.
- Another unusual feature of the dark spot is the absence of bright companion clouds around it, which were present in Hubble images taken when the vortex was discovered in 2018. Apparently, the clouds disappeared when the vortex halted its southward journey. The bright clouds form when the flow of air is perturbed and diverted upward over the vortex, causing gases to likely freeze into methane ice crystals. The lack of clouds could be revealing information on how spots evolve, say researchers.
Weather Eye on the Outer Planets
- Hubble snapped many of the images of the dark spots as part of 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 dark spots on Neptune or potentially Uranus. 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.
- "We wouldn't know anything about these latest dark spots if it wasn't for Hubble," Simon said. "We can now follow the large storm for years and watch its complete life cycle. If we didn't have Hubble, then we might think the Great Dark Spot seen by Voyager in 1989 is still there on Neptune, just like Jupiter's Great Red Spot. And, we wouldn't have known about the four other spots Hubble discovered." Wong will present the team's findings Dec. 15 at the fall meeting of the American Geophysical Union.
- 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.
• December 11, 2020: Nestled in the constellation of Columba (The Dove), NGC 1792 is both a spiral galaxy, and a starburst galaxy. Within starburst galaxies, stars are forming at comparatively exorbitant rates. The rate of star formation can be more than 10 times faster in a starburst galaxy than in the Milky Way. When galaxies have a large reservoir of gas, like NGC 1792, these short lived starburst phases can be sparked by galactic events such as mergers and tidal interactions. One might think that these starburst galaxies would easily consume all of their gas in a large forming event. However, supernova explosions and intense stellar winds produced in these powerful starbursts can inject energy into the gas and disperse it. This halts the star formation before it can completely deplete the galaxy of all its fuel. Scientists are actively working to understand this complex interplay between the dynamics that drive and quench these fierce bursts of star formation. 3)
Figure 3: An orange glow radiates from the center of NGC 1792, the heart of this stellar forge. Captured by the NASA/ESA Hubble Space Telescope, this intimate view of NGC 1792 gives us some insight into this galactic powerhouse. The vast swathes of tell-tale blue seen throughout the galaxy indicate areas that are full of young, hot stars, and it is in the shades of orange, seen nearer the centre, that the older, cooler stars reside (image credit: ESA/Hubble & NASA, J. Lee; CC BY 4.0 Acknowledgement: Leo Shatz)
• December 10, 2020: Though every planet in our solar system has been visited by spacecraft over nearly the past 60 years, the outer frontier of the solar system, beyond Neptune, has been barely explored. There is circumstantial evidence that a planet five times Earth's mass – dubbed Planet Nine – may be lurking out there in the abyss. If real, it is creeping along a very wide orbit taking it 800 times farther from the Sun than Earth is. Though astronomers have yet to find this legendary world – if it exists at all – they have found another clue 336 light-years away. 4)
Figure 4: This Hubble Space Telescope image shows one possible orbit (dashed ellipse) of the 11-Jupiter-mass exoplanet HD 106906 b. This remote world is widely separated from its host stars, whose brilliant light is masked here to allow the planet to be seen. The planet resides outside its system's circumstellar debris disk, which is akin to our own Kuiper Belt of small, icy bodies beyond Neptune. The disk itself is asymmetric and distorted, perhaps due to the gravitational tug of the wayward planet. Other points of light in the image are background stars [image credit: NASA/ESA, M. Nguyen (University of California, Berkeley), R. De Rosa (European Southern Observatory), and P. Kalas (University of California, Berkeley and SETI Institute)]
- Astronomers analyzing Hubble images of the double star, HD 106906, have discovered a planet in a huge 15,000-year-long orbit that sweeps it as far from its stellar duo as Planet Nine would be from our Sun. This is observational evidence that similarly far-flung worlds may exist around other stars. Researchers hypothesize that the planet wound up there in a game of planetary pinball where the gravitational pull of a passing star modified the orbit's shape. Perhaps a passing star had a similar influence on our solar system 4.6 billion years ago.
- A planet in an unlikely orbit around a double star 336 light-years away may offer a clue to a mystery much closer to home: a hypothesized, distant body in our solar system dubbed "Planet Nine."
- This is the first time that astronomers have been able to measure the motion of a massive Jupiter-like planet that is orbiting very far away from its host stars and visible debris disk. This disk is similar to our Kuiper Belt of small, icy bodies beyond Neptune. In our own solar system, the suspected Planet Nine would also lie far outside of the Kuiper Belt on a similarly strange orbit. Though the search for a Planet Nine continues, this exoplanet discovery is evidence that such oddball orbits are possible.
- "This system draws a potentially unique comparison with our solar system," explained the paper's lead author, Meiji Nguyen of the University of California, Berkeley. "It's very widely separated from its host stars on an eccentric and highly misaligned orbit, just like the prediction for Planet Nine. This begs the question of how these planets formed and evolved to end up in their current configuration."
- The system where this gas giant resides is only 15 million years old. This suggests that our Planet Nine—if it does exist—could have formed very early on in the evolution of our 4.6-billion-year-old solar system.
An Extreme Orbit
- The 11-Jupiter-mass exoplanet called HD 106906 b was discovered in 2013 with the Magellan Telescopes at the Las Campanas Observatory in the Atacama Desert of Chile. However, astronomers did not know anything about the planet's orbit. This required something only the Hubble Space Telescope could do: collect very accurate measurements of the vagabond's motion over 14 years with extraordinary precision. The team used data from the Hubble archive that provided evidence for this motion.
- The exoplanet resides extremely far from its host pair of bright, young stars—more than 730 times the distance of the Earth from the Sun, or nearly 68 billion miles. This wide separation made it enormously challenging to determine the 15,000-year-long orbit in such a relatively short time span of Hubble observations. The planet is creeping very slowly along its orbit, given the weak gravitational pull of its very distant parent stars.
- The Hubble team was surprised to find that the remote world has an extreme orbit that is very misaligned, elongated and external to the debris disk that surrounds the exoplanet's twin host stars. The debris disk itself is very unusual-looking, perhaps due to the gravitational tug of the wayward planet.
How Did It Get There?
- So how did the exoplanet arrive at such a distant and strangely inclined orbit? The prevailing theory is that it formed much closer to its stars, about three times the distance that the Earth is from the Sun. But drag within the system's gas disk caused the planet's orbit to decay, forcing it to migrate inward toward its stellar pair. The gravitational effects from the whirling twin stars then kicked it out onto an eccentric orbit that almost threw it out of the system and into the void of interstellar space. Then a passing star from outside the system stabilized the exoplanet's orbit and prevented it from leaving its home system.
- Using precise distance and motion measurements from the European Space Agency's Gaia survey satellite, candidate passing stars were identified in 2019 by team members Robert De Rosa of the European Southern Observatory in Santiago, Chile and Paul Kalas of the University of California.
A Messy Disk
- In a study published in 2015, Kalas led a team that found circumstantial evidence for the runaway planet's behavior: the system's debris disk is strongly asymmetric, rather than being a circular "pizza pie" distribution of material. One side of the disk is truncated relative to the opposite side, and it is also disturbed vertically rather than being restricted to a narrow plane as seen on the opposite side of the stars.
- "The idea is that every time the planet comes to its closest approach to the binary star, it stirs up the material in the disk," explains De Rosa. "So every time the planet comes through, it truncates the disk and pushes it up on one side. This scenario has been tested with simulations of this system with the planet on a similar orbit—this was before we knew what the orbit of the planet was."
- "It's like arriving at the scene of a car crash, and you're trying to reconstruct what happened," explained Kalas. "Is it passing stars that perturbed the planet, then the planet perturbed the disk? Is it the binary in the middle that first perturbed the planet, and then it perturbed the disk? Or did passing stars disturb both the planet and disk at the same time? This is astronomy detective work, gathering the evidence we need to come up with some plausible storylines about what happened here."
A Planet Nine Proxy?
- This scenario for HD 106906 b’s bizarre orbit is similar in some ways to what may have caused the hypothetical Planet Nine to end up in the outer reaches of our own solar system, well beyond the orbit of the other planets and beyond the Kuiper Belt. Planet Nine could have formed in the inner solar system and been kicked out by interactions with Jupiter. However, Jupiter—the proverbial 800-pound gorilla in our solar system—would very likely have flung Planet Nine far beyond Pluto. Passing stars may have stabilized the orbit of the kicked-out planet by pushing the orbit path away from Jupiter and the other planets in the inner solar system.
- "It's as if we have a time machine for our own planetary system going back 4.6 billion years to see what may have happened when our young solar system was dynamically active and everything was being jostled around and rearranged," said Kalas.
- To date, astronomers only have circumstantial evidence for Planet Nine. They've found a cluster of small celestial bodies beyond Neptune that move in unusual orbits compared with the rest of the solar system. This configuration, some astronomers say, suggests these objects were shepherded together by the gravitational pull of a huge, unseen planet. An alternative theory is that there is not one giant perturbing planet, but instead the imbalance is due to the combined gravitational influence of multiple, much smaller objects. Another theory is that Planet Nine does not exist at all and the clustering of smaller bodies may be just a statistical anomaly.
• December 10, 2020: The exoplanet HD106906 b was discovered in 2013 with the Magellan Telescopes at the Las Campanas Observatory in Chile’s Atacama Desert. However, astronomers did not then know anything about the planet’s orbit. This required something only the Hubble Space Telescope could do: collect very accurate measurements of the vagabond’s motion over 14 years with extraordinary precision. 5)
- The exoplanet resides extremely far from its host pair of bright, young stars — more than 730 times the distance of Earth from the Sun. This wide separation made it enormously challenging to determine the 15,000-year-long orbit in such a short time span of Hubble observations. The planet is creeping very slowly along its orbit, given the weak gravitational pull of its very distant parent stars.
- The Hubble team behind this new result  was surprised to find that the remote world has an extreme orbit that is very inclined, elongated and external to a dusty debris disc that surrounds the exoplanet’s twin host stars. The debris disc itself is very extraordinary, perhaps due to the gravitational tug of the rogue planet. This study was led by Meiji Nguyen of the University of California, Berkeley.
- Note : The data used in this study were taken as part the following Hubble Space Telescope observing programs GO-10330 (PI: Ford), GO-14670 (PI: Kalas), GO-14241 (PI: Apai) and GO-14241 (PI: Apai). These results have been published in the Astronomical Journal.
- “To highlight why this is weird, we can just look at our own Solar System and see that all of the planets lie roughly in the same plane,” explained Nguyen. “It would be bizarre if, say, Jupiter just happened to be inclined 30 degrees relative to the plane that every other planet orbits in. This raises all sorts of questions about how HD 106906 b ended up so far out on such an inclined orbit.”
- The prevailing theory to explain how the exoplanet arrived at such a distant and strangely inclined orbit is that it formed much closer to its stars, about three times the distance that Earth is from the Sun. However, drag within the system’s gas disc caused the planet’s orbit to decay, forcing it to migrate inward toward its stellar hosts. The gravitational forces from the whirling twin stars then kicked it out onto an eccentric orbit that almost threw it out of the system and into the void of interstellar space. Then a star passed very close by to this system, stabilizing the exoplanet’s orbit and preventing it from leaving its home system. Candidate passing stars had been previously identified using precise distance and motion measurements from the European Space Agency’s Gaia survey satellite.
- This scenario to explain HD106906 b’s bizarre orbit is similar in some ways to what may have caused the hypothetical Planet Nine to end up in the outer reaches of our own Solar System, beyond the Kuiper Belt. Planet Nine could have formed in the inner Solar System and was then kicked out by interactions with Jupiter. However, Jupiter would very likely have flung Planet Nine far beyond Pluto. Passing stars may have stabilized the orbit of the kicked-out planet by pushing the orbit path away from Jupiter and the other planets in the inner Solar System.
- “It’s as if we have a time machine for our own Solar System going back 4.6 billion years to see what may have happened when our young Solar System was dynamically active and everything was being jostled around and rearranged,” explained team member Paul Kalas of the University of California, Berkeley.
- To date, astronomers have only circumstantial evidence for the existence of Planet Nine. They’ve found a cluster of small celestial bodies beyond Neptune that move in unusual orbits compared to the rest of the Solar System. This configuration, some astronomers think, suggests that these objects were shepherded together by the gravitational pull of a huge, unseen planet. An alternative hypothesis is that there is not one giant perturber, but instead the imbalance is due to the combined gravitational influence of much smaller objects.
Figure 5: Artist’s Impression of Exoplanet HD 106906b. This 11-Jupiter-mass exoplanet called HD106906 b occupies an unlikely orbit around a double star 336 light-years away and it may be offering clues to something that might be much closer to home: a hypothesized distant member of our Solar System dubbed “Planet Nine.” This is the first time that astronomers have been able to measure the motion of a massive Jupiter-like planet that is orbiting very far away from its host stars and visible debris disc (image credit: ESA/Hubble, M. Kornmesser)
- “Despite the lack of detection of Planet Nine to date, the orbit of the planet can be inferred based on its effect on the various objects in the outer Solar System,” explained team member Robert De Rosa of the European Southern Observatory in Santiago, Chile who led the study’s analysis. “This suggests that if a planet was indeed responsible for what we observe in the orbits of trans-Neptunian objects it should have an eccentric orbit inclined relative to the plane of the Solar System. This prediction of the orbit of Planet Nine is similar to what we are seeing with HD 106906b."
- Scientists using the upcoming NASA/ESA/CSA James Webb Space Telescope plan to get additional data on HD106906 b to better understand the planet’s system. Astronomers want to know where and how the planet formed and whether the planet has its own debris system around it, among other questions.
- “There are still a lot of open questions about this system,” added De Rosa. “For example, we do not conclusively know where or how the planet formed. Although we have made the first measurement of orbital motion, there are still large uncertainties on the various orbital parameters. It is likely that both observers and theorists alike will be studying HD 106906 for years to come, unraveling the many mysteries of this remarkable planetary system.”
Figure 6: Artist’s Impression of the Hypothesized “Planet Nine”. An 11-Jupiter-mass exoplanet called HD106906 b occupies an unlikely orbit around a double star 336 light-years away and may be offering clues to something that might be much closer to home: a hypothesized distant member of our Solar System dubbed “Planet Nine.” This is the first time that astronomers have been able to measure the motion of a massive Jupiter-like planet that is orbiting very far away from its host stars and visible debris disc (image credit: ESA/Hubble, M. Kornmesser)
• December 4, 2020: This is a post-starburst galaxy, which is a product of galaxies that have merged within the past billion years. This merger event drove gas to the galaxy's center and created an abundance of new stars, of which the brightest are visible in the remnant galaxy's core. The visible shells and tails surrounding the galaxies are also a result of this merger event. 6)
- Residing far beyond our own Milky Way, it is almost impossible to glimpse this galaxy without the assistance of Hubble, and, like thousands of similar faint and very distant galaxies, it was only discovered and catalogued in recent years. Many millions of galaxies still await our discovery as we build a wondrous picture of our night sky.
Figure 7: This large expanse of space captured with the Hubble Space Telescope features the galaxy SDSSJ225506.80+005839.9. Unlike many other extravagant galaxies and stunning nebulae imaged by Hubble, this galaxy does not have a short popular name, and is only known by its long SDSS name, which refers to its coordinates in the sky. This galaxy - visible in the center right portion of the image - and its many wondrous neighboring galaxies lie in the constellation of Pisces (The Fish), image credit: ESA/Hubble & NASA, A. Zabludoff; CC BY 4.0
• December 03, 2020: Even though the Universe is constantly changing, most processes are too slow to be observed within a human lifespan. However, the Stingray Nebula is now offering scientists a special opportunity to observe a system’s evolution in real time. 7)
Figure 8: Archival data from the NASA/ESA Hubble Space Telescope reveal that the nebula Hen 3-1357, nicknamed the Stingray nebula, has faded precipitously over just the past two decades. Witnessing such a swift rate of change in a planetary nebula is exceedingly rare, say researchers. - These images captured by Hubble in 1996 (left), when compared to Hubble images taken in 2016 (right), show a nebula that has drastically dimmed in brightness and changed shape. Bright blue shells of gas near the center of the nebula have all but disappeared, and the wavy edges that earned this nebula its aquatic-themed name are virtually gone. The young nebula no longer pops against the black velvet background of the distant Universe [image credit: NASA, ESA, B. Balick (University of Washington), M. Guerrero (Instituto de Astrofisica de Andalucia), and G. Ramos-Larios (Universidad de Guadalajara)]
- Researchers discovered unprecedented changes in the light emitted by glowing gas — nitrogen, hydrogen and oxygen — that is being blasted off by the dying star at the center of the nebula. The oxygen emission, in particular, dropped in brightness by a factor of nearly 1000.
- “In most studies, the nebula usually gets bigger," said Bruce Balick of the University of Washington, USA, who led the new research. "Here, it’s fundamentally changing its shape and getting fainter, and doing so on an unprecedented timescale.”
- “Because of Hubble’s optical stability, we are very, very confident that this nebula is changing in brightness,” said team member Martin Guerrero of the Instituto de Astrofisica de Andalucia in Granada, Spain. "That easy to see since, unlike the nebula, all of the other stars in the Hubble image - including a distant companion star - stayed constant in brightness."
- The researchers note that while speculating on causes for this surprising finding, it’s important to explore the properties of the dying star at the center of the Stingray nebula, which influences the structure and brightness of the nebula.
- A 2016 study by Nicole Reindl of the University of Leicester, UK, and a team of international researchers, also using Hubble data, noted that the star at the centre of the Stingray nebula, SAO 244567, is special in its own right.
- Observations from 1971 to 2002 showed the temperature of the star skyrocketing to almost ten times hotter than the surface of our Sun. Reindl speculates the temperature jump was caused by a brief flash of helium fusion that occurred outside the core of the central star. After that the star began to cool again, returning to its previous stage of stellar evolution.
- The team studying the rapid fading of the Stingray nebula can only speculate at this time what’s in store for the future of this young nebula. 8)
• November 26, 2020: In 2018 an international team of researchers using the NASA/ESA Hubble Space Telescope and several other observatories uncovered, for the first time, a galaxy in our cosmic neighborhood that is missing most of its dark matter. This discovery of the galaxy NGC 1052-DF2 was a surprise to astronomers, as it was understood that Dark matter (DM) is a key constituent in current models of galaxy formation and evolution. In fact, without the presence of DM, the primordial gas would lack enough gravity pull to start collapsing and forming new galaxies. A year later, another galaxy that misses dark matter was discovered, NGC 1052-DF4, which further triggered intense debates among astronomers about the nature of these objects. 9)
Figure 9: Ground-based view of the sky around the galaxies NGC 1052-DF4 NGC & 1052-DF2. This image shows the sky around the ultra diffuse galaxies NGC 1052-DF4 and NGC 1052-DF2. It was created from images forming part of the Digitized Sky Survey 2. NGC 1052-DF2 is basically invisible in this image (image credit: ESA/Hubble, NASA, Digitized Sky Survey 2, Acknowledgement: Davide de Martin)
- Now, new Hubble data  have been used to explain the reason behind the missing dark matter in NGC 1052-DF4, which resides 45 million light-years away. Mireia Montes of the University of New South Wales in Australia led an international team of astronomers to study the galaxy using deep optical imaging. They discovered that the missing dark matter can be explained by the effects of tidal disruption. The gravity forces of the neighboring massive galaxy NGC 1035 are tearing NGC 1052-DF4 apart. During this process, the dark matter is removed, while the stars feel the effects of the interaction with another galaxy at a later stage.
- Notes:  These results were achieved using data from Hubble Space Telescope programs GO-14644 and GO-15695 (PI: van Dokkum).
- Until now, the removal of dark matter in this way has remained hidden from astronomers as it can only be observed using extremely deep images that can reveal extremely faint features. “We used Hubble in two ways to discover that NGC 1052-DF4 is experiencing an interaction,” explained Montes. “This includes studying the galaxy’s light and the galaxy’s distribution of globular clusters.”
Figure 10: This image presents the region around the galaxy NGC 1052-DF4, taken by the IAC80 telescope at the Teide Observatory in Tenerife. The figure highlights the main galaxies in the field-of-view, including NGC 1052-DF4 (center of the image), and its neighbor NGC 1035 (center left), image credit: M. Montes et al.
- Thanks to Hubble’s high resolution, the astronomers could identify the galaxy’s globular clusters population. The 10.4 meter Gran Telescopio Canarias (GTC) telescope and the IAC80 telescope in the Canaries, Spain, were also used to complement Hubble’s observations by further studying the data.
- “It is not enough just to spend a lot of time observing the object, but a careful treatment of the data is vital,” explained team member Raúl Infante-Sainz of the Instituto de Astrofísica de Canarias in Spain. “It was therefore important that we use not just one telescope/instrument, but several (both ground- and space-based) to conduct this research. With the high resolution of Hubble, we can identify the globular clusters, and then with GTC photometry we obtain the physical properties.”
- Globular clusters are thought to form in the episodes of intense star formation that shaped galaxies. Their compact sizes and luminosity make them easily observable and they are therefore good tracers of the properties of their host galaxy. In this way, by studying and characterizing the spatial distribution of the clusters in NGC 1052-DF4, astronomers can develop insight into the present state of the galaxy itself. The alignment of these clusters suggests they are being “stripped” from their host galaxy, and this supports the conclusion that tidal disruption is occurring.
- By studying the galaxy’s light, the astronomers also found evidence of tidal tails, which are formed of material moving away from NGC1052-DF4 — this further supports the conclusion that this is a disruption event. Additional analysis concluded that the central parts of the galaxy remain untouched and only ~7% of the stellar mass of the galaxy is hosted in these tidal tails. This means that dark matter, which is less concentrated than stars, was previously and preferentially stripped from the galaxy, and now the outer stellar component is starting to be stripped as well.
- This result is a good indicator that, while the dark matter of the galaxy was evaporated from the system, the stars are only now starting to suffer the disruption mechanism,” explained team member Ignacio Trujillo of the Instituto de Astrofísica de Canarias in Spain. “In time, NGC1052-DF4 will be cannibalized by the large system around NGC1035, with at least some of their stars floating free in deep space.”
- The discovery of evidence to support the mechanism of tidal disruption as the explanation for the galaxy’s missing dark matter has not only solved an astronomical conundrum, but has also brought a sigh of relief to astronomers. Without it, scientists would be faced with having to revise our understanding of the laws of gravity.
- “This discovery reconciles existing knowledge of how galaxies form and evolve with the most favorable cosmological model,” added Montes. 10)
• November 20, 2020: Lying in the constellation of Andromeda in the Northern hemisphere, this galaxy is classified as a spiral galaxy. Unlike the classic image of a spiral galaxy, however, the huge arms of stars and gas in UGC 12588 are very faint, undistinguished, and tightly wound around its center. The clearest view of the spiral arms comes from the bluer stars sprinkled around the edges of the galaxy that highlight the regions where new star formation is most likely taking place. 11)
Figure 11: Observed with the NASA/ESA Hubble Space Telescope, the faint galaxy featured in this image is known as UGC 12588. Unlike many spiral galaxies, UGC 12588 displays neither a bar of stars across its centre nor the classic prominent spiral arm pattern. Instead, to a viewer, its circular, white and mostly unstructured centre makes this galaxy more reminiscent of a cinnamon bun than a mega-structure of stars and gas in space (image credit: ESA/Hubble & NASA, R. Tully; CC BY 4.0 - Acknowledgement: Gagandeep Anand)
• November 19, 2020: Some of the most stunning views of our sky occur at sunset, when sunlight pierces the clouds, creating a mixture of bright and dark rays formed by the clouds' shadows and the beams of light scattered by the atmosphere. 12)
- Astronomers studying nearby galaxy IC 5063 are tantalized by a similar effect in images taken by NASA's Hubble Space Telescope. In this case, a collection of narrow bright rays and dark shadows is seen beaming out of the blazingly bright center of the active galaxy.
Figure 12: This Hubble Space Telescope image of the heart of nearby active galaxy IC 5063 reveals a mixture of bright rays and dark shadows coming from the blazing core, home of a supermassive black hole. Astronomers suggest that a ring of dusty material surrounding the black hole may be casting its shadow into space. According to their scenario, this interplay of light and shadow may occur when light blasted by the monster black hole strikes the dust ring, which is buried deep inside the core. Light streams through gaps in the ring, creating the brilliant cone-shaped rays. However, denser patches in the disk block some of the light, casting long, dark shadows through the galaxy. - This phenomenon is similar to sunlight piercing our Earthly clouds at sunset, creating a mixture of bright rays and dark shadows formed by beams of light scattered by the atmosphere. However, the bright rays and dark shadows appearing in IC 5063 are happening on a vastly larger scale, shooting across at least 36,000 light-years. IC 5063 resides 156 million light-years from Earth. The observations were taken on March 7 and Nov. 25, 2019 by Hubble's Wide Field Camera 3 and Advanced Camera for Surveys [image credit: NASA, ESA, STScI and W. P. Maksym (CfA)]
- A team of astronomers, led by Peter Maksym of the Center for Astrophysics | Harvard & Smithsonian (CfA), in Cambridge, Massachusetts, has traced the rays back to the galaxy's core, the location of an active supermassive black hole. A black hole is a dense, compact region of space that swallows light and matter under the crushing pull of gravity. The monster object is frenetically feeding on infalling material, producing a powerful gusher of light from superheated gas near it.
- Although the researchers have developed several plausible theories for the lightshow, the most intriguing idea suggests that an inner-tube-shaped ring, or torus, of dusty material surrounding the black hole is casting its shadow into space.
- According to Maksym's proposed scenario, the dust disk around the black hole doesn't block all of the light. Gaps in the disk allow light to beam out, creating brilliant cone-shaped rays similar to the fingers of light sometimes seen at sunset. However, the rays in IC 5063 are happening on a vastly larger scale, shooting across at least 36,000 light-years.
- Some of the light hits dense patches in the ring, casting the ring's shadow into space. These shadows appear as dark finger shapes interspersed with bright rays. These beams and shadows are visible because the black hole and its ring are tipped sideways relative to the plane of the galaxy. This alignment allows the light beams to extend far outside the galaxy.
- This interplay of light and shadow offers a unique insight into the distribution of material encircling the black hole. In some areas, the material may resemble scattered clouds. If this interpretation is correct, the observations may provide an indirect probe of the disk's mottled structure.
- "I'm most excited by the shadow of the torus idea because it's a really cool effect that I don't think we've seen before in images, although it has been hypothesized," Maksym said. "Scientifically, it's showing us something that is hard—usually impossible—to see directly. We know this phenomenon should happen, but in this case, we can see the effects throughout the galaxy. Knowing more about the geometry of the torus will have implications for anybody trying to understand the behavior of supermassive black holes and their environments. As a galaxy evolves, it is shaped by its central black hole."
- Studying the torus is important because it funnels material toward the black hole. If the "shadow" interpretation is accurate, the dark rays provide indirect evidence that the disk in IC 5063 could be very thin, which explains why light is leaking out all around the structure.
- Observations of similar black holes by NASA's Chandra X-ray Observatory detected X-rays leaking out of holes in the torus, making the structure appear like Swiss cheese. The holes may be caused by the disk being torqued by internal forces, causing it to warp, Maksym said. "It's possible that the warping creates big enough gaps for some of the light to shine through, and as the torus rotates, beams of light could sweep across the galaxy like lighthouse beams through fog," he added.
Citizen Science Serendipity
- Although astronomers have been studying the galaxy for decades, it took a non-scientist to make the surprising discovery. Judy Schmidt, an artist and amateur astronomer based in Modesto, California, uncovered the dark shadows when she reprocessed Hubble exposures of the galaxy in December 2019. Schmidt routinely culls the Hubble archive for interesting observations that she can turn into beautiful images. She shares those images on her Twitter feed with her many followers, who include astronomers such as Maksym.
- Schmidt selected the Hubble observations of IC 5063 from the archive because she is interested in galaxies that have active cores. The cone-shaped shadows were not apparent in the original exposures, so she was surprised to see them in her reprocessed image. "I had no idea they were there, and even after I'd processed it, I kept blinking my eyes wondering if I was seeing what I thought I was seeing," she said.
- She immediately posted her image to her Twitter account. "It was something I'd never seen before, and even though I had strong suspicions about them being shadow rays or 'crepuscular rays,' as Peter has dubbed them, it's easy to let one's imagination and wishful thinking run wild," she explained. "I figured if I was wrong, someone would come to ground me."
- The image prompted a lively Twitter discussion among her astronomer followers, including Maksym, who debated the rays' origin. Maksym had already been analyzing Hubble images of the jets produced by the galaxy's black hole. So he took the lead in studying the rays and writing a science paper. His study is based on near-infrared observations made by Hubble's Wide Field Camera 3 and Advanced Camera for Surveys in March and November 2019. Red and near-infrared light pierces the dusty galaxy to reveal the details that may be enshrouded in dust.
- This discovery would not have been possible without Hubble's sharp vision. The galaxy is also relatively nearby, only 156 million light-years from Earth. "Older images from telescopes on the ground showed maybe hints of this kind of structure, but the galaxy itself is such a mess that you'd never guess that this is what's going on without Hubble," Maksym explained. "Hubble has sharp pictures, is sensitive to faint things, and has a big enough field of view to image the entire galaxy."
- Maksym hopes to continue his study of the galaxy to determine whether his scenario is correct. "We will want to keep investigating, and it will be great if other scientists try to test our conclusions, too, with new observations and modeling," he said. "This is a project that is just begging for new data because it raises more questions than it answers."
- The team's results were published in The Astrophysical Journal Letters. 13)
• November 13, 2020: Gravitational lensing occurs when a large distribution of matter, such as a galaxy cluster, sits between Earth and a distant light source. As space is warped by massive objects, the light from the distant object bends as it travels to us and we see a distorted image of it. This effect was first predicted by Einstein’s general theory of relativity. 14)
- Strong gravitational lenses provide an opportunity for studying properties of distant galaxies, since Hubble can resolve details within the multiple arcs that are one of the main results of gravitational lensing. An important consequence of lensing distortion is magnification, allowing us to observe objects that would otherwise be too far away and too faint to be seen. Hubble makes use of this magnification effect to study objects beyond the sensitivity of its 2.4-meter-diameter primary mirror, showing us the most distant galaxies humanity has ever encountered.
- This lensed galaxy was found as part of the Sloan Bright Arcs Survey, which discovered some of the brightest gravitationally lensed high-redshift galaxies in the night sky.
Figure 13: This NASA/ESA Hubble Space Telescope image features the galaxy LRG-3-817, also known as SDSS J090122.37+181432.3. The galaxy, its image distorted by the effects of gravitational lensing, appears as a long arc to the left of the central galaxy cluster (image credit: ESA/Hubble & NASA, S. Allam et al.)
• November 12, 2020: Long ago and far across the universe, an enormous burst of gamma rays unleashed more energy in a half-second than the Sun will produce over its entire 10-billion-year lifetime. In May of 2020, light from the flash finally reached Earth and was first detected by NASA's Neil Gehrels Swift Observatory. Scientists quickly enlisted other telescopes — including NASA's Hubble Space Telescope, the Very Large Array radio observatory, the W. M. Keck Observatory, and the Las Cumbres Observatory Global Telescope network — to study the explosion's aftermath and the host galaxy. It was Hubble that provided the surprise. 15)
- Based on X-ray and radio observations from the other observatories, astronomers were baffled by what they saw with Hubble: the near-infrared emission was 10 times brighter than predicted. These results challenge conventional theories of what happens in the aftermath of a short gamma-ray burst. One possibility is that the observations might point to the birth of a massive, highly magnetized neutron star called a magnetar.
- "These observations do not fit traditional explanations for short gamma-ray bursts," said study leader Wen-fai Fong of Northwestern University in Evanston, Illinois. "Given what we know about the radio and X-rays from this blast, it just doesn't match up. The near-infrared emission that we're finding with Hubble is way too bright. In terms of trying to fit the puzzle pieces of this gamma-ray burst together, one puzzle piece is not fitting correctly."
Figure 14: This image shows the glow from a kilonova caused by the merger of two neutron stars. The kilonova, whose peak brightness reaches up to 10,000 times that of a classical nova, appears as a bright spot (indicated by the arrow) to the upper left of the host galaxy. The merger of the neutron stars is believed to have produced a magnetar, which has an extremely powerful magnetic field. The energy from that magnetar brightened the material ejected from the explosion [image credits: NASA, ESA, W. Fong (Northwestern University), and T. Laskar (University of Bath, UK)]
- Without Hubble, the gamma-ray burst would have appeared like many others, and Fong and her team would not have known about the bizarre infrared behavior. "It's amazing to me that after 10 years of studying the same type of phenomenon, we can discover unprecedented behavior like this," said Fong. "It just reveals the diversity of explosions that the universe is capable of producing, which is very exciting."
- The intense flashes of gamma rays from these bursts appear to come from jets of material that are moving extremely close to the speed of light. The jets do not contain a lot of mass — maybe a millionth of the mass of the Sun — but because they're moving so fast, they release a tremendous amount of energy across all wavelengths of light. This particular gamma-ray burst was one of the rare instances in which scientists were able to detect light across the entire electromagnetic spectrum.
Figure 15: This illustration shows the sequence for forming a magnetar-powered kilonova, whose peak brightness reaches up to 10,000 times that of a classical nova. 1) Two orbiting neutron stars spiral closer and closer together. 2) They collide and merge, triggering an explosion that unleashes more energy in a half-second than the Sun will produce over its entire 10-billion-year lifetime. 3) The merger forms an even more massive neutron star called a magnetar, which has an extraordinarily powerful magnetic field. 4) The magnetar deposits energy into the ejected material, causing it to glow unexpectedly bright at infrared wavelengths[image credits: NASA, ESA, and D. Player (STScI)]
- "As the data were coming in, we were forming a picture of the mechanism that was producing the light we were seeing," said the study's co-investigator, Tanmoy Laskar of the University of Bath in the United Kingdom. "As we got the Hubble observations, we had to completely change our thought process, because the information that Hubble added made us realize that we had to discard our conventional thinking, and that there was a new phenomenon going on. Then we had to figure out what that meant for the physics behind these extremely energetic explosions."
- Gamma-ray bursts — the most energetic, explosive events known — live fast and die hard. They are split into two classes based on the duration of their gamma rays.
- If the gamma-ray emission is greater than two seconds, it's called a long gamma-ray burst. This event is known to result directly from the core collapse of a massive star. Scientists expect a supernova to accompany this longer type of burst.
- If the gamma-ray emission lasts less than two seconds, it's considered a short burst. This is thought to be caused by the merger of two neutron stars, extremely dense objects about the mass of the Sun compressed into the volume of a city. A neutron star is so dense that on Earth, one teaspoonful would weigh a billion tons! A merger of two neutron stars is generally thought to produce a black hole.
- Neutron star mergers are very rare but are extremely important because scientists think that they are one of the main sources of heavy elements in the universe, such as gold and uranium.
- Accompanying a short gamma-ray burst, scientists expect to see a "kilonova" whose peak brightness typically reaches 1,000 times that of a classical nova. Kilonovae are an optical and infrared glow from the radioactive decay of heavy elements and are unique to the merger of two neutron stars, or the merger of a neutron star with a small black hole.
- Fong and her team have discussed several possibilities to explain the unusual brightness that Hubble saw. While most short gamma-ray bursts probably result in a black hole, the two neutron stars that merged in this case may have combined to form a magnetar, a supermassive neutron star with a very powerful magnetic field.
- "You basically have these magnetic field lines that are anchored to the star that are whipping around at about a thousand times a second, and this produces a magnetized wind," explained Laskar. "These spinning field lines extract the rotational energy of the neutron star formed in the merger, and deposit that energy into the ejecta from the blast, causing the material to glow even brighter."
Figure 16: These two images taken on May 26 and July 16, 2020, show the fading light of a kilonova located in a distant galaxy. The kilonova appears as a spot to the upper left of the host galaxy. The glow is prominent in the May 26 image but fades in the July 16 image. The kilonova's peak brightness reaches up to 10,000 times that of a classical nova. A merger of two neutron stars—the source of the kilonova—is believed to have produced a magnetar, which has an extremely powerful magnetic field. The energy from that magnetar brightened the material ejected from the explosion, causing it to become unusually bright at infrared wavelengths of light [video credits: NASA, ESA, W. Fong (Northwestern University), T. Laskar (University of Bath, UK) and A. Pagan (STScI)]
- If the extra brightness came from a magnetar that deposited energy into the kilonova material, then within a few years, the team expects the ejecta from the burst to produce light that shows up at radio wavelengths. Follow-up radio observations may ultimately prove that this was a magnetar, and this may explain the origin of such objects.
- "With its amazing sensitivity at near-infrared wavelengths, Hubble really sealed the deal with this burst," explained Fong. "Amazingly, Hubble was able to take an image only three days after the burst. Through a series of later images, Hubble showed that a source faded in the aftermath of the explosion. This is as opposed to being a static source that remains unchanged. With these observations, we knew we had not only nabbed the source, but we had also discovered something extremely bright and very unusual. Hubble's angular resolution was also key in pinpointing the position of the burst and precisely measuring the light coming from the merger."
- NASA's upcoming James Webb Space Telescope is particularly well-suited for this type of observation. "Webb will completely revolutionize the study of similar events," said Edo Berger of Harvard University in Cambridge, Massachusetts, and principal investigator of the Hubble program. "With its incredible infrared sensitivity, it will not only detect such emission at even larger distances, but it will also provide detailed spectroscopic information that will resolve the nature of the infrared emission."
- The team's findings appear in an upcoming issue of The Astrophysical Journal. 16)
• November 6, 2020: The galaxy UGCA 193, seen here by the NASA/ESA Hubble Space Telescope, is a galaxy in the constellation of Sextans (The Sextant). Looking rather like a waterfall, UGCA 193 appears to host many young stars, especially in its lower portion, creating a striking blue haze and the sense that the stars are falling from “above”. 17)
Figure 17: The blue color of UGCA 193 indicates the stars that we see are hot — some with temperatures exceeding 6 times that of our Sun. We know that cooler stars appear to our eyes more red, and hotter stars appear more blue. As the mass and surface temperature of a star, and therefore its color, are linked, heavier stars are able to “burn” at higher temperatures resulting in a blue glow from their surface (image credit: ESA/Hubble & NASA, R. Tully, CC BY 4.0)
• October 29,2020: Hubble Finds ‘Greater Pumpkin’ Galaxy Pair. In our infinite universe, if you can imagine something, you may eventually find it out there. And, that even goes for celestial objects that look like some creepy incarnation straight out of a Halloween tale. Hubble's holiday offering is a pair of colliding galaxies that resemble the cartoon Peanuts character Linus's imagining of the elusive Great Pumpkin. "Great" is an understatement in this case because the galaxy pair spans 100,000 light-years. The "pumpkin’s" glowing "eyes" are the bright, star-filled cores of each galaxy that contain supermassive black holes. An arm of newly forming stars embracing the pair gives the imaginary pumpkin a wry smirk. In about 6 billion years our Milky Way galaxy will collide with the neighboring Andromeda galaxy. When viewed from an extraterrestrial civilization far away, our collision may take on a spooky appearance too. That is, assuming they also have fertile imaginations for seeing ghostly entities among the stars. 18)
- Sorry Charlie Brown, NASA's Hubble Space Telescope is taking a peek at what might best be described as the "Greater Pumpkin," that looks like a Halloween decoration tucked away in a patch of sky cluttered with stars. What looks like two glowing eyes and a crooked carved smile is a snapshot of the early stages of a collision between two galaxies. The entire view is nearly 109,000 light-years across, approximately the diameter of our Milky Way.
- The overall pumpkin-ish color corresponds to the glow of aging red stars in two galaxies, cataloged as NGC 2292 and NGC 2293, which only have a hint of spiral structure. Yet the smile is bluish due to newborn star clusters, spread out like pearls on a necklace, along a newly forming dusty arm. The glowing eyes are concentrations of stars around a pair of supermassive black holes. The scattering of blue foreground stars makes the "pumpkin" look like it got all glittery for a Halloween party.
- What's going on in this pumpkin-like pair?
- If you mix two fried eggs together, you get something resembling scrambled eggs. The same goes for galaxy collisions throughout the universe. They lose their flattened spiral disk and the stars are scrambled into a football-shaped volume of space, forming an elliptical galaxy. But this interacting pair is a very rare example of what may turn out to result in a bigger fried egg—the construction of a giant spiral galaxy. It may depend on the specific trajectory the colliding galaxy pair is following. The encounter scenario must be rare because there's only a handful of other examples in the universe, say astronomers.
- The ghostly arm making the "smile" may be just the beginning of the process of rebuilding a spiral galaxy, say researchers. The arm embraces both galaxies. It most likely formed when interstellar gas was compressed as the two galaxies began to merge. The higher density precipitates new star formation.
- The dynamic duo hides out 120 million light-years away in the constellation Canis Major, so it is seen far behind the star-filled foreground plane of our Milky Way galaxy. Therefore, it's a difficult area to pinpoint far-flung distant background galaxies from the plethora of stars seen in the field.
Figure 18: This is a Hubble Space Telescope snapshot of the early stages of a collision between two galaxies that resembles a Halloween carved pumpkin. The "pumpkin's" glowing “eyes” are the bright, star-filled cores of each galaxy that contain supermassive black holes. An arm of newly forming stars give the imaginary pumpkin a wry smirk. The two galaxies, cataloged as NGC 2292 and NGC 2293, are located about 120 million light-years away in the constellation Canis Major [image credit: NASA/ESA, and W. Keel (University of Alabama)]
Figure 19: Halloween is scarier with Hubble! What looks like two glowing eyes and a crooked carved smile is a snapshot of the early stages of a collision between two galaxies. This new image is just one of several spooky views Hubble has captured in the universe (video credits: NASA's Goddard Space Flight Center)
- The galaxy pair was similar to objects tagged by the citizen-science project Galaxy Zoo, where volunteers go hunting for oddball-looking galaxies. Astronomer William Keel, of the University of Alabama in Tuscaloosa, included several of these in the "Gems of the Galaxy Zoos" Hubble program, which is observing several kinds of rare galaxies during short gaps between other scheduled Hubble observations. The Hubble image brought out new details of the close encounter.
- Keel speculates that the ultimate destiny for this pair will be to merge into a giant luminous spiral galaxy like UGC 2885, Rubin's Galaxy, which is over twice the diameter of our Milky Way. Hubble has caught a snapshot of the groundbreaking early stages of a galactic makeover.
• October 23, 2020: Hubble views a galactic waterfall. 19)
- Interacting galaxies, such as these, are so named because of the influence they have on each other, which may eventually result in a merger or a unique formation. Already, these two galaxies have seemingly formed a sideways waterspout, with stars from NGC 2799 appearing to fall into NGC 2798 almost like drops of water.
- Galactic mergers can take place over several hundred million to over a billion years. While one might think the merger of two galaxies would be catastrophic for the stellar systems within, the sheer amount of space between stars means that stellar collisions are unlikely and stars typically drift past each other.
Figure 20: In this spectacular image captured by the NASA/ESA Hubble Space Telescope, the galaxy NGC 2799 (on the left) is seemingly being pulled into the center of the galaxy NGC 2798 (on the right), [image credit: ESA/Hubble & NASA, SDSS, J. Dalcanton, CC BY 4.0; Acknowledgement: Judy Schmidt (Geckzilla)]
• October 16, 2020: When a massive new star starts to shine while still within the cool molecular cloud from which it formed, its energetic radiation can ionize the cloud’s hydrogen and create a large, hot bubble of ionized gas. Amazingly, located within this bubble of hot gas around a nearby massive star are the frEGGs (Free-floating Evaporating Gaseous Globules) : dark compact globules of dust and gas, some of which are giving birth to low-mass stars. The boundary between the cool, dusty frEGG and the hot gas bubble is seen as the glowing purple/blue edges in this fascinating image. 20)
Figure 21: This image, taken with the NASA/ESA Hubble Space Telescope, depicts a special class of star-forming nursery known as frEGGs (Free-floating Evaporating Gaseous Globules). This object is formally known as J025157.5+600606 (image credit: ESA/Hubble & NASA, R. Sahai; CC BY 4.0)
• 09 October 2020: At around 60 million light-years from Earth, the Great Barred Spiral Galaxy NGC 1365 is captured beautifully in this image by the NASA/ESA Hubble Space Telescope. Located in the constellation of Fornax (The Furnace), the blue and fiery orange swirls show us where stars have just formed and the dusty sites of future stellar nurseries. 21) 22)
- At the outer edge of the image, enormous star-forming regions within NGC 1365 can be seen. The bright, light-blue regions indicate the presence of hundreds of baby stars that formed from coalescing gas and dust within the galaxy's outer arms.
Figure 22: This Hubble image was captured as part of a joint survey with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The survey will help scientists understand how the diversity of galaxy environments observed in the nearby Universe, including NGC 1365 and previous ESA/Hubble Pictures of the Week such as NGC 2835 and NGC 2775, influence the formation of stars and star clusters. Expected to image over 100,000 gas clouds and star-forming regions beyond our Milky Way, the PHANGS survey is expected to uncover and clarify many of the links between cold gas clouds, star formation and the overall shape and morphology of galaxies [image credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team; CC BY 4.0 - Acknowledgement: Judy Schmidt (Geckzilla)]
• 02 October 2020: NGC 5643 is about 60 million light-years away from Earth and has been the host of a recent supernova event (not visible in this latest image). This supernova (2017cbv) was a specific type in which a white dwarf steals so much mass from a companion star that it becomes unstable and explodes. The explosion releases significant amounts of energy and lights up that part of the galaxy. 23)
- The observation was proposed by Adam Riess, who was awarded a Nobel Laureate in physics 2011 for his contributions to the discovery of the accelerating expansion of the Universe, alongside Saul Perlmutter and Brian Schmidt.
Figure 23: This stunning image by the NASA/ESA Hubble Space Telescope features the spiral galaxy NGC 5643 in the constellation of Lupus (The Wolf). Looking this good isn’t easy; thirty different exposures, for a total of 9 hours observation time, together with the high resolution and clarity of Hubble, were needed to produce an image of such high level of detail and of beauty (image credit: ESA/Hubble & NASA, A. Riess et al.; CC BY 4.0; Acknowledgement: Mahdi Zamani)
• 01 October 2020: When a star unleashes as much energy in a matter of days as our Sun does in several billion years, you know it's not going to remain visible for long. 24)
- Like intergalactic paparazzi, NASA's Hubble Space Telescope captured the quick, fading celebrity status of a supernova, the self-detonation of a star. The Hubble snapshots have been assembled into a telling movie of the titanic stellar blast disappearing into oblivion in the spiral galaxy NGC 2525, located 70 million light-years away.
Figure 24: This video zooms into the barred spiral galaxy NGC 2525, located 70 million light-years away in the southern constellation Puppis. Roughly half the diameter of our Milky Way, it was discovered by British astronomer William Herschel in 1791 as a "spiral nebula." The sharpness of the image increases as we zoom into the Hubble view. As we approach an outer spiral arm a Hubble time-lapse video is inserted that shows the fading light of supernova 2018gv. Hubble didn't record the initial blast in January 2018, but for nearly one year took consecutive photos, from 2018 to 2019, that have been assembled into a time-lapse sequence. At its peak, the exploding star was as bright as 5 billion Suns [video credits: NASA, ESA, J. DePasquale (STScI), M. Kornmesser and M. Zamani (ESA/Hubble), A. Riess (STScI/JHU) and the SH0ES team, and the Digitized Sky Survey]
- Hubble began observing SN 2018gv in February 2018, after the supernova was first detected by amateur astronomer Koichi Itagaki a few weeks earlier in mid-January. Hubble astronomers were using the supernova as part of a program to precisely measure the expansion rate of the universe — a key value in understanding the physical underpinnings of the cosmos. The supernova serves as a milepost marker to measure galaxy distances, a fundamental value needed for measuring the expansion of space.
- In the time-lapse sequence, spanning nearly a year, the supernova first appears as a blazing star located on the galaxy's outer edge. It initially outshines the brightest stars in the galaxy before fading out of sight.
- "No Earthly fireworks display can compete with this supernova, captured in its fading glory by the Hubble Space Telescope," said Nobel laureate Adam Riess, of the Space Telescope Science Institute (STScI) and Johns Hopkins University in Baltimore, leader of the High-z Supernova Search Team and the Supernovae H0 for the Equation of State (SH0ES) Team to measure the universe's expansion rate.
- The type of supernova seen in this sequence originated from a burned-out star — a white dwarf located in a close binary system — that is accreting material from its companion star. When the white dwarf reaches a critical mass, its core becomes hot enough to ignite nuclear fusion, turning it into a giant atomic bomb. This thermonuclear runaway process tears the dwarf apart. The opulence is short-lived as the fireball fades away.
- Because supernovae of this type all peak at the same brightness, they are known as "standard candles," which act as cosmic tape measures. Knowing the actual brightness of the supernova and observing its brightness in the sky, astronomers can calculate the distances of their host galaxies. This allows astronomers to measure the expansion rate of the universe. Over the past 30 years Hubble has helped dramatically improve the precision of the universe's expansion rate.
Figure 25: Astronomers using NASA's Hubble Space Telescope captured the quick, fading celebrity status of a supernova, the self-detonation of a star. The supernova, called SN 2018gv, appears in the lower left portion of the frame as a blazing star located on the outer edge of spiral galaxy NGC 2525, located 70 million light-years away [image credits: NASA, ESA, and A. Riess (STScI/JHU) and the SH0ES team; acknowledgment: M. Zamani (ESA/Hubble)]
Figure 26: The Hubble snapshots have been assembled into a telling movie of the titanic stellar blast disappearing into oblivion in the spiral galaxy NGC 2525, located 70 million light-years away. The supernova, named SN 2018gv, appears as a blazing star located on the galaxy's outer edge. It initially outshines the brightest stars in the galaxy before fading out of sight. The time-lapse video consists of observations taken from February 2018 to February 2019 [image credits: NASA, ESA, and A. Riess (STScI/JHU) and the SH0ES team; acknowledgment: M. Zamani (ESA/Hubble)]
• 25 September 2020: Resting on the tail of the Great Bear in the constellation of Ursa Major, lies NGC 5585, a spiral galaxy that is more than it appears. 25)
- The stellar disc of the galaxy extends over 35,000 light-years across. When compared with galaxies of a similar shape and size, NGC 5585 stands out by having a notably different composition: Contributing to the total mass of the galaxy, it contains a far higher proportion of dark matter.
- Hotspots of star formation can be seen along the galaxy’s faint spiral arms. These regions shine a brilliant blue, contrasting strikingly against the ever-black background of space.
Figure 27: The many stars, and dust and gas clouds that make up NGC 5585, shown here in this Hubble image, contribute only a small fraction of the total mass of the galaxy. As in many galaxies, this discrepancy can be explained by the abundant yet seemingly invisible presence of dark matter (image credit: ESA/Hubble & NASA, R. Tully; CC BY 4.0 Acknowledgement: Gagandeep Anand)
• 18 September 2020: The twisting patterns created by the multiple spiral arms of NGC 2835 create the illusion of an eye. This is a fitting description, as this magnificent galaxy resides near the head of the southern constellation of Hydra, the water snake. This stunning barred spiral galaxy, with a width of just over half that of the Milky Way, is brilliantly featured in this image taken by the NASA/ESA Hubble Space Telescope. Although it cannot be seen in this image, a supermassive black hole with a mass millions of times that of our Sun is known to nestle in the very centre of NGC 2835. 26) 27)
- Note: PHANGS (Physics at High Angular resolution in Nearby GalaxieS) is the principal ALMA Large Program for nearby galaxies, which has obtained CO (2-1) maps for a complete sample of 74 spiral galaxies. PHANGS-HST will build the first astronomical dataset charting the connections between young stars and cold molecular gas throughout a diversity of galactic environments by imaging the 38 galaxies from PHANGS sample best suited for study of resolved stars, stellar associations, and star clusters. 28)
- Expected to image over 100,000 gas clouds and star-forming regions outside our Milky Way, this survey hopes to uncover and clarify many of the links between cold gas clouds, star formation, and the overall shape and morphology of galaxies. This initiative is a collaboration with the international Atacama Large Millimeter/submillimeter Array (ALMA) and the European Southern Observatory's Very Large Telescope's MUSE instrument, through the greater PHANGS program.
Figure 28: This galaxy was imaged as part of PHANGS-HST, a large galaxy survey with Hubble that aims to study the connections between cold gas and young stars in a variety of galaxies in the local Universe. Within NGC 2835, this cold, dense gas produces large numbers of young stars within large star formation regions. The bright blue areas, commonly observed in the outer spiral arms of many galaxies, show where near-ultraviolet light is being emitted more strongly , indicating recent or ongoing star formation (image credit: ESA/Hubble & NASA, J. Lee, and the PHANGS-HST Team Acknowledgement: Judy Schmidt (Geckzilla); CC BY 4.0)
• 17 September 2020: A unique and exciting detail of Hubble’s new snapshot appears at mid-northern latitudes as a bright, white, stretched-out storm moving at 560 km/hr. This single plume erupted on 18 August 2020 and another has since appeared. 29) 30)
- While it’s common for storms to pop up in this region, often several at once, this particular disturbance appears to have more structure behind it than observed in previous storms. Trailing behind the plume are small, counterclockwise dark clumps also not witnessed in the past. Researchers speculate this may be the beginning of a longer-lasting northern hemisphere spot, perhaps to rival the legendary Great Red Spot that dominates the southern hemisphere.
- Hubble shows that the Great Red Spot, rolling counterclockwise in the planet’s southern hemisphere, is ploughing into the clouds ahead of it, forming a cascade of white and beige ribbons. The Great Red Spot is currently an exceptionally rich red color, with its core and outermost band appearing deeper red.
- Researchers say the Great Red Spot now measures about 15,800 km across, big enough to swallow the Earth. The super-storm is still shrinking, as noted in telescopic observations dating back to 1930, but its rate of shrinkage appears to have slowed. The reason for its dwindling size is a complete mystery.
- Researchers are noticing that another feature has changed: the Oval BA, nicknamed by astronomers as Red Spot Jr., which appears just below the Great Red Spot in this image. For the past few years, Red Spot Jr. has been fading in color to its original shade of white after appearing red in 2006. However, now the core of this storm appears to be darkening to a reddish hue. This could hint that Red Spot Jr. is on its way to reverting to a color more similar to that of its cousin.
- Hubble’s image shows that Jupiter is clearing out its higher-altitude white clouds, especially along the planet’s equator, which is enveloped in an orangish hydrocarbon smog.
- Jupiter’s icy moon Europa is visible to the left of the gas giant. Europa is already thought to harbor a liquid ocean beneath its icy crust, making this moon one of the main targets in the search for habitable worlds beyond Earth. In 2013 it was announced that the Hubble Space Telescope discovered water vapor erupting from the frigid surface of Europa, in one or more localized plumes near its south pole. ESA's JUpiter ICy moons Explorer (JUICE), a mission planned for launch in 2022, aims to explore both Jupiter and three of its largest moons: Ganymede, Callisto, and Europa.
- Hubble also captured a new multiwavelength observation in ultraviolet/visible/near-infrared light of Jupiter on 25 August 2020, which is giving researchers an entirely new view of the giant planet. Hubble’s near infrared imaging, combined with ultraviolet views, provides a unique panchromatic look that offers insights into the altitude and distribution of the planet’s haze and particles. This complements Hubble’s visible-light picture that shows the ever-changing cloud patterns.
- These new Hubble images form part of yearly maps of the entire planet taken under the OPAL (Outer Planets Atmospheres Legacy) program. The program provides yearly Hubble global views of the outer planets to look for changes in their storms, winds, and clouds.
Figure 29: This latest image of Jupiter, taken by the NASA/ESA Hubble Space Telescope on 25 August 2020, was captured when the planet was 653 million km from Earth. Hubble’s sharp view is giving researchers an updated weather report on the monster planet’s turbulent atmosphere, including a remarkable new storm brewing, and a cousin of the Great Red Spot changing color — again. The new image also features Jupiter’s icy moon Europa (image credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M. H. Wong (University of California, Berkeley) and the OPAL team; CC BY 4.0)
• 11 September 2020: Many colorful stars are packed close together in this image of the globular cluster NGC 1805, taken by the NASA/ESA Hubble Space Telescope. This tight grouping of thousands of stars is located near the edge of the Large Magellanic Cloud, a satellite galaxy of our own Milky Way. The stars orbit closely to one another, like bees swarming around a hive. In the dense centre of one of these clusters, stars are 100 to 1000 times closer together than the nearest stars are to our Sun, making planetary systems around them unlikely. 31)
- This young globular cluster can be seen from the southern hemisphere, in the Dorado constellation, which is Portuguese for dolphinfish. Usually, globular clusters contain stars which are born at the same time; however, NGC 1805 is unusual as it appears to host two different populations of stars with ages millions of years apart. Observing such clusters of stars can help astronomers understand how stars evolve, and what factors determine whether they end their lives as white dwarfs, or explode as supernovae.
Figure 30: The striking difference in star colors is illustrated beautifully in this image, which combines two different types of light: blue stars, shining brightest in near-ultraviolet light, and red stars, illuminated in red and near-infrared. Space telescopes like Hubble can observe in the ultraviolet because they are positioned above Earth’s atmosphere, which absorbs most of this wavelength, making it inaccessible to ground-based facilities (image credit: ESA/Hubble & NASA, J. Kalirai; CC BY 4.0)
• 10 September 2020: Astronomers have discovered that there may be a missing ingredient in our cosmic recipe of how dark matter behaves. They have uncovered a discrepancy between the theoretical models of how dark matter should be distributed in galaxy clusters, and observations of dark matter's grip on clusters. 32)
Figure 31: Astronomers seem to have revealed a puzzling detail in the way dark matter behaves. They found small, dense concentrations of dark matter that bend and magnify light much more strongly than expected (video credit: NASA's Goddard Space Flight Center)
- Dark matter does not emit, absorb, or reflect light. Its presence is only known through its gravitational pull on visible matter in space. Therefore, dark matter remains as elusive as Alice in Wonderland's Cheshire Cat – where you only see its grin (in the form of gravity) but not the animal itself.
- One way astronomers can detect dark matter is by measuring how its gravity distorts space, an effect called gravitational lensing.
- Researchers found that small-scale concentrations of dark matter in clusters produce gravitational lensing effects that are 10 times stronger than expected. This evidence is based on unprecedentedly detailed observations of several massive galaxy clusters by NASA's Hubble Space Telescope and the European Southern Observatory's Very Large Telescope (VLT) in Chile.
- Galaxy clusters, the most massive structures in the universe composed of individual member galaxies, are the largest repositories of dark matter. Not only are they held together largely by dark matter's gravity, the individual cluster galaxies are themselves replete with dark matter. Dark matter in clusters is therefore distributed on both large and small scales.
- "Galaxy clusters are ideal laboratories to understand if computer simulations of the universe reliably reproduce what we can infer about dark matter and its interplay with luminous matter," said Massimo Meneghetti of the INAF (National Institute for Astrophysics)-Observatory of Astrophysics and Space Science of Bologna in Italy, the study's lead author.
- "We have done a lot of careful testing in comparing the simulations and data in this study, and our finding of the mismatch persists," Meneghetti continued. "One possible origin for this discrepancy is that we may be missing some key physics in the simulations."
- Priyamvada Natarajan of Yale University in New Haven, Connecticut, one of the senior theorists on the team, added, "There's a feature of the real universe that we are simply not capturing in our current theoretical models. This could signal a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data have permitted us to probe the detailed distribution of dark matter on the smallest scales."
- The team's paper is in the September 11 issue of the journal Science. 33)
- The distribution of dark matter in clusters is mapped via the bending of light, or the gravitational lensing effect, they produce. The gravity of dark matter magnifies and warps light from distant background objects, much like a funhouse mirror, producing distortions and sometimes multiple images of the same distant galaxy. The higher the concentration of dark matter in a cluster, the more dramatic its light bending.
- Hubble's crisp images, coupled with spectra from the VLT, helped the team produce an accurate, high-fidelity dark-matter map. They identified dozens of multiply imaged, lensed, background galaxies. By measuring the lensing distortions, astronomers could trace out the amount and distribution of dark matter.
- The three key galaxy clusters used in the analysis, MACS J1206.2-0847, MACS J0416.1-2403, and Abell S1063, were part of two Hubble surveys: The Frontier Fields and the Cluster Lensing And Supernova survey with Hubble (CLASH) programs.
- To the team's surprise, the Hubble images also revealed smaller-scale arcs and distorted images nested within the larger-scale lens distortions in each cluster's core, where the most massive galaxies reside.
- The researchers believe that the embedded lenses are produced by the gravity of dense concentrations of dark matter associated with individual cluster galaxies. Dark matter's distribution in the inner regions of individual galaxies is known to enhance the cluster's overall lensing effect.
Figure 32: This Hubble Space Telescope image shows the massive galaxy cluster MACS J1206. Embedded within the cluster are the distorted images of distant background galaxies, seen as arcs and smeared features. These distortions are caused by the amount of dark matter in the cluster, whose gravity bends and magnifies the light from faraway galaxies. This effect, called gravitational lensing, allows astronomers to study remote galaxies that would otherwise be too faint to see. Several of the cluster galaxies are sufficiently massive and dense to also distort and magnify faraway sources. The galaxies in the three pullouts represent examples of such effects. In the snapshots at upper right and bottom, two distant, blue galaxies are lensed by the foreground, redder cluster galaxies, forming rings and multiple images of the remote objects. The red blobs around the galaxy at upper left denote emission from clouds of hydrogen in a single distant source. The source, seen four times because of lensing, may be a faint galaxy. These blobs were detected by the Multi-Unit Spectroscopic Explorer (MUSE) at the European Southern Observatory's Very Large Telescope (VLT) in Chile. The blobs do not appear in the Hubble images. MACS J1206 is part of the Cluster Lensing And Supernova survey with Hubble (CLASH) and is one of three galaxy clusters the researchers studied with Hubble and the VLT. The Hubble image is a combination of visible- and infrared-light observations taken in 2011 by the Advanced Camera for Surveys and Wide Field Camera 3 (image credits: NASA, ESA, P. Natarajan (Yale University), G. Caminha (University of Groningen), M. Meneghetti (INAF-Observatory of Astrophysics and Space Science of Bologna), the CLASH-VLT/Zooming teams; acknowledgment: NASA, ESA, M. Postman (STScI), the CLASH team)
- Follow-up spectroscopic observations added to the study by measuring the velocity of the stars orbiting inside several of the cluster galaxies. "Based on our spectroscopic study, we were able to associate the galaxies with each cluster and estimate their distances," said team member Piero Rosati of the University of Ferrara in Italy.
- "The stars' speed gave us an estimate of each individual galaxy's mass, including the amount of dark matter," added team member Pietro Bergamini of the INAF-Observatory of Astrophysics and Space Science in Bologna, Italy.
- The team compared the dark-matter maps with samples of simulated galaxy clusters with similar masses, located at roughly the same distances as the observed clusters. The clusters in the computer simulations did not show the same level of dark-matter concentration on the smallest scales – the scales associated with individual cluster galaxies as seen in the universe.
- The team looks forward to continuing their stress-testing of the standard dark-matter model to pin down its intriguing nature.
- The team looks forward to continuing their stress-testing of the standard dark-matter model to pin down its intriguing nature.
- 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 conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
• 04 September 2020: The blue and orange stars of the faint galaxy named NGC 2188 sparkle in this image taken with the NASA/ESA Hubble Space Telescope. Although NGC 2188 appears at first glance to consist solely of a narrow band of stars, it is classified by astronomers as a barred-spiral galaxy. It appears this way from our viewpoint on Earth as the center and spiral arms of the galaxy are tilted away from us, with only the very narrow outer edge of the galaxy’s disc visible to us. Astronomers liken this occurrence to turning a dinner plate in your hands so you see only its outer edge. The true shape of the galaxy was identified by studying the distribution of the stars in the inner central bulge and outer disc and by observing the stars’ colors. 34)
Figure 33: NGC 2188 is estimated to be just half the size of our Milky Way, at 50,000 light-years across, and it is situated in the northern hemisphere constellation of Columba (The Dove). Named in the late 1500s after Noah’s dove in biblical stories, the small constellation consists of many faint yet beautiful stars and astronomical objects (image credit: ESA/Hubble & NASA, R. Tully; CC BY 4.0)
• 28 August 2020: The original supernova explosion blasted apart a dying star about 20 times more massive than our Sun between 10,000 and 20,000 years ago. Since then, the remnant has expanded 60 light-years from its center. The shockwave marks the outer edge of the supernova remnant and continues to expand at around 350 km/s. The interaction of the ejected material and the low-density interstellar material swept up by the shockwave forms the distinctive veil-like structure seen in this image. 35)
Figure 34: While appearing as a delicate and light veil draped across the sky, this image from the NASA/ESA Hubble Space Telescope actually depicts a small section of the Cygnus supernova blast wave, located around 2400 light-years away. The name of the supernova remnant comes from its position in the northern constellation of Cygnus (The Swan), where it covers an area 36 times larger than the full moon (image credit: SA/Hubble & NASA, W. Blair; CC BY 4.0; Acknowledgement: Leo Shatz)
• 27 August 2020: In a landmark study, scientists using NASA’s Hubble Space Telescope have mapped the immense envelope of gas, called a halo, surrounding the Andromeda galaxy, our nearest large galactic neighbor. Scientists were surprised to find that this tenuous, nearly invisible halo of diffuse plasma extends 1.3 million light-years from the galaxy—about halfway to our Milky Way—and as far as 2 million light-years in some directions. This means that Andromeda’s halo is already bumping into the halo of our own galaxy. 36)
- They also found that the halo has a layered structure, with two main nested and distinct shells of gas. This is the most comprehensive study of a halo surrounding a galaxy.
- “Understanding the huge halos of gas surrounding galaxies is immensely important,” explained co-investigator Samantha Berek of Yale University in New Haven, Connecticut. “This reservoir of gas contains fuel for future star formation within the galaxy, as well as outflows from events such as supernovae. It’s full of clues regarding the past and future evolution of the galaxy, and we’re finally able to study it in great detail in our closest galactic neighbor.”
- “We find the inner shell that extends to about a half million light-years is far more complex and dynamic,” explained study leader Nicolas Lehner of the University of Notre Dame in Indiana. “The outer shell is smoother and hotter. This difference is a likely result from the impact of supernova activity in the galaxy’s disk more directly affecting the inner halo.”
- A signature of this activity is the team’s discovery of a large amount of heavy elements in the gaseous halo of Andromeda. Heavier elements are cooked up in the interiors of stars and then ejected into space—sometimes violently as a star dies. The halo is then contaminated with this material from stellar explosions.
- The Andromeda galaxy, also known as M31, is a majestic spiral of perhaps as many as 1 trillion stars and comparable in size to our Milky Way. At a distance of 2.5 million light-years, it is so close to us that the galaxy appears as a cigar-shaped smudge of light high in the autumn sky. If its gaseous halo could be viewed with the naked eye, it would be about three times the width of the Big Dipper. This would easily be the biggest feature on the nighttime sky.
- Through a program called Project AMIGA (Absorption Map of Ionized Gas in Andromeda), the study examined the light from 43 quasars—the very distant, brilliant cores of active galaxies powered by black holes—located far beyond Andromeda. The quasars are scattered behind the halo, allowing scientists to probe multiple regions. Looking through the halo at the quasars’ light, the team observed how this light is absorbed by the Andromeda halo and how that absorption changes in different regions. The immense Andromeda halo is made of very rarified and ionized gas that doesn’t emit radiation that is easily detectable. Therefore, tracing the absorption of light coming from a background source is a better way to probe this material.
- The researchers used the unique capability of Hubble’s Cosmic Origins Spectrograph (COS) to study the ultraviolet light from the quasars. Ultraviolet light is absorbed by Earth’s atmosphere, which makes it impossible to observe with ground-based telescopes. The team used COS to detect ionized gas from carbon, silicon, and oxygen. An atom becomes ionized when radiation strips one or more electrons from it.
Figure 35: This illustration shows the location of the 43 quasars scientists used to probe Andromeda’s gaseous halo. These quasars—the very distant, brilliant cores of active galaxies powered by black holes—are scattered far behind the halo, allowing scientists to probe multiple regions. Looking through the immense halo at the quasars’ light, the team observed how this light is absorbed by the halo and how that absorption changes in different regions. By tracing the absorption of light coming from the background quasars, scientists are able to probe the halo’s material [image credits: NASA, ESA, and E. Wheatley (STScI)]
- Andromeda’s halo has been probed before by Lehner’s team. In 2015, they discovered that the Andromeda halo is large and massive. But there was little hint of its complexity; now, it’s mapped out in more detail, leading to its size and mass being far more accurately determined.
- “Previously, there was very little information—only six quasars—within 1 million light-years of the galaxy. This new program provides much more information on this inner region of Andromeda’s halo,” explained co-investigator J. Christopher Howk, also of Notre Dame. “Probing gas within this radius is important, as it represents something of a gravitational sphere of influence for Andromeda.”
- Because we live inside the Milky Way, scientists cannot easily interpret the signature of our own galaxy’s halo. However, they believe the halos of Andromeda and the Milky Way must be very similar since these two galaxies are quite similar. The two galaxies are on a collision course, and will merge to form a giant elliptical galaxy beginning about 4 billion years from now.
- Scientists have studied gaseous halos of more distant galaxies, but those galaxies are much smaller on the sky, meaning the number of bright enough background quasars to probe their halo is usually only one per galaxy. Spatial information is therefore essentially lost. With its close proximity to Earth, the gaseous halo of Andromeda looms large on the sky, allowing for a far more extensive sampling.
- “This is truly a unique experiment because only with Andromeda do we have information on its halo along not only one or two sightlines, but over 40,” explained Lehner. “This is groundbreaking for capturing the complexity of a galaxy halo beyond our own Milky Way.”
- In fact, Andromeda is the only galaxy in the universe for which this experiment can be done now, and only with Hubble. Only with an ultraviolet-sensitive future space telescope will scientists be able to routinely undertake this type of experiment beyond the approximately 30 galaxies comprising the Local Group.
- “So Project AMIGA has also given us a glimpse of the future,” said Lehner.
- The team’s findings appear in the Aug. 27 edition of The Astrophysical Journal. 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 conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
Figure 36: This illustration depicts the gaseous halo of the Andromeda galaxy if it could be seen with the naked eye. At a distance of 2.5 million light-years, the majestic spiral Andromeda galaxy is so close to us that it appears as a cigar-shaped smudge of light high in the autumn sky. If its gaseous halo could be seen with the naked eye, it would be about three times the width of the Big Dipper—easily the biggest feature on the nighttime sky [image credits: NASA, ESA, J. DePasquale and E. Wheatley (STScI), and Z. Levay (background image)]
• 21 August 2020: Comet NEOWISE is the brightest comet visible from the Northern Hemisphere since 1997’s Hale-Bopp comet. It’s estimated to be travelling at over 60 km/s. The comet’s closest approach to the Sun was on 3 July and it’s now heading back to the outer reaches of the Solar System, not to pass through our neighborhood again for another 7000 years. 38)
- Hubble’s observation of NEOWISE is the first time a comet of this brightness has been photographed at such high resolution after its pass by the Sun. Earlier attempts to photograph other bright comets (such as comet ATLAS) proved unsuccessful as they disintegrated in the searing heat.
- Comets often break apart due to thermal and gravitational stresses at such close encounters, but Hubble's view suggests that NEOWISE's solid nucleus stayed intact. This heart of the comet is too small to be seen directly by Hubble. The ball of ice may be no more than 4.8 km across. But the Hubble image does captures a portion of the vast cloud of gas and dust enveloping the nucleus, which measures about 18,000 km across in this image.
- Hubble's observation also resolves a pair of jets from the nucleus shooting out in opposite directions. They emerge from the comet's core as cones of dust and gas, and then are curved into broader fan-like structures by the rotation of the nucleus. Jets are the result of ice sublimating beneath the surface with the resulting dust/gas being squeezed out at high velocity.
- The Hubble photos may also help reveal the color of the comet’s dust and how that color changes as the comet moves away from the Sun. This, in turn, may explain how solar heat affects the contents and structure of that dust and the comet’s coma. The ultimate goal here would be to determine the original properties of the dust. Researchers who used Hubble to observe the comet are currently delving further into the data to see what they’re able to find.
- Hubble has captured other well-known comet visitors throughout the past year. This includes snapping images of the breakup of comet ATLAS in April 2020 and impressive images of the interstellar comet 2I BORISOV in October 2019 and December 2019.
Figure 37: The NASA/ESA Hubble Space Telescope has captured the closest images yet of the sky’s latest visitor to make the headlines, comet C/2020 F3 NEOWISE, after it passed by the Sun. This color image of the comet was taken on 8 August 2020. The two structures appearing on the left and right sides of the comet's center are jets of sublimating ice from beneath the surface of the nucleus, with the resulting dust and gas bring squeezed through at a high velocity. The jets emerge as cone-like structures, then are fanned out by the rotation of the comet's nucleus [image credit: NASA, ESA, Q. Zhang (California Institute of Technology), A. Pagan (STScI)]
• 21 August 2020: This galaxy was host to a supernova explosion, known as SN2015F, that was created by a white dwarf star. The white dwarf was part of a binary star system and syphoned mass from its companion, eventually becoming too greedy and taking on more than it could handle. This unbalanced the star and triggered runaway nuclear fusion that eventually led to an intensely violent supernova explosion. 39)
- SN2015F was spotted in March 2015 in the galaxy named NGC 2442, nicknamed the Meathook Galaxy owing to its extremely asymmetrical and irregular shape. The supernova shone brightly for quite some time and was easily visible from Earth through even a small telescope until later that summer.
Figure 38: This image from the NASA/ESA Hubble Space Telescope features the spectacular galaxy NGC 2442 (image credit: ESA/Hubble & NASA, S. Smartt et al.; CC BY 4.0)
• 14 August 2020: NGC 1614 is the result of a past galactic merger which created its peculiar appearance. The cosmic collision also drove a turbulent flow of interstellar gas from the smaller of the two galaxies involved into the nucleus of the larger one, resulting in a burst of star formation which started in the core and slowly spread outwards through the galaxy. 40)
- Owing to its turbulent past and its current appearance, astronomers classify NGC 1614 as a peculiar galaxy, a starburst galaxy, and a luminous infrared galaxy. Luminous infrared galaxies are among the most luminous objects in the local Universe — and NGC 1614 is, in fact, the second most luminous galaxy within 250 million light-years.
Figure 39: NGC 1614 is the result of a past galactic merger which created its peculiar appearance. The cosmic collision also drove a turbulent flow of interstellar gas from the smaller of the two galaxies involved into the nucleus of the larger one, resulting in a burst of star formation which started in the core and slowly spread outwards through the galaxy (image credit: ESA/Hubble & NASA, A. Adamo; CC BY 4.0)
• 13 August 2020: New observations by the NASA/ESA Hubble Space Telescope suggest that the unexpected dimming of the supergiant star Betelgeuse was most likely caused by an immense amount of hot material that was ejected into space, forming a dust cloud that blocked starlight coming from the star’s surface. 41)
- Betelgeuse is an aging, red supergiant star that has swelled in size as a result of complex, evolving changes in the nuclear fusion processes in its core. The star is so large that if it replaced the Sun at the center of our Solar System, its outer surface would extend past the orbit of Jupiter. The unprecedented phenomenon of Betelgeuse’s great dimming, eventually noticeable to even the naked eye, began in October 2019. By mid-February 2020, the brightness of this monster star had dropped by more than a factor of three.
- This sudden dimming has mystified astronomers, who sought to develop theories to account for the abrupt change. Thanks to new Hubble observations , a team of researchers now suggest that a dust cloud formed when superhot plasma was unleashed from an upwelling of a large convection cell on the star’s surface and passed through the hot atmosphere to the colder outer layers, where it cooled and formed dust. The resulting cloud blocked light from about a quarter of the star’s surface, beginning in late 2019. By April 2020, the star had returned to its normal brightness.
- Several months of Hubble’s ultraviolet-light spectroscopic observations of Betelgeuse, beginning in January 2019, produced an insightful timeline leading up to the star’s dimming. These observations provided important new clues to the mechanism behind the dimming. Hubble saw dense, heated material moving through the star’s atmosphere in September, October, and November 2019. Then, in December, several ground-based telescopes observed the star decreasing in brightness in its southern hemisphere.
- “With Hubble, we see the material as it left the star’s visible surface and moved out through the atmosphere, before the dust formed that caused the star appear to dim,” said lead researcher Andrea Dupree, associate director of The Center for Astrophysics | Harvard & Smithsonian. “We could see the effect of a dense, hot region in the southeast part of the star moving outward.”
- “This material was two to four times more luminous than the star’s normal brightness,” she continued. “And then, about a month later, the southern hemisphere of Betelgeuse dimmed conspicuously as the star grew fainter. We think it is possible that a dark cloud resulted from the outflow that Hubble detected. Only Hubble gives us this evidence of what led up to the dimming.”
- The team began using Hubble early last year to analyze the massive star. Their observations are part of a three-year Hubble study to monitor variations in the star’s outer atmosphere. The telescope’s sensitivity to ultraviolet light allowed researchers to probe the layers above the star’s surface, which are so hot that they emit mostly in the ultraviolet region of the spectrum and are not seen in visible light. These layers are heated partly by the star’s turbulent convection cells bubbling up to the surface.
- “Spatially resolving a stellar surface is only possible in favorable cases and only with the best available equipment,” said Klaus Strassmeier of the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany. “In that respect, Betelgeuse and Hubble are made for each other.”
- Hubble spectra, taken in early and late 2019 and in 2020, probed the star’s outer atmosphere by measuring spectral lines of ionized magnesium. From September to November 2019, the researchers measured material passing from the star’s surface into its outer atmosphere. This hot, dense material continued to travel beyond Betelgeuse’s visible surface, reaching millions of kilometers from the star. At that distance, the material cooled down enough to form dust, the researchers said.
Figure 40: Betelgeuse’s Dust Cloud. This artist's impression was generated using an image of Betelgeuse from late 2019 taken with the SPHERE instrument on the European Southern Observatory's Very Large Telescope (image credit: ESO, ESA/Hubble, M. Kornmesser)
- This interpretation is consistent with Hubble ultraviolet-light observations in February 2020, which showed that the behavior of the star’s outer atmosphere returned to normal, even though in visible light it was still dimming.
- Although Dupree does not know the cause of the outburst, she thinks it was aided by the star’s pulsation cycle, which continued normally though the event, as recorded by visible-light observations. Strassmeier used an automated telescope of the Leibniz Institute for Astrophysics called STELLar Activity (STELLA) to measure changes in the velocity of the gas on the star’s surface as it rose and fell during the pulsation cycle. The star was expanding in its cycle at the same time as the convective cell was upwelling. The pulsation rippling outward from Betelgeuse may have helped propel the outflowing plasma through the atmosphere.
- The red supergiant is destined to end its life in a supernova blast and some astronomers think the sudden dimming may be a pre-supernova event. The star is relatively nearby, about 725 light-years away, so the dimming event would have happened around the year 1300, as its light is just reaching Earth now.
- Dupree and her collaborators will get another chance to observe the star with Hubble in late August or early September. Right now, Betelgeuse is in the daytime sky, too close to the Sun for Hubble observations. 42)
• 07 August 2020: The barred spiral galaxy known as NGC 4907 shows its best side from 270 million light-years away to anyone who can see it from the northern hemisphere. This is a new image from the NASA/ESA Hubble Space Telescope of the face-on the galaxy, displaying its beautiful spiral arms, wound loosely around its central bright bar of stars. 43)
- NGC 4907 is also part of the Coma Cluster, a group of over 1000 galaxies, some of which can be seen around NGC 4907 in this image. This massive cluster of galaxies lies within the constellation of Coma Berenices, which is named for the locks of Queen Berenice II of Egypt: the only constellation named after a historical person.
Figure 41: Shining brightly below the galaxy is a star that is actually within our own Milky Way galaxy. This star appears much brighter than the many millions of stars in NGC 4907 as it is 100,000 times closer, residing only 2500 light-years away (image credit: ESA/Hubble & NASA, M. Gregg; CC BY 4.0)
• 06 August 2020: Astronauts who have gazed at Earth from space have been awestruck at our blue marble planet's majesty and diversity. Mike Massimino, who helped service the Hubble Space Telescope in orbit, said, "I think of our planet as a paradise. We are very lucky to be here." 44)
- What's mind-blowing is that astronomers estimate there could be as many as 1 billion other planets like Earth in our Milky Way galaxy alone. Just imagine, one billion – not million – other "paradise planets." But it's paradise lost if nothing is living there to marvel at sunsets in azure blue skies. And, as 19th century philosopher Thomas Carlyle mused, "... what a waste of space."
- It is sobering that our home planet is the only known place in the universe where life as we know it exists and thrives. And so, we gaze outward to the stars, imprisoned by space and time, into a cosmic loneliness. That's why scientists are dedicated to building ever-larger telescopes to search for potentially habitable planets. But how will they know life is present without traveling there and watching creatures walk, fly, or slither around?
- One way is by probing a planet's atmosphere. An atmosphere with the right mix of chemical elements is necessary to nurture and sustain life. Earth's atmosphere includes oxygen, nitrogen, methane, and carbon dioxide that have helped support life for billions of years. Earth's abundance of oxygen, especially, is a clue that our atmosphere's oxygen content is being replenished by biological processes.
- Astronomers have been using a variety of ground- and space-based telescopes to analyze how the ingredients of Earth's atmosphere look from space, using our planet as a proxy for studying extrasolar planets' atmospheres. They hope to eventually compare Earth's atmospheric composition with those of other worlds to note similarities and differences. Taking advantage of a total lunar eclipse, astronomers using the Hubble telescope have detected ozone in Earth's atmosphere by looking at Earthlight reflected off the Moon. Our Moon came in handy as a giant mirror in space.
- Ozone is a key ingredient in our planet's atmosphere. It forms naturally when oxygen is exposed to strong concentrations of ultraviolet light, which triggers chemical reactions. Ozone is Earth's security blanket, protecting life from deadly ultraviolet rays.
- This is the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope. This method simulates how astronomers will search for circumstantial evidence of life beyond Earth by looking for potential biosignatures on extrasolar planets.
- Using a space telescope for eclipse observations reproduces the conditions under which future telescopes would measure atmospheres of extrasolar planets that pass in front of their stars. These atmospheres may contain chemical signatures very similar to Earth, and pique our curiosity to wonder if we are not alone in the universe.
- Taking advantage of a total lunar eclipse, astronomers using NASA's Hubble Space Telescope have detected Earth's own brand of sunscreen – ozone – in our atmosphere. This method simulates how astronomers and astrobiology researchers will search for evidence of life beyond Earth by observing potential "biosignatures" on exoplanets (planets around other stars).
- Hubble did not look at Earth directly. Instead, the astronomers used the Moon as a mirror to reflect sunlight, which had passed through Earth's atmosphere, and then reflected back towards Hubble. Using a space telescope for eclipse observations reproduces the conditions under which future telescopes would measure atmospheres of transiting exoplanets. These atmospheres may contain chemicals of interest to astrobiology, the study of and search for life.
- Though numerous ground-based observations of this kind have been done previously, this is the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope. Hubble detected the strong spectral fingerprint of ozone, which absorbs some of the sunlight. Ozone is important to life because it is the source of the protective shield in Earth's atmosphere.
- On Earth, photosynthesis over billions of years is responsible for our planet's high oxygen levels and thick ozone layer. That's one reason why scientists think ozone or oxygen could be a sign of life on another planet, and refer to them as biosignatures.
- "Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life," explained Allison Youngblood of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, lead researcher of Hubble's observations.
- Although ozone in Earth's atmosphere had been detected in previous ground-based observations during lunar eclipses, Hubble's study represents the strongest detection of the molecule to date because ozone – as measured from space with no interference from other chemicals in the Earth's atmosphere – absorbs ultraviolet light so strongly.
- Hubble recorded ozone absorbing some of the Sun's ultraviolet radiation that passed through the edge of Earth's atmosphere during a lunar eclipse that occurred on January 20 to 21, 2019. Several other ground-based telescopes also made spectroscopic observations at other wavelengths during the eclipse, searching for more of Earth's atmospheric ingredients, such as oxygen and methane.
- "One of NASA's major goals is to identify planets that could support life," Youngblood said. "But how would we know a habitable or an uninhabited planet if we saw one? What would they look like with the techniques that astronomers have at their disposal for characterizing the atmospheres of exoplanets? That's why it's important to develop models of Earth's spectrum as a template for categorizing atmospheres on extrasolar planets." 45)
Figure 42: Hubble Observes the Total Lunar Eclipse. Taking advantage of a total lunar eclipse in January 2019, astronomers using NASA's Hubble Space Telescope have detected ozone in Earth's atmosphere. This method serves as a proxy for how they will observe Earth-like planets transiting in front of other stars in search of life. Our planet's perfect alignment with the Sun and Moon during a total lunar eclipse mimics the geometry of a transiting terrestrial planet with its star. In a new study, Hubble did not look at Earth directly. Instead, astronomers used the Moon as a mirror that reflects the sunlight transmitted through Earth's atmosphere, which was then captured by Hubble. This is the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope (image credit: M. Kornmesser (ESA/Hubble), NASA and ESA)
Sniffing Out Planetary Atmospheres
- The atmospheres of some extrasolar planets can be probed if the alien world passes across the face of its parent star, an event called a transit. During a transit, starlight filters through the backlit exoplanet's atmosphere. (If viewed close up, the planet's silhouette would look like it had a thin, glowing "halo" around it caused by the illuminated atmosphere, just as Earth does when seen from space.)
- Chemicals in the atmosphere leave their telltale signature by filtering out certain colors of starlight. Astronomers using Hubble pioneered this technique for probing exoplanets. This is particularly remarkable because extrasolar planets had not yet been discovered when Hubble was launched in 1990 and the space observatory was not initially designed for such experiments.
- So far, astronomers have used Hubble to observe the atmospheres of gas giant planets and super-Earths (planets several times Earth's mass) that transit their stars. But terrestrial planets about the size of Earth are much smaller objects and their atmospheres are thinner, like the skin on an apple. Therefore, teasing out these signatures from Earth-sized exoplanets will be much harder.
- That's why researchers will need space telescopes much larger than Hubble to collect the feeble starlight passing through these small planets' atmospheres during a transit. These telescopes will need to observe planets for a longer period, many dozens of hours, to build up a strong signal.
- To prepare for these bigger telescopes, astronomers decided to conduct experiments on a much closer and only known inhabited terrestrial planet: Earth. Our planet's perfect alignment with the Sun and Moon during a total lunar eclipse mimics the geometry of a terrestrial planet transiting its star.
- But the observations were also challenging because the Moon is very bright, and its surface is not a perfect reflector because it is mottled with bright and dark areas. The Moon is also so close to Earth that Hubble had to try and keep a steady eye on one select region, despite the Moon's motion relative to the space observatory. So, Youngblood's team had to account for the Moon's drift in their analysis.
Where There's Ozone, There's Life?
- Finding ozone in the skies of a terrestrial extrasolar planet does not guarantee that life exists on the surface. "You would need other spectral signatures in addition to ozone to conclude that there was life on the planet, and these signatures cannot necessarily be seen in ultraviolet light," Youngblood said.
- On Earth, ozone is formed naturally when oxygen in the Earth's atmosphere is exposed to strong concentrations of ultraviolet light. Ozone forms a blanket around Earth, protecting it from harsh ultraviolet rays.
- "Photosynthesis might be the most productive metabolism that can evolve on any planet, because it is fueled by energy from starlight and uses cosmically abundant elements like water and carbon dioxide," said Giada Arney of NASA's Goddard Space Flight Center in Greenbelt, Maryland, a co-author of the science paper. "These necessary ingredients should be common on habitable planets."
- Seasonal variability in the ozone signature also could indicate seasonal biological production of oxygen, just as it does with the growth seasons of plants on Earth.
- But ozone can also be produced without the presence of life when nitrogen and oxygen are exposed to sunlight. To increase confidence that a given biosignature is truly produced by life, astronomers must search for combinations of biosignatures. A multiwavelength campaign is needed because each of the many biosignatures are more easily detected at wavelengths specific to those signatures.
- "Astronomers will also have to take the developmental stage of the planet into account when looking at younger stars with young planets. If you wanted to detect oxygen or ozone from a planet similar to the early Earth, when there was less oxygen in our atmosphere, the spectral features in optical and infrared light aren't strong enough," Arney explained. "We think Earth had low concentrations of ozone before the mid-Proterozoic geological period (between roughly 2.0 billion to 0.7 billion years ago) when photosynthesis contributed to the build up of oxygen and ozone in the atmosphere to the levels we see today. But because the ultraviolet-light signature of ozone features is very strong, you would have a hope of detecting small amounts of ozone. The ultraviolet may therefore be the best wavelength for detecting photosynthetic life on low-oxygen exoplanets."
- NASA has a forthcoming observatory called the James Webb Space Telescope that could make similar kinds of measurements in infrared light, with the potential to detect methane and oxygen in exoplanet atmospheres. Webb is currently scheduled to launch in 2021.
• 31 July 2020: A main-sequence star, like our Sun, is the term applied to a star during the longest period of its life, when it burns fuel steadily. Our Sun’s fuel will run out in approximately 6 billion years, and it will then move on to the next stage of its life when it will turn into a red giant. Astronomers studying NGC 2203, which contains stars that are roughly twice as massive as our Sun, found that rotation rates might be a factor as to why some of the stars stay longer than usual in this main-sequence phase of their life. 46)
Figure 43: Looking its best ever is the star cluster NGC 2203, here imaged by the NASA/ESA Hubble Space Telescope. Aside from its dazzling good looks, this cluster of stars contains lots of astronomical treats that have helped astronomers puzzle together the lifetimes of stars (image credit: ESA/Hubble & NASA, L. Girardi)
• 24 July, 2020: A notable feature of most spiral galaxies is the multitude of arching spiral arms that seemingly spin out from the galaxy’s center. In this image, taken with the NASA/ESA Hubble Space Telescope, the stunning silvery-blue spiral arms of the galaxy NGC 4848 are observed in immense detail. Not only do we see the inner section of the spiral arms containing hundreds of thousands of young, bright, blue stars, but Hubble has also captured the extremely faint wispy tails of the outer spiral arms. 47) Myriad more distant and delightfully diverse galaxies appear in the background. 48)
- If you are situated in the Northern Hemisphere with a large telescope, you might just be able to observe the ghost-like appearance of this faint galaxy within the faint constellation of Coma Berenices (Berenice’s Hair).
Figure 44: Hubble snaps a ghostly galaxy. This wispy barred spiral galaxy was first discovered in 1865 by the German astronomer Heinrich Louis d’Arrest. In his career, Heinrich also notably discovered the asteroid 76 Freia and many other galaxies and he also contributed to the discovery of Neptune (image credit: ESA/Hubble & NASA, M. Gregg; CC BY 4.0)
• 23 July 2020: Saturn is truly the lord of the rings in this latest snapshot from NASA's Hubble Space Telescope, taken on July 4, 2020, when the opulent giant world was 839 million miles (1.35 billion km, or 9.5 AU) from Earth. This new Saturn image was taken during summer in the planet's northern hemisphere. 49)
- Hubble found a number of small atmospheric storms. These are transient features that appear to come and go with each yearly Hubble observation. The banding in the northern hemisphere remains pronounced as seen in Hubble's 2019 observations, with several bands slightly changing color from year to year. The ringed planet's atmosphere is mostly hydrogen and helium with traces of ammonia, methane, water vapor, and hydrocarbons that give it a yellowish-brown color.
- Hubble photographed a slight reddish haze over the northern hemisphere in this color composite. This may be due to heating from increased sunlight, which could either change the atmospheric circulation or perhaps remove ices from aerosols in the atmosphere. Another theory is that the increased sunlight in the summer months is changing the amounts of photochemical haze produced. "It's amazing that even over a few years, we're seeing seasonal changes on Saturn," said lead investigator Amy Simon of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Conversely, the just-now-visible south pole has a blue hue, reflecting changes in Saturn's winter hemisphere.
- Hubble's sharp view resolves the finely etched concentric ring structure. The rings are mostly made of pieces of ice, with sizes ranging from tiny grains to giant boulders. Just how and when the rings formed remains one of our solar system's biggest mysteries. Conventional wisdom is that they are as old as the planet, over 4 billion years. But because the rings are so bright – like freshly fallen snow – a competing theory is that they may have formed during the age of the dinosaurs. Many astronomers agree that there is no satisfactory theory that explains how rings could have formed within just the past few hundred million years. "However, NASA's Cassini spacecraft measurements of tiny grains raining into Saturn's atmosphere suggest the rings can only last for 300 million more years, which is one of the arguments for a young age of the ring system," said team member Michael Wong of the University of California, Berkeley.
- Two of Saturn's icy moons are clearly visible in this exposure: Mimas at right, and Enceladus at bottom.
- This image is taken as part of the Outer Planets Atmospheres Legacy (OPAL) project. OPAL is helping scientists understand the atmospheric dynamics and evolution of our solar system's gas giant planets. In Saturn's case, astronomers continue tracking shifting weather patterns and storms.
- 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 conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
Figure 45: NASA's Hubble Space Telescope captured this image of Saturn on July 4, 2020. Two of Saturn's icy moons are clearly visible in this exposure: Mimas at right, and Enceladus at bottom. This image is taken as part of the Outer Planets Atmospheres Legacy (OPAL) project. OPAL is helping scientists understand the atmospheric dynamics and evolution of our solar system's gas giant planets. In Saturn's case, astronomers continue tracking shifting weather patterns and storms (image credits: NASA, ESA, A. Simon (Goddard Space Flight Center), M.H. Wong (University of California, Berkeley), and the OPAL Team)
Figure 46: These images are a composite of separate exposures acquired by the WFC3 instrument on the Hubble Space Telescope. Several filters were used to sample narrow wavelength ranges. The color results from assigning different hues (colors) to each monochromatic (grayscale) image associated with an individual filter. In this case, the assigned colors are: Blue: F395N Green: F502N Red: F631N (Saturn 2020 compass image)
• 17 July 2020: Galaxies are well known as the birthplaces of stars and planets thanks to the overwhelmingly large amount of dust and gas within them. Over time, cold gas coalesces into molecular clouds, leading to the further emergence of star-forming regions. 50)
- When a massive new star (or stars) starts to shine while still within the cool molecular cloud from which it formed, its energetic radiation can ionize the cloud’s hydrogen and create a large, hot bubble of ionized gas. Amazingly, located within this bubble of hot gas around a nearby massive star are the frEGGs (Free-floating Evaporating Gaseous Globules): dark compact globules of dust and gas, some of which are also giving birth to low-mass stars. The boundary between the cool, dusty frEGG and hot gas bubble is seen as the glowing purple/blue edges in this fascinating image (Figure 47).
- Learning more about these odd objects can help astronomers understand how stars like our Sun form under external influences. In fact, our Sun may have even been born in a frEGG!
Figure 47: This image taken with the NASA/ESA Hubble Space Telescope depicts a fantastic new class of star-forming nursery, known as frEGGs. This object, known as J025027.7+600849, is located in the constellation of Cassiopeia (image credit: ESA/Hubble & NASA, R. Sahai; CC BY 4.0)
• 17 July 2020: As beautiful as the surrounding space may be, the sparkling galaxy in the foreground of this image from the NASA/ESA Hubble Space Telescope undeniably steals the show. 51)
Figure 48: This spotlight-hogging galaxy, seen set against a backdrop of more distant galaxies of all shapes and sizes, is known as PGC 29388. Although it dominates in this image, this galaxy is a small player on the cosmic stage and is known as a dwarf elliptical galaxy. As the “dwarf” moniker suggests, the galaxy is on the smaller side, and boasts a “mere” 100 million to a few billion stars — a very small number indeed when compared to the Milky Way's population of around 250 billion to 400 billion stellar residents (image credit: ESA/Hubble & NASA, T. Armandroff)
• 10 July 2020: Hubble image of the week. The sculpted galaxy (NGC 7513) is moving at the astounding speed of 1564 km/s, and it is heading away from us. For context, the Earth orbits the Sun at about 30 km/s. Though NGC 7513’s apparent movement away from the Milky Way might seem strange, it is not that unusual. 52)
- While some galaxies, like the Milky Way and the Andromeda galaxy, are caught in each other’s gravitational pull and will eventually merge together, the vast majority of galaxies in our Universe appear to be moving away from each other. This phenomenon is due to the expansion of the Universe, and it is the space between galaxies that is stretching, rather than the galaxies themselves moving.
Figure 49: Captured by the NASA/ESA Hubble Space Telescope, this image shows NGC 7513, a barred spiral galaxy. Located approximately 60 million light-years away, NGC 7513 lies within the Sculptor constellation in the southern hemisphere (image credit: ESA/Hubble & NASA, M. Stiavelli; CC BY 4.0)
• 03 July 2020: The spiral pattern shown by the galaxy in this image from the NASA/ESA Hubble Space Telescope is striking because of its delicate, feathery nature. These "flocculent" spiral arms indicate that the recent history of star formation of the galaxy, known as NGC 2775, has been relatively quiet. There is virtually no star formation in the central part of the galaxy, which is dominated by an unusually large and relatively empty galactic bulge, where all the gas was converted into stars long ago. 53)
- Millions of bright, young, blue stars shine in the complex, feather-like spiral arms, interlaced with dark lanes of dust. Complexes of these hot, blue stars are thought to trigger star formation in nearby gas clouds. The overall feather-like spiral patterns of the arms are then formed by shearing of the gas clouds as the galaxy rotates. The spiral nature of flocculents stands in contrast to the grand design spirals, which have prominent, well defined-spiral arms.
Figure 50: NGC 2275 is classified as a flocculent spiral galaxy, located 67 million light-years away in the constellation of Cancer [image credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team; CC BY 4.0; Acknowledgement: Judy Schmidt (Geckzilla)]
• 30 June, 2020: At first sight, this image from the NASA/ESA Hubble Space Telescope portrays the sparkling stars of AGC111977, a dwarf galaxy located around 15 million light years away and visible in the lower left part of the image. Other galaxies appear sprinkled across the frame, along with foreground stars from our own galaxy, the Milky Way. 54)
Figure 51: After closer inspection, something else comes to sight, much closer to home. Towards the lower right corner of the frame, two elongated streaks are faintly visible: the trails of asteroids – small rocky bodies in our Solar System – crossing their ways in the foreground of the stars and galaxies that Hubble was observing [image credit: ESA/Hubble & NASA; J. Cannon (Macalester College)]
- The image combines observations obtained on 16 November 2012 with Hubble’s ACS (Advanced Camera for Surveys) instrument using two different filters (606 nm, shown in blue, and 814 nm, shown in red). As the asteroids moved relative to Hubble during the observation, both trails have been imaged subsequently in each filter and thus appear part red and part blue.
- The two asteroids are located at different distances from us, so they did not actually collide as their intersecting streaks might suggest. They were uncovered by citizen scientists Sovan Acharya, Graeme Aitken, Claude Cornen, Abe Hoekstra and Edmund Perozzi, some of the volunteers who have been inspecting images from the iconic space telescope in search for rocky interlopers as part of the Hubble Asteroid Hunter citizen science project.
- Launched one year ago, on International Asteroid Day 2019, the Hubble Asteroid Hunter is a collaboration between ESA and the Zooniverse, inviting members of the public to identify asteroids that had been serendipitously observed by the Hubble Space Telescope. Since then, 9000 volunteers from all over the world provided 2 million classifications of 140 000 composite Hubble images, finding 1500 asteroid trails – about one every hundred images.
- In the project’s first phase, volunteers could explore a collection of archival Hubble images where calculations by the Solar System Objects pipeline of ESASky, ESA's discovery portal for astronomy, indicated that an asteroid might have been crossing the space telescope’s field of view at the time of the observations. The sheer number and enthusiasm of volunteers led the team to expand the project, including more images of the sky collected by Hubble over the years.
- Besides asteroids, the volunteers have also identified trails left by satellites in orbits higher than Hubble’s, intriguing instances of gravitational lensing, and ring-shaped features that arise when galaxies collide.
- The project experienced a surge in participation during the past few months, as many people around the world were staying at home due to the COVID-19 pandemic, leading to a threefold increase in the number of classifications. Thanks to the continued efforts of the volunteers, this citizen science project is now nearing completion, with only the infrared images left to explore.
- Meanwhile, the team is working to identify the asteroids that were uncovered as part of the project – including the two pictured in this image – to possibly match them with known asteroids in the Minor Planet Center database, and calculate their distances from us. Stay tuned!
- Asteroid Day is a UN-endorsed awareness campaign day to mark the anniversary of the 30 June 1908 Tunguska impact, and this year ESA is taking part in Asteroid Day TV distributed by the Luxembourg-based Asteroid Foundation. To watch programming by ESA as well as top content makers including Discovery Science, TED, IMAX, BBC, CNN, ESO and other educational producers, access https://asteroidday.org/
• 26 June 2020: The galaxy known as NGC 5907 stretches wide across this image. Appearing as an elongated line of stars and dark dust, the galaxy is categorized as a spiral galaxy just like our own Milky Way. In this new image from the NASA/ESA Hubble Space Telescope, we don’t see the beautiful spiral arms because we are viewing it edge-on, like looking at the rim of a plate. It is for this reason that NGC 5907 is also known as the Knife Edge Galaxy. 55)
Figure 52: The Knife Edge Galaxy is about 50 million light-years from Earth, lying in the northern constellation of Draco. Although not visible in this image, ghostly streams of stars on large arching loops extend into space, circling around the galaxy; they are believed to be remnants of a small dwarf galaxy, torn apart by the Knife Edge Galaxy and merged with it over four billion years ago [image credit: ESA/Hubble & NASA, R. de Jong; CC BY 4.0; Acknowledgement: Judy Schmidt (Geckzilla)]
• 18 June 2020: The NASA/ESA Hubble Space Telescope demonstrates its full range of imaging capabilities with two new images of planetary nebulae. The images depict two nearby young planetary nebulae, NGC 6302, dubbed the Butterfly Nebula, and NGC 7027. Both are among the dustiest planetary nebulae known and both contain unusually large masses of gas, which made them an interesting pair for study in parallel by a team of researchers. 56)
Figure 53: These two new images from the Hubble Space Telescope depict two nearby young planetary nebulae, NGC 6302, dubbed the Butterfly Nebula, and NGC 7027, which resembles a jewel bug. Both are among the dustiest planetary nebulae known and both contain unusually large masses of gas [image credit: NASA, ESA, and J. Kastner (RIT)]
- As nuclear fusion engines, most stars live placid lives for hundreds of millions to billions of years. But near the end of their lives they can turn into crazy whirligigs, puffing off shells and jets of hot gas. Astronomers have used Hubble to dissect such crazy fireworks happening in these two planetary nebulae. The researchers have found unprecedented levels of complexity and rapid changes in the jets and gas bubbles blasting off of the stars at the center of each nebula. 57) Hubble is now allowing the researchers to converge on an understanding of the mechanisms underlying this chaos.
- The Hubble Space Telescope has imaged these objects before, but not for many years and never before with the Wide Field Camera 3 instrument across its full wavelength range — making observations in near-ultraviolet to near-infrared light. “These new multi-wavelength Hubble observations provide the most comprehensive view to date of both of these spectacular nebulae,” said Joel Kastner of the Rochester Institute of Technology, Rochester, New York, leader of the new study. “As I was downloading the resulting images, I felt like a kid in a candy store.”
- The new Hubble images reveal in vivid detail how both nebulae are splitting themselves apart on extremely short timescales — allowing astronomers to see changes over the past couple of decades. In particular, Hubble’s broad multi-wavelength views of each nebula are helping the researchers to trace the histories of shock waves in them. Such shocks are typically generated when fresh, fast stellar winds slam into and sweep up more slowly expanding gas and dust ejected by the star in its recent past, generating bubble-like cavities with well-defined walls.
- Researchers suspect that at the heart of each nebula were two stars orbiting around each other. Evidence for such a central “dynamic duo” comes from the bizarre shapes of these nebulas. Each has a pinched, dusty waist and polar lobes or outflows, as well as other, more complex symmetrical patterns.
- A leading theory for the generation of such structures in planetary nebulae is that the mass-losing star is one of two stars in a binary system. The two stars orbit one another closely enough that they eventually interact, producing a gas disc around one or both stars. The disc then launches jets that inflate polar-directed lobes of outflowing gas.
- Another, related, popular hypothesis is that the smaller star of the pair may merge with its bloated, more rapidly evolving stellar companion. This very short-lived “common envelope” binary star configuration can also generate wobbling jets, forming the trademark bipolar outflows commonly seen in planetary nebulae. However, the suspect companion stars in these planetary nebulae have not been directly observed. Researchers suggest this may be because these companions are next to, or have already been swallowed by, far larger and brighter red giant stars.
- NGC 6302, commonly known as the Butterfly Nebula, exhibits a distinct S-shaped pattern seen in reddish-orange in the image. Imagine a lawn sprinkler spinning wildly, throwing out two S-shaped streams. In this case it is not water in the air, but gas blown out at high speed by a star. And the “S” only appears when captured by the Hubble camera filter that records near-infrared emission from singly ionized iron atoms. This iron emission is indicative of energetic collisions between both slow and fast winds, which is most commonly observed in active galactic nuclei and supernova remnants.
- "This is very rarely seen in planetary nebulae,” explained team member Bruce Balick of the University of Washington in Seattle. “Importantly, the iron emission image shows that fast, off-axis winds penetrate far into the nebula like tsunamis, obliterating former clumps in their paths and leaving only long tails of debris.”
- The accompanying image of NGC 7027, which resembles a jewel bug, indicates that it had been slowly puffing away its mass in quiet, spherically symmetric or perhaps spiral patterns for centuries — until relatively recently. “Something recently went haywire at the very center, producing a new cloverleaf pattern, with bullets of material shooting out in specific directions,” Kastner explained.
• 05 June 2020: Almost like snowflakes, the stars of the globular cluster NGC 6441 sparkle peacefully in the night sky, about 13,000 light-years from the Milky Way’s galactic center. Like snowflakes, the exact number of stars in such a cluster is difficult to discern. It is estimated that together the stars weigh 1.6 million times the mass of the Sun, making NGC 6441 one of the most massive and luminous globular clusters in the Milky Way. 58)
- NGC 6441 is host to four pulsars that each complete a single rotation in a few milliseconds. Also hidden within this cluster is JaFu 2, a planetary nebula. Despite its name, this has little to do with planets. A phase in the evolution of intermediate-mass stars, planetary nebulae last for only a few tens of thousands of years, the blink of an eye on astronomical timescales.
- There are about 150 known globular clusters in the Milky Way. Globular clusters contain some of the first stars to be produced in a galaxy, but the details of their origins and evolution still elude astronomers.
Figure 54: Hubble image of the week: snowflakes (image credit: ESA/Hubble & NASA, G. Piotto)
• 03 June 2020: New results from the NASA/ESA Hubble Space Telescope suggest the formation of the first stars and galaxies in the early universe took place sooner than previously thought. A European team of astronomers have found no evidence of the first generation of stars, known as Population III stars, as far back as when the universe was just 500 million years old. 59) 60)
- The exploration of the very first galaxies remains a significant challenge in modern astronomy. We do not know when or how the first stars and galaxies in the universe formed. These questions can be addressed with the Hubble Space Telescope through deep imaging observations. Hubble allows astronomers to view the universe back to within 500 million years of the big bang.
- A team of European researchers, led by Rachana Bhatawdekar of the European Space Agency, set out to study the first generation of stars in the early Universe. Known as Population III stars , these stars were forged from the primordial material that emerged from the Big Bang. Population III stars must have been made solely out of hydrogen, helium and lithium, the only elements that existed before processes in the cores of these stars could create heavier elements, such as oxygen, nitrogen, carbon and iron.
- Bhatawdekar and her team probed the early Universe from about 500 million to 1 billion years after the Big Bang by studying the cluster MACSJ0416 and its parallel field with the Hubble Space Telescope [with supporting data from NASA's Spitzer Space Telescope and the ground-based VLT (Very Large Telescope) of the European Southern Observatory]. "We found no evidence of these first-generation Population III stars in this cosmic time interval" said Bhatawdekar of the new results.
- The result was achieved using the Hubble's Space Telescope's Wide Field Camera 3 and Advanced Camera for Surveys , as part of the Hubble Frontier Fields program. This program (which observed six distant galaxy clusters from 2012 to 2017) produced the deepest observations ever made of galaxy clusters and the galaxies located behind them which were magnified by the gravitational lensing effect, thereby revealing galaxies 10 to 100 times fainter than any previously observed. The masses of foreground galaxy clusters are large enough to bend and magnify the light from the more distant objects behind them. This allows Hubble to use these cosmic magnifying glasses to study objects that are beyond its nominal operational capabilities.
- Bhatawdekar and her team developed a new technique that removes the light from the bright foreground galaxies that constitute these gravitational lenses. This allowed them to discover galaxies with lower masses than ever previously observed with Hubble, at a distance corresponding to when the Universe was less than a billion years old. At this point in cosmic time, the lack of evidence for exotic stellar populations and the identification of many low-mass galaxies supports the suggestion that these galaxies are the most likely candidates for the re-ionization of the Universe. This period of re-ionization in the early Universe is when the neutral intergalactic medium was ionized by the first stars and galaxies.
- "These results have profound astrophysical consequences as they show that galaxies must have formed much earlier than we thought," said Bhatawdekar. "This also strongly supports the idea that low-mass/faint galaxies in the early Universe are responsible for re-ionization."
- These results  also suggest that the earliest formation of stars and galaxies occurred much earlier than can be probed with the Hubble Space Telescope. This leaves an exciting area of further research for the upcoming NASA/ESA/CSA James Webb Space Telescope – to study the Universe's earliest galaxies.
Figure 55: Artist's impression of the early Universe (image credit: ESA/Hubble, M. Kornmesser, CC BY 4.0)
 The name Population III arose because astronomers had already classified the stars of the Milky Way as Population I (stars like the Sun, which are rich in heavier elements) and Population II (older stars with a low heavy-element content, found in the Milky Way bulge and halo, and in globular star clusters).
 Owing to the expansion of the Universe, the light from the distant galaxies is shifted from ultraviolet and optical wavelengths into the infrared part of the electromagnetic spectrum. Hubble's Wide Field Camera 3 is well equipped to probe this part of the spectrum. In addition, the telescope's Advanced Camera for Surveys is optimized for visible/ light observations.
 These results are based on a previous 2019 paper by Bhatawdekar et al., and a paper that will appear in an upcoming issue of the Monthly Notices of the Royal Astronomical Society (MNRAS). These results are also being presented at a press conference during the 236th meeting of the American Astronomical Society on 3 June 2020.
Figure 56: This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACS J0416. This is one of six being studied by the Hubble Frontier Fields program, which together have produced the deepest images of gravitational lensing ever made. Scientists used intracluster light (visible in blue) to study the distribution of dark matter within the cluster [image credit: NASA, ESA, and M. Montes (University of New South Wales, Sydney, Australia)]
• 29 May 2020: Far away in the Ursa Major constellation is a swirling galaxy that would not look out of place on a coffee made by a starry-eyed barista. NGC 3895 is a barred spiral galaxy that was first spotted by William Herschel in 1790 and was later observed by the NASA/ESA Hubble Space Telescope. 61)
Figure 57: Hubble's orbit high above the Earth's distorting atmosphere allows astronomers to make the very high resolution observations that are essential to opening new windows on planets, stars and galaxies — such as this beautiful view of NGC 3895. The telescope is positioned approximately 570 km above the ground, where it whirls around Earth at 28,000 km/hr and takes 96 minutes to complete one orbit (image credit: ESA/Hubble, NASA, and R. Barrows; CC BY 4.0)
• 28 May 2020: The NASA/ESA Hubble Space Telescope was used to conduct a three-year study of the crowded, massive and young star cluster Westerlund 2. The research found that the material encircling stars near the cluster’s center is mysteriously devoid of the large, dense clouds of dust that would be expected to become planets in a few million years. Their absence is caused by the cluster’s most massive and brightest stars that erode and disperse the discs of gas and dust of neighboring stars. This is the first time that astronomers have analyzed an extremely dense star cluster to study which environments are favorable to planet formation. 62)
Figure 58: The star cluster Westerlund 2 (Milky Way). This image shows the sparkling centerpiece of Hubble's 25th anniversary tribute. Westerlund 2 is a giant cluster of about 3000 stars located 20,000 light-years away in the constellation Carina. Hubble's near-infrared imaging camera pierces through the dusty veil enshrouding the stellar nursery, giving astronomers a clear view of the dense concentration of stars in the central cluster (image credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team)
The Westerlund 2 star cluster's raucous core is no place to form planets 63)
- Astronomers using NASA's Hubble Space Telescope are finding that planets have a tough time forming in the rough-and-tumble central region of the massive, crowded star cluster Westerlund 2. Located 20,000 light-years away, Westerlund 2 is a unique laboratory to study stellar evolutionary processes because it's relatively nearby, quite young, and contains a large stellar population.
- A three-year Hubble study of stars in Westerlund 2 revealed that the precursors to planet-forming disks encircling stars near the cluster's center are mysteriously devoid of large, dense clouds of dust that in a few million years could become planets.
- However, the observations show that stars on the cluster's periphery do have the immense planet-forming dust clouds embedded in their disks. Researchers think our solar system followed this recipe when it formed 4.6 billion years ago.
- So why do some stars in Westerlund 2 have a difficult time forming planets while others do not? It seems that planet formation depends on location, location, location. The most massive and brightest stars in the cluster congregate in the core, which is verified by observations of other star-forming regions. The cluster's center contains at least 30 extremely massive stars, some weighing up to 80 times the mass of the Sun. Their blistering ultraviolet radiation and hurricane-like stellar winds of charged particles blowtorch disks around neighboring lower-mass stars, dispersing the giant dust clouds.
- "Basically, if you have monster stars, their energy is going to alter the properties of the disks around nearby, less massive stars," explained Elena Sabbi, of the Space Telescope Science Institute in Baltimore, Maryland, lead researcher of the Hubble study. "You may still have a disk, but the stars change the composition of the dust in the disks, so it's harder to create stable structures that will eventually lead to planets. We think the dust either evaporates away in 1 million years, or it changes in composition and size so dramatically that planets don't have the building blocks to form."
- The Hubble observations represent the first time that astronomers analyzed an extremely dense star cluster to study which environments are favorable to planet formation. Scientists, however, are still debating whether bulky stars are born in the center or whether they migrate there. Westerlund 2 already has massive stars in its core, even though it is a comparatively young 2-million-year-old system.
- Using Hubble's Wide Field Camera 3, the researchers found that of the nearly 5,000 stars in Westerlund 2 with masses between 0.1 to 5 times the Sun's mass, 1,500 of them show fluctuations in their light as the stars accrete material from their disks. Orbiting material clumped within the disk would temporarily block some of the starlight, causing brightness fluctuations.
- However, Hubble detected the signature of such orbiting material only around stars outside the cluster's packed central region. The telescope witnessed large drops in brightness for as much as 10 to 20 days around 5% of the stars before they returned to normal brightness. They did not detect these dips in brightness in stars residing within four light-years of the center. These fluctuations could be caused by large clumps of dust passing in front of the star. The clumps would be in a disk tilted nearly edge-on to the view from Earth. "We think they are planetesimals or structures in formation," Sabbi explained. "These could be the seeds that eventually lead to planets in more evolved systems. These are the systems we don't see close to very massive stars. We see them only in systems outside the center."
- Thanks to Hubble, astronomers can now see how stars are accreting in environments that are like the early universe, where clusters were dominated by monster stars. So far, the best known nearby stellar environment that contains massive stars is the starbirth region in the Orion Nebula. However, Westerlund 2 is a richer target because of its larger stellar population.
- "Hubble's observations of Westerlund 2 give us a much better sense of how stars of different masses change over time, and how powerful winds and radiation from very massive stars affect nearby lower-mass stars and their disks," Sabbi said. "We see, for example, that lower-mass stars, like our Sun, that are near extremely massive stars in the cluster still have disks and still can accrete material as they grow. But the structure of their disks (and thus their planet-forming capability) seems to be very different from that of disks around stars forming in a calmer environment farther away from the cluster core. This information is important for building models of planet formation and stellar evolution."
- This cluster will be an excellent laboratory for follow-up observations with NASA's upcoming James Webb Space Telescope, an infrared observatory. Hubble has helped astronomers identify the stars that have possible planetary structures. With Webb, researchers can study which disks around stars are not accreting material and which disks still have material that could build up into planets. This information on 1,500 stars will allow astronomers to map a path on how star systems grow and evolve. Webb also can study the chemistry of the disks in different evolutionary phases and watch how they change, and help astronomers determine what influence environment plays in their evolution.
- NASA's planned infrared observatory, the Nancy Grace Roman Space Telescope, will be able to perform Sabbi's study on a much larger area. Westerlund 2 is just a small slice of an immense star-formation region. These vast regions contain clusters of stars with different ages and different densities. Astronomers could use Roman Space Telescope observations to start to build up statistics on how a star's characteristics, like its mass or outflows, affect its own evolution or the nature of stars that form nearby. The observations could also provide more information on how planets form in tough environments. 64)
• 22 May 2020: Unlike a spiral or elliptical galaxy, the galaxy KK 246 looks like glitter spilled across a black velvet sheet. KK 246, also known as ESO 461-036, is a dwarf irregular galaxy residing within the Local Void, a vast region of empty space. This lonely galaxy is the only one known for certain to reside in this enormous volume, along with 15 others that have been tentatively identified. 65)
Figure 59: Although the picture appears to be full of galaxies, they are actually beyond this void, and instead form part of other galaxy groups or clusters. Cosmic voids, such as this one, are the spaces within the web-like structure of the Universe wherein very few or no galaxies exist. Adjacent to the Local Group, this region of empty space is at least 150 million light-years across. For perspective, our own Milky Way galaxy is estimated to be 150,000 light-years across, making this void immense in its nothingness (image credit: ESA/Hubble & NASA, E. Shaya, L. Rizzi, B. Tully, et al.; CC BY 4.0)
• 20 May 2020: Today, NASA announced that it is naming its next-generation space telescope, the Wide Field Infrared Survey Telescope (WFIRST), in honor of Dr. Nancy Grace Roman, NASA’s first Chief Astronomer, who paved the way for space telescopes focused on the broader universe. The newly named Nancy Grace Roman Space Telescope (or Roman Space Telescope, for short), is set to launch in the mid-2020s. 66)
Figure 60: An artist's illustration of the Roman Space Telescope against a starry background, formerly WFIRST. It will revolutionize astronomy by building on the science discoveries and technological leaps of the Hubble and James Webb space telescopes (image credit: NASA/GSFC)
- Dr. Roman is credited with making the Hubble Space Telescope a reality, leading to her nickname "mother of Hubble." In the mid-1960s, she set up a committee of astronomers and engineers to envision a telescope that could accomplish important scientific goals. She convinced NASA and Congress that it was a priority to launch the most powerful space telescope the world had ever seen. She argued that, for the price of a movie ticket, each American could be given years of scientific discoveries.
- Her vision was realized when Hubble launched in 1990. Hubble turned out to be the most scientifically revolutionary space telescope of all time.
- The Space Telescope Science Institute (STScI) in Baltimore, Maryland, is the science operations center for Hubble, and will house the science and mission operations centers for the upcoming James Webb Space Telescope. In 2019, NASA announced that STScI would serve as the science operations center for the Roman Space Telescope. In that role, the Institute will plan, schedule, and carry out observations, process and archive mission datasets, and engage and inform the astronomical community and the public.
• 08 May 2020: Galaxy Galaxy, Burning Bright! Hubble Spots Source of Two Brilliant Supernovae. 67)
- There are a few different ways that supernova can form. In the case of these two supernovae, the explosions evolved from two independent binary star systems in which the stellar remnant of a Sun-like star, known as a white dwarf, was collecting material from its companion star. Feeding off of its partner, the white dwarf gorged on the material until it reached a maximum mass. At this point, the star collapsed inward before exploding outward in a brilliant supernova.
- Two of these events were spotted in NGC 3583, and though not visible in this picture of the week, we can still marvel at the galaxy’s fearful symmetry.
Figure 61: In the forests of the night lies a barred spiral galaxy called NGC 3583, imaged here by the NASA/ESA Hubble Space Telescope. This is a barred spiral galaxy with two arms that twist out into the Universe. This galaxy is located 98 million light-years away from the Milky Way. Two supernovae exploded in this galaxy, one in 1975 and another, more recently, in 2015 (image credit: ESA/Hubble & NASA, A. Riess et al.; CC BY 4.0)
• 01 May 2020: This sparkling spiral galaxy looks almost stretched across the sky in this new image from the NASA/ESA Hubble Space Telescope. Known as NGC 4100, the galaxy boasts a neat spiral structure and swirling arms speckled with the bright blue hue of newly formed stars. 68)
Figure 62: Like so many of the stunning images of galaxies we enjoy today, this image was captured by Hubble’s Advanced Camera for Surveys (ACS). This remarkable instrument was installed in 2002, and, with some servicing over the years by intrepid astronauts, is still going strong. You can access many of the stunning images captured by the ACS here, featuring objects from out-of-this-world spiral galaxies to dark, imposing nebulae, bizarre cosmic phenomena, and sparkling clusters made up of thousands upon thousands of stars (image credit: ESA/Hubble & NASA, L. Ho; CC BY 4.0)
• 30 April 2020: During its 30 years in orbit around Earth, the NASA/ESA Hubble Space Telescope has witnessed the changing nature of spaceflight as the skies have filled with greater numbers of satellites, the International Space Station was born and in-space crashes and explosions have created clouds of fast-moving space debris. 69)
- Hubble itself has felt the impact of this debris, accumulating tiny impact craters across its solar panels that evidence a long and eventful life in space. So what can we learn from these impacts, and what does the future hold for Hubble?
- In 1993, the first Shuttle mission to ‘spruce up’ Hubble was conducted. By providing the space observatory with corrective optics, it was suddenly able to take the incredibly sharp images of the Universe loved by the world over.
- While the astronauts were there, they replaced the observatory’s solar arrays which had been ‘jittering’ due to temperature fluctuations. One of the panels was disposed of in orbit, later burning up in Earth’s atmosphere, but the other was brought back down to Earth.
- Part of ESA’s contribution to Hubble was to design, manufacture and provide its solar arrays in exchange for observation time, meaning the returned array was available for the Agency to inspect.
- This was one of the earliest opportunities in the history of space exploration to see the impact of more than two years in space on an orbiting satellite. The team discovered hundreds of impact craters pocketing the surface of just a small section of the solar array, ranging from µm to millimeters in diameter.
Figure 63: Astronaut Kathy Thornton threw the damaged Hubble array into free space once it became clear its failed compensation mechanism had bent it into a bow-like shape, making refurling for return to Earth at the end of STS-61 on Servicing Mission in December 1993 impossible (image credit: NASA)
- Nine years later, the solar panels were again replaced and returned to Earth this time having accumulated almost a decade of impact craters.
Figure 64: Hubble solar cell impact damage. Post-flight analysis of an impact crater on one of the solar wings deployed by the Space Shuttle Endeavour in 1993 and retrieved by Space Shuttle Columbia in 2002 (image credit: ESA)
Figure 65: ESA built-solar cells retrieved from the Hubble Space Telescope in 2002. Solar cells in space undergo various kinds of degradation over time – meteorite impacts being the single most violent. Note two front impacts and one from the rear side seen on bottom right (image credit: ESA)
Array of evidence of Hubble’s early bombardment
- Although we don't know exactly when each impact crater was formed, they must have occurred during the solar array’s period in orbit. As such, imprinted on them, is unique evidence of spaceflight activity during their time in space.
- The impact craters were studied to determine their size and depth, but also to seek out potential new residues. Given that the chemical composition of the solar cell was known, ‘alien’ materials or elements could have been brought into the crater by the impactor.
- Metals like iron and nickel would suggest an impact from a natural source – fragments of asteroids and comets known as micrometeoroids. The craters found in Hubble’s solar arrays however contained small amounts of aluminum and oxygen, a strong indication of human activity in the form of ‘solid rocket motor’ firing residues.
- The space debris team, as part of a larger effort with partners in industry and academia, were able to match the shape and size of these craters to models of rocket firings that were known to have happened at the time, finding a match between craters observed and craters expected.
Was Hubble hurt?
- These tiny particles, ranging from micrometers up to a millimeter in size, would have struck Hubble at huge relative speeds of 10 km/s, however they didn't have a major impact on the craft which continues to take incredible images of our Universe.
- Such impacts occur quite frequently for all satellites, the main effect being a continuous but gradual degradation in the amount of power the solar arrays can produce.
- New missions make use of a model created by the space debris team, based on early Hubble impact data, to predict how many impacts can be expected for each mission and what effect this will have on solar power.
Hubble still lives with the threat of collision
- Imagine the Hubble spacecraft in orbit, residing inside a 1 km x 1 km x 1km cube. On average, at any moment, a single piece of µm-sized debris shares that cube with Hubble, because for every cubic kilometer of space around Earth, there is about one tiny debris object.
- This doesn't sound like a lot, but Hubble itself is travelling at 7.6 km/s relative to Earth and so are these tiny fragments of debris. A large fraction of collisions between the two don't happen head on, but at an angle, leading to relative impact speeds of about 10 km/s.
- For simplicity, imagine these particles are travelling at 10 km/s relative to a still Hubble. This is the same as ten of these fast-moving objects crossing in and out of Hubble’s cubic space every second. Because Hubble’s solar panels take up a large surface area, measuring approximately 7 x 2 m, they are more likely to come face-to-face with large numbers of these projectiles.
Figure 66: This animation shows different types of space debris objects and different debris sizes in orbit around Earth. For debris objects bigger than 10 cm the data come from the US Space Surveillance Catalog. (video credit: ESA)
- Hubble today faces a similar threat from small debris fragments as it did soon after it was launched. While µm-sized particles are still being created today, the atmosphere at this low altitude, 547 km above Earth’s surface, also sweeps a number of them away.
- However, the risk from larger objects is unfortunately also increasing. Debris fragments ranging from about 1-10 cm in size are too small to be catalogued and tracked from ground, but have enough energy to destroy an entire satellite. At Hubble's altitude, the probability of a collision with one of these objects has doubled since the early 2000s, from a 0.15% chance per year to a 0.3% today.
Hubble lives where mega-constellations plan to reside
- Some satellites are launched today without the capability to change their orbit. Instead of maneuvering at the end of their life, they can be inserted into relatively low altitudes so that over time Earth’s atmosphere pulls them down to burn up, including the region that Hubble calls home.
- In addition, the total number of operational satellites being put into this region looks set to soon rapidly increase. Some broadband internet constellations, the largest of which are planned to contain thousands of satellites, have their sights set on these heights.
Space Safety at ESA
- To help prevent the build-up of new debris through collisions, ESA's Space Safety program is developing ‘automated collision avoidance’ technologies that will make the process of avoiding collisions more efficient, by automating the decision processes on the ground.
- But what about the debris that’s already out there? In a world first, ESA has commissioned an active debris removal mission that will safely dispose of an item of debris currently in orbit. The ClearSpace-1 mission will target a 100 kg Vespa rocket part, left in orbit after the second flight of ESA’s Vega launcher back in 2013.
- With a mass of 100 kg, the Vespa is close in size to a small satellite. Its relatively simple shape and sturdy construction make it a suitable first goal, before progressing to larger, more challenging captures by follow-up missions – eventually including multi-object capture.
Figure 67: This image shows the results of a lab test impact between a small sphere of aluminum travelling at approximately 6.8 km per sec and a block of aluminum 18 cm thick. This test simulates what can happen when a small space debris object hits a spacecraft. - In such an impact, the pressure and temperature can exceed those found at the center of the Earth e.g. greater than 365 GPa and more than 6,000 K (image credit: ESA) 70)
Figure 68: These two Hubble Space Telescope images of comet C/2019 Y4 (ATLAS), taken on April 20 (left) and April 23, 2020, provide the sharpest views yet of the breakup of the solid nucleus of the comet. Hubble's eagle-eye view identifies as many as 30 separate fragments. Hubble distinguishes pieces that are roughly the size of a house. Before the breakup, the entire nucleus of the comet may have been the length of one or two football fields. Astronomers aren't sure why this comet broke apart. The comet was approximately 91 million miles (146 million kilometers) from Earth when the images were taken [image credit: NASA, ESA, STScI and D. Jewitt (UCLA)]
- Hubble identified about 30 fragments on April 20, and 25 pieces on April 23. They are all enveloped in a sunlight-swept tail of cometary dust. "Their appearance changes substantially between the two days, so much so that it's quite difficult to connect the dots," said David Jewitt, professor of planetary science and astronomy at UCLA, Los Angeles, and leader of one of two teams that photographed the doomed comet with Hubble. "I don't know whether this is because the individual pieces are flashing on and off as they reflect sunlight, acting like twinkling lights on a Christmas tree, or because different fragments appear on different days."
- "This is really exciting — both because such events are super cool to watch and because they do not happen very often. Most comets that fragment are too dim to see. Events at such scale only happen once or twice a decade," said the leader of a second Hubble observing team, Quanzhi Ye, of the University of Maryland, College Park.
- The results are evidence that comet fragmentation is actually fairly common, say researchers. It might even be the dominant mechanism by which the solid, icy nuclei of comets die. Because this happens quickly and unpredictably, astronomers remain largely uncertain about the cause of fragmentation. Hubble's crisp images may yield new clues to the breakup. Hubble distinguishes pieces as small as the size of a house. Before the breakup, the entire nucleus may have been no more than the length of two football fields.
- One idea is that the original nucleus spun itself into pieces because of the jet action of outgassing from sublimating ices. Because such venting is probably not evenly dispersed across the comet, it enhances the breakup. "Further analysis of the Hubble data might be able to show whether or not this mechanism is responsible," said Jewitt. "Regardless, it's quite special to get a look with Hubble at this dying comet."
- The comet was discovered on Dec. 29, 2019, by the ATLAS (Asteroid Terrestrial-impact Last Alert System) robotic astronomical survey system based in Hawaii. This NASA-supported survey project for Planetary Defense operates two autonomous telescopes that look for Earth-approaching comets and asteroids.
- The comet brightened quickly until mid-March, and some astronomers anticipated that it might be visible to the naked eye in May to become one of the most spectacular comets seen in the last 20 years.
- However, the comet abruptly started to get dimmer instead of brighter. Astronomers speculated that the icy core may be fragmenting, or even disintegrating. ATLAS' fragmentation was confirmed by amateur astronomer Jose de Queiroz, who was able to photograph around three pieces of the comet on 11 April.
- The disintegrating comet was approximately 91 million miles (146 million km) from Earth when the latest Hubble observations were taken. If any of it survives, the comet will make its closest approach to Earth on May 23 at a distance of about 72 million miles (116 million km), and eight days later it will skirt past the Sun at 25 million miles (40 million km).
- 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 conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
• 24 April 2020: NASA is celebrating the Hubble Space Telescope's 30 years of unlocking the beauty and mystery of space by unveiling a stunning new portrait of a firestorm of starbirth in a neighboring galaxy. Thirty years ago, on 24 April 1990, Hubble was carried aloft from NASA's Kennedy Space Center in Florida aboard the space shuttle Discovery, along with a five-astronaut crew. Deployed into Earth orbit a day later, the telescope opened a new eye onto the cosmos that has been transformative for our civilization. 73)
- "Hubble has given us stunning insights about the universe, from nearby planets to the farthest galaxies we have seen so far," said Thomas Zurbuchen, associate administrator for science at NASA Headquarters in Washington, D.C. "It was revolutionary to launch such a large telescope 30 years ago, and this astronomy powerhouse is still delivering revolutionary science today. Its spectacular images have captured the imagination for decades, and will continue to inspire humanity for years to come."
- The NASA/ESA Hubble Space Telescope’s iconic images and scientific breakthroughs have redefined our view of the Universe. To commemorate three decades of scientific discoveries, this image is one of the most photogenic examples of the many turbulent stellar nurseries the telescope has observed during its 30-year lifetime. 74)
- The portrait features the giant nebula NGC 2014 and its neighbor NGC 2020 which together form part of a vast star-forming region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, approximately 163,000 light-years away. The image is nicknamed the ‘Cosmic Reef’ because it resembles an undersea world.
- On 24 April 1990, the Hubble Space Telescope was launched on the Space Shuttle Discovery, along with a five-astronaut crew. Deployed into low Earth orbit a day later, the telescope has since opened our eyes to the cosmos and transformed our collective knowledge of the Universe.
- Hubble has revolutionized modern astronomy not only for astronomers, but also for the public, taking them on a journey of exploration and discovery. Unlike any other telescope before it, Hubble has made astronomy relevant, engaging and accessible for people of all ages.
- The mission has yielded to date 1.4 million observations and provided data that astronomers around the world have used to write more than 17 000 peer-reviewed scientific publications, making it one of the most prolific space observatories in history. Its rich data archive alone will fuel future astronomy research for generations to come.
Figure 69: Hubble Space Telescope’s iconic images and scientific breakthroughs have redefined our view of the Universe. To commemorate three decades of scientific discoveries, this image is one of the most photogenic examples of the many turbulent stellar nurseries the telescope has observed during its 30-year lifetime. The portrait features the giant nebula NGC 2014 and its neighbor NGC 2020 which together form part of a vast star-forming region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, approximately 163 000 light-years away. The image is nicknamed the “Cosmic Reef” because it resembles an undersea world (image credit: NASA, ESA, and STScI; CC BY 4.0)
- Each year, Hubble has a small portion of its precious observing time dedicated to taking a special anniversary image, showcasing particularly beautiful and meaningful objects. These observations continue to challenge scientists with surprising new findings and to fascinate the public with ever more evocative images.
Figure 70: Hubble’s collection of anniversary images: To celebrate Hubble’s 30th anniversary, let’s look back at the beauty and science behind each of the anniversary images unveiled as of 2005. In this video, we will also feature the very special 2020 Hubble Space Telescope 30th anniversary image [video credit: ESA/Hubble, Directed by: Bethany Downer; Visual design and editing: Martin Kornmesser; Written by: Bethany Downer; Narration: Sara Mendes da Costa; Images & Videos: NASA, ESA, M. Kornmesser, L. Calçada, ESO, NAOJ, G. Bacon, L. Frattare, Z. Levay and F. Summers (STScI/AURA), D. Lennon and E. Sabbi (ESA/STScI), J. Anderson, S. E. de Mink, R. van der Marel, T. Sohn, and N. Walborn (STScI), L. Bedin (INAF, Padua), C. Evans (STFC), H. Sana (Amsterdam), N. Langer (Bonn), P. Crowther (Sheffield), A. Herrero (IAC, Tenerife), N. Bastian (USM, Munich), and E. Bressert (ESO), the Hubble Heritage Team, T. Davis, L. Frattare, Z. Levay, (Viz 3D team, STScI), J. Anderson (STScI), the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team, Eckhard Slawik (email@example.com); Music: Johan B. Monell (www.johanmonell.com Web and technical support: Raquel Yumi Shida Executive producer: Mariya Lyubenova]
- This year, Hubble is celebrating this new milestone with a portrait of two colorful nebulae that reveals how energetic, massive stars sculpt their homes of gas and dust. Although NGC 2014 and NGC 2020 appear to be separate in this visible-light image, they are actually part of one giant star formation complex.
- The star-forming regions seen here are dominated by the glow of stars at least 10 times more massive than our Sun. These stars have short lives of only a few million years, compared to the 10-billion-year lifetime of our Sun.
- The sparkling centerpiece of NGC 2014 is a grouping of bright, massive stars near the center of the image that has blown away its cocoon of hydrogen gas (colored red) and dust in which it was born. A torrent of ultraviolet radiation from the star cluster is illuminating the landscape around it.
- These massive stars also unleash fierce winds that are eroding the gas cloud above and to the right of them. The gas in these areas is less dense, making it easier for the stellar winds to blast through them, creating bubble-like structures reminiscent of coral, that have earned the nebula the nickname ‘Brain Coral’.
Figure 71: Pan across the Cosmic Reef: This image is one of the most photogenic examples of the many turbulent stellar nurseries the NASA/ESA Hubble Space Telescope has observed during its 30-year lifetime. The portrait features the giant nebula NGC 2014 and its neighbor NGC 2020 which together form part of a vast star-forming region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, approximately 163,000 light-years away (video credit: NASA, ESA, and STScI)
- By contrast, the blue-colored nebula below NGC 2014 has been shaped by one mammoth star that is roughly 200,000 times more luminous than our Sun. It is an example of a rare class of stars called Wolf-Rayet stars, thought to be the descendants of the most massive stars. Wolf-Rayet stars are very luminous and have a high rate of mass loss through powerful winds.
- The star in this Hubble image is 15 times more massive than our Sun and is unleashing powerful winds, which have cleared out the area around it. It has ejected its outer layers of gas, sweeping them around into a cone-like shape, and exposing its searing hot core.
- The behemoth appears offset from the center because the telescope is viewing the cone from a slightly tilted angle. In a few million years, the star might become a supernova. The brilliant blue color of the nebula comes from oxygen gas that is heated to roughly 11,000 °C, which is much hotter than the hydrogen gas surrounding it.
Figure 72: A 3D animation of the Cosmic Reef, exploring the star-forming region featured in the Hubble's 30th anniversary image in impressive detail [video credit: NASA, ESA, G. Bacon, J. DePasquale, L. Hustak, J. Olmstead, A. Pagan, D. Player, and F. Summers (STScI). Music: "Cosmic Reef" by J. DePasquale (STScI)]
- Stars, both big and small, are born when clouds of dust and gas collapse because of gravity. As more material falls onto the forming star, it finally becomes hot and dense enough at its center to trigger the nuclear fusion reactions that make stars, including our Sun, shine.
- Massive stars make up only a few percent of the billions of stars in our Universe. Yet they play a crucial role in shaping our Universe, through stellar winds, supernova explosions, and the production of heavy elements.
- “The Hubble Space Telescope has shaped the imagination of truly a whole generation, inspiring not only scientists, but almost everybody,” said Prof. Günther Hasinger, ESA Director of Science. “It is paramount for the excellent and long-lasting cooperation between NASA and ESA.”
• 20 April 2020: On Aug. 30, 2019, when amateur astronomer Gennady Borisov gazed upward with his homemade telescope, he spotted an object moving in an unusual direction. Now called 2I/Borisov, this runaway point of light turned out to be the first confirmed comet to enter our solar system from some unknown place beyond our Sun’s influence. Astronomers everywhere rushed to take a look with some of the most powerful instruments in the world, hoping to learn as much as they could about the mysterious visitor. 75) 76)
Figure 73: This is a time-lapse sequence compressing Hubble Space Telescope observations of comet 2I/Borisov, spanning a seven-hour period. As the second known interstellar object to enter our solar system, the comet is moving along at a breakneck speed of 110,000 miles per hour (180,000 kilometers per hour). To photograph the comet Hubble has to track it, like a photographer tracking a racetrack horse. Therefore, background stars are streaked in the exposure frames. An artificial satellite also crosses the field of view. Hubble reveals a central concentration of dust around an unseen nucleus. Comet 2I/Borisov is only the second such interstellar object known to have passed through the solar system. In 2017, the first identified interstellar visitor, an object formally named 'Oumuamua', swung within 24 million miles (39 million kilometers) of the Sun before racing out of the solar system [image credit: NASA, ESA and J. DePasquale (STScI)]
- Now, thanks to observations with NASA’s Hubble Space Telescope and the National Radio Astronomy Observatory’s ALMA (Atacama Large Millimeter/submillimeter Array), astronomers have figured out that 2I/Borisov has an unusual composition. Specifically, it has a higher concentration of carbon monoxide than any comet seen at a similar distance; that is, within about 200 million miles (300 million kilometers) of the Sun.
- This suggests to scientists that the comet could have formed around a red dwarf — a smaller, fainter type of star than the Sun — though other kinds of stars are possible. Another idea is that 2I/Borisov could be a carbon monoxide-rich fragment of a small planet.
What is an interstellar comet?
- Comets are snowballs of ice, dust and frozen gas. When totally frozen (or “inactive”), they’re approximately the diameter of a small town, but when heated by the Sun their tails can extend for millions of miles. 2I/Borisov is about the length of nine football fields, or 0.61 miles (0.98 kilometers), making it relatively small. The new results on the comet’s composition are published in the journal Nature Astronomy.
- All comets form in the primordial disk of material that encircles a young star, preserving remnants of a planetary system’s ancient past. Comets from our own solar neighborhood reveal the history of materials, including water, that made Earth the planet we know today, as well as our other planetary neighbors. An interstellar comet, on the other hand, is a chemical ambassador from an entirely different star system — containing a treasure trove of clues to worlds too far to reach with modern technology.
- “With an interstellar comet passing through our own solar system, it’s like we get a sample of a planet orbiting another star showing up in our own backyard,” said John Noonan of the Lunar and Planetary Laboratory at the University of Arizona, Tucson, and a member of the Hubble research team led by Dennis Bodewits of Auburn University in Alabama.
What scientists found
- Bodewits and colleagues used Hubble to look at 2I/Borisov from Dec. 11, 2019 to Jan. 13, 2020. Separately, a team of international scientists led by Martin Cordiner and Stefanie Milam at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, studied the comet on December 15 and 16, 2019, with ALMA, an array of radio telescopes at an altitude of 5,050 meters in northern Chile. Radio telescopes are especially useful for looking at cold, low-energy gas in objects like comets.
- Results from both Hubble and ALMA estimate that 2I/Borisov’s carbon monoxide concentration is higher than that of the average solar system comet.
Where did it come from?
- A high carbon-monoxide-to-water ratio suggests that the comet has traveled from a very cold place — as cold as the area where Pluto is in relation to our Sun, called the Kuiper Belt. The group using Hubble additionally theorizes 2I/Borisov may have originated around the most common type of star in the Milky Way: a red dwarf. Red dwarfs are much smaller and dimmer than the Sun, so the planet-forming material around them would be colder than the building blocks of our solar system.
- “These stars have exactly the low temperatures and luminosities where a comet could form with the type of composition found in comet 2I/Borisov,” said Noonan.
- Scientists using ALMA say it’s possible that 2I/Borisov could be a fragment of a dwarf planet that had a lot of carbon monoxide near its surface, regardless of which type of star it came from. “If that object collided with another, then the carbon monoxide-rich fragments could be released into space,” said Cordiner.
- But 2I/Borisov may have simply formed as a comet with a high concentration of carbon monoxide, the ALMA team points out. Alternatively, it may have an unusually thick outer layer that insulates frozen gases like hydrogen cyanide and water. As the more volatile carbon monoxide evaporates or “outgasses,” it may appear more abundant than other cometary gases. 2I/Borisov’s unusual properties may also suggest a wider diversity of carbon monoxide in comets in our own solar system than previously thought.
- “Whatever the answer is, 2I/Borisov opens up a whole new can of worms for cometary science,” says Milam, one of the scientists using ALMA.
- In our own solar system, there are two places where most comets reside: The Kuiper Belt, an area that includes Pluto; and the Oort Cloud, which is much farther away. All of these comets likely formed closer to the Sun, but may have been booted outward by the erratic movements of Jupiter and Saturn billions of years ago. These giant planets, because of their immense gravity, could have even sent comets flying toward other stars, escaping the influence of the Sun’s gravity altogether.
- Given this history, scientists using Hubble theorize that a massive planet in a red dwarf system, in an environment with frozen carbon monoxide, may have punted 2I/Borisov our way.
- “If a Jupiter-sized planet migrates inward, it could kick out a lot of these comets,” Bodewits said.
- The team using ALMA agrees that a young moving planet likely sent the comet on its way. “Then, after a cold, lonely voyage, 2I/Borisov made its close encounter with our solar system and started outgassing and showing us what it’s got inside,” Cordiner says.
Figure 74: When the scientists peeked inside the halo of gas that formed around comet 2I/Borisov as it came closer to the Sun, they detected something peculiar: 2I/Borisov was releasing gas with a greater concentration of carbon monoxide than anyone had detected in any comet at a similar distance from the Sun (video credits: NASA/ALMA)
More to come
- 2I/Borisov is only the second object astronomers have detected that definitely came from a different star system. The first was 'Oumuamua, discovered in October 2017, that whizzed by too quickly for scientists to pin down its chemistry. Whether it too is a comet, an asteroid, or something else — we may never know.
- 2I/Borisov is continuing on its path through the solar system, and will eventually head out. As more advanced telescopes and other instruments turn on and gaze out in the coming years, astronomers expect to find more interstellar objects, though they will still be rare.
- “Our solar system is so tiny compared to the distances between star systems,” Cordiner says. “For an interstellar comet to come in and hit the bullseye like Borisov did is incredible.”
- ALMA is a partnership of ESO (representing its Member States), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (South Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.
• 20 April 2020: The Hubble Space Telescope offers insight into the nature of exoplanet Fomalhaut b. What astronomers thought was a planet beyond our solar system, has now seemingly vanished from sight. Astronomers now suggest that a full-grown planet never existed in the first place. The NASA/ESA Hubble Space Telescope had instead observed an expanding cloud of very fine dust particles caused by a titanic collision between two icy asteroid-sized bodies orbiting the bright star Fomalhaut, about 25 light-years from Earth. 77)
Figure 75: Data from the NASA/ESA Hubble Space Telescope have revealed an expanding cloud of dust produced in a collision between two large bodies orbiting the bright nearby star Fomalhaut. This is the first time such a catastrophic event around another star has been imaged (image credit: ESA/NASA, M. Kornmesser)
- “The Fomalhaut system is the ultimate test lab for all of our ideas about how exoplanets and star systems evolve,” said George Rieke of the University of Arizona’s Steward Observatory. “We do have evidence of such collisions in other systems, but none of this magnitude has ever been observed. This is a blueprint for how planets destroy each other.”
- The object was previously believed to be a planet, called Fomalhaut b, and was first announced in 2008 based on data taken in 2004 and 2006. It was clearly visible in several years of Hubble observations that revealed it as a moving dot. Unlike other directly imaged exoplanets, nagging puzzles with Fomalhaut b arose early on. The object was unusually bright in visible light, but did not have any detectable infrared heat signature. Astronomers proposed that the added brightness came from a huge shell or ring of dust encircling the object that may have been collision-related. Also, early Hubble observations suggested the object might not be following an elliptical orbit, as planets usually do.
- “These collisions are exceedingly rare and so this is a big deal that we actually get to see one,” said András Gáspár of the University of Arizona. “We believe that we were at the right place at the right time to have witnessed such an unlikely event with the Hubble Space Telescope.”
- “Our study, which analyzed all available archival Hubble data on Fomalhaut b, including the most recent images taken by Hubble, revealed several characteristics that together paint a picture that the planet-sized object may never have existed in the first place,” said Gáspár. 78) 79)
- Hubble images from 2014 showed the object had vanished, to the disbelief of the astronomers. Adding to the mystery, earlier images showed the object to continuously fade over time. “Clearly, Fomalhaut b was doing things a bona fide planet should not be doing,” said Gáspár.
Figure 76: Illustration from the Hubble Space Telescope’s observations of Fomalhaut b’s expanding dust cloud from 2004 to 2013. The cloud was produced in a collision between two large bodies orbiting the bright nearby star Fomalhaut. This is the first time such a catastrophic event around another star has been imaged [image credit: NASA, ESA, and A. Gáspár and G. Rieke (University of Arizona)]
- The resulting interpretation is that Fomalhaut b is not a planet, but a slowly expanding cloud blasted into space as a result of a collision between two large bodies. Researchers believe the collision occurred not too long prior to the first observations taken in 2004. By now the debris cloud, consisting of dust particles around 1 micron (1/50th the diameter of a human hair), is below Hubble’s detection limit. The dust cloud is estimated to have expanded by now to a size larger than the orbit of Earth around our Sun.
- Equally confounding is that the object is not on an elliptical orbit, as expected for planets, but on an escape trajectory, or hyperbolic path. “A recently created massive dust cloud, experiencing considerable radiative forces from the central star Fomalhaut, would be placed on such a trajectory” Gáspár said, “Our model is naturally able to explain all independent observable parameters of the system: its expansion rate, its fading and its trajectory.”
- Because Fomalhaut b is presently inside a vast ring of icy debris encircling the star, the colliding bodies were likely a mixture of ice and dust, like the cometary bodies that exist in the Kuiper belt on the outer fringe of our solar system. Gáspár and Rieke estimate that each of these comet-like bodies measured about 200 kilometers across. The also suggest that the Fomalhaut system may experience one of these collision events only every 200,000 years.
- Gáspár, Rieke, and other astronomers will also be observing the Fomalhaut system with the upcoming NASA/ESA/CSA James Webb Space Telescope, which is scheduled to launch in 2021.
• 17 April 2020: This image depicts a swirling spiral galaxy named NGC 2906. - The blue speckles seen scattered across this galaxy are massive young stars, which emit hot, blue-tinged radiation as they burn through their fuel at an immense rate. The swathes of orange are a mix of older stars that have swollen and cooled, and low-mass stars that were never especially hot to begin with. Owing to their lower temperatures, these stars emit a cooler, reddish, radiation. 80)
Figure 77: This image of NGC 2906 was captured by the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3, an instrument installed on Hubble in 2009 during the telescope’s fourth servicing mission. Hubble observed this galaxy on the hunt for fading light from recent, nearby occurrences of objects known as supernovae (image credit: ESA/Hubble & NASA, A Filippenko; CC BY 4.0)
• 10 April 2020: At first glance, the subject of this NASA/ESA Hubble Space Telescope image looks to be a simple spiral galaxy, with two pinwheeling arms emerging from a central bar of stars and material that cuts through the galactic center. In fact, there are rings within these spiral arms, too: spirals within a spiral. 81)
- This kind of morphology is known as a multiring structure. As this description suggests, this galaxy, named NGC 2273, hosts an inner ring and two outer “pseudorings” — having so many distinct rings is rare, and makes NGC 2273 unusual. Rings are created when a galaxy’s spiral arms appear to loop around to nearly close upon one another, combined with a trick of cosmic perspective. NGC 2273’s two pseudorings are formed by two swirling sets of spiral arms coming together, and the inner ring by two arcing structures nearer to the galactic center, which seem to connect in a similar way.
Figure 78: These rings are not the only unique feature of this galaxy. NGC 2273 is also a Seyfert galaxy, a galaxy with an extremely luminous core. In fact, the center of a galaxy such as this is powered by a supermassive black hole, and can glow brightly enough to outshine an entire galaxy like the Milky Way (image credit: ESA/Hubble & NASA, J. Greene; CC BY 4.0)
• 03 April 2020: This remarkable spiral galaxy, known as NGC 4651, may look serene and peaceful as it swirls in the vast, silent emptiness of space, but don’t be fooled — it keeps a violent secret. It is believed that this galaxy consumed another smaller galaxy to become the large and beautiful spiral that we observe today. 82)
Figure 79: Although only a telescope like the NASA/ESA Hubble Space Telescope, which captured this image, could give us a picture this clear, NGC 4651 can also be observed with an amateur telescope — so if you have a telescope at home and a star-gazing eye, look out for this glittering carnivorous spiral (image credit: ESA/Hubble & NASA, D. Leonard; CC BY 4.0)
Hubble Space Telescope 2020 continued
• 31 March 2020: Astronomers have found the best evidence for the perpetrator of a cosmic homicide: a black hole of an elusive class known as "intermediate-mass," which betrayed its existence by tearing apart a wayward star that passed too close. 83)
- Weighing in at about 50,000 times the mass of our Sun, the black hole is smaller than the supermassive black holes (at millions or billions of solar masses) that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of a massive star.
- These so-called intermediate-mass black holes (IMBHs) are a long-sought "missing link" in black hole evolution. Though there have been a few other IMBH candidates, researchers consider these new observations the strongest evidence yet for mid-sized black holes in the universe.
- It took the combined power of two X-ray observatories and the keen vision of NASA's Hubble Space Telescope to nail down the cosmic beast.
Figure 80: Astronomers have found the best evidence for a black hole of an elusive class known as “intermediate-mass,” which betrayed its existence by tearing apart a wayward star that passed too close. This exciting discovery opens the door to the possibility of many more lurking undetected in the dark, waiting to be given away by a star passing too close (video credit: NASA's Goddard Space Flight Center)
- "Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate. That is what Hubble has allowed us to do for our candidate," said Dacheng Lin of the University of New Hampshire, principal investigator of the study. The results are published on March 31, 2020, in The Astrophysical Journal Letters. 84)
- The story of the discovery reads like a Sherlock Holmes story, involving the meticulous step-by-step case-building necessary to catch the culprit.
- Lin and his team used Hubble to follow up on leads from NASA's Chandra X-ray Observatory and ESA's (the European Space Agency) X-ray Multi-Mirror Mission (XMM-Newton). In 2006 these satellites detected a powerful flare of X-rays, but they could not determine whether it originated from inside or outside of our galaxy. Researchers attributed it to a star being torn apart after coming too close to a gravitationally powerful compact object, like a black hole.
- Surprisingly, the X-ray source, named 3XMM J215022.4-055108, was not located in a galaxy's center, where massive black holes normally would reside. This raised hopes that an IMBH was the culprit, but first another possible source of the X-ray flare had to be ruled out: a neutron star in our own Milky Way galaxy, cooling off after being heated to a very high temperature. Neutron stars are the crushed remnants of an exploded star.
- Hubble was pointed at the X-ray source to resolve its precise location. Deep, high-resolution imaging provides strong evidence that the X-rays emanated not from an isolated source in our galaxy, but instead in a distant, dense star cluster on the outskirts of another galaxy — just the type of place astronomers expected to find an IMBH. Previous Hubble research has shown that the mass of a black hole in the center of a galaxy is proportional to that host galaxy's central bulge. In other words, the more massive the galaxy, the more massive its black hole. Therefore, the star cluster that is home to 3XMM J215022.4-055108 may be the stripped-down core of a lower-mass dwarf galaxy that has been gravitationally and tidally disrupted by its close interactions with its current larger galaxy host.
Figure 81: This Hubble Space Telescope image identified the location of an intermediate-mass black hole, weighing 50,000 times the mass of our Sun (making it much smaller than supermassive black holes found in the centers of galaxies). The black hole, named 3XMM J215022.4-055108, is indicated by the white circle. The elusive type of black hole was first identified in a burst of telltale X-rays emitted by hot gas from a star as it was captured and destroyed by the black hole. Hubble was needed to pinpoint the black hole's location in visible light. Hubble's deep, high-resolution imaging shows that the black hole resides inside a dense cluster of stars that is far beyond our Milky Way galaxy. The star cluster is in the vicinity of the galaxy at the center of the image. Much smaller-looking background galaxies appear sprinkled around the image, including a face-on spiral just above the central foreground galaxy. This photo was taken with Hubble's Advanced Camera for Surveys [image credits: NASA, ESA and D. Lin (University of New Hampshire)]
- IMBHs have been particularly difficult to find because they are smaller and less active than supermassive black holes; they do not have readily available sources of fuel, nor as strong a gravitational pull to draw stars and other cosmic material which would produce telltale X-ray glows. Astronomers essentially have to catch an IMBH red-handed in the act of gobbling up a star. Lin and his colleagues combed through the XMM-Newton data archive, searching hundreds of thousands of observations to find one IMBH candidate.
- The X-ray glow from the shredded star allowed astronomers to estimate the black hole's mass of 50,000 solar masses. The mass of the IMBH was estimated based on both X-ray luminosity and the spectral shape. "This is much more reliable than using X-ray luminosity alone as typically done before for previous IMBH candidates," said Lin. "The reason why we can use the spectral fits to estimate the IMBH mass for our object is that its spectral evolution showed that it has been in the thermal spectral state, a state commonly seen and well understood in accreting stellar-mass black holes."
- This object isn't the first to be considered a likely candidate for an intermediate-mass black hole. In 2009 Hubble teamed up with NASA's Swift observatory and ESA's XMM-Newton to identify what is interpreted as an IMBH, called HLX-1, located towards the edge of the galaxy ESO 243-49. It too is in the center of a young, massive cluster of blue stars that may be a stripped-down dwarf galaxy core. The X-rays come from a hot accretion disk around the black hole. "The main difference is that our object is tearing a star apart, providing strong evidence that it is a massive black hole, instead of a stellar-mass black hole as people often worry about for previous candidates including HLX-1," Lin said.
- Finding this IMBH opens the door to the possibility of many more lurking undetected in the dark, waiting to be given away by a star passing too close. Lin plans to continue his meticulous detective work, using the methods his team has proved successful. Many questions remain to be answered. Does a supermassive black hole grow from an IMBH? How do IMBHs themselves form? Are dense star clusters their favored home?
- 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 82: This illustration depicts a cosmic homicide in action. A wayward star is being shredded by the intense gravitational pull of a black hole that contains tens of thousands of solar masses. The stellar remains are forming an accretion disk around the black hole. Flares of X-ray light from the super-heated gas disk alerted astronomers to the black hole's location; otherwise it lurked unknown in the dark. The elusive object is classified as an intermediate mass black hole (IMBH), as it is much less massive than the monster black holes that dwell in the centers of galaxies. Therefore, IMBHs are mostly quiescent because they do not pull in as much material, and are hard to find. Hubble observations provide evidence that the IMBH dwells inside a dense star cluster. The cluster itself may be the stripped-down core of a dwarf galaxy [image credit: NASA, ESA and D. Player (STScI)]
• 27 March 2020: NGC 4618 was discovered on 9 April 1787 by the German-British astronomer, Wilhelm Herschel, who also discovered Uranus in 1781. Only a year before discovering NGC 4618, Herschel theorized that the “foggy” objects astronomers were seeing in the night sky were likely to be large star clusters located much further away then the individual stars he could easily discern. 85)
- Since Herschel proposed his theory, astronomers have come to understand that what he was seeing was a galaxy. NGC 4618, classified as a barred spiral galaxy, has the special distinction amongst other spiral galaxies of only having one arm rotating around the center of the galaxy.
Figure 83: Located about 21 million light-years from our galaxy in the constellation Canes Venatici, NGC 4618 has a diameter of about one third that of the Milky Way. Together with its neighbor, NGC 4625, it forms an interacting galaxy pair, which means that the two galaxies are close enough to influence each other gravitationally. These interactions may result in the two (or more) galaxies merging together to form a new formation, such as a ring galaxy (image credit: ESA/Hubble & NASA, I. Karachentsev; CC BY 4.0)
• 19 March 2020: Using the unique capabilities of NASA's Hubble Space Telescope, a team of astronomers has discovered the most energetic outflows ever witnessed in the universe. They emanate from quasars and tear across interstellar space like tsunamis, wreaking havoc on the galaxies in which the quasars live. 86)
Figure 84: This is an artist's concept of a distant galaxy with an active quasar at its center. A quasar emits exceptionally large amounts of energy generated by a supermassive black hole fueled by infalling matter. Using the unique capabilities of the Hubble Space Telescope, astronomers have discovered that blistering radiation pressure from the vicinity of the black hole pushes material away from the galaxy's center at a fraction of the speed of light. The "quasar winds" are propelling hundreds of solar masses of material each year. This affects the entire galaxy as the material snowplows into surrounding gas and dust [image credit: NASA/ESA, and J. Olmsted (STScI)]
- Quasars are extremely remote celestial objects, emitting exceptionally large amounts of energy. Quasars contain supermassive black holes fueled by infalling matter that can shine 1,000 times brighter than their host galaxies of hundreds of billions of stars.
- As the black hole devours matter, hot gas encircles it and emits intense radiation, creating the quasar. Winds, driven by blistering radiation pressure from the vicinity of the black hole, push material away from the galaxy's center. These outflows accelerate to breathtaking velocities that are a few percent of the speed of light.
- "No other phenomena carries more mechanical energy. Over the lifetime of 10 million years, these outflows produce a million times more energy than a gamma-ray burst," explained principal investigator Nahum Arav of Virginia Tech in Blacksburg, Virginia. "The winds are pushing hundreds of solar masses of material each year. The amount of mechanical energy that these outflows carry is up to several hundreds of times higher than the luminosity of the entire Milky Way galaxy."
- The quasar winds snowplow across the galaxy's disk. Material that otherwise would have formed new stars is violently swept from the galaxy, causing star birth to cease. Radiation pushes the gas and dust to far greater distances than scientists previously thought, creating a galaxy-wide event.
- As this cosmic tsunami slams into interstellar material, the temperature at the shock front spikes to billions of degrees, where material glows largely in X-rays, but also widely across the light spectrum. Anyone witnessing this event would see a brilliant celestial display. "You'll get lots of radiation first in X-rays and gamma rays, and afterwards it will percolate to visible and infrared light," said Arav. "You'd get a huge light show—like Christmas trees all over the galaxy."
- Numerical simulations of galaxy evolution suggest that such outflows can explain some important cosmological puzzles, such as why astronomers observe so few large galaxies in the universe, and why there is a relationship between the mass of the galaxy and the mass of its central black hole. This study shows that such powerful quasar outflows should be prevalent in the early universe.
- "Both theoreticians and observers have known for decades that there is some physical process that shuts off star formation in massive galaxies, but the nature of that process has been a mystery. Putting the observed outflows into our simulations solves these outstanding problems in galactic evolution," explained eminent cosmologist Jeremiah P. Ostriker, of Columbia University in New York and Princeton University in New Jersey.
- Astronomers studied 13 quasar outflows, and they were able to clock the breakneck speed of gas being accelerated by the quasar wind by looking at spectral "fingerprints" of light from the glowing gas. The Hubble ultraviolet data show that these light absorption features created from material along the path of the light were shifted in the spectrum because of the fast motion of the gas across space. This is due to the Doppler effect, where the motion of an object compresses or stretches wavelengths of light depending on whether it is approaching or receding from us. Only Hubble has the specific range of ultraviolet sensitivity that allows for astronomers to obtain the necessary observations leading to this discovery.
- Aside from measuring the most energetic quasars ever observed, the team also discovered another outflow accelerating faster than any other. It increased from nearly 43 million miles per hour to roughly 46 million miles per hour in a three-year period. The scientists believe its acceleration will continue to increase over time.
- "Hubble's ultraviolet observations allow us to follow the whole range of energy output from quasars, from cooler gas to the extremely hot, highly ionized gas in the more massive winds," added team member Gerard Kriss of the Space Telescope Science Institute in Baltimore, Maryland. "These were previously only visible with much more difficult X-ray observations. Such powerful outflows may yield new insights into the link between the growth of a central supermassive black hole and the development of its entire host galaxy."
- The team also includes graduate student Xinfeng Xu and postdoctoral researcher Timothy Miller, both of Virginia Tech, as well as Rachel Plesha of the Space Telescope Science Institute in Baltimore, Maryland. The findings were published in a series of six papers in March 2020, as a focus issue of The Astrophysical Journal Supplements.
• 19 March 2020: As reports continue about the spread of COVID-19 (coronavirus disease), this Announcement details the updates for the planned Hubble 30th Anniversary Image Unveiling events. 87)
- The health and safety of visitors and organizers of Hubble 30 celebrations events remain our top priority. The original plan was to have unveiling events taking place on or shortly after the anniversary date of 24 April. ESA/Hubble is now shifting its vision to instead hold events in the coming months that are a general celebration of the Hubble Space Telescope’s splendid 30 years.
- We understand the spread of the coronavirus may impact the feasibility of hosting public events, particularly those with large audiences. We are therefore flexible with regard to the new dates for the Hubble Space Telescope’s 30th anniversary image to be featured at various European facilities. The showcasing of the image may form part of a general Hubble celebration event any time after the 24 April public image release, and before 30 September 2020. Instead of only display events, ESA/Hubble is encouraging a broad style of events and activities that celebrate Hubble in general, and its 30 years of scientific discoveries. As events become more widespread throughout the year, ESA/Hubble will also support activities in whatever way possible, including the provision of additional materials and possible on-site support, such as qualified representatives from ESA/Hubble who can speak at various events.
- Public health should remain paramount in this situation and ESA/Hubble is confident that the presence of the Hubble 30th anniversary image at various European locations will continue to be a source of amazement to public guests. Updates will be provided at a later date regarding the new event dates and plans, and general Hubble 30 updates will be posted on this page.
• 18 March 2020: This scene of stellar creation, captured by the NASA/ESA Hubble Space Telescope, sits near the outskirts of the famous Tarantula Nebula (Figure 85). This cloud of gas and dust, as well as the many young and massive stars surrounding it, is the perfect laboratory to study the origin of massive stars. 88)
- The bright pink cloud and the young stars surrounding it in this image taken with the NASA/ESA Hubble Space Telescope have the uninspiring name LHA 120-N 150. This region of space is located on the outskirts of the Tarantula Nebula, which is the largest known stellar nursery in the local Universe. The nebula is situated over 160,000 light-years away in the Large Magellanic Cloud, a neighboring irregular dwarf galaxy that orbits the Milky Way.
- The Large Magellanic Cloud has had one or more close encounters in the past, possibly with the Small Magellanic Cloud. These interactions have caused an episode of energetic star formation in our tiny neighbor – part of which is visible as the Tarantula Nebula.
- Also known as 30 Doradus or NGC 2070, the Tarantula Nebula owes its name to the arrangement of bright patches that somewhat resemble the legs of a tarantula. It measures nearly 1000 light-years across. Its proximity, the favorable inclination of the Large Magellanic Cloud, and the absence of intervening dust make the Tarantula Nebula one of the best laboratories in which to study the formation of stars, in particular massive stars. This nebula has an exceptionally high concentration of massive stars, often referred to as super star clusters.
- Astronomers have studied LHA 120-N 150 to learn more about the environment in which massive stars form. Theoretical models of the formation of massive stars suggest that they should form within clusters of stars; but observations indicate that up to ten percent of them also formed in isolation. The giant Tarantula Nebula with its numerous substructures is the perfect laboratory in which to resolve this puzzle as in it massive stars can be found both as members of clusters and in isolation.
Figure 85: A massive laboratory. This image shows a region of space called LHA 120-N150. It is a substructure of the gigantic Tarantula Nebula. The latter is the largest known stellar nursery in the local Universe. The nebula is situated more than 160,000 light-years away in the Large Magellanic Cloud, a neighboring dwarf irregular galaxy that orbits the Milky Way (image credit: ESA/Hubble, NASA, I. Stephens; CC BY 4.0)
- With the help of Hubble, astronomers try to find out whether the isolated stars visible in the nebula truly formed alone or just moved away from their stellar siblings. However, such a study is not an easy task; young stars, before they are fully formed – especially massive ones – look very similar to dense clumps of dust.
- LHA 120-N 150 contains several dozen of these objects. They are a mix of unclassified sources – some probably young stellar objects and others probably dust clumps. Only detailed analysis and observations will reveal their true nature and that will help to finally solve the unanswered question of the origin of massive stars.
- Hubble has observed the Tarantula Nebula and its substructures in the past – always being interested in the formation and evolution of stars.
• 10 March 2020: A simple single-cell organism that may be growing on your lawn is helping astronomers probe the largest structures in the universe. — These organisms, called slime mold, feed on dead plant material, and they have an uncanny ability to seek out food sources. Although brainless, the organism's "genius" at creating efficient networks to reach their food goal has caught the attention of scientists. Researchers have recreated the slime mold's behavior in computer algorithms to help solve large-scale engineering problems such as finding the most efficient traffic routes in large cities, solving mazes, and pinpointing crowd evacuation routes. 89)
Figure 86: Cosmic Web and Slime Mold: Feasting Behavior of Brainless Organisms Shows Astronomers Where to Point the Hubble Telescope (image credit: NASA)
- These organisms, called slime mold, feed on dead plant material, and they have an uncanny ability to seek out food sources. Although brainless, the organism's "genius" at creating efficient networks to reach their food goal has caught the attention of scientists. Researchers have recreated the slime mold's behavior in computer algorithms to help solve large-scale engineering problems such as finding the most efficient traffic routes in large cities, solving mazes, and pinpointing crowd evacuation routes.
- A team of astronomers has now turned to slime mold to help them trace the universe's large-scale network of filaments. Built by gravity, these vast cobweb structures, called the cosmic web, tie galaxies and clusters of galaxies together along faint bridges of gas and dark matter hundreds of millions of light-years long.
- To trace the filaments, the research team designed a computer algorithm informed by slime-mold behavior. The team seeded the algorithm with the charted positions of 37,000 galaxies and ran it to generate a filamentary map. The astronomers then used archival observations from the Hubble Space Telescope to detect and study the faint gas permeating the web at the predicted locations.
- The behavior of one of nature's humblest creatures is helping astronomers probe the largest structures in the universe.
- The single-cell organism, known as slime mold (Physarum polycephalum), builds complex filamentary networks in search of food, finding near-optimal pathways to connect different locations. In shaping the universe, gravity builds a vast cobweb structure of filaments tying galaxies and clusters of galaxies together along faint bridges hundreds of millions of light-years long. There is an uncanny resemblance between the two networks: one crafted by biological evolution, and the other by the primordial force of gravity.
- The cosmic web is the large-scale backbone of the cosmos, consisting primarily of the mysterious substance known as dark matter and laced with gas, upon which galaxies are built. Dark matter cannot be seen, but it makes up the bulk of the universe's material. The existence of a web-like structure to the universe was first hinted at in the 1985 Redshift Survey conducted at the Harvard-Smithsonian Center for Astrophysics. Since those studies, the grand scale of this filamentary structure has grown in subsequent sky surveys. The filaments form the boundaries between large voids in the universe.
- But astronomers have had a difficult time finding these elusive strands, because the gas is so dim it is hard to detect. Now a team of researchers has turned to slime mold to help them build a map of the filaments in the local universe (within 500 million light-years from Earth) and find the gas within them.
- They designed a computer algorithm, inspired by slime-mold behavior, and tested it against a computer simulation of the growth of dark matter filaments in the universe. A computer algorithm is similar to a recipe that tells a computer precisely what steps to take to solve a problem.
- The researchers then applied the slime mold algorithm to data containing the locations of 37,000 galaxies mapped by the Sloan Digital Sky Survey at distances corresponding to 300 million light-years. The algorithm produced a three-dimensional map of the underlying cosmic web structure.
- They then analyzed the ultraviolet light from 350 quasars (at much farther distances of billions of light-years) catalogued in the Hubble Spectroscopic Legacy Archive, which holds the data from NASA's Hubble Space Telescope's spectrographs. These distant cosmic flashlights are the brilliant black-hole powered cores of active galaxies, whose light shines across space and through the foreground cosmic web. Imprinted on that light was the telltale absorption signature of otherwise undetected hydrogen gas that the team analyzed at specific points along the filaments. These target locations are far from the galaxies, which allowed the research team to link the gas to the universe's large-scale structure.
- "It's really fascinating that one of the simplest forms of life actually enables insight into the very largest-scale structures in the universe," said lead researcher Joseph Burchett of the University of California (UC), Santa Cruz. "By using the slime-mold simulation to find the location of the cosmic web filaments, including those far from galaxies, we could then use the Hubble Space Telescope's archival data to detect and determine the density of the cool gas on the very outskirts of those invisible filaments. Scientists have detected signatures of this gas for several decades, and we have proven the theoretical expectation that this gas comprises the cosmic web."
- The survey further validates research that denser regions of intergalactic gas is organized into filaments that the team found stretches over 10 million light-years from galaxies. (That distance is more than 100 times the diameter of our Milky Way galaxy.)
- The researchers turned to slime mold simulations when they were searching for a way to visualize the theorized connection between the cosmic web structure and the cool gas detected in previous Hubble spectroscopic studies.
- Then team member Oskar Elek, a computational media scientist at UC Santa Cruz, discovered online the work of Sage Jenson, a Berlin-based media artist. Among Jenson's works were mesmerizing artistic visualizations showing the growth of a slime mold's tentacle-like network of food-seeking structures. Jenson's art was based on outside scientific research which detailed an algorithm for simulating the growth of slime mold.
- The research team noted a striking similarity between how the slime mold builds complex filaments to capture new food, and how gravity, in shaping the universe, constructs the cosmic web strands between galaxies and galaxy clusters.
- Based on the simulation, Elek developed a three-dimensional computer model of the buildup of slime mold to estimate the location of the cosmic web's filamentary structure.
- Although using a slime-mold-inspired simulation to pinpoint the universe's largest structures may sound bizarre at first, scientists have used computer models of these humble microorganisms, as well as grown them in petri dishes in a lab, to solve such complex problems as finding the most efficient traffic routes in large cities, solving mazes, and pinpointing crowd evacuation routes. "These are hard problems to solve for a human, let alone a computer algorithm," Elek said.
- "You can almost see, especially in the map of galaxies in the local universe from the Sloan data, where the filaments should be," Burchett explained. "The slime-mold model fits that intuition impressively. The structure that you know should be there is all of a sudden found by the computer algorithm. There was no other known method that was well suited to this problem for our research."
- The researchers say that it is very difficult to design a reliable algorithm for finding the filaments in such a large survey of galaxies. "So it's quite amazing to see that the virtual slime mold gives you a very close approximation in just minutes," Elek explained. "You can literally watch it grow." Just for comparison, growing the organism in a petri dish takes days. Slime mold actually has a very special kind of intelligence for solving this one spatial task. After all, it's critical to its survival. 90)
• 09 March 2020: Hubble investigates hungry Galaxy. The astronomers are now using the NASA/ESA Hubble Space Telescope to test this interpretation. Hubble has observed such events before, so the scientists are confident that Hubble will be able to provide smoking gun evidence in the form of stellar debris that was ejected during the disruption event. 91)
Figure 87: The subject of this image taken by the NASA/ESA Hubble Space Telescope, a spiral galaxy named NGC 1589, was once the scene of a violent bout of cosmic hunger pangs. As astronomers looked on, a poor, hapless star was seemingly torn apart and devoured by the ravenous supermassive black hole at the center of the galaxy (image credit: ESA/Hubble & NASA; CC BY 4.0)
• 21 February 2020: In recognition of the NASA Hubble Space Telescope's 30 years of civilization-changing astronomical discoveries, the Smithsonian National Air and Space Museum (NASM) in Washington, D.C., has awarded its 2020 Collins Trophy for Current Achievement to the Hubble operations team at the Space Telescope Science Institute (STScI) in Baltimore and NASA's Goddard Space Flight Center in Greenbelt, Maryland. 92)
- In giving the award, the museum lauded the Hubble team’s achievements in keeping the venerable telescope a highly productive and scientifically viable space observatory. Over 30 years, the team's collective efforts have made Hubble history's most scientifically viable and celebrated telescope.
- "Through the efforts of the Hubble team the observatory has continued to produce research unachievable with any other instrument. System engineers in Hubble’s control center and science operations facility have continued to find creative ways to operate the 30-year-old spacecraft to make this revolutionary science possible and ensuring its capabilities will continue for years to come," reports NASM.
- “During its 30 years in space, Hubble has brought the universe down to Earth for all of humankind to explore. Hubble excites the imagination, inspires the soul, and teaches us that there is still much to learn about our place in the cosmos,” said STScI Director Ken Sembach. “Hubble's success would not have been possible without the close linkage of science and NASA's human exploration program, so the awarding of the Collins Trophy is especially meaningful. Hubble and Apollo are both superb examples of teamwork at its finest. It is an honor for the Space Telescope Science Institute and our employees to be an integral part of the Hubble Team, and it is with deep appreciation and great joy that we thank NASM for this recognition.”
- “The incredible images and groundbreaking science achievements of Hubble are possible only because of this extraordinary operations team of engineers, managers, technical experts and support scientists working tirelessly behind the scenes," said NASA Goddard Senior Hubble Project Scientist Jennifer Wiseman. "Every snapshot and spectrum of a planet, stellar nebula or distant galaxy is achieved because of this attentive and intertwined oversight of telescope hardware, control software, science instruments and data management. This team is keeping humanity’s premiere “eye on the sky” — Hubble — in fantastic shape for profound scientific discoveries.”
- Hubble was originally planned to operate through 2005. However, five astronaut space shuttle servicing missions to Hubble, from 1993 to 2009, continued to upgrade the telescope with advanced instruments, new electronics and on-orbit repairs.
- The Hubble operations team, consisting of engineers and scientists at NASA Goddard and STScI, is the backbone behind keeping the observatory viable between these space tune-ups, and tackling day-to-day operational challenges that are common with such a complex science facility remotely controlled in space.
- “The Hubble operations team continues to rise to the challenges presented to them, developing innovative long-term measures to ensure this national asset continues to unlock secrets of our universe,” said Greg Goulet, Lockheed Martin Hubble Project Engineering Manager at NASA Goddard. “The team has used its broad range of experience to keep the spacecraft and its instruments operating at a high level for the future. Our vision is to keep Hubble operating at its peak scientific productivity for many more years.”
- The ingenuity and dedication of Hubble's staff drive its success in two critical ways: first, they overcome the ongoing impacts of the harsh space environment, and second, they ensure Hubble continues to be a unique and powerful asset for tackling our most pressing mysteries in the next decade,” said Tom Brown, STScI Hubble mission head.
- Servicing thousands of astronomers worldwide with science planning, scheduling and archiving, the team's efforts have yielded to date 1.4 million observations and provided data that astronomers have used to write more than 17,000 peer-reviewed scientific publications on a broad range of topics: from solar system investigation, to characterizing exoplanets, to probing galaxy evolution throughout 97% of the lifetime of the observable universe.
- "Hubble has changed humans’ fundamental understanding of the universe," the NASM award cites. -Hubble’s jaw-dropping iconic images are a visual shorthand for Hubble’s achievements. A true trailblazer, Hubble has made astronomy very relevant, engaging and accessible for people of all ages.
- NASM awards this trophy to recognize achievements involving the management or execution of a scientific or technological project, a distinguished career of service in air and space technology, or a significant contribution in chronicling the history of air and space technology. The award was established in 1985 as the National Air and Space Museum Trophy, but renamed in 2020 in honor of Michael Collins, the Apollo 11 astronaut who flew the command module around the Moon while Neil Armstrong and Buzz Aldrin walked on the surface.
• 20 February 2020: Surprising new data from NASA's Hubble Space Telescope suggests the smooth, settled "brim" of the Sombrero galaxy's disk may be concealing a turbulent past. Hubble's sharpness and sensitivity resolves tens of thousands of individual stars in the Sombrero's vast, extended halo, the region beyond a galaxy's central portion, typically made of older stars. These latest observations of the Sombrero are turning conventional theory on its head, showing only a tiny fraction of older, metal-poor stars in the halo, plus an unexpected abundance of metal-rich stars typically found only in a galaxy's disk, and the central bulge. Past major galaxy mergers are a possible explanation, though the stately Sombrero shows none of the messy evidence of a recent merger of massive galaxies. 93)
- "The Sombrero has always been a bit of a weird galaxy, which is what makes it so interesting," said Paul Goudfrooij of the Space Telescope Science Institute (STScI), Baltimore, Maryland. "Hubble's metallicity measurements (i.e., the abundance of heavy elements in the stars) are another indication that the Sombrero has a lot to teach us about galaxy assembly and evolution."
- ”Hubble's observations of the Sombrero's halo are turning our generally accepted understanding of galaxy makeup and metallicity on its head," added co-investigator Roger Cohen of STScI.
- Long a favorite of astronomers and amateur sky watchers alike for its bright beauty and curious structure, the Sombrero galaxy (M104) now has a new chapter in its strange story — an extended halo of metal-rich stars with barely a sign of the expected metal-poor stars that have been observed in the halos of other galaxies. Researchers, puzzling over the data from Hubble, turned to sophisticated computer models to suggest explanations for the perplexing inversion of conventional galactic theory. Those results suggest the equally surprising possibility of major mergers in the galaxy's past, though the Sombrero's majestic structure bears no evidence of recent disruption. The unusual findings and possible explanations are published in the Astrophysical Journal.
- "The absence of metal-poor stars was a big surprise," said Goudfrooij, "and the abundance of metal-rich stars only added to the mystery."
- In a galaxy's halo astronomers expect to find earlier generations of stars with less heavy elements, called metals, as compared to the crowded stellar cities in the main disk of a galaxy. Elements are created through the stellar "lifecycle" process, and the longer a galaxy has had stars going through this cycle, the more element-rich the gas and the higher-metallicity the stars that form from that gas. These younger, high-metallicity stars are typically found in the main disk of the galaxy where the stellar population is denser — or so goes the conventional wisdom.
- Complicating the facts is the presence of many old, metal-poor globular clusters of stars. These older, metal-poor stars are expected to eventually move out of their clusters and become part of the general stellar halo, but that process seems to have been inefficient in the Sombrero galaxy. The team compared their results with recent computer simulations to see what could be the origin of such unexpected metallicity measurements in the galaxy's halo.
- The results also defied expectations, indicating that the unperturbed Sombrero had undergone major accretion, or merger, events billions of years ago. Unlike our Milky Way galaxy, which is thought to have swallowed up many small satellite galaxies in so-called "minor" accretions over billions of years, a major accretion is the merger of two or more similarly massive galaxies that are rich in later-generation, higher-metallicity stars.
- The satellite galaxies only contained low-metallicity stars that were largely hydrogen and helium from the big bang. Heavier elements had to be cooked up in stellar interiors through nucleosynthesis and incorporated into later-generation stars. This process was rather ineffective in dwarf galaxies such as those around our Milky Way, and more effective in larger, more evolved galaxies.
- The results for the Sombrero are surprising because its smooth disk shows no signs of disruption. By comparison, numerous interacting galaxies, like the iconic Antennae galaxies, get their name from the distorted appearance of their spiral arms due to the tidal forces of their interaction. Mergers of similarly massive galaxies typically coalesce into large, smooth elliptical galaxies with extended halos — a process that takes billions of years. But the Sombrero has never quite fit the traditional definition of either a spiral or an elliptical galaxy. It is somewhere in between — a hybrid.
- For this particular project, the team chose the Sombrero mainly for its unique morphology. They wanted to find out how such "hybrid" galaxies might have formed and assembled over time. Follow-up studies for halo metallicity distributions will be done with several galaxies at distances similar to that of the Sombrero.
Figure 88: On the left is an image of the Sombrero galaxy (M104) that includes a portion of the much fainter halo far outside its bright disk and bulge. Hubble photographed two regions in the halo (one of which is shown by the white box). The images on the right zoom in to show the level of detail Hubble captured. The orange box, a small subset of Hubble's view, contains myriad halo stars. The stellar population increases in density closer to the galaxy's disk (bottom blue box). Each frame contains a bright globular cluster of stars, of which there are many in the galaxy's halo. The Sombrero's halo contained more metal-rich stars than expected, but even stranger was the near-absence of old, metal-poor stars typically found in the halos of massive galaxies. Many of the globular clusters, however, contain metal-poor stars. A possible explanation for the Sombrero's perplexing features is that it is the product of the merger of massive galaxies billions of years ago, even though the smooth appearance of the galaxy's disk and halo show no signs of such a huge disruption [image credits: NASA/Digitized Sky Survey/P. Goudfrooij (STScI)/The Hubble Heritage Team (STScI/AURA)]
• 14 February 2020: The spiral galaxy NGC 2008 sits center stage, its ghostly spiral arms spreading out towards us, in this image captured by the NASA/ESA Hubble Space Telescope. 94)
Figure 89: This galaxy is located about 425 million light-years from Earth in the constellation of Pictor (The Painter’s Easel). Discovered in 1834 by astronomer John Herschel, NGC 2008 is categorized as a type Sc galaxy in the Hubble sequence, a system used to describe and classify the various morphologies of galaxies. The “S” indicates that NGC 2008 is a spiral, while the “c” means it has a relatively small central bulge and more open spiral arms. Spiral galaxies with larger central bulges tend to have more tightly wrapped arms, and are classified as Sa galaxies, while those in between are classified as type Sb (image credit: ESA/Hubble & NASA, A. Bellini; CC BY 4.0)
- Spiral galaxies are ubiquitous across the cosmos, comprising over 70% of all observed galaxies — including our own, the Milky Way. However, their ubiquity does not detract from their beauty. These grand, spiralling collections of billions of stars are among the most wondrous sights that have been captured by telescopes such as Hubble, and are firmly embedded in astronomical iconography.
• 31 January 2020: The NGC 7541 galaxy is a barred spiral galaxy with whirling, pinwheeling, spiral arms, and a bright center that is intersected by a bar of gas and stars. This bar cuts directly through the galaxy’s central region, and is thought to invigorate the region somewhat, sparking activity and fuelling myriad processes that may otherwise have never occurred or have previously ground to a halt (star formation and active galactic nuclei being key examples). We think bars exist in up to two-thirds of all spiral galaxies, including our own home, the Milky Way. 95)
- NGC 7541 is actually observed to have a higher-than-usual star formation rate, adding weight to the theory that spiral bars act as stellar nurseries, corralling and funnelling inwards the material and fuel needed to create and nurture new baby stars. Along with its nearby companion NGC 7537, the galaxy makes up a pair of galaxies located about 110 million light-years away from us.
Figure 90: The galaxy depicted in this Picture of the Week is a barred spiral known as NGC 7541, seen here as viewed by the NASA/ESA Hubble Space Telescope, in the constellation of Pisces (The Fishes), image credit: ESA/Hubble & NASA, A. Riess et al.; CC BY 4.0
• 08 January 2020: Using NASA's Hubble Space Telescope and a new observing technique, astronomers have found that dark matter forms much smaller clumps than previously known. This result confirms one of the fundamental predictions of the widely accepted "cold dark matter" theory. — The mysterious material makes up most of the mass in the universe, yet scientists don't understand its fundamental properties. Hubble observations have provided new clues. 96) 97)
Figure 91: Each snapshot shows four distorted images of a background quasar (an extremely bright region in the center of some distant galaxies), surrounding the core of a massive foreground galaxy. The gravity of the foreground galaxy magnifies the quasar, an effect called gravitational lensing (image credit: NASA, ESA, A. Nierenberg, T. Treu)
- All galaxies, according to this theory, form and are embedded within clouds of dark matter. Dark matter itself consists of slow-moving, or "cold," particles that come together to form structures ranging from hundreds of thousands of times the mass of the Milky Way galaxy to clumps no more massive than the heft of a commercial airplane. (In this context, "cold" refers to the particles' speed.)
- The Hubble observation yields new insights into the nature of dark matter and how it behaves. "We made a very compelling observational test for the cold dark matter model and it passes with flying colors," said Tommaso Treu of the University of California, Los Angeles (UCLA), a member of the observing team.
Figure 92: This graphic illustrates how a faraway quasar (an extremely bright region in the center of some distant galaxies) is altered by a massive foreground galaxy. The galaxy's powerful gravity warps and magnifies the quasar's light, producing four distorted images of the quasar [image credit: NASA, ESA and D. Player (STScI)]
- Dark matter is an invisible form of matter that makes up the bulk of the universe's mass and creates the scaffolding upon which galaxies are built. Although astronomers cannot see dark matter, they can detect its presence indirectly by measuring how its gravity affects stars and galaxies. Detecting the smallest dark matter formations by looking for embedded stars can be difficult or impossible because they contain very few stars.
- While dark matter concentrations have been detected around large- and medium-sized galaxies, much smaller clumps of dark matter have not been found until now. In the absence of observational evidence for such small-scale clumps, some researchers have developed alternative theories, including "warm dark matter." This idea suggests that dark matter particles are fast moving, zipping along too quickly to merge and form smaller concentrations. The new observations do not support this scenario, finding that dark matter is "colder" than it would have to be in the warm dark matter alternative theory.
- "Dark matter is colder than we knew at smaller scales," said Anna Nierenberg of NASA's Jet Propulsion Laboratory in Pasadena, California, leader of the Hubble survey. "Astronomers have carried out other observational tests of dark matter theories before, but ours provides the strongest evidence yet for the presence of small clumps of cold dark matter. By combining the latest theoretical predictions, statistical tools and new Hubble observations, we now have a much more robust result than was previously possible."
- Hunting for dark matter concentrations devoid of stars has proved challenging. The Hubble research team, however, used a technique in which they did not need to look for the gravitational influence of stars as tracers of dark matter. The team targeted eight powerful and distant cosmic "streetlights," called quasars (regions around active black holes that emit enormous amounts of light). The astronomers measured how the light emitted by oxygen and neon gas orbiting each of the quasars' black holes is warped by the gravity of a massive foreground galaxy, which acts as a magnifying lens.
- Using this method, the team uncovered dark matter clumps along the telescope's line of sight to the quasars, as well as in and around the intervening lensing galaxies. The dark matter concentrations detected by Hubble are 1/10,000th to 1/100,000th times the mass of the Milky Way's dark matter halo. Many of these tiny groupings most likely do not contain even small galaxies, and therefore would have been impossible to detect by the traditional method of looking for embedded stars.
- The eight quasars and galaxies were aligned so precisely that the warping effect, called gravitational lensing, produced four distorted images of each quasar. The effect is like looking at a funhouse mirror. Such quadruple images of quasars are rare because of the nearly exact alignment needed between the foreground galaxy and background quasar. However, the researchers needed the multiple images to conduct a more detailed analysis.
- The presence of the dark matter clumps alters the apparent brightness and position of each distorted quasar image. Astronomers compared these measurements with predictions of how the quasar images would look without the influence of the dark matter. The researchers used the measurements to calculate the masses of the tiny dark matter concentrations. To analyze the data, the researchers also developed elaborate computing programs and intensive reconstruction techniques.
- "Imagine that each one of these eight galaxies is a giant magnifying glass," explained team member Daniel Gilman of UCLA. "Small dark matter clumps act as small cracks on the magnifying glass, altering the brightness and position of the four quasar images compared to what you would expect to see if the glass were smooth."
- The researchers used Hubble's Wide Field Camera 3 to capture the near-infrared light from each quasar and disperse it into its component colors for study with spectroscopy. Unique emissions from the background quasars are best seen in infrared light. "Hubble's observations from space allow us to make these measurements in galaxy systems that would not be accessible with the lower resolution of ground-based telescopes - and Earth's atmosphere is opaque to the infrared light we needed to observe," explained team member Simon Birrer of UCLA.
- Treu added: "It's incredible that after nearly 30 years of operation, Hubble is enabling cutting-edge views into fundamental physics and the nature of the universe that we didn't even dream of when the telescope was launched."
- The gravitational lenses were discovered by sifting through ground-based surveys such as the Sloan Digital Sky Survey and Dark Energy Survey, which provide the most detailed three-dimensional maps of the universe ever made. The quasars are located roughly 10 billion light-years from Earth; the foreground galaxies, about 2 billion light-years.
- The number of small structures detected in the study offers more clues about dark matter's nature. "The particle properties of dark matter affect how many clumps form," Nierenberg explained. "That means you can learn about the particle physics of dark matter by counting the number of small clumps."
- However, the type of particle that makes up dark matter is still a mystery. "At present, there's no direct evidence in the lab that dark matter particles exist," Birrer said. "Particle physicists would not even talk about dark matter if the cosmologists didn't say it's there, based on observations of its effects. When we cosmologists talk about dark matter, we're asking 'how does it govern the appearance of the universe, and on what scales?'"
- Astronomers will be able to conduct follow-up studies of dark matter using future NASA space telescopes such as the James Webb Space Telescope and the Wide Field Infrared Survey Telescope (WFIRST), both infrared observatories. Webb will be capable of efficiently obtaining these measurements for all known quadruply lensed quasars. WFIRST's sharpness and large field of view will help astronomers make observations of the entire region of space affected by the immense gravitational field of massive galaxies and galaxy clusters. This will help researchers uncover many more of these rare systems.
- The team will present its results at the 235th meeting of the American Astronomical Society in Honolulu (4-8 January 2020).
- 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.
• 06 January 2020: To kickstart the 30th anniversary year of the NASA/ESA Hubble Space Telescope, Hubble has imaged a majestic spiral galaxy. Galaxy UGC 2885 may be the largest known in the local universe. It is 2.5 times wider than our Milky Way and contains 10 times as many stars. This galaxy is 232 million light-years away, located in the northern constellation of Perseus. 98) 99)
- Despite its gargantuan size, researchers are calling it a “gentle giant” because it looks as if it has been sitting quietly over billions of years, possibly sipping hydrogen from the filamentary structure of intergalactic space. This is fuelling modest ongoing star birth at a rate half that of our Milky Way. In fact, its supermassive central black hole is also a sleeping giant; because the galaxy does not appear to be feeding on much smaller satellite galaxies, it is starved of infalling gas.
- A number of foreground stars in our Milky Way can be seen in the image, identified by their diffraction spikes. The brightest appears to sit on top of the galaxy’s disc, though UGC 2885 is really 232 million light-years farther away. The giant galaxy is located in the northern constellation Perseus.
- The galaxy has also been nicknamed “Rubin’s galaxy”, after astronomer Vera Rubin (1928–2016), by Benne Holwerda of the University of Louisville, Kentucky, who observed the galaxy with the Hubble Space Telescope.
- “My research was in large part inspired by Vera Rubin’s work in 1980 on the size of this galaxy,” said Holwerda. Rubin measured the galaxy’s rotation, providing evidence for dark matter that makes up most of the galaxy’s mass. “We consider this a commemorative image. The goal of citing Dr. Rubin in our observation was very much part of our original Hubble proposal.”
- Researchers are still seeking to understand what led to the galaxy’s monstrous size. “It’s as big as you can make a disk galaxy without hitting anything else in space,” added Holwerda.
- One clue is that the galaxy is fairly isolated in space and doesn’t have any nearby galaxies to crash into and disrupt the shape of its disc.
- Did the monster galaxy gobble up much smaller satellite galaxies over time? Or did it just slowly accrete gas to make new stars? “It seems like it’s been puttering along, slowly growing,” Holwerda said. Using Hubble’s exceptional resolution, his team is counting the number of globular star clusters in the galaxy’s halo — a vast shell of faint stars surrounding the galaxy. An excess of clusters would yield evidence that they were captured from smaller infalling galaxies over many billions of years.
- The upcoming NASA/ESA/CSA James Webb Space Telescope could be used to explore the center of this galaxy as well as the globular cluster population. The infrared capability of this telescope will give researchers a less impeded view of the underlying stellar populations that will complement Hubble’s visible-light ability to track wispy star formation throughout the galaxy.
Figure 93: This Hubble Space Telescope photograph showcases the majestic spiral galaxy UGC 2885, located 232 million light-years away in the northern constellation Perseus. The galaxy is 2.5 times wider than our Milky Way and contains 10 times as many stars. A number of foreground stars in our Milky Way can be seen in the image, identified by their diffraction spikes. The brightest star photobombs the galaxy's disk. The galaxy has been nicknamed "Rubin's galaxy," after astronomer Vera Rubin (1928 – 2016), who studied the galaxy's rotation rate in search of dark matter (image credit: NASA, ESA, and B. Holwerda (University of Louisville)
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21) ”Survey of the stars,” ESA Science & Exploration, 09 October 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_05-09_October_2020
22) Hubble Sees Swirls of Forming Stars,” NASA, 09 October 2020, URL: https://www.nasa.gov
23) ”Spirals are in this season,” ESA, Science & Exploration, 02 October 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_28_September_-_02_October_2020
24) ”Hubble Watches Exploding Star Fade Into Oblivion,” NASA Feature, 01 October 2020, URL: https://www.nasa.gov/feature/goddard/2020/hubble-watches-exploding-star-fade-into-oblivion/
25) ”Dark matter surplus,” ESA Science & Exploration, 25 September 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_21-25_September_2020
26) ”Eye of the serpent,” ESA Science & Exploration, 18 September 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_14-18_September_2020
27) ”Hubble Locks Eyes With a Snake,” NASA, Hubble, 18 September 2020, URL: https://www.nasa.gov/image-feature/goddard/2020/hubble-locks-eyes-with-a-snake
30) ”Hubble's new views of Jupiter,” ESA, 17 September 2020, URL: https://sci.esa.int/web/hubble/-/hubble-s-new-views-of-jupiter
31) ”A pocketful of stars,” ESA Science & Exploration, 11 September 2020, URL: https://www.esa.int
Claire Andreoli, Donna Weaver, Ray Villard, Priyamvada Natarajan,
Massimo Meneghetti, ”Hubble Observations Suggest a Missing
Ingredient in Dark Matter Theories,” NASA Feature, 10 September
2020, URL: https://www.nasa.gov/feature/goddard/2020/
33) Massimo Meneghetti, Guido Davoli,Pietro Bergamini, Piero Rosati, Priyamvada Natarajan, Carlo Giocoli, Gabriel B. Caminha, R. Benton Metcalf, Elena Rasia, Stefano Borgani, Francesco Calura, Claudio Grillo, Amata Mercurio, Eros Vanzella, ”An excess of small-scale gravitational lenses observed in galaxy clusters,” Science, Vol. 369, Issue 6509, pp. 1347-1351, https://doi.org/10.1126/science.aax5164
34) ”A tilted wonder,” ESA Science & Exploration, 31 August 2020, URL: https://www.esa.int
35) ”At the edge of the blast,” ESA Science & Exploration, 28 August 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_24-28_August_2020
36) Claire Andreoli, Ann Jenkins, Ray Villard,Nicolas Lehner, ”Hubble Maps a Giant Halo Around the Andromeda Galaxy,” NASA Feature, 27 August 2020, URL: https://www.nasa.gov/feature/goddard/2020/hubble-maps-giant-halo-around-andromeda-galaxy
37) Nicolas Lehner,Samantha C. Berek, J. Christopher Howk,Bart P. Wakker,Jason Tumlinson, Edward B. Jenkins, J. Xavier Prochaska, Ramona Augustin, Suoqing Ji, Claude-André Faucher-Gigu`ere,Zachary Hafen, Molly S. Peeples, Kat A. Barger, Michelle A. Berg,Rongmon Bordoloi,Thomas M. Brown,Andrew J. Fox,Karoline M. Gilbert, Puragra Guhathakurta, Jason S. Kalirai, Felix J. Lockman, John M. O’Meara, D. J. Pisano, Joseph Ribaudo, and Jessica K. Werk, ”Project AMIGA: The Circumgalactic Medium of Andromeda,” Draft version July 1, 2020, URL: https://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_attachment/601/new.ms.pdf
38) ”Hubble Snaps Close-Up of Comet NEOWISE,” HST, heic2015 — Photo Release, 21 August 2020, URL: https://www.spacetelescope.org/news/heic2015/?lang
39) ”Cosmic fireworks,” ESA Science & Exploration, 21 August 2020, Picture of the week, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_17-21_August_2020
40) ”Ring of stellar wildfire,” ESA Science & Exploration, 14 August 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_10-14_August_2020
41) ”Hubble Helps Uncover the Mystery of the Dimming of Betelgeuse,” Hubble Space Telescope, heic2014 — Science Release, 13 August 2020, URL: https://www.spacetelescope.org/news/heic2014/?lang
42) Andrea K. Dupree, Klaus G. Strassmeier, Lynn D. Matthews, Han Uitenbroek, Thomas Calderwood, Thomas Granzer, Edward F. Guinan, Reimar Leike, Miguel Montarg`es, Anita M. S. Richards, ”Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse,” The Astrophysical Journal, Draft version August 13, 202, URL: https://arxiv.org/pdf/2008.04945.pdf
43) ”Seeing near and far,” ESA Science & Exploration, 07 August 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_03-07_August_2020
44) ”Hubble Uses Our Moon to Probe Earth’s Atmosphere During a Lunar Eclipse,” NASA Hubblesite, 6 August 2020, URL: https://hubblesite.org/contents/news-releases/2020/news-2020-30
45) Allison Youngblood, Giada N. Arney, Antonio García Muñoz, John T. Stocke, Kevin France, and Aki Roberge, ”The Hubble Space Telescope's Near-UV and Optical Transmission Spectrum of Earth as an Exoplanet,” The Astronomical Journal, Volume 160, Number 3, Published: 6 August 2020, https://doi.org/10.3847/1538-3881/aba0b4
46) ”Stellar sweet shop,” ESA Science & Exploration, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_27-31_July_2020
47) ”A discovery of ghostly arms,” ESA Science & Exploration, 24 July 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_20-24_July_2020
48) Rob Garner, ”Hubble Snaps Ghostly Galaxy,” NASA Feature, 24 July 2020, URL: https://www.nasa.gov/image-feature/goddard/2020/hubble-snaps-ghostly-galaxy
Claire Andreroli, Ray Villard, Amy Simon, Michael H. Wong, Rob Garner,
”Hubble Sees Summertime on Saturn,” NASA Feature, 23 July
2020, URL: https://www.nasa.gov
50) ”A frEGGs-cellent discovery,” ESA Science & Exploration, 17 July 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_13-17_July_2020
51) ”Hubble Spies Sparkling Galaxy,” NASA, 17 July 2020, URL: https://www.nasa.gov
52) ”The sculpted galaxy,” ESA Science & Exploration, 10 July 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_6-10_July_2020
53) ”Birds of a feather,” ESA Science & Exploration, 03 July 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_29_June_-_3_July_2020
54) ”One galaxy, two asteroids,” ESA Science & Exploration, 30 June 2020, URL: http://www.esa.int/ESA_Multimedia/Images/2020/06/One_galaxy_two_asteroids
55) ”A galaxy on edge,” ESA Science & Exploration, 26 June 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_22-26_June_20202
57) Joel H. Kastner, Jesse Bublitz, Bruce Balick, Rodolfo Montez, Jr., Adam Frank and Eric Blackman, ”First Results from a Panchromatic HST/WFC3 Imaging Study of the Young, Rapidly Evolving Planetary Nebulae NGC 7027 and NGC 6302,” Galaxies 2020, Volume 8 No 2, Published: 15 June 2020, https://doi.org/10.3390/galaxies8020049
58) ”Stellar snowflakes,” ESA Science & Exploration, 05 June 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_1-5_June_2020
Claire Andreoli, Bethany Downer, Rachana Bhatawdekar, Ray Villard,
”Hubble Makes Surprising Find in Early Universe,” NASA
Feature, 03 June 2020, URL: https://www.nasa.gov
60) ”Hubble makes surprising find in the early Universe [heic2010],” ESA, 03 June 2020, URL: https://sci.esa.int/web/hubble/-/hubble-makes-surprising-find-in-the-early-universe-2010
61) ”One large stellar latte to go,” ESA Science & Exploration, 29 May 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_25-29_May_2020
62) ”Hubble Finds that "Distance" From the Brightest Stars is Key to Preserving Primordial Discs,” Hubble Space Telescope, Release No.: heic2009 - Science Release, 28 May 2020, URL: https://www.spacetelescope.org/news/heic2009/
63) ”In Planet Formation, It's Location, Location, Location,” NASA Hubblesite, Release ID: 2020-15, 28 May 2020, URL: https://hubblesite.org/contents/news-releases/2020/news-2020-15
64) E. Sabbi, M. Gennaro, J. Anderson, V. Bajaj, N. Bastian, J. S. Gallagher III, M. Gieles, D. J. Lennon, A. Nota, K. C. Sahu, and P. Zeidler, ”Time-domain Study of the Young Massive Cluster Westerlund 2 with the Hubble Space Telescope. I,” The Astrophysical Journal, Volume 891, No 2, Published: 18 March 2020, https://doi.org/10.3847/1538-4357/ab7372, URL: https://hubblesite.org/uploads/science_paper/file_attachment/585/published_ApJ_paper.pdf
65) ”Stellar glitter in a field of black,” ESA Science & Exploration, 22 May 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_18-22_May_2020
66) ”WFIRST Telescope Named For ‘Mother of Hubble’ Nancy Grace Roman,” Hubblesite Release ID: 2020-35, 20 May 2020, URL: https://hubblesite.org/contents/news-releases/2020/news-2020-35
67) ”Galaxy Galaxy, Burning Bright!,” ESA Science & Exploration, 8 May 2020, URL: https://www.esa.int/ESA_Multimedia/Images/2020/05/Galaxy_galaxy_burning_bright
68) ”A stretched spiral,” ESA Science & Exploration, 01 May 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_27_April_-_1_May_2020
69) ”Hubble's impactful life alongside space debris,” ESA / Safety & Security, 30 April 2020, URL: http://www.esa.int/Safety_Security/Hubble_s_impactful_life_alongside_space_debris
70) ”High-velocity impact sample,” ESA, 16 March, 2005, URL: http://www.esa.int/Safety_Security/Hubble_s_impactful_life_alongside_space_debris
Claire Andreoli, Ray Villard, Davide Jewitt, Quanzhi Ye, ”Hubble
Watches Comet ATLAS Disintegrate Into More Than Two Dozen
Pieces,” NASA Hubble, 28 April 2020, URL: https://www.nasa.gov/
72) ”Hubble Watches Comet ATLAS Disintegrate Into More Than Two Dozen Pieces,” Hubblesite Release ID: 2020-28, 28 April 2020, URL: https://hubblesite.org/contents/news-releases/2020/news-2020-28
73) ”Hubble Marks 30 Years in Space With Tapestry of Blazing Starbirth,” NASA Feature, 24 April 2020, URL: https://www.nasa.gov/feature/goddard/2020/
”Hubble celebrates its 30th anniversary with a tapestry of
blazing starbirth,” ESA Science & Exploration, 24 April 2020,
Elizabeth Landau, Tricia Talbert, ”Interstellar Comet Borisov
Reveals Its Chemistry and Possible Origins,” NASA Hubble, 20
April 2020, URL: https://www.nasa.gov/feature
76) D. Bodewits, J. W. Noonan, P. D. Feldman, M. T. Bannister, D. Farnocchia,W. M. Harris, J.-Y. Li, K. E. Mandt, J. Wm. Parker, and Z.-X. Xing, ” The carbon monoxide-rich interstellar comet 2I/Borisov,” Nature Astronomy Letters, Published: 20 April 2020, , https://doi.org/10.1038/s41550-020-1095-2 ,URL: https://tinyurl.com/y74do8kh
78) András Gáspár and George H. Rieke, ”New HST data and modeling reveal a massive planetesimal collision around Fomalhaut,” PNAS ( Proceedings of the National Academy of Sciences), first published: 20 April 2020, https://doi.org/10.1073/pnas.1912506117
79) ”Exoplanet Apparently Disappears in Latest Hubble Observations,” NASA, 20 April 2020, URL: https://www.nasa.gov/feature/goddard/2020/
80) ”Hunting for dead stars,” ESA Science & Exploration, 17 April 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_13-17_April_2020
81) ”Rings upon rings,” ESA Science & Exploration, 10 April 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_6-10_April_2020
82) ”Feeding time,” ESA Science & Exploration, 03 April 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_30_March_-_3_April_2020
83) ”Hubble Finds Best Evidence for Elusive Mid-Sized Black Hole,” NASA, Hubble, 31 March 2020, URL: https://www.nasa.gov/feature/goddard/2020/hubble-finds-best-evidence-for-elusive-mid-sized-black-hole
84) Dacheng Lin, Jay Strader, Aaron J. Romanowsky, Jimmy A. Irwin, Olivier Godet, Didier Barret, Natalie A. Webb, Jeroen Homan, and Ronald A. Remillard, ”Multiwavelength Follow-up of the Hyperluminous Intermediate-mass Black Hole Candidate 3XMM J215022.4-055108,” The Astrophysical Journal Letters, Volume 892, Number 2, Published: 31 March 2020, https://doi.org/10.3847/2041-8213/ab745b, URL: https://iopscience.iop.org/article/10.3847/2041-8213/ab745b/pdf
85) ”Single arm galaxy,” ESA Science & Exploration, 27 March 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_23-27_March_2020
86) ”Quasar Tsunamis Rip Across Galaxies,” NASA Hubblesite, Release ID: 2020-10, 19 March 2020, URL: https://hubblesite.org/contents/news-releases/2020/news-2020-10
87) ”Update to Hubble Anniversary Image Unveiling Events due to COVID-19,” Hubble Space Telescope, ann2004 — Announcement, 19 March 2020, URL: https://www.spacetelescope.org/announcements/ann2004/
88) ”On the origin of massive stars [heic2004],” ESA, 18 March 2020, URL: https://sci.esa.int/web/hubble/-/on-the-origin-of-massive-stars-heic2004
”Slime Mold Simulations Used to Map the Dark Matter Holding the
Universe Together,” NASA Hubblesite, Release ID: 2020-11, 10
March 2020, URL: https://hubblesite.org
90) Joseph N. Burchett, Oskar Elek, Nicolas Tejos, J. Xavier Prochaska, Todd M. Tripp, Rongmon Bordoloi, and Angus G. Forbes, ”Revealing the Dark Threads of the Cosmic Web,” The Astrophysical Journal Letters, Volume 891, Number 2, Published: 10 March 2020, https://doi.org/10.3847/2041-8213/ab700c
91) ”Hunger pangs,” ESA Science & Exploration, 09 March 2020, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_9-13_March_2020
Claire Andreoli , Rob Gutro, Ray Villard, ”Hubble Team Wins 2020
Michael Collins Trophy,” NASA Feature, 21 February 2020, URL: https://www.nasa.gov/feature
Claire Andreoli, Rob Gutro, Leah Ramsay, Ray Villard, Roger Cohen ,
Paul Godfry, ”Beyond the Brim, Sombrero Galaxy’s Halo
Suggests Turbulent Past,” NASA Feature, 20 February 2020, URL: https://www.nasa.gov/feature/goddard/2020/
94) ”Open arms,” ESA Science & Exploration, Image of the week for 14 February 2020, URL: http://www.esa.int/About_Us/Week_in_images/Week_in_images_10-14_February_2020
95) ”Bars and baby stars,” ESA Science & Application, Image of the week for 27-31 January 2020, URL: https://www.esa.int/About_Us/Corporate_news/Week_in_images_27-31_January_2020
96) Donna Weaver / Ray Villard (STSCI), Calla Cofield (NASA/JPL), ”Cosmic Magnifying Glasses Find Dark Matter in Small Clumps,” NASA/JPL News, 8 January 2020, URL: https://www.jpl.nasa.gov/news/news.php?release=2020-005
97) ”Cosmic Magnifying Glasses Yield Independent Measure of Universe's Expansion,” NASA Hubblesite, 8 January 2020, URL: https://hubblesite.org/contents/news-releases/2020/news-2020-04
99) ”NASA's Hubble Surveys Gigantic Galaxy,” NASA Hubblesite, 5 January 2020, URL: https://hubblesite.org/contents/news-releases/2020/news-2020-01
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).