VLT (Very Large Telescope) of ESO on Cerro ParanalMission Status Instruments ESPRESSO Hawk-I FLAMES FORS2 MATISSE GRAVITY MUSE References
VLT is a telescope facility operated by ESO (European Southern Observatory) on the Cerro Paranal mountain in the Atacama Desert of northern Chile at an elevation of 2,635 m (coordinates: 24°37'38''S, 70°24'17''W).
VLT is the flagship facility for European ground-based astronomy at the beginning of the third Millennium. It is the world's most advanced optical instrument, consisting of four Unit Telescopes with main mirrors of 8.2m diameter and four movable 1.8m diameter ATs (Auxiliary Telescopes). The telescopes can work together, to form a giant ‘interferometer', the ESO VLTI (Very Large Telescope Interferometer), allowing astronomers to see details up to 25 times finer than with the individual telescopes. The VLTI functions like a telescope with a mirror 200 m in diameter. The light beams are combined in the VLTI using a complex system of mirrors in underground tunnels where the light paths must be kept equal to distances less than 1/1000 mm over a hundred meters. With this kind of precision, the VLTI can reconstruct images with an angular resolution of milliarcseconds (marcsec), equivalent to distinguishing the two headlights of a car at the distance of the Moon. 1)
ESO is the foremost intergovernmental astronomy organization in Europe and the world's most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and by Australia as a strategic partner. ESO carries out an ambitious program focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organizing cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: , Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world's most advanced visible-light astronomical observatory and two survey telescopes. VISTA (Visible and Infrared Survey Telescope for Astronomy) works in the infrared and is the world's largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-meter ELT (Extremely Large Telescope), which will become "the world's biggest eye on the sky".
Figure 1: Aerial view of the observing platform on the top of Cerro Paranal, with the four enclosures for the 8.2-m UTs (Unit Telescopes) and various installations for the VLT Interferometer (VLTI). Three 1.8 m VLTI ATs (Auxiliary Telescopes) and paths of the light beams have been superimposed on the photo. Also seen are some of the 30 "stations" where the ATs will be positioned for observations and from where the light beams from the telescopes can enter the Interferometric Tunnel below. The straight structures are supports for the rails on which the telescopes can move from one station to another. The Interferometric Laboratory (partly subterranean) is at the center of the platform (image credit: ESO)
The 8.2m diameter Unit Telescopes can also be used individually. With one such telescope, images of celestial objects as faint as magnitude 30 can be obtained in a one-hour exposure. This corresponds to seeing objects that are four billion (four thousand million) times fainter than what can be seen with the unaided eye.
The large telescopes are named Antu, Kueyen, Melipal and Yepun, which are the names for the Sun, the Moon, the Southern Cross, and Venus in the language of the Mapuche people.
Figure 2: Alternate view of ESO's Paranal Observatory hosting several world-class telescopes; among them are the Very Large Telescope, the Visible and Infrared Survey Telescope for Astronomy, and the VLT Survey Telescope. Other scientific and support facilities are also located at Paranal, including several smaller telescopes and an innovative accommodation facility known as the Residencia (image credit: ESO) 2)
ESO (European Southern Observatory) is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky. The organization employs about 730 staff members and receives annual member state contributions of approximately €131 million. Its observatories are located in northern Chile. 3)
ESO has built and operated some of the largest and most technologically advanced telescopes. These include the NTT (New Technology Telescope), an early pioneer in the use of active optics, and the VLT (Very Large Telescope), which consists of four individual telescopes, each with a primary mirror 8.2 m across, and four smaller auxiliary telescopes. The ALMA (Atacama Large Millimeter Array) observes the universe in the millimeter and su-millimeter wavelength ranges, and is the world's largest ground-based astronomy project to date. It was completed in March 2013 in an international collaboration by Europe (represented by ESO), North America, East Asia and Chile.
Currently under construction is the ELT (Extremely Large Telescope). It will use a 39.3 meter diameter segmented mirror, and become the world's largest optical reflecting telescope when operational in 2024. Its light-gathering power will allow detailed studies of planets around other stars, the first objects in the universe, supermassive black holes, and the nature and distribution of the dark matter and dark energy which dominate the universe.
ESO's observing facilities have made astronomical discoveries and produced several astronomical catalogs. Its findings include the discovery of the most distant gamma-ray burst and evidence for a black hole at the center of the Milky Way.
In 2004, the VLT allowed astronomers to obtain the first picture of an extrasolar planet (2M1207b) orbiting a brown dwarf 173 light-years away. The HARPS (High Accuracy Radial Velocity Planet Searcher) instrument installed in another ESO telescope led to the discovery of extrasolar planets, including Gliese 581c—one of the smallest planets seen outside the solar system.
Construction of the VLT began in 1991, and its first observations were made in 1998. Among the VLT's notable discoveries are the first direct spectrum of an extrasolar planet, HR 8799c, and the first direct measurement of the mass of an extrasolar planet, HD 209458b. The VLT also discovered the most massive star known, R136a1, which has a mass 265 times that of the Sun. The VLT is operated by the European Southern Observatory. 4)
Chilean observation sites:
Although ESO is headquartered in Garching, Germany, its telescopes and observatories are in northern Chile, where the organization operates advanced ground-based astronomical facilities:
• La Silla, which hosts the New Technology Telescope (NTT)
• Paranal, where the VLT (Very Large Telescope) is located
These are among the best locations for astronomical observations in the southern hemisphere. An ESO project is the ELT (Extremely Large Telescope), a 40 m class telescope based on a five-mirror design and the formerly planned Overwhelmingly Large Telescope. The ELT will be the largest optical near-infrared telescope in the world. ESO began its design in early 2006, and aimed to begin construction in 2012. Construction work at the ELT site started in June 2014. As decided by the ESO council on 26 April 2010, a fourth site (Cerro Armazones) is to be home to ELT.
Each year about 2,000 requests are made for the use of ESO telescopes, for four to six times more nights than are available. Observations made with these instruments appear in a number of peer-reviewed publications annually; in 2009, more than 650 reviewed papers based on ESO data were published.
ESO telescopes generate large amounts of data at a high rate, which are stored in a permanent archive facility at ESO headquarters. The archive contains more than 1.5 million images (or spectra) with a total volume of about 65 TB of data.
Table 1: ESO telescopes
Figure 3: Aerial view of Paranal with VISTA in the foreground and the VLT (Very Large Telescope) in the background (image credit: ESO/G.Hüdepohl)
Sample Observations with VLT (Very Large Telescope) and some status
• April 20, 2022: A team of astronomers, with the help of the European Southern Observatory's Very Large Telescope (ESO's VLT), have observed a new type of stellar explosion — a micronova. These outbursts happen on the surface of certain stars, and can each burn through around 3.5 billion Great Pyramids of Giza of stellar material in only a few hours. 5)
Figure 4: This artist's impression shows a two-star system where micronovae may occur. The blue disc swirling around the bright white dwarf in the centre of the image is made up of material, mostly hydrogen, stolen from its companion star. Towards the centre of the disc, the white dwarf uses its strong magnetic fields to funnel the hydrogen towards its poles. As the material falls on the hot surface of the star, it triggers a micronova explosion, contained by the magnetic fields at one of the white dwarf's poles (image credit: ESO/M. Kornmesser, L. Calçada)
- We have discovered and identified for the first time what we are calling a micronova," explains Simone Scaringi, an astronomer at Durham University in the UK who led the study on these explosions published today in Nature. "The phenomenon challenges our understanding of how thermonuclear explosions in stars occur. We thought we knew this, but this discovery proposes a totally new way to achieve them," he adds.
- Micronovae are extremely powerful events, but are small on astronomical scales; they are much less energetic than the stellar explosions known as novae, which astronomers have known about for centuries. Both types of explosions occur on white dwarfs, dead stars with a mass about that of our Sun, but as small as Earth.
- A white dwarf in a two-star system can steal material, mostly hydrogen, from its companion star if they are close enough together. As this gas falls onto the very hot surface of the white dwarf star, it triggers the hydrogen atoms to fuse into helium explosively. In novae, these thermonuclear explosions occur over the entire stellar surface. "Such detonations make the entire surface of the white dwarf burn and shine brightly for several weeks," explains co-author Nathalie Degenaar, an astronomer at the University of Amsterdam, the Netherlands.
- Micronovae are similar explosions that are smaller in scale and faster, lasting just several hours. They occur on some white dwarfs with strong magnetic fields, which funnel material towards the star's magnetic poles. "For the first time, we have now seen that hydrogen fusion can also happen in a localised way. The hydrogen fuel can be contained at the base of the magnetic poles of some white dwarfs, so that fusion only happens at these magnetic poles," says Paul Groot, an astronomer at Radboud University in the Netherlands and co-author of the study.
- "This leads to micro-fusion bombs going off, which have about one millionth of the strength of a nova explosion, hence the name micronova," Groot continues. Although ‘micro' may imply these events are small, do not be mistaken: just one of these outbursts can burn through about 20,000,000 trillion kg, or about 3.5 billion Great Pyramids of Giza, of material.
- These new micronovae challenge astronomers' understanding of stellar explosions and may be more abundant than previously thought. "It just goes to show how dynamic the Universe is. These events may actually be quite common, but because they are so fast they are difficult to catch in action," Scaringi explains.
- The team first came across these mysterious micro-explosions when analysing data from NASA's Transiting Exoplanet Survey Satellite (TESS). "Looking through astronomical data collected by NASA's TESS, we discovered something unusual: a bright flash of optical light lasting for a few hours. Searching further, we found several similar signals," says Degenaar.
- The team observed three micronovae with TESS: two were from known white dwarfs, but the third required further observations with the X-shooter instrument on ESO's VLT to confirm its white dwarf status.
- "With help from ESO's Very Large Telescope, we found that all these optical flashes were produced by white dwarfs," says Degenaar. "This observation was crucial in interpreting our result and for the discovery of micronovae," Scaringi adds.
- The discovery of micronovae adds to the repertoire of known stellar explosions. The team now want to capture more of these elusive events, requiring large scale surveys and quick follow-up measurements. "Rapid response from telescopes such as the VLT or ESO's New Technology Telescope and the suite of available instruments will allow us to unravel in more detail what these mysterious micronovae are," Scaringi concludes. 6)
• April 11, 2022: An international team of astronomers have used ground-based telescopes, including the European Southern Observatory's Very Large Telescope (ESO's VLT), to track Neptune's atmospheric temperatures over a 17-year period. They found a surprising drop in Neptune's global temperatures followed by a dramatic warming at its south pole. 7)
Figure 5: This composite shows thermal images of Neptune taken between 2006 and 2020. The first three images (2006, 2009, 2018) were taken with the VISIR instrument on ESO's Very Large Telescope while the 2020 image was captured by the COMICS instrument on the Subaru Telescope (VISIR wasn't in operation in mid-late 2020 because of the pandemic). After the planet's gradual cooling, the south pole appears to have become dramatically warmer in the past few years, as shown by a bright spot at the bottom of Neptune in the images from 2018 and 2020 (image credit: ESO/M. Roman, NAOJ/Subaru/COMICS)
- "This change was unexpected," says Michael Roman, a postdoctoral research associate at the University of Leicester, UK, and lead author of the study published today in The Planetary Science Journal. "Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder."
- Like Earth, Neptune experiences seasons as it orbits the Sun. However, a Neptune season lasts around 40 years, with one Neptune year lasting 165 Earth years. It has been summertime in Neptune's southern hemisphere since 2005, and the astronomers were eager to see how temperatures were changing following the southern summer solstice.
- Astronomers looked at nearly 100 thermal-infrared images of Neptune, captured over a 17-year period, to piece together overall trends in the planet's temperature in greater detail than ever before.
- These data showed that, despite the onset of southern summer, most of the planet had gradually cooled over the last two decades. The globally averaged temperature of Neptune dropped by 8ºC between 2003 and 2018.
- The astronomers were then surprised to discover a dramatic warming of Neptune's south pole during the last two years of their observations, when temperatures rapidly rose 11ºC between 2018 and 2020. Although Neptune's warm polar vortex has been known for many years, such rapid polar warming has never been previously observed on the planet.
- "Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes," says co-author Glenn Orton, senior research scientist at Caltech's Jet Propulsion Laboratory (JPL) in the US.
- The astronomers measured Neptune's temperature using thermal cameras that work by measuring the infrared light emitted from astronomical objects. For their analysis the team combined all existing images of Neptune gathered over the last two decades by ground-based telescopes. They investigated infrared light emitted from a layer of Neptune's atmosphere called the stratosphere. This allowed the team to build up a picture of Neptune's temperature and its variations during part of its southern summer.
- Because Neptune is roughly 4.5 billion kilometres away and is very cold, the planet's average temperature reaching around –220ºC, measuring its temperature from Earth is no easy task. "This type of study is only possible with sensitive infrared images from large telescopes like the VLT that can observe Neptune clearly, and these have only been available for the past 20 years or so," says co-author Leigh Fletcher, a professor at the University of Leicester.
- Around one third of all the images taken came from the VLT Imager and Spectrometer for mid-InfraRed (VISIR) instrument on ESO's VLT in Chile's Atacama Desert. Because of the telescope's mirror size and altitude, it has a very high resolution and data quality, offering the clearest images of Neptune. The team also used data from NASA's Spitzer Space Telescope and images taken with the Gemini South telescope in Chile, as well as with the Subaru Telescope, the Keck Telescope, and the Gemini North telescope, all in Hawai‘i.
- Because Neptune's temperature variations were so unexpected, the astronomers do not know yet what could have caused them. They could be due to changes in Neptune's stratospheric chemistry, or random weather patterns, or even the solar cycle. More observations will be needed over the coming years to explore the reasons for these fluctuations. Future ground-based telescopes like ESO's Extremely Large Telescope (ELT) could observe temperature changes like these in greater detail, while the NASA/ESA/CSA James Webb Space Telescope will provide unprecedented new maps of the chemistry and temperature in Neptune's atmosphere.
- "I think Neptune is itself very intriguing to many of us because we still know so little about it," says Roman. "This all points towards a more complicated picture of Neptune's atmosphere and how it changes with time."
- This research was presented in the paper "Sub-Seasonal Variation in Neptune's Mid-Infrared Emission" published today in The Planetary Science Journal. 8)
• March 2, 2022: In 2020 a team led by European Southern Observatory (ESO) astronomers reported the closest black hole to Earth, located just 1000 light-years away in the HR 6819 system. But the results of their study were contested by other researchers, including by an international team based at KU Leuven, Belgium. In a paper published today, these two teams have united to report that there is in fact no black hole in HR 6819, which is instead a "vampire" two-star system in a rare and short-lived stage of its evolution. 9)
Figure 6: Artist's impression of HR 6819. New research using data from ESO's Very Large Telescope and Very Large Telescope Interferometer has revealed that HR 6819, previously believed to be a triple system with a black hole, is in fact a system of two stars with no black hole. The scientists, a KU Leuven-ESO team, believe they have observed this binary system in a brief moment after one of the stars sucked the atmosphere off its companion, a phenomenon often referred to as "stellar vampirism". This artist's impression shows what the system might look like; it's composed of an oblate star with a disc around it (a Be "vampire" star; foreground) and B-type star that has been stripped of its atmosphere (background), image credit: ESO/L. Calçada
- The original study on HR 6819 received significant attention from both the press and scientists. Thomas Rivinius, a Chile-based ESO astronomer and lead author on that paper, was not surprised by the astronomy community's reception to their discovery of the black hole. "Not only is it normal, but it should be that results are scrutinised," he says, "and a result that makes the headlines even more so."
- Rivinius and his colleagues were convinced that the best explanation for the data they had, obtained with the MPG/ESO 2.2-metre telescope, was that HR 6819 was a triple system, with one star orbiting a black hole every 40 days and a second star in a much wider orbit. But a study led by Julia Bodensteiner, then a PhD student at KU Leuven, Belgium, proposed a different explanation for the same data: HR 6819 could also be a system with only two stars on a 40-day orbit and no black hole at all. This alternative scenario would require one of the stars to be "stripped", meaning that, at an earlier time, it had lost a large fraction of its mass to the other star.
- "We had reached the limit of the existing data, so we had to turn to a different observational strategy to decide between the two scenarios proposed by the two teams," says KU Leuven researcher Abigail Frost, who led the new study published today in Astronomy & Astrophysics. 10)
- To solve the mystery, the two teams worked together to obtain new, sharper data of HR 6819 using ESO's Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI). "The VLTI was the only facility that would give us the decisive data we needed to distinguish between the two explanations," says Dietrich Baade, author on both the original HR 6819 study and the new Astronomy & Astrophysics paper. Since it made no sense to ask for the same observation twice, the two teams joined forces, which allowed them to pool their resources and knowledge to find the true nature of this system.
- "The scenarios we were looking for were rather clear, very different and easily distinguishable with the right instrument," says Rivinius. "We agreed that there were two sources of light in the system, so the question was whether they orbit each other closely, as in the stripped-star scenario, or are far apart from each other, as in the black hole scenario."
- "MUSE confirmed that there was no bright companion in a wider orbit, while GRAVITY's high spatial resolution was able to resolve two bright sources separated by only one-third of the distance between the Earth and the Sun," says Frost. "These data proved to be the final piece of the puzzle, and allowed us to conclude that HR 6819 is a binary system with no black hole."
- "Our best interpretation so far is that we caught this binary system in a moment shortly after one of the stars had sucked the atmosphere off its companion star. This is a common phenomenon in close binary systems, sometimes referred to as "stellar vampirism" in the press," explains Bodensteiner, now a fellow at ESO in Germany and an author on the new study. "While the donor star was stripped of some of its material, the recipient star began to spin more rapidly."
- "Catching such a post-interaction phase is extremely difficult as it is so short," adds Frost. "This makes our findings for HR 6819 very exciting, as it presents a perfect candidate to study how this vampirism affects the evolution of massive stars, and in turn the formation of their associated phenomena including gravitational waves and violent supernova explosions."
- The newly formed Leuven-ESO joint team now plans to monitor HR 6819 more closely using the VLTI's GRAVITY instrument. The researchers will conduct a joint study of the system over time, to better understand its evolution, constrain its properties, and use that knowledge to learn more about other binary systems.
- As for the search for black holes, the team remains optimistic. "Stellar-mass black holes remain very elusive owing to their nature," says Rivinius. "But order-of-magnitude estimates suggest there are tens to hundreds of millions of black holes in the Milky Way alone," Baade adds. It is just a matter of time until astronomers discover them.
• February 16, 2022: The European Southern Observatory's Very Large Telescope Interferometer (ESO's VLTI) has observed a cloud of cosmic dust at the centre of the galaxy Messier 77 that is hiding a supermassive black hole. The findings have confirmed predictions made around 30 years ago and are giving astronomers new insight into "active galactic nuclei", some of the brightest and most enigmatic objects in the universe. 11)
Figure 7: The left panel of this image shows a dazzling view of the active galaxy Messier 77 captured with the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) instrument on ESO's Very Large Telescope. The right panel shows a blow-up view of the very inner region of this galaxy, its active galactic nucleus, as seen with the MATISSE instrument on ESO's Very Large Telescope Interferometer (image credit: ESO/Jaffe, Gámez-Rosas et al.)
- Active galactic nuclei (AGNs) are extremely energetic sources powered by supermassive black holes and found at the centre of some galaxies. These black holes feed on large volumes of cosmic dust and gas. Before it is eaten up, this material spirals towards the black hole and huge amounts of energy are released in the process, often outshining all the stars in the galaxy.
- Astronomers have been curious about AGNs ever since they first spotted these bright objects in the 1950s. Now, thanks to ESO's VLTI, a team of researchers, led by Violeta Gámez Rosas from Leiden University in the Netherlands, have taken a key step towards understanding how they work and what they look like up close. The results are published today in Nature. 12)
- By making extraordinarily detailed observations of the centre of the galaxy Messier 77, also known as NGC 1068, Gámez Rosas and her team detected a thick ring of cosmic dust and gas hiding a supermassive black hole. This discovery provides vital evidence to support a 30-year-old theory known as the Unified Model of AGNs.
- Astronomers know there are different types of AGN. For example, some release bursts of radio waves while others don't; certain AGNs shine brightly in visible light, while others, like Messier 77, are more subdued. The Unified Model states that despite their differences, all AGNs have the same basic structure: a supermassive black hole surrounded by a thick ring of dust.
- According to this model, any difference in appearance between AGNs results from the orientation at which we view the black hole and its thick ring from Earth. The type of AGN we see depends on how much the ring obscures the black hole from our view point, completely hiding it in some cases.
- Astronomers had found some evidence to support the Unified Model before, including spotting warm dust at the centre of Messier 77. However, doubts remained about whether this dust could completely hide a black hole and hence explain why this AGN shines less brightly in visible light than others.
- "The real nature of the dust clouds and their role in both feeding the black hole and determining how it looks when viewed from Earth have been central questions in AGN studies over the last three decades," explains Gámez Rosas. "Whilst no single result will settle all the questions we have, we have taken a major step in understanding how AGNs work."
- The observations were made possible thanks to the Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) mounted on ESO's VLTI, located in Chile's Atacama Desert. MATISSE combined infrared light collected by all four 8.2-metre telescopes of ESO's Very Large Telescope (VLT) using a technique called interferometry. The team used MATISSE to scan the centre of Messier 77, located 47 million light-years away in the constellation Cetus.
- "MATISSE can see a broad range of infrared wavelengths, which lets us see through the dust and accurately measure temperatures. Because the VLTI is in fact a very large interferometer, we have the resolution to see what's going on even in galaxies as far away as Messier 77. The images we obtained detail the changes in temperature and absorption of the dust clouds around the black hole," says co-author Walter Jaffe, a professor at Leiden University.
- Combining the changes in dust temperature (from around room temperature to about 1200 °C) caused by the intense radiation from the black hole with the absorption maps, the team built up a detailed picture of the dust and pinpointed where the black hole must lie. The dust — in a thick inner ring and a more extended disc — with the black hole positioned at its centre supports the Unified Model. The team also used data from the Atacama Large Millimeter/submillimeter Array, co-owned by ESO, and the National Radio Astronomy Observatory's Very Long Baseline Array to construct their picture.
- "Our results should lead to a better understanding of the inner workings of AGNs," concludes Gámez Rosas. "They could also help us better understand the history of the Milky Way, which harbours a supermassive black hole at its centre that may have been active in the past."
- The researchers are now looking to use ESO's VLTI to find more supporting evidence of the Unified Model of AGNs by considering a larger sample of galaxies.
- Team member Bruno Lopez, the MATISSE Principal Investigator at the Observatoire de la Côte d'Azur in Nice, France, says: "Messier 77 is an important prototype AGN and a wonderful motivation to expand our observing programme and to optimise MATISSE to tackle a wider sample of AGNs."
- ESO's Extremely Large Telescope (ELT), set to begin observing later this decade, will also aid the search, providing results that will complement the team's findings and allow them to explore the interaction between AGNs and galaxies.
• February 10, 2022: A team of astronomers using the European Southern Observatory's Very Large Telescope (ESO's VLT) in Chile have found evidence of another planet orbiting Proxima Centauri, the closest star to our Solar System. This candidate planet is the third detected in the system and the lightest yet discovered orbiting this star. At just a quarter of Earth's mass, the planet is also one of the lightest exoplanets ever found. 13)
Figure 8: This artist's impression shows a close-up view of Proxima d, a planet candidate recently found orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The planet is believed to be rocky and to have a mass about a quarter that of Earth. Two other planets known to orbit Proxima Centauri are visible in the image too: Proxima b, a planet with about the same mass as Earth that orbits the star every 11 days and is within the habitable zone, and candidate Proxima c, which is on a longer five-year orbit around the star (image credit: ESO, L. Calçada)
- "The discovery shows that our closest stellar neighbour seems to be packed with interesting new worlds, within reach of further study and future exploration," explains João Faria, a researcher at the Instituto de Astrofísica e Ciências do Espaço, Portugal and lead author of the study published today in Astronomy & Astrophysics. Proxima Centauri is the closest star to the Sun, lying just over four light-years away. 14)
- The newly discovered planet, named Proxima d, orbits Proxima Centauri at a distance of about four million kilometres, less than a tenth of Mercury's distance from the Sun. It orbits between the star and the habitable zone — the area around a star where liquid water can exist at the surface of a planet — and takes just five days to complete one orbit around Proxima Centauri.
- The star is already known to host two other planets: Proxima b, a planet with a mass comparable to that of Earth that orbits the star every 11 days and is within the habitable zone, and candidate Proxima c, which is on a longer five-year orbit around the star.
- Proxima b was discovered a few years ago using the HARPS instrument on ESO's 3.6-metre telescope. The discovery was confirmed in 2020 when scientists observed the Proxima system with a new instrument on ESO's VLT that had greater precision, the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO). It was during these more recent VLT observations that astronomers spotted the first hints of a signal corresponding to an object with a five-day orbit. As the signal was so weak, the team had to conduct follow-up observations with ESPRESSO to confirm that it was due to a planet, and not simply a result of changes in the star itself.
- "After obtaining new observations, we were able to confirm this signal as a new planet candidate," Faria says. "I was excited by the challenge of detecting such a small signal and, by doing so, discovering an exoplanet so close to Earth."
- At just a quarter of the mass of Earth, Proxima d is the lightest exoplanet ever measured using the radial velocity technique, surpassing a planet recently discovered in the L 98-59 planetary system. The technique works by picking up tiny wobbles in the motion of a star created by an orbiting planet's gravitational pull. The effect of Proxima d's gravity is so small that it only causes Proxima Centauri to move back and forth at around 40 cm/s (1.44 km/hour).
- "This achievement is extremely important," says Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile. "It shows that the radial velocity technique has the potential to unveil a population of light planets, like our own, that are expected to be the most abundant in our galaxy and that can potentially host life as we know it."
- "This result clearly shows what ESPRESSO is capable of and makes me wonder about what it will be able to find in the future," Faria adds.
- ESPRESSO's search for other worlds will be complemented by ESO's Extremely Large Telescope (ELT), currently under construction in the Atacama Desert, which will be crucial to discovering and studying many more planets around nearby stars.
• December 22, 2021: Rogue planets are elusive cosmic objects that have masses comparable to those of the planets in our Solar System but do not orbit a star, instead roaming freely on their own. Not many were known until now, but a team of astronomers, using data from several European Southern Observatory (ESO) telescopes and other facilities, have just discovered at least 70 new rogue planets in our galaxy. This is the largest group of rogue planets ever discovered, an important step towards understanding the origins and features of these mysterious galactic nomads. 15)
Figure 9: This artist's impression shows an example of a rogue planet with the Rho Ophiuchi cloud complex visible in the background. Rogue planets have masses comparable to those of the planets in our Solar System but do not orbit a star, instead roaming freely on their own (image credit: ESO, M. Kornmesser)
- "We did not know how many to expect and are excited to have found so many," says Núria Miret-Roig, an astronomer at the Laboratoire d'Astrophysique de Bordeaux, France and the University of Vienna, Austria, and the first author of the new study published today in Nature Astronomy.
- Rogue planets, lurking far away from any star illuminating them, would normally be impossible to image. However, Miret-Roig and her team took advantage of the fact that, in the few million years after their formation, these planets are still hot enough to glow, making them directly detectable by sensitive cameras on large telescopes. They found at least 70 new rogue planets with masses comparable to Jupiter's in a star-forming region close to our Sun, located within the Scorpius and Ophiuchus constellations .
Notes:  The exact number of rogue planets found by the team is hard to pin down because the observations don't allow the researchers to measure the masses of the probed objects. Objects with masses higher than about 13 times the mass of Jupiter are most likely not planets, so they cannot be included in the count. However, since the team didn't have values for the mass, they had to rely on studying the planets' brightness to provide an upper limit to the number of rogue planets observed. The brightness is, in turn, related to the age of the planets themselves, as the older the planet, the longer it has been cooling down and reducing in brightness. If the studied region is old, then the brightest objects in the sample are likely above 13 Jupiter masses, and below if the region is on the younger side. Given the uncertainty in the age of the study region, this method gives a rogue planet count of between 70 and 170.
- To spot so many rogue planets, the team used data spanning about 20 years from a number of telescopes on the ground and in space. "We measured the tiny motions, the colors and luminosities of tens of millions of sources in a large area of the sky," explains Miret-Roig. "These measurements allowed us to securely identify the faintest objects in this region, the rogue planets."
- The team used observations from ESO's Very Large Telescope (VLT), the Visible and Infrared Survey Telescope for Astronomy (VISTA), the VLT Survey Telescope (VST) and the MPG/ESO 2.2-meter telescope located in Chile, along with other facilities. "The vast majority of our data come from ESO observatories, which were absolutely critical for this study. Their wide field of view and unique sensitivity were keys to our success," explains Hervé Bouy, an astronomer at the Laboratoire d'Astrophysique de Bordeaux, France, and project leader of the new research. "We used tens of thousands of wide-field images from ESO facilities, corresponding to hundreds of hours of observations, and literally tens of terabytes of data."
- The team also used data from the European Space Agency's Gaia satellite, marking a huge success for the collaboration of ground- and space-based telescopes in the exploration and understanding of our Universe.
- The study suggests there could be many more of these elusive, starless planets that we have yet to discover. "There could be several billions of these free-floating giant planets roaming freely in the Milky Way without a host star," Bouy explains.
- By studying the newly found rogue planets, astronomers may find clues to how these mysterious objects form. Some scientists believe rogue planets can form from the collapse of a gas cloud that is too small to lead to the formation of a star, or that they could have been kicked out from their parent system. But which mechanism is more likely remains unknown.
- Further advances in technology will be key to unlocking the mystery of these nomadic planets. The team hopes to continue to study them in greater detail with ESO's forthcoming Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert and due to start observations later this decade. "These objects are extremely faint and little can be done to study them with current facilities," says Bouy. "The ELT will be absolutely crucial to gathering more information about most of the rogue planets we have found."
• December 14, 2021: The European Southern Observatory's Very Large Telescope Interferometer (ESO's VLTI) has obtained the deepest and sharpest images to date of the region around the supermassive black hole at the centre of our galaxy. The new images zoom in 20 times more than what was possible before the VLTI and have helped astronomers find a never-before-seen star close to the black hole. By tracking the orbits of stars at the centre of our Milky Way, the team has made the most precise measurement yet of the black hole's mass.16)
Table 2: Definition of ESO's VLTI 17)
Figure 10: These annotated images, obtained with the GRAVITY instrument on ESO's Very Large Telescope Interferometer (VLTI) between March and July 2021, show stars orbiting very close to Sgr A*, the supermassive black hole at the heart of the Milky Way. One of these stars, named S29, was observed as it was making its closest approach to the black hole at 13 billion km, just 90 times the distance between the Sun and Earth. Another star, named S300, was detected for the first time in the new VLTI observations. - To obtain the new images, the astronomers used a machine-learning technique, called Information Field Theory. They made a model of how the real sources may look, simulated how GRAVITY would see them, and compared this simulation with GRAVITY observations. This allowed them to find and track stars around Sagittarius A* with unparalleled depth and accuracy (image credit: ESO/GRAVITY collaboration)
- "We want to learn more about the black hole at the centre of the Milky Way, Sagittarius A*: How massive is it exactly? Does it rotate? Do stars around it behave exactly as we expect from Einstein's general theory of relativity? The best way to answer these questions is to follow stars on orbits close to the supermassive black hole. And here we demonstrate that we can do that to a higher precision than ever before," explains Reinhard Genzel, a director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany who was awarded a Nobel Prize in 2020 for Sagittarius A* research. Genzel and his team's latest results, which expand on their three-decade-long study of stars orbiting the Milky Way's supermassive black hole, are published today in two papers in Astronomy & Astrophysics. 18)
- On a quest to find even more stars close to the black hole, the team, known as the GRAVITY collaboration, developed a new analysis technique that has allowed them to obtain the deepest and sharpest images yet of our Galactic Centre. "The VLTI gives us this incredible spatial resolution and with the new images we reach deeper than ever before. We are stunned by their amount of detail, and by the action and number of stars they reveal around the black hole," explains Julia Stadler, a researcher at the Max Planck Institute for Astrophysics in Garching who led the team's imaging efforts during her time at MPE. Remarkably, they found a star, called S300, which had not been seen previously, showing how powerful this method is when it comes to spotting very faint objects close to Sagittarius A*.
- With their latest observations, conducted between March and July 2021, the team focused on making precise measurements of stars as they approached the black hole. This includes the record-holder star S29, which made its nearest approach to the black hole in late May 2021. It passed it at a distance of just 13 billion km, about 90 times the Sun-Earth distance, at the stunning speed of 8740 km/s. No other star has ever been observed to pass that close to, or travel that fast around, the black hole.
- The team's measurements and images were made possible thanks to GRAVITY, a unique instrument that the collaboration developed for ESO's VLTI, located in Chile. GRAVITY combines the light of all four 8.2-meter telescopes of ESO's Very Large Telescope (VLT) using a technique called interferometry. This technique is complex, "but in the end you arrive at images 20 times sharper than those from the individual VLT telescopes alone, revealing the secrets of the Galactic Centre," says Frank Eisenhauer from MPE, principal investigator of GRAVITY.
- "Following stars on close orbits around Sagittarius A* allows us to precisely probe the gravitational field around the closest massive black hole to Earth, to test General Relativity, and to determine the properties of the black hole," explains Genzel. The new observations, combined with the team's previous data, confirm that the stars follow paths exactly as predicted by General Relativity for objects moving around a black hole of mass 4.30 million times that of the Sun. This is the most precise estimate of the mass of the Milky Way's central black hole to date. The researchers also managed to fine-tune the distance to Sagittarius A*, finding it to be 27,000 light-years away.
- To obtain the new images, the astronomers used a machine-learning technique, called Information Field Theory. They made a model of how the real sources may look, simulated how GRAVITY would see them, and compared this simulation with GRAVITY observations. This allowed them to find and track stars around Sagittarius A* with unparalleled depth and accuracy. In addition to the GRAVITY observations, the team also used data from NACO and SINFONI, two former VLT instruments, as well as measurements from the Keck Observatory and NOIRLab's Gemini Observatory in the US.
- GRAVITY will be updated later this decade to GRAVITY+, which will also be installed on ESO's VLTI and will push the sensitivity further to reveal fainter stars even closer to the black hole. The team aims to eventually find stars so close that their orbits would feel the gravitational effects caused by the black hole's rotation. ESO's upcoming Extremely Large Telescope (ELT), under construction in the Chilean Atacama Desert, will further allow the team to measure the velocity of these stars with very high precision. "With GRAVITY+'s and the ELT's powers combined, we will be able to find out how fast the black hole spins," says Eisenhauer. "Nobody has been able to do that so far."
• December 8, 2021: he European Southern Observatory's Very Large Telescope (ESO's VLT) has captured an image of a planet orbiting b Centauri, a two-star system that can be seen with the naked eye. This is the hottest and most massive planet-hosting star system found to date, and the planet was spotted orbiting it at 100 times the distance Jupiter orbits the Sun. Some astronomers believed planets could not exist around stars this massive and this hot — until now. 19)
Figure 11: This image shows the most massive planet-hosting star pair to date, b Centauri, and its giant planet b Centauri b. This is the first time astronomers have directly observed a planet orbiting a star pair this massive and hot. - The star pair, which has a total mass of at least six times that of the Sun, is the bright object in the top left corner of the image, the bright and dark rings around it being optical artefacts. The planet, visible as a bright dot in the lower right of the frame, is ten times as massive as Jupiter and orbits the pair at 100 times the distance Jupiter orbits the Sun. The other bright dot in the image (top right) is a background star. By taking different images at different times, astronomers were able to distinguish the planet from the background stars. - The image was captured by the SPHERE instrument on ESO's Very Large Telescope and using a coronagraph, which blocked the light from the massive star system and allowed astronomers to detect the faint planet (image credit: ESO/Janson et al.)
- "Finding a planet around b Centauri was very exciting since it completely changes the picture about massive stars as planet hosts," explains Markus Janson, an astronomer at Stockholm University, Sweden and first author of the new study published online today in Nature. 20)
- Located approximately 325 light-years away in the constellation Centaurus, the b Centauri two-star system (also known as HIP 71865) has at least six times the mass of the Sun, making it by far the most massive system around which a planet has been confirmed. Until now, no planets had been spotted around a star more than three times as massive as the Sun.
- Most massive stars are also very hot, and this system is no exception: its main star is a so-called B-type star that is over three times as hot as the Sun. Owing to its intense temperature, it emits large amounts of ultraviolet and X-ray radiation.
- The large mass and the heat from this type of star have a strong impact on the surrounding gas, that should work against planet formation. In particular, the hotter a star is, the more high-energy radiation it produces, which causes the surrounding material to evaporate faster. "B-type stars are generally considered as quite destructive and dangerous environments, so it was believed that it should be exceedingly difficult to form large planets around them," Janson says.
- But the new discovery shows planets can in fact form in such severe star systems. "The planet in b Centauri is an alien world in an environment that is completely different from what we experience here on Earth and in our Solar System," explains co-author Gayathri Viswanath, a PhD student at Stockholm University. "It's a harsh environment, dominated by extreme radiation, where everything is on a gigantic scale: the stars are bigger, the planet is bigger, the distances are bigger."
- Indeed, the planet discovered, named b Centauri (AB)b or b Centauri b, is also extreme. It is 10 times as massive as Jupiter, making it one of the most massive planets ever found. Moreover, it moves around the star system in one of the widest orbits yet discovered, at a distance a staggering 100 times greater than the distance of Jupiter from the Sun. This large distance from the central pair of stars could be key to the planet's survival.
- These results were made possible thanks to the sophisticated Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) mounted on ESO's VLT in Chile. SPHERE has successfully imaged several planets orbiting stars other than the Sun before, including taking the first ever-image of two planets orbiting a Sun-like star.
- However, SPHERE was not the first instrument to image this planet. As part of their study, the team looked into archival data on the b Centauri system and discovered that the planet had actually been imaged more than 20 years ago by the ESO 3.6-m telescope, though it was not recognized as a planet at the time.
- With ESO's Extremely Large Telescope (ELT), due to start observations later this decade, and with upgrades to the VLT, astronomers may be able to unveil more about this planet's formation and features. "It will be an intriguing task to try to figure out how it might have formed, which is a mystery at the moment," concludes Janson.
• November 30, 2021: Using the European Southern Observatory's Very Large Telescope (ESO's VLT), astronomers have revealed the closest pair of supermassive black holes to Earth ever observed. The two objects also have a much smaller separation than any other previously spotted pair of supermassive black holes and will eventually merge into one giant black hole. 21)
- Located in the galaxy NGC 7727 in the constellation Aquarius, the supermassive black hole pair is about 89 million light-years away from Earth. Although this may seem distant, it beats the previous record of 470 million light-years by quite some margin, making the newfound supermassive black hole pair the closest to us yet.
Figure 12: This image shows close-up (left) and wide (right) views of the two bright galactic nuclei, each housing a supermassive black hole, in NGC 7727, a galaxy located 89 million light-years away from Earth in the constellation Aquarius. Each nucleus consists of a dense group of stars with a supermassive black hole at its center. The two black holes are on a collision course and form the closest pair of supermassive black holes found to date. It is also the pair with the smallest separation between two supermassive black holes found to date — observed to be just 1600 light-years apart in the sky. — The image on the left was taken with the MUSE instrument on ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile while the one on the right was taken with ESO's VLT Survey Telescope (image credit: ESO/Voggel et al.; ESO/VST ATLAS team. Acknowledgement: Durham University/CASU/WFAU)
- Supermassive black holes lurk at the center of massive galaxies and when two such galaxies merge, the black holes end up on a collision course. The pair in NGC 7727 beat the record for the smallest separation between two supermassive black holes, as they are observed to be just 1600 light-years apart in the sky. "It is the first time we find two supermassive black holes that are this close to each other, less than half the separation of the previous record holder," says Karina Voggel, an astronomer at the Strasbourg Observatory in France and lead author of the study published online today in Astronomy & Astrophysics. 22)
- "The small separation and velocity of the two black holes indicate that they will merge into one monster black hole, probably within the next 250 million years," adds co-author Holger Baumgardt, a professor at the University of Queensland, Australia. The merging of black holes like these could explain how the most massive black holes in the Universe come to be.
- Voggel and her team were able to determine the masses of the two objects by looking at how the gravitational pull of the black holes influences the motion of the stars around them. The bigger black hole, located right at the core of NGC 7727, was found to have a mass almost 154 million times that of the Sun, while its companion is 6.3 million solar masses.
- It is the first time the masses have been measured in this way for a supermassive black hole pair. This feat was made possible thanks to the close proximity of the system to Earth and the detailed observations the team obtained at the Paranal Observatory in Chile using the Multi-Unit Spectroscopic Explorer (MUSE) on ESO's VLT, an instrument Voggel learnt to work with during her time as a student at ESO. Measuring the masses with MUSE, and using additional data from the NASA/ESA Hubble Space Telescope, allowed the team to confirm that the objects in NGC 7727 were indeed supermassive black holes.
- Astronomers suspected that the galaxy hosted the two black holes, but they had not been able to confirm their presence until now since we do not see large amounts of high-energy radiation coming from their immediate surroundings, which would otherwise give them away. "Our finding implies that there might be many more of these relics of galaxy mergers out there and they may contain many hidden massive black holes that still wait to be found," says Voggel. "It could increase the total number of supermassive black holes known in the local Universe by 30 percent."
- The search for similarly hidden supermassive black hole pairs is expected to make a great leap forward with ESO's Extremely Large Telescope (ELT), set to start operating later this decade in Chile's Atacama Desert. "This detection of a supermassive black hole pair is just the beginning," says co-author Steffen Mieske, an astronomer at ESO in Chile and Head of ESO Paranal Science Operations. "With the HARMONI instrument on the ELT we will be able to make detections like this considerably further than currently possible. ESO's ELT will be integral to understanding these objects."
• November 11, 2021: Using the European Southern Observatory's Very Large Telescope (ESO's VLT), astronomers have discovered a small black hole outside the Milky Way by looking at how it influences the motion of a star in its close vicinity. This is the first time this detection method has been used to reveal the presence of a black hole outside of our galaxy. The method could be key to unveiling hidden black holes in the Milky Way and nearby galaxies, and to help shed light on how these mysterious objects form and evolve. 23)
Figure 13: This artist's impression shows a compact black hole 11 times as massive as the Sun and the five-solar-mass star orbiting it. The two objects are located in NGC 1850, a cluster of thousands of stars roughly 160,000 light-years away in the Large Magellanic Cloud, a Milky Way neighbor. The distortion of the star's shape is due to the strong gravitational force exerted by the black hole. - Not only does the black hole's gravitational force distort the shape of the star, but it also influences its orbit. By looking at these subtle orbital effects, a team of astronomers were able to infer the presence of the black hole, making it the first small black hole outside of our galaxy to be found this way. For this discovery, the team used the Multi Unit Spectroscopic Explorer (MUSE) instrument at ESO's VLT (Very Large Telescope) in Chile (image credit: ESO, M. Kornmesser)
- The newly found black hole was spotted lurking in NGC 1850, a cluster of thousands of stars roughly 160 000 light-years away in the Large Magellanic Cloud, a neighbor galaxy of the Milky Way.
- "Similar to Sherlock Holmes tracking down a criminal gang from their missteps, we are looking at every single star in this cluster with a magnifying glass in one hand trying to find some evidence for the presence of black holes but without seeing them directly," says Sara Saracino from the Astrophysics Research Institute of Liverpool John Moores University in the UK, who led the research now accepted for publication in Monthly Notices of the Royal Astronomical Society. "The result shown here represents just one of the wanted criminals, but when you have found one, you are well on your way to discovering many others, in different clusters."
- This first "criminal" tracked down by the team turned out to be roughly 11 times as massive as our Sun. The smoking gun that put the astronomers on the trail of this black hole was its gravitational influence on the five-solar-mass star orbiting it.
- Astronomers have previously spotted such small, "stellar-mass" black holes in other galaxies by picking up the X-ray glow emitted as they swallow matter, or from the gravitational waves generated as black holes collide with one another or with neutron stars.
- However, most stellar-mass black holes don't give away their presence through X-rays or gravitational waves. "The vast majority can only be unveiled dynamically," says Stefan Dreizler, a team member based at the University of Göttingen in Germany. "When they form a system with a star, they will affect its motion in a subtle but detectable way, so we can find them with sophisticated instruments."
- This dynamical method used by Saracino and her team could allow astronomers to find many more black holes and help unlock their mysteries. "Every single detection we make will be important for our future understanding of stellar clusters and the black holes in them," says study co-author Mark Gieles from the University of Barcelona, Spain.
- The detection in NGC 1850 marks the first time a black hole has been found in a young cluster of stars (the cluster is only around 100 million years old, a blink of an eye on astronomical scales). Using their dynamical method in similar star clusters could unveil even more young black holes and shed new light on how they evolve. By comparing them with larger, more mature black holes in older clusters, astronomers would be able to understand how these objects grow by feeding on stars or merging with other black holes. Furthermore, charting the demographics of black holes in star clusters improves our understanding of the origin of gravitational wave sources.
- To carry out their search, the team used data collected over two years with the Multi Unit Spectroscopic Explorer (MUSE) mounted at ESO's VLT, located in the Chilean Atacama Desert. "MUSE allowed us to observe very crowded areas, like the innermost regions of stellar clusters, analyzing the light of every single star in the vicinity. The net result is information about thousands of stars in one shot, at least 10 times more than with any other instrument," says co-author Sebastian Kamann, a long-time MUSE expert based at Liverpool's Astrophysics Research Institute. This allowed the team to spot the odd star out whose peculiar motion signalled the presence of the black hole. Data from the University of Warsaw's Optical Gravitational Lensing Experiment and from the NASA/ESA Hubble Space Telescope enabled them to measure the mass of the black hole and confirm their findings.
- ESO's Extremely Large Telescope in Chile, set to start operating later this decade, will allow astronomers to find even more hidden black holes. "The ELT will definitely revolutionize this field," says Saracino. "It will allow us to observe stars considerably fainter in the same field of view, as well as to look for black holes in globular clusters located at much greater distances."
• October 12, 2021: Using the European Southern Observatory's Very Large Telescope (ESO's VLT) in Chile, astronomers have imaged 42 of the largest objects in the asteroid belt, located between Mars and Jupiter. Never before had such a large group of asteroids been imaged so sharply. The observations reveal a wide range of peculiar shapes, from spherical to dog-bone, and are helping astronomers trace the origins of the asteroids in our Solar System. 25)
Figure 14: This image depicts 42 of the largest objects in the asteroid belt, located between Mars and Jupiter. Most of them are larger than 100 km, with the two biggest asteroids being Ceres and Vesta, which are around 940 and 520 km in diameter, and the two smallest ones being Urania and Ausonia, each only about 90 km. - The images of the asteroids have been captured with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO's Very Large Telescope [image credit: ESO/M. Kornmesser/Vernazza et al./MISTRAL algorithm (ONERA/CNRS)]
- The detailed images of these 42 objects are a leap forward in exploring asteroids, made possible thanks to ground-based telescopes, and contribute to answering the ultimate question of life, the Universe, and everything.
- "Only three large main belt asteroids, Ceres, Vesta and Lutetia, have been imaged with a high level of detail so far, as they were visited by the space missions Dawn and Rosetta of NASA and the European Space Agency, respectively," explains Pierre Vernazza, from the Laboratoire d'Astrophysique de Marseille in France, who led the asteroid study published today in Astronomy & Astrophysics. "Our ESO observations have provided sharp images for many more targets, 42 in total."
- The previously small number of detailed observations of asteroids meant that, until now, key characteristics such as their 3D shape or density had remained largely unknown. Between 2017 and 2019, Vernazza and his team set out to fill this gap by conducting a thorough survey of the major bodies in the asteroid belt.
- Most of the 42 objects in their sample are larger than 100 km in size; in particular, the team imaged nearly all of the belt asteroids larger than 200 km, 20 out of 23. The two biggest objects the team probed were Ceres and Vesta, which are around 940 and 520 km in diameter, whereas the two smallest asteroids are Urania and Ausonia, each only about 90 km.
- By reconstructing the objects' shapes, the team realized that the observed asteroids are mainly divided into two families. Some are almost perfectly spherical, such as Hygiea and Ceres, while others have a more peculiar, "elongated" shape, their undisputed queen being the "dog-bone" asteroid Kleopatra.
- By combining the asteroids' shapes with information on their masses, the team found that the densities change significantly across the sample. The four least dense asteroids studied, including Lamberta and Sylvia, have densities of about 1.3 gram/cm3, approximately the density of coal. The highest, Psyche and Kalliope, have densities of 3.9 and 4.4 gram/cm3, respectively, which is higher than the density of diamond (3.5 gram/cm3).
- This large difference in density suggests the asteroids' composition varies significantly, giving astronomers important clues about their origin. "Our observations provide strong support for substantial migration of these bodies since their formation. In short, such tremendous variety in their composition can only be understood if the bodies originated across distinct regions in the Solar System," explains Josef Hanuš of the Charles University, Prague, Czech Republic, one of the authors of the study. In particular, the results support the theory that the least dense asteroids formed in the remote regions beyond the orbit of Neptune and migrated to their current location.
- These findings were made possible thanks to the sensitivity of the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument mounted on ESO's VLT. "With the improved capabilities of SPHERE, along with the fact that little was known regarding the shape of the largest main belt asteroids, we were able to make substantial progress in this field," says co-author Laurent Jorda, also of the Laboratoire d'Astrophysique de Marseille.
- Astronomers will be able to image even more asteroids in fine detail with ESO's upcoming Extremely Large Telescope (ELT), currently under construction in Chile and set to start operations later this decade. "ELT observations of main-belt asteroids will allow us to study objects with diameters down to 35 to 80 km, depending on their location in the belt, and craters down to approximately 10 to 25 km in size," says Vernazza. "Having a SPHERE-like instrument at the ELT would even allow us to image a similar sample of objects in the distant Kuiper Belt. This means we'll be able to characterize the geological history of a much larger sample of small bodies from the ground."
• September 9, 2021: Using the European Southern Observatory's Very Large Telescope (ESO's VLT), a team of astronomers have obtained the sharpest and most detailed images yet of the asteroid Kleopatra. The observations have allowed the team to constrain the 3D shape and mass of this peculiar asteroid, which resembles a dog bone, to a higher accuracy than ever before. Their research provides clues as to how this asteroid and the two moons that orbit it formed. 26) 27)
Figure 15: Asteroid Kleopatra from different angles. These eleven images are of the asteroid Kleopatra, viewed at different angles as it rotates. The images were taken at different times between 2017 and 2019 with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO's VLT. - Kleopatra orbits the Sun in the Asteroid Belt between Mars and Jupiter. Astronomers have called it a "dog-bone asteroid" ever since radar observations around 20 years ago revealed it has two lobes connected by a thick "neck"[image credit: ESO/Vernazza, Marchis et al./MISTRAL algorithm (ONERA/CNRS)]
- "Kleopatra is truly a unique body in our Solar System," says Franck Marchis, an astronomer at the SETI Institute in Mountain View, USA and at the Laboratoire d'Astrophysique de Marseille, France, who led a study on the asteroid — which has moons and an unusual shape — published today in Astronomy & Astrophysics. "Science makes a lot of progress thanks to the study of weird outliers. I think Kleopatra is one of those and understanding this complex, multiple asteroid system can help us learn more about our Solar System."
- Kleopatra orbits the Sun in the Asteroid Belt between Mars and Jupiter. Astronomers have called it a "dog-bone asteroid" ever since radar observations around 20 years ago revealed it has two lobes connected by a thick "neck". In 2008, Marchis and his colleagues discovered that Kleopatra is orbited by two moons, named AlexHelios and CleoSelene, after the Egyptian queen's children.
- To find out more about Kleopatra, Marchis and his team used snapshots of the asteroid taken at different times between 2017 and 2019 with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO's VLT. As the asteroid was rotating, they were able to view it from different angles and to create the most accurate 3D models of its shape to date. They constrained the asteroid's dog-bone shape and its volume, finding one of the lobes to be larger than the other, and determined the length of the asteroid to be about 270 km half the length of the English Channel.
- In a second study, also published in Astronomy & Astrophysics and led by Miroslav Brož of Charles University in Prague, Czech Republic, the team reported how they used the SPHERE observations to find the correct orbits of Kleopatra's two moons. Previous studies had estimated the orbits, but the new observations with ESO's VLT showed that the moons were not where the older data predicted them to be.
- "This had to be resolved," says Brož. "Because if the moons' orbits were wrong, everything was wrong, including the mass of Kleopatra." Thanks to the new observations and sophisticated modelling, the team managed to precisely describe how Kleopatra's gravity influences the moons' movements and to determine the complex orbits of AlexHelios and CleoSelene. This allowed them to calculate the asteroid's mass, finding it to be 35% lower than previous estimates.
- Combining the new estimates for volume and mass, astronomers were able to calculate a new value for the density of the asteroid, which, at less than half the density of iron, turned out to be lower than previously thought . The low density of Kleopatra, which is believed to have a metallic composition, suggests that it has a porous structure and could be little more than a "pile of rubble". This means it likely formed when material reaccumulated following a giant impact.
- Kleopatra's rubble-pile structure and the way it rotates also give indications as to how its two moons could have formed. The asteroid rotates almost at a critical speed, the speed above which it would start to fall apart, and even small impacts may lift pebbles off its surface. Marchis and his team believe that those pebbles could subsequently have formed AlexHelios and CleoSelene, meaning that Kleopatra has truly birthed its own moons.
- The new images of Kleopatra and the insights they provide are only possible thanks to one of the advanced adaptive optics systems in use on ESO's VLT, which is located in the Atacama Desert in Chile. Adaptive optics help to correct for distortions caused by the Earth's atmosphere which cause objects to appear blurred — the same effect that causes stars viewed from Earth to twinkle. Thanks to such corrections, SPHERE was able to image Kleopatra — located 200 million km away from Earth at its closest — even though its apparent size on the sky is equivalent to that of a golf ball about 40 km away.
- ESO's upcoming Extremely Large Telescope (ELT), with its advanced adaptive optics systems, will be ideal for imaging distant asteroids such as Kleopatra. "I can't wait to point the ELT at Kleopatra, to see if there are more moons and refine their orbits to detect small changes," adds Marchis.
• August 5, 2021: A team of astronomers have used the European Southern Observatory's Very Large Telescope (ESO's VLT) in Chile to shed new light on planets around a nearby star, L 98-59, that resemble those in the inner Solar System. Amongst the findings are a planet with half the mass of Venus — the lightest exoplanet ever to be measured using the radial velocity technique — an ocean world, and a possible planet in the habitable zone. 28)
Figure 16: This artist's impression shows L 98-59b, one of the planets in the L 98-59 system 35 light-years away. The system contains four confirmed rocky planets with a potential fifth, the furthest from the star, being unconfirmed. In 2021, astronomers used data from the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument on ESO's VLT to measure the mass of L 98-59b, finding it to be half that of Venus. This makes it the lightest planet measured to date using the radial velocity technique (image credit: ESO, M. Kornmesser)
- The planet in the habitable zone may have an atmosphere that could protect and support life," says María Rosa Zapatero Osorio, an astronomer at the Centre for Astrobiology in Madrid, Spain, and one of the authors of the study published today in Astronomy & Astrophysics. 29)
- The results are an important step in the quest to find life on Earth-sized planets outside the Solar System. The detection of biosignatures on an exoplanet depends on the ability to study its atmosphere, but current telescopes are not large enough to achieve the resolution needed to do this for small, rocky planets. The newly studied planetary system, called L 98-59 after its star, is an attractive target for future observations of exoplanet atmospheres. Its orbits a star only 35 light-years away and has now been found to host rocky planets, like Earth or Venus, which are close enough to the star to be warm.
- With the contribution of ESO's VLT, the team was able to infer that three of the planets may contain water in their interiors or atmospheres. The two planets closest to the star in the L 98-59 system are probably dry, but might have small amounts of water, while up to 30% of the third planet's mass could be water, making it an ocean world.
- Furthermore, the team found "hidden" exoplanets that had not previously been spotted in this planetary system. They discovered a fourth planet and suspect there is a fifth, in a zone at the right distance from the star for liquid water to exist on its surface. "We have hints of the presence of a terrestrial planet in the habitable zone of this system," explains Olivier Demangeon, a researcher at the Instituto de Astrofísica e Ciências do Espaço, University of Porto in Portugal and lead author of the new study.
- The study represents a technical breakthrough, as astronomers were able to determine, using the radial velocity method, that the innermost planet in the system has just half the mass of Venus. This makes it the lightest exoplanet ever measured using this technique, which calculates the wobble of the star caused by the tiny gravitational tug of its orbiting planets.
- The team used the ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) instrument on ESO's VLT to study L 98-59. "Without the precision and stability provided by ESPRESSO this measurement would have not been possible," says Zapatero Osorio. "This is a step forward in our ability to measure the masses of the smallest planets beyond the Solar System."
- The astronomers first spotted three of L 98-59's planets in 2019, using NASA's Transiting Exoplanet Survey Satellite (TESS). This satellite relies on a technique called the transit method — where the dip in the light coming from the star caused by a planet passing in front of it is used to infer the properties of the planet — to find the planets and measure their sizes. However, it was only with the addition of radial velocity measurements made with ESPRESSO and its predecessor, the HARPS (High Accuracy Radial velocity Planet Searcher) at the ESO La Silla 3.6-meter telescope, that Demangeon and his team were able to find extra planets and measure the masses and radii of the first three. "If we want to know what a planet is made of, the minimum that we need is its mass and its radius," Demangeon explains.
- The team hopes to continue to study the system with the forthcoming NASA/ESA/CSA JWST ( James Webb Space Telescope), while ESO's Extremely Large Telescope (ELT), under construction in the Chilean Atacama Desert and set to start observations in 2027, will also be ideal for studying these planets. "The HIRES instrument on the ELT may have the power to study the atmospheres of some of the planets in the L 98-59 system, thus complementing the JWST from the ground," says Zapatero Osorio.
- "This system announces what is to come," adds Demangeon. "We, as a society, have been chasing terrestrial planets since the birth of astronomy and now we are finally getting closer and closer to the detection of a terrestrial planet in the habitable zone of its star, of which we could study the atmosphere."
• July 16, 2021: A team of astronomers has released new observations of nearby galaxies that resemble colorful cosmic fireworks. The images, obtained with the European Southern Observatory's Very Large Telescope (ESO's VLT), show different components of the galaxies in distinct colors, allowing astronomers to pinpoint the locations of young stars and the gas they warm up around them. By combining these new observations with data from the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, the team is helping shed new light on what triggers gas to form stars. 30)
Figure 17: This image combines observations of the nearby galaxies NGC 1300, NGC 1087, NGC 3627 (top, from left to right), NGC 4254 and NGC 4303 (bottom, from left to right) taken with the Multi-Unit Spectroscopic Explorer (MUSE) on ESO's Very Large Telescope (VLT). Each individual image is a combination of observations conducted at different wavelengths of light to map stellar populations and warm gas. The golden glows mainly correspond to clouds of ionized hydrogen, oxygen and sulphur gas, marking the presence of newly born stars, while the bluish regions in the background reveal the distribution of slightly older stars. -The images were taken as part of the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) project, which is making high-resolution observations of nearby galaxies with telescopes operating across the electromagnetic spectrum (image credit: ESO/PHANGS)
- Astronomers know that stars are born in clouds of gas, but what sets off star formation, and how galaxies as a whole play into it, remains a mystery. To understand this process, a team of researchers has observed various nearby galaxies with powerful telescopes on the ground and in space, scanning the different galactic regions involved in stellar births.
- "For the first time we are resolving individual units of star formation over a wide range of locations and environments in a sample that well represents the different types of galaxies," says Eric Emsellem, an astronomer at ESO in Germany and lead of the VLT-based observations conducted as part of the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) project. "We can directly observe the gas that gives birth to stars, we see the young stars themselves, and we witness their evolution through various phases."
- Emsellem, who is also affiliated with the University of Lyon, France, and his team have now released their latest set of galactic scans, taken with the Multi-Unit Spectroscopic Explorer (MUSE) instrument on ESO's VLT in the Atacama Desert in Chile. They used MUSE to trace newborn stars and the warm gas around them, which is illuminated and heated up by the stars and acts as a smoking gun of ongoing star formation.
- The new MUSE images are now being combined with observations of the same galaxies taken with ALMA and released earlier this year. ALMA, which is also located in Chile, is especially well suited to mapping cold gas clouds — the parts of galaxies that provide the raw material out of which stars form.
- By combining MUSE and ALMA images astronomers can examine the galactic regions where star formation is happening, compared to where it is expected to happen, so as to better understand what triggers, boosts or holds back the birth of new stars. The resulting images are stunning, offering a spectacularly colorful insight into stellar nurseries in our neighboring galaxies.
- "There are many mysteries we want to unravel," says Kathryn Kreckel from the University of Heidelberg in Germany and PHANGS team member. "Are stars more often born in specific regions of their host galaxies — and, if so, why? And after stars are born how does their evolution influence the formation of new generations of stars?"
- Astronomers will now be able to answer these questions thanks to the wealth of MUSE and ALMA data the PHANGS team have obtained. MUSE collects spectra — the "bar codes" astronomers scan to unveil the properties and nature of cosmic objects — at every single location within its field of view, thus providing much richer information than traditional instruments. For the PHANGS project, MUSE observed 30 000 nebulae of warm gas and collected about 15 million spectra of different galactic regions. The ALMA observations, on the other hand, allowed astronomers to map around 100 000 cold-gas regions across 90 nearby galaxies, producing an unprecedentedly sharp atlas of stellar nurseries in the close Universe.
- In addition to ALMA and MUSE, the PHANGS project also features observations from the NASA/ESA Hubble Space Telescope. The various observatories were selected to allow the team to scan our galactic neighbors at different wavelengths (visible, near-infrared and radio), with each wavelength range unveiling distinct parts of the observed galaxies. "Their combination allows us to probe the various stages of stellar birth — from the formation of the stellar nurseries to the onset of star formation itself and the final destruction of the nurseries by the newly born stars — in more detail than is possible with individual observations," says PHANGS team member Francesco Belfiore from INAF-Arcetri in Florence, Italy. "PHANGS is the first time we have been able to assemble such a complete view, taking images sharp enough to see the individual clouds, stars, and nebulae that signify forming stars."
- The work carried out by the PHANGS project will be further honed by upcoming telescopes and instruments, such as NASA's James Webb Space Telescope. The data obtained in this way will lay further groundwork for observations with ESO's future Extremely Large Telescope (ELT), which will start operating later this decade and will enable an even more detailed look at the structures of stellar nurseries.
- "As amazing as PHANGS is, the resolution of the maps that we produce is just sufficient to identify and separate individual star-forming clouds, but not good enough to see what's happening inside them in detail," pointed out Eva Schinnerer, a research group leader at the Max Planck Institute for Astronomy in Germany and principal investigator of the PHANGS project, under which the new observations were conducted. "New observational efforts by our team and others are pushing the boundary in this direction, so we have decades of exciting discoveries ahead of us."
- The international PHANGS team is composed of over 90 scientists ranging from Master students to retirees working at 30 institutions across four continents. The MUSE data reduction working group within PHANGS is being led by Eric Emsellem (European Southern Observatory, Garching, Germany and Centre de Recherche Astrophysique de Lyon, Université de Lyon, ENS de Lyon, Saint-Genis Laval, France) and includes Francesco Belfiore (INAF Osservatorio Astrofisico di Arcetri, Florence, Italy), Guillermo Blanc (Carnegie Observatories, Pasadena, US), Enrico Congiu (Universidad de Chile, Santiago, Chile and Las Campanas Observatory, Carnegie Institution for Science, Atacama Region, Chile), Brent Groves (The University of Western Australia, Perth, Australia), I-Ting Ho (Max Planck Institute for Astronomy, Heidelberg, Germany [MPIA]), Kathryn Kreckel (Heidelberg University, Heidelberg, Germany), Rebecca McElroy (Sydney Institute for Astronomy, Sydney, Australia), Ismael Pessa (MPIA), Patricia Sanchez-Blazquez (Complutense University of Madrid, Madrid, Spain), Francesco Santoro (MPIA), Fabian Scheuermann (Heidelberg University, Heidelberg, Germany) and Eva Schinnerer (MPIA).
- Go to the ESO public image archive to see a sample of PHANGS images.
• July 14, 2021: An international team of astronomers have become the first in the world to detect isotopes in the atmosphere of an exoplanet. It concerns different forms of carbon in the gaseous giant planet TYC 8998-760-1 b at a distance of 300 light years in the constellation Musca (Fly). The weak signal was measured with ESO's Very Large Telescope in Chile and seems to indicate that the planet is relatively rich in carbon-13. The astronomers speculate that this is because the planet formed at a great distance from its parent star. The research will be published in the scientific journal Nature on Thursday. 31)
Figure 18: Cartoon about the discovery of carbon-13 in the atmosphere of an exoplanet [image credit: Daniëlle Futselaar (Artsource)]
- Isotopes are different forms of the same atom, but with varying number of neutrons in the nucleus. For example, carbon with six protons typically has six neutrons (carbon-12), but occasionally seven (carbon-13) or eight (carbon-14). This does not change much the chemical properties of carbon, but isotopes are formed in different ways and often react slightly differently to the prevailing conditions. Isotopes are therefore used in a wide range of research fields: from detecting cardiovascular disease or cancer to studying climate change and determining the age of fossils and rocks.
- The astronomers were able to distinguish carbon-13 from carbon-12 because it absorbs radiation at slightly different colors. "It is really quite special that we can measure this in an exoplanet atmosphere, at such a large distance," says Leiden PhD student Yapeng Zhang, first author of the article.
- The astronomers had expected to detect about one in 70 carbon atoms to be carbon-13, but for this planet it seems to be twice as much. The idea is that the higher carbon-13 is somehow related to the formation of the exoplanet.
- Co-author Paul Mollière, from the Max Planck Institute for Astronomy in Heidelberg, Germany, explains: "The planet is more than one hundred and fifty times further away from its parent star than our Earth is from our Sun. At such a great distance, ices have possibly formed with more carbon-13, causing the higher fraction of this isotope in the planet's atmosphere today."
Figure 19: Isotopes In Exoplanets Explained. Twelve-year-old Kevin explains about isotopes in the atmosphere of an exoplanets, now measured by astronomers for the first time. What does this mean? What do we learn? Based on "The 13CO-rich atmosphere of a young accreting super-Jupiter" by Yapeng Zhang et al., Nature - July 2021 (video credit: Astronomie.nl) 32)
- The planet itself, TYC 8998-760-1 b, was discovered only two years ago by Leiden PhD student Alexander Bohn, co-author of the article. "It' s awesome that this discovery has been made close to 'my' planet. It will probably be the first of many."
- Ignas Snellen, professor in Leiden and for many years the driving force behind this subject, is above all proud. "The expectation is that in the future isotopes will further help to understand exactly how, where and when planets form. This is just the beginning."
• June 16, 2021: When Betelgeuse, a bright orange star in the constellation of Orion, became visibly darker in late 2019 and early 2020, the astronomy community was puzzled. A team of astronomers have now published new images of the star's surface, taken using the European Southern Observatory's Very Large Telescope (ESO's VLT), that clearly show how its brightness changed. The new research reveals that the star was partially concealed by a cloud of dust, a discovery that solves the mystery of the "Great Dimming" of Betelgeuse. 33)
Figure 20: Betelgeuse's surface before and during its 2019–2020 Great Dimming. These images, taken with the SPHERE instrument on ESO's Very Large Telescope, show the surface of the red supergiant star Betelgeuse during its unprecedented dimming, which happened in late 2019 and early 2020. The image on the far left, taken in January 2019, shows the star at its normal brightness, while the remaining images, from December 2019, January 2020 and March 2020, were all taken when the star's brightness had noticeably dropped, especially in its southern region. The brightness returned to normal in April 2020 (image credit: ESO/M. Montargès et al.)
- Betelgeuse's dip in brightness — a change noticeable even to the naked eye — led Miguel Montargès and his team to point ESO's VLT towards the star in late 2019. An image from December 2019, when compared to an earlier image taken in January of the same year, showed that the stellar surface was significantly darker, especially in the southern region. But the astronomers weren't sure why.
- The team continued observing the star during its Great Dimming, capturing two other never-before-seen images in January 2020 and March 2020. By April 2020, the star had returned to its normal brightness.
- "For once, we were seeing the appearance of a star changing in real time on a scale of weeks," says Montargès, from the Observatoire de Paris, France, and KU Leuven, Belgium. The images now published are the only ones we have that show Betelgeuse's surface changing in brightness over time.
- In their new study, published today in Nature, the team revealed that the mysterious dimming was caused by a dusty veil shading the star, which in turn was the result of a drop in temperature on Betelgeuse's stellar surface.
- Betelgeuse's surface regularly changes as giant bubbles of gas move, shrink and swell within the star. The team concludes that some time before the Great Dimming, the star ejected a large gas bubble that moved away from it. When a patch of the surface cooled down shortly after, that temperature decrease was enough for the gas to condense into solid dust.
- "We have directly witnessed the formation of so-called stardust," says Montargès, whose study provides evidence that dust formation can occur very quickly and close to a star's surface. "The dust expelled from cool evolved stars, such as the ejection we've just witnessed, could go on to become the building blocks of terrestrial planets and life," adds Emily Cannon, from KU Leuven, who was also involved in the study.
- Rather than just the result of a dusty outburst, there was some speculation online that Betelgeuse's drop in brightness could signal its imminent death in a spectacular supernova explosion. A supernova hasn't been observed in our galaxy since the 17th century, so present-day astronomers aren't entirely sure what to expect from a star in the lead-up to such an event. However, this new research confirms that Betelgeuse's Great Dimming was not an early sign that the star was heading towards its dramatic fate.
- Witnessing the dimming of such a recognizable star was exciting for professional and amateur astronomers alike, as summed up by Cannon: "Looking up at the stars at night, these tiny, twinkling dots of light seem perpetual. The dimming of Betelgeuse breaks this illusion."
- The team used the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on ESO's VLT to directly image the surface of Betelgeuse, alongside data from the GRAVITY instrument on ESO's Very Large Telescope Interferometer (VLTI), to monitor the star throughout the dimming. The telescopes, located at ESO's Paranal Observatory in Chile's Atacama Desert, were a "vital diagnostic tool in uncovering the cause of this dimming event," says Cannon. "We were able to observe the star not just as a point but could resolve the details of its surface and monitor it throughout the event," Montargès adds.
- Montargès and Cannon are looking forward to what the future of astronomy, in particular what ESO's Extremely Large Telescope (ELT), will bring to their study of Betelgeuse, a red supergiant star. "With the ability to reach unparalleled spatial resolutions, the ELT will enable us to directly image Betelgeuse in remarkable detail," says Cannon. "It will also significantly expand the sample of red supergiants for which we can resolve the surface through direct imaging, further helping us to unravel the mysteries behind the winds of these massive stars."
- This research was presented in the paper "A dusty veil shading Betelgeuse during its Great Dimming" to appear in Nature.
• May 19, 2021: A new study by a Belgian team using data from the European Southern Observatory's Very Large Telescope (ESO's VLT) has shown that iron and nickel exist in the atmospheres of comets throughout our Solar System, even those far from the Sun. A separate study by a Polish team, who also used ESO data, reported that nickel vapor is also present in the icy interstellar comet 2I/Borisov. This is the first time heavy metals, usually associated with hot environments, have been found in the cold atmospheres of distant comets. 34)
Figure 21: The detection of the heavy metals iron (Fe) and nickel (Ni) in the fuzzy atmosphere of a comet are illustrated in this image, which features the spectrum of light of C/2016 R2 (PANSTARRS) on the top left superimposed to a real image of the comet taken with the SPECULOOS telescope at ESO's Paranal Observatory. Each white peak in the spectrum represents a different element, with those for iron and nickel indicated by blue and orange dashes, respectively. Spectra like these are possible thanks to the UVES instrument on ESO's VLT, a high-resolution spectrograph that spreads the line so much they can be individually identified. In addition, UVES remains sensitive down to wavelengths of 300nm. Most of the important iron and nickel lines appear at wavelengths of around 350nm, meaning that the capabilities of UVES were essential in making this discovery. (image credit: ESO/L. Calçada, SPECULOOS Team/E. Jehin, Manfroid et al.)
- "It was a big surprise to detect iron and nickel atoms in the atmosphere of all the comets we have observed in the last two decades, about 20 of them, and even in ones far from the Sun in the cold space environment," says Jean Manfroid from the University of Liège, Belgium, who lead the new study on Solar System comets published today in Nature. 35)
- Astronomers know that heavy metals exist in comets' dusty and rocky interiors. But, because solid metals don't usually "sublimate" (become gaseous) at low temperatures, they did not expect to find them in the atmospheres of cold comets that travel far from the Sun. Nickel and iron vapors have now even been detected in comets observed at more than 480 million kilometers from the Sun, more than three times the Earth-Sun distance.
- The Belgian team found iron and nickel in comets' atmospheres in approximately equal amounts. Material in our Solar System, for example that found in the Sun and in meteorites, usually contains about ten times more iron than nickel. This new result therefore has implications for astronomers' understanding of the early Solar System, though the team is still decoding what these are.
- "Comets formed around 4.6 billion years ago, in the very young Solar System, and haven't changed since that time. In that sense, they're like fossils for astronomers," says study co-author Emmanuel Jehin, also from the University of Liège.
- While the Belgian team has been studying these "fossil" objects with ESO's VLT for nearly 20 years, they had not spotted the presence of nickel and iron in their atmospheres until now. "This discovery went under the radar for many years," Jehin says.
- The team used data from the Ultraviolet and Visual Echelle Spectrograph (UVES) instrument on ESO's VLT, which uses a technique called spectroscopy, to analyze the atmospheres of comets at different distances from the Sun. This technique allows astronomers to reveal the chemical makeup of cosmic objects: each chemical element leaves a unique signature — a set of lines — in the spectrum of the light from the objects.
- The Belgian team had spotted weak, unidentified spectral lines in their UVES data and on closer inspection noticed that they were signalling the presence of neutral atoms of iron and nickel. A reason why the heavy elements were difficult to identify is that they exist in very small amounts: the team estimates that for each 100 kg of water in the comets' atmospheres there is only 1 g of iron, and about the same amount of nickel.
- "Usually there is 10 times more iron than nickel, and in those comet atmospheres we found about the same quantity for both elements. We came to the conclusion they might come from a special kind of material on the surface of the comet nucleus, sublimating at a rather low temperature and releasing iron and nickel in about the same proportions," explains Damien Hutsemékers, also a member of the Belgian team from the University of Liège.
- Although the team aren't sure yet what material this might be, advances in astronomy — such as the Mid-infrared ELT Imager and Spectrograph (METIS) on ESO's upcoming Extremely Large Telescope (ELT) — will allow researchers to confirm the source of the iron and nickel atoms found in the atmospheres of these comets.
- The Belgian team hope their study will pave the way for future research. "Now people will search for those lines in their archival data from other telescopes," Jehin says. "We think this will also trigger new work on the subject."
• March 30, 2021: New observations with the European Southern Observatory's Very Large Telescope (ESO's VLT) indicate that the rogue comet 2I/Borisov, which is only the second and most recently detected interstellar visitor to our Solar System, is one of the most pristine ever observed. Astronomers suspect that the comet most likely never passed close to a star, making it an undisturbed relic of the cloud of gas and dust it formed from. 36)
Figure 22: This image was taken with the FORS2 instrument on ESO's VLT in late 2019, when comet 2I/Borisov passed near the Sun. Since the comet was travelling at breakneck speed, around 175,000 km/hr, the background stars appeared as streaks of light as the telescope followed the comet's trajectory. The colors in these streaks give the image some disco flair and are the result of combining observations in different wavelength bands, highlighted by the various colors in this composite image (image credit: ESO, O. Hainaut)
- 2I/Borisov was discovered by amateur astronomer Gennady Borisov in August 2019 and was confirmed to have come from beyond the Solar System a few weeks later. “2I/Borisov could represent the first truly pristine comet ever observed,” says Stefano Bagnulo of the Armagh Observatory and Planetarium, Northern Ireland, UK, who led the new study published today in Nature Communications. The team believes that the comet had never passed close to any star before it flew by the Sun in 2019.
- Bagnulo and his colleagues used the FORS2 instrument on ESO's VLT, located in northern Chile, to study 2I/Borisov in detail using a technique called polarimetry. Since this technique is regularly used to study comets and other small bodies of our Solar System, this allowed the team to compare the interstellar visitor with our local comets.
- The team found that 2I/Borisov has polarimetric properties distinct from those of Solar System comets, with the exception of Hale–Bopp. Comet Hale–Bopp received much public interest in the late 1990s as a result of being easily visible to the naked eye, and also because it was one of the most pristine comets astronomers had ever seen. Prior to its most recent passage, Hale–Bopp is thought to have passed by our Sun only once and had therefore barely been affected by solar wind and radiation. This means it was pristine, having a composition very similar to that of the cloud of gas and dust it — and the rest of the Solar System — formed from some 4.5 billion years ago.
- By analyzing the polarization together with the color of the comet to gather clues on its composition, the team concluded that 2I/Borisov is in fact even more pristine than Hale–Bopp. This means it carries untarnished signatures of the cloud of gas and dust it formed from.
- “The fact that the two comets are remarkably similar suggests that the environment in which 2I/Borisov originated is not so different in composition from the environment in the early Solar System,” says Alberto Cellino, a co-author of the study, from the Astrophysical Observatory of Torino, National Institute for Astrophysics (INAF), Italy.
- Olivier Hainaut, an astronomer at ESO in Germany who studies comets and other near-Earth objects but was not involved in this new study, agrees. “The main result — that 2I/Borisov is not like any other comet except Hale–Bopp — is very strong,” he says, adding that “it is very plausible they formed in very similar conditions.”
- “The arrival of 2I/Borisov from interstellar space represented the first opportunity to study the composition of a comet from another planetary system and check if the material that comes from this comet is somehow different from our native variety,” explains Ludmilla Kolokolova, of the University of Maryland in the US, who was involved in the Nature Communications research.
- Bagnulo hopes astronomers will have another, even better, opportunity to study a rogue comet in detail before the end of the decade. “ESA is planning to launch Comet Interceptor in 2029, which will have the capability of reaching another visiting interstellar object, if one on a suitable trajectory is discovered,” he says, referring to an upcoming mission by the European Space Agency.
An origin story hidden in the dust
- Even without a space mission, astronomers can use Earth’s many telescopes to gain insight into the different properties of rogue comets like 2I/Borisov. “Imagine how lucky we were that a comet from a system light-years away simply took a trip to our doorstep by chance,” says Bin Yang, an astronomer at ESO in Chile, who also took advantage of 2I/Borisov’s passage through our Solar System to study this mysterious comet. Her team’s results are published in Nature Astronomy. 37) 38)
- Yang and her team used data from the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, as well as from ESO’s VLT, to study 2I/Borisov’s dust grains to gather clues about the comet’s birth and conditions in its home system.
- They discovered that 2I/Borisov’s coma — an envelope of dust surrounding the main body of the comet — contains compact pebbles, grains about one millimeter in size or larger. In addition, they found that the relative amounts of carbon monoxide and water in the comet changed drastically as it neared the Sun. The team, which also includes Olivier Hainaut, says this indicates that the comet is made up of materials that formed in different places in its planetary system.
- The observations by Yang and her team suggest that matter in 2I/Borisov’s planetary home was mixed from near its star to further out, perhaps because of the existence of giant planets, whose strong gravity stirs material in the system. Astronomers believe that a similar process occurred early in the life of our Solar System.
- While 2I/Borisov was the first rogue comet to pass by the Sun, it was not the first interstellar visitor. The first interstellar object to have been observed passing by our Solar System was 'Oumuamua, another object studied with ESO’s VLT back in 2017. Originally classified as a comet, 'Oumuamua was later reclassified as an asteroid as it lacked a coma.
• March 8, 2021: With the help of the European Southern Observatory’s Very Large Telescope (ESO’s VLT), astronomers have discovered and studied in detail the most distant source of radio emission known to date. The source is a “radio-loud” quasar — a bright object with powerful jets emitting at radio wavelengths — that is so far away its light has taken 13 billion years to reach us. The discovery could provide important clues to help astronomers understand the early Universe. 39)
Figure 23: This artist’s impression shows how the distant quasar P172+18 and its radio jets may have looked. To date (early 2021), this is the most distant quasar with radio jets ever found and it was studied with the help of ESO’s Very Large Telescope. It is so distant that light from it has travelled for about 13 billion years to reach us: we see it as it was when the Universe was only about 780 million years old (image credit: ESO, M. Kornmesser)
- Quasars are very bright objects that lie at the centre of some galaxies and are powered by supermassive black holes. As the black hole consumes the surrounding gas, energy is released, allowing astronomers to spot them even when they are very far away.
- The newly discovered quasar,
nicknamed P172+18, is so distant that light from it has travelled for
about 13 billion years to reach us: we see it as it was when the
Universe was just around 780 million years old. While more distant
quasars have been discovered, this is the first time astronomers have
been able to identify the telltale signatures of radio jets in a quasar
this early on in the history of the Universe. Only about 10% of quasars
— which astronomers classify as “radio-loud” —
have jets, which shine brightly at radio frequencies .
- P172+18 is powered by a black hole about 300 million times more massive than our Sun that is consuming gas at a stunning rate. “The black hole is eating up matter very rapidly, growing in mass at one of the highest rates ever observed,” explains astronomer Chiara Mazzucchelli, Fellow at ESO in Chile, who led the discovery together with Eduardo Bañados of the Max Planck Institute for Astronomy in Germany.
- The astronomers think that there’s a link between the rapid growth of supermassive black holes and the powerful radio jets spotted in quasars like P172+18. The jets are thought to be capable of disturbing the gas around the black hole, increasing the rate at which gas falls in. Therefore, studying radio-loud quasars can provide important insights into how black holes in the early Universe grew to their supermassive sizes so quickly after the Big Bang.
- “I find it very exciting to discover ‘new’ black holes for the first time, and to provide one more building block to understand the primordial Universe, where we come from, and ultimately ourselves,” says Mazzucchelli.
- P172+18 was first recognized as a far-away quasar, after having been previously identified as a radio source, at the Magellan Telescope at Las Campanas Observatory in Chile by Bañados and Mazzucchelli. “As soon as we got the data, we inspected it by eye, and we knew immediately that we had discovered the most distant radio-loud quasar known so far,” says Bañados.
- However, owing to a short observation time, the team did not have enough data to study the object in detail. A flurry of observations with other telescopes followed, including with the X-shooter instrument on ESO’s VLT, which allowed them to dig deeper into the characteristics of this quasar, including determining key properties such as the mass of the black hole and how fast it’s eating up matter from its surroundings. Other telescopes that contributed to the study include the National Radio Astronomy Observatory's Very Large Array and the Keck Telescope in the US.
- While the team are excited about their discovery, to appear in The Astrophysical Journal, they believe this radio-loud quasar could be the first of many to be found, perhaps at even larger cosmological distances. “This discovery makes me optimistic and I believe — and hope — that the distance record will be broken soon,” says Bañados. 40)
- Observations with facilities such as ALMA, in which ESO is a partner, and with ESO’s upcoming Extremely Large Telescope (ELT) could help uncover and study more of these early-Universe objects in detail.
• March 2, 2021: The European Astronomical Society (EAS) awards the 2021 Tycho Brahe Medal to Dr Frank Eisenhauer (Max Planck Institute for Extraterrestrial Physics, MPE) for his leadership of the SINFONI and GRAVITY instruments on the ESO VLT, which revolutionized the study of exoplanets, supermassive black holes, and star forming galaxies in the early universe. Both SINFONI and GRAVITY are part of the instrument suite employed in the discovery and characterization of the Galactic Center Black Hole, which led to the Nobel Prize 2020 in Physics. 41)
- SINFONI revolutionized the spectroscopy of stars close to the Galactic Center black hole and the kinematic studies of galaxies at the peak of galaxy formation a few billion years after the Big Bang. It consists of an integral field spectrometer coupled to an adaptive optics system. Integral filed spectroscopy is a technique to simultaneously record an image and the spectrum for every pixel of the image, and adaptive optics corrects the blurring due to the Earth’s atmosphere. The development of SINFONI catapulted integral-field units to the leading design choice of imaging spectroscopy for all large telescopes, including also the ESO-ELT, a 40 m-class telescope currently under construction.
- The next major technological breakthrough came with GRAVITY, which combines the light of all four VLT telescopes interferometrically. This allows astronomers to use the four telescopes simultaneously as a virtual 130m telescope achieving milli-arcsecond resolution – sharp enough to detect houses on the moon, if there were any.
- After a mere three years of science operation, GRAVITY has already provided several breakthroughs: Combining GRAVITY astrometry and SINFONI spectroscopy, the team was able to measure the gravitational redshift and relativistic precession in the orbit of the star S2 around the Galactic Center black hole SgrA*. The astronomers also used GRAVITY to observe gas swirl at around 30% the speed of light close to the innermost stable orbit around SgrA*. This provides very strong support that SgrA* indeed is a massive black hole. GRAVITY can also look outside our home galaxy: in the quasar 3C273, at a distance of ca. 1.4 billion light years, the instrument directly measured the rotation of the Broad Line Region, a set of clouds rotating around the black hole in the center of this galaxy. Further, GRAVITY can zoom onto exoplanets around nearby stars, and for example, could reveal the details of an exoplanet's atmosphere in the constellation of Pegasus.
- With the Tycho Brahe Medal, which is awarded in recognition of the development or exploitation of European instruments or major discoveries based largely on such instruments, the EAS recognizes that Frank Eisenhauer was the leader and driver in the design and development of these complex and innovative instruments and has been a key figure in their scientific exploitation. Frank Eisenhauer studied physics at the Technical University of Munich (TUM) and obtained his PhD in 1998 from the Ludwig Maximilian University Munich. He is currently a Senior Research Scientist at MPE and Adjunct Teaching Professor at TUM. From 1998-2004 Dr Eisenhauer was leading the development on the world’s first adaptive optics assisted integral field spectrometer on an 8m-class telescope, SINFONI. He then became the leader of the GRAVITY instrument to combine the light from the four 8m VLT telescopes, which is operated at the VLT since 2016. The instrument is currently further developed into GRAVITY+, enhancing its capabilities and sensitivity. The leader of this project is again Dr Eisenhauer.
Figure 24: This collage shows some of the results obtained with GRAVITY in its first three years of observation showing the wide range of possible applications (image credit: ESA, MPE)
• February 17, 2021: An international research team with members from ETH Zürich has developed a new method for directly imaging smaller planets in the habitable zone of a neighboring star system. This opens up new possibilities in the search for extraterrestrial life. 42)
Figure 25: Alpha Centauri A (left) and Alpha Centauri B are located in the constellation of Centaurus (The Centaur), at a distance of 4.3 light-years. The star pair orbits a common center of gravity once every 80 years (image credit: NASA/ESA/Hubble)
- In the search for planets capable of sustaining life, an international research team with members from ETH has taken a significant step forward. As the researchers reported recently in the journal Nature Communications, they found signs of a Neptune-sized planet in the Alpha Centauri star system, a mere 4.4 light years away from Earth. This exoplanet is located in a zone that may offer suitable conditions for life. The team was able to collect data with unprecedented sensitivity, thus registering even very weak signals. 43)
Earth is a disruptive factor
- Thanks to the new process, the researchers have advanced one step closer to a major goal of exoplanet research: the discovery of Earth-like planets capable of supporting life. Direct imaging of planets delivers information about the composition of their atmospheres and possibly even signs of life. To date, however, direct measurements have mostly found exoplanets that are larger than Jupiter and orbit far away from very young host stars. In other words, these planets fall outside the habitable zone where liquid water could form.
- One reason that the search for Earth-like planets has so far proved fruitless is that it has been conducted in the near-infrared range, even though Earth-like planets that might have water are brightest in the mid-infrared range. Yet it is precisely in that range that measurements with normal telescopes are difficult, because that is where the Earth and its atmosphere are also at their brightest. This means the faint signals from exoplanets are lost in particularly strong background noise.
Figure 26: To the naked eye, Alpha Centauri is a single bright star. But it is actually made up of a pair of binary stars, Alpha Centauri A and Alpha Centauri B, plus the faint red dwarf Alpha Centauri C, Proxima Centauri (image credit: Yuri Beletsky/LCO/ESO)
100 hours of observations
- As reported in their study, the researchers have now been able to overcome this difficulty and take measurements in the mid-infrared range. They used the Very Large Telescope at the European Southern Observatory in Chile to examine Alpha Centauri stars A and B, logging nearly 100 hours over the course of a month. “Keeping the telescope pointed at the same star for such a long time is highly unusual,” explains Anna Boehle, a postdoc in ETH Professor Sascha Quanz’s group. As second author of the study, Boehle was heavily involved in evaluating the data. “We assessed more than five million images,” she says.
- To be able to detect the faint signals from potential planets, the researchers not only processed a huge volume of data, they also employed two sophisticated measurement techniques: one was to use a new deformable secondary telescope mirror, which made it possible to correct for distortions in the light coming through the Earth’s atmosphere; and the other was to use a coronagraph to alternately block the light from each of the stars in turn at very short intervals. This let the scientists further reduce signal noise while examining the surroundings of both stars.
Figure 27: Imaging habitable-zone exoplanets around Alpha Centauri (video credit: Kevin Wagner)
Signs of a planet
- “Our findings indicate that in principle, this process enables us to discover smaller terrestrial planets capable of hosting life,” Boehle explains, “and it represents a clear improvement over previous observation methods.” Indeed, in their data the researchers found a light signal that may originate from a Neptune-sized planet. Boehle says, “Whether or not this signal is actually from a planet requires further study. To that end, we plan to combine the infrared measurements with other measurement methods."
• January 12, 2021: An international team led by Dutch astronomers has, after years of searching and defying the boundaries of a telescope, for the first time directly captured polarized light from an exoplanet. They can deduct from the light that a disk of dust and gas is orbiting around the exoplanet in which moons are possibly forming. The researchers will soon publish their findings in the journal Astronomy & Astrophysics. 44)
- The discovery concerns the exoplanet DH Tau b. This is a very young planet of only 2 million years old at 437 light years from Earth in the constellation Taurus. Exoplanet DH Tau b does not resemble our Earth. The planet is at least eleven times more massive than Jupiter, the most massive planet in our solar system. The planet is also located ten times further away from its star than our furthest planet Neptune. The planet is still glowing after its formation. As a result, it emits heat in the form of infrared radiation.
- The researchers discovered that the infrared radiation of the planet is polarized. This means that the light waves vibrate in a preferential direction. And that, according to the researchers, is because the infrared radiation of the planet is scattered by a disk of dust and gas that orbits the planet. In such a disk, moons may form.
Figure 28: Three images of the exoplanet DH Tau b. The left image shows all light, both unpolarized and polarized. The middle image shows only polarized light. The right image additionally shows the direction of the polarized light. In polarized light the planet DH Tau b is visible, which points to a disk of dust and gas around this planet. The disk around the star is also visible (image credit: ESO/VLT/SPHERE/Van Holstein et al.)
- Furthermore, the disk around the planet appears to have a different orientation from the disk around the star. Such a tilted disk indicates that the planet has likely formed at a large distance from the star. This is contrary to the theory that planets are formed close to their star and then migrate outward.
- For the observations, the astronomers used the SPHERE instrument on the VLT (Very Large Telescope) of the European Southern Observatory (ESO) in Chile. This instrument can, among other things, block the overwhelming light of the associated star and determine the polarization of the remaining light.
- First author and research leader Rob van Holstein (Leiden University, the Netherlands) has been working with the SPHERE instrument since his university study in 2014: "Because we fully understood the instrument, we were able to make it perform better than it was designed for. In the end, we were able to capture the light from twenty exoplanets, one of which had polarized light."
- Co-author Frans Snik (Leiden University) has been trying to capture polarized light from planets since 2012: "It's already very special that we can see a planet separated from the star around which it orbits. And now we can also deduce that material is orbiting this planet as well, and that this material does so at a completely different angle than the disk that orbits the star. This gives us unique insights into how such a planet and possible moons are formed." 45)
- In the future, the researchers aim to carry out similar research on the ELT (Extremely Large Telescope) that is under construction. This telescope should make it possible to study the light of rocky, Earth-like planets. From the polarization of the light it will be possible to obtain more information about the atmosphere of such planets and whether there are possible signs of life.
• September 3, 2020: A team of astronomers have identified the first direct evidence that groups of stars can tear apart their planet-forming disc, leaving it warped and with tilted rings. This new research suggests exotic planets, not unlike Tatooine in Star Wars, may form in inclined rings in bent discs around multiple stars. The results were made possible thanks to observations with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA). 46)
Figure 29: ALMA and the Sphere instrument on the VLT have imaged GW Orionis, a triple star system with a peculiar inner region. The new observations revealed that this object has a warped planet-forming disc with a misaligned ring. In particular, the SPHERE image (right panel) allowed astronomers to see, for the first time, the shadow that this ring casts on the rest of the disc. This helped them figure out the 3D shape of the ring and the overall disc. The left panel shows an artistic impression of the inner region of the disc, including the ring, which is based on the 3D shape reconstructed by the team (image credit: ESO, L. Calçada, Exeter/Kraus, et al.)
- Our Solar System is remarkably flat, with the planets all orbiting in the same plane. But this is not always the case, especially for planet-forming discs around multiple stars, like the object of the new study: GW Orionis. This system, located just over 1300 light-years away in the constellation of Orion, has three stars and a deformed, broken-apart disc surrounding them.
- “Our images reveal an extreme case where the disc is not flat at all, but is warped and has a misaligned ring that has broken away from the disc,” says Stefan Kraus, a professor of astrophysics at the University of Exeter in the UK who led the research published today in the journal Science. The misaligned ring is located in the inner part of the disc, close to the three stars. 47)
- The new research also reveals that this inner ring contains 30 Earth-masses of dust, which could be enough to form planets. “Any planets formed within the misaligned ring will orbit the star on highly oblique orbits and we predict that many planets on oblique, wide-separation orbits will be discovered in future planet imaging campaigns, for instance with the ELT,” says team member Alexander Kreplin of the University of Exeter, referring to ESO’s Extremely Large Telescope, which is planned to start operating later this decade. Since more than half the stars in the sky are born with one or more companions, this raises an exciting prospect: there could be an unknown population of exoplanets that orbit their stars on very inclined and distant orbits.
- To reach these conclusions, the team observed GW Orionis for over 11 years. Starting in 2008, they used the AMBER and later the GRAVITY instruments on ESO’s VLT Interferometer in Chile, which combines the light from different VLT telescopes, to study the gravitational dance of the three stars in the system and map their orbits. “We found that the three stars do not orbit in the same plane, but their orbits are misaligned with respect to each other and with respect to the disc,” says Alison Young of the Universities of Exeter and Leicester and a member of the team.
- They also observed the system with the SPHERE instrument on ESO’s VLT and with ALMA, in which ESO is a partner, and were able to image the inner ring and confirm its misalignment. ESO’s SPHERE also allowed them to see, for the first time, the shadow that this ring casts on the rest of the disc. This helped them figure out the 3D shape of the ring and the overall disc.
- The international team, which includes researchers from the UK, Belgium, Chile, France and the US, then combined their exhaustive observations with computer simulations to understand what had happened to the system. For the first time, they were able to clearly link the observed misalignments to the theoretical “disc-tearing effect”, which suggests that the conflicting gravitational pull of stars in different planes can warp and break their discs.
- Their simulations showed that the misalignment in the orbits of the three stars could cause the disc around them to break into distinct rings, which is exactly what they see in their observations. The observed shape of the inner ring also matches predictions from numerical simulations on how the disc would tear.
- Interestingly, another team who studied the same system using ALMA believe another ingredient is needed to understand the system. “We think that the presence of a planet between these rings is needed to explain why the disc tore apart,” says Jiaqing Bi of the University of Victoria in Canada who led a study of GW Orionis published in The Astrophysical Journal Letters in May this year. His team identified three dust rings in the ALMA observations, with the outermost ring being the largest ever observed in planet-forming discs.
- Future observations with ESO’s ELT and other telescopes may help astronomers fully unravel the nature of GW Orionis and reveal young planets forming around its three stars.
• August 28, 2020: A team including researchers from the Institute for Astrophysics of the University of Cologne has for the first time directly observed the columns of matter that build up newborn stars. This was observed in the young star TW Hydrae system located approximately 163 light years from Earth. This result was obtained with the VLTI (Very Large Telescope Interferometer) and its GRAVITY instrument of the European Southern Observatory (ESO) in Chile. The article 'A measure of the size of the magnetospheric accretion region in TW Hydrae' has been published in a recent issue of Nature. 48) 49)
- The formation of stars in the Galaxy involves processes in which primordial matter such as gas and dust present in the giant molecular clouds is rapidly aggregated via gravity to form a protostar. This 'accretion' of gas occurs through the disk that forms around the newborn star and represents the major mechanism of supply of material to the growing central baby star. These so-called protoplanetary disks are one of the key ingredients to explain the formation of very diverse exoplanets that are to date frequently discovered orbiting our closest neighbors.
- Based on theoretical and observational evidence, many scenarios were hypothesized to describe the mechanism of interaction between the star and the parent circumstellar disk, like for instance the funnelling and accretion of host gas onto the central star along the local magnetic field. But this could never be directly observed and proven so far with any telescope. The main reason is that the level of details of the image - astronomers talk about angular resolution - necessary to observe what happens very close to the star was simply out of reach. For comparison, detecting these events would be like discerning a small one-cubic meter box on the surface of the Moon. With a normal telescope, this is not possible. However, with an interferometer like the VLTI in Chile and its instrument GRAVITY, which delivers unprecedented angular resolution in the infrared, such a precise observation has now become possible. An interferometer collects and combines the light from different telescopes a few hundred meters apart, which provides the same level of accuracy as a hypothetical giant telescope with a comparable diameter.
- With the contribution of members of Cologne's Institute for Astrophysics, astrophysicists from several European institutions exploited the GRAVITY instrument at the VLTI to probe the closest regions around the young solar analog TW Hydrae, which is thought to be the most representative example of what our Sun may have looked like at the time of its formation, more than 5 billion years ago. By measuring very precisely the typical angular size of the very inner gaseous regions - using a particular infrared atomic transition of the hot hydrogen gas - the scientists were able to directly prove that the hot gas emission was indeed resulting from magnetospheric accretion taking place very close to the stellar surface. 'This is an important milestone in our attempt to confirm the mechanisms at work in the field of star formation', said Professor Lucas Labadie, co-author of the paper. 'We now want to extend such exploration to other young stars of different nature to understand how the evolution of the circumstellar disk, the birthplace of planets, goes.'
- The team is part of the GRAVITY collaboration, named after the instrument that was co-developed by the University of Cologne and which combines interferometrically the four large VLT 8-m telescopes of ESO in Chile. The team members include Lucas Labadie, Rebekka Grellmann, Andreas Eckart, Matthew Horrobin, Christian Straubmeier and Michael Wiest. 'This result illustrates what is the unique potential of interferometry at the VLTI', added Dr Christian Straubmeier, team member and co-investigator of the GRAVITY instrument in Cologne. 'This is why we decided to look ahead and develop the upgrade GRAVITY+ in the hope of being able to observe and image even fainter objects than what GRAVITY currently does.'
Figure 30: Artist's Impression of the Streams of Hot Gas that Build up Stars. Matter from the surrounding protoplanetary disk, the birthplace of planets, is channeled onto the stellar surface by magnetic fields shocking the surface at supersonic velocity (image credit: University of Cologne, Mark A. Garlick)
• July 30, 2020: Resembling a butterfly with its symmetrical structure, beautiful colors, and intricate patterns, this striking bubble of gas — known as NGC 2899 — appears to float and flutter across the sky in this new picture from ESO’s Very Large Telescope (VLT). This object has never before been imaged in such striking detail, with even the faint outer edges of the planetary nebula glowing over the background stars. 50)
- NGC 2899’s vast swathes of gas extend up to a maximum of two light-years from its center, glowing brightly in front of the stars of the Milky Way as the gas reaches temperatures upwards of ten thousand degrees. The high temperatures are due to the large amount of radiation from the nebula’s parent star, which causes the hydrogen gas in the nebula to glow in a reddish halo around the oxygen gas, in blue.
- This object, located between 3000 and 6500 light-years away in the Southern constellation of Vela (The Sails), has two central stars, which are believed to give it its nearly symmetric appearance. After one star reached the end of its life and cast off its outer layers, the other star now interferes with the flow of gas, forming the two-lobed shape seen here. Only about 10–20% of planetary nebulae  display this type of bipolar shape.
- Astronomers were able to capture this highly detailed image of NGC 2899 using the FORS (FOcal Reducer and low dispersion Spectrograph) instrument installed on UT1 (Antu), one of the four 8.2-meter telescopes that make up ESO’s VLT in Chile. Standing for FOcal Reducer and low dispersion Spectrograph, this high-resolution instrument was one of the first to be installed on ESO’s VLT and is behind numerous beautiful images and discoveries from ESO. FORS has contributed to observations of light from a gravitational wave source, has researched the first known interstellar asteroid, and has been used to study in depth the physics behind the formation of complex planetary nebulae.
- This image (Figure 31) was created under the ESO Cosmic Gems program, an outreach initiative to produce images of interesting, intriguing or visually attractive objects using ESO telescopes, for the purposes of education and public outreach. The program makes use of telescope time that cannot be used for science observations. All data collected may also be suitable for scientific purposes, and are made available to astronomers through ESO’s science archive.
- Notes:  Unlike what their common name suggests, planetary nebulae have nothing to do with planets. The first astronomers to observe them merely described them as planet-like in appearance. They are instead formed when ancient stars with up to 6 times the mass of our Sun reach the end of their lives, collapse, and blow off expanding shells of gas, rich in heavy elements. Intense ultraviolet radiation energizes and lights up these moving shells, causing them to shine brightly for thousands of years until they ultimately disperse slowly through space, making planetary nebulae relatively short-lived phenomena on astronomical timescales.
Figure 31: This highly detailed image of the fantastic NGC 2899 planetary nebula was captured using the FORS instrument on ESO’s VLT (Very Large Telescope) in northern Chile. This object has never before been imaged in such striking detail, with even the faint outer edges of the planetary nebula glowing over the background stars (image credit: ESO)
• July 22, 2020: ESO's VLT has taken the first-ever image of a young, Sun-like star accompanied by two giant exoplanets. Images of systems with multiple exoplanets are extremely rare, and — until now — astronomers had never directly observed more than one planet orbiting a star similar to the Sun. The observations can help astronomers understand how planets formed and evolved around our own Sun. 51) 52)
- Just a few weeks ago, ESO revealed a planetary system being born in a new, stunning VLT image. Now, the same telescope, using the same instrument, has taken the first direct image of a planetary system around a star like our Sun, located about 300 light-years away and known as TYC 8998-760-1.
- “This discovery is a snapshot of an environment that is very similar to our Solar System, but at a much earlier stage of its evolution,” says Alexander Bohn, a PhD student at Leiden University in the Netherlands, who led the new research published today in The Astrophysical Journal Letters. 53)
- “Even though astronomers have indirectly detected thousands of planets in our galaxy, only a tiny fraction of these exoplanets have been directly imaged,” says co-author Matthew Kenworthy, Associate Professor at Leiden University, adding that “direct observations are important in the search for environments that can support life.” The direct imaging of two or more exoplanets around the same star is even more rare; only two such systems have been directly observed so far, both around stars markedly different from our Sun. The new ESO’s VLT image is the first direct image of more than one exoplanet around a Sun-like star. ESO’s VLT was also the first telescope to directly image an exoplanet, back in 2004, when it captured a speck of light around a brown dwarf, a type of ‘failed’ star.
- “Our team has now been able to take the first image of two gas giant companions that are orbiting a young, solar analogue,” says Maddalena Reggiani, a postdoctoral researcher from KU Leuven, Belgium, who also participated in the study. The two planets can be seen in the new image as two bright points of light distant from their parent star, which is located in the upper left of the frame (click on the image to view the full frame). By taking different images at different times, the team were able to distinguish these planets from the background stars.
- The two gas giants orbit their host star at distances of 160 and about 320 times the Earth-Sun distance. This places these planets much further away from their star than Jupiter or Saturn, also two gas giants, are from the Sun; they lie at only 5 and 10 times the Earth-Sun distance, respectively. The team also found the two exoplanets are much heavier than the ones in our Solar System, the inner planet having 14 times Jupiter’s mass and the outer one six times.
- Bohn’s team imaged this system during their search for young, giant planets around stars like our Sun but far younger. The star TYC 8998-760-1 is just 17 million years old and located in the Southern constellation of Musca (The Fly). Bohn describes it as a “very young version of our own Sun.”
- These images were possible thanks to the high performance of the SPHERE instrument on ESO’s VLT in the Chilean Atacama desert. SPHERE blocks the bright light from the star using a device called coronagraph, allowing the much fainter planets to be seen. While older planets, such as those in our Solar System, are too cool to be found with this technique, young planets are hotter, and so glow brighter in infrared light. By taking several images over the past year, as well as using older data going back to 2017, the research team have confirmed that the two planets are part of the star’s system.
- Further observations of this system, including with the future ESO Extremely Large Telescope (ELT), will enable astronomers to test whether these planets formed at their current location distant from the star or migrated from elsewhere. ESO’s ELT will also help probe the interaction between two young planets in the same system. Bohn concludes: “The possibility that future instruments, such as those available on the ELT, will be able to detect even lower-mass planets around this star marks an important milestone in understanding multi-planet systems, with potential implications for the history of our own Solar System.”
Figure 32: This image, captured by the SPHERE instrument on ESO’s Very Large Telescope, shows the star TYC 8998-760-1 accompanied by two giant exoplanets, TYC 8998-760-1b and TYC 8998-760-1c. This is the first time astronomers have directly observed more than one planet orbiting a star similar to the Sun. The two planets are visible as two bright dots in the center (TYC 8998-760-1b) and bottom right (TYC 8998-760-1c) of the frame. Other bright dots, which are background stars, are visible in the image as well. By taking different images at different times, the team were able to distinguish these planets from the background stars. The image was captured by blocking the light from the young, Sun-like star (top-left of center) using a coronagraph, which allows for the fainter planets to be detected. The bright and dark rings we see on the star’s image are optical artefacts (image credit: ESO/Bohn et al.)
• June 30, 2020: Using the European Southern Observatory’s Very Large Telescope (VLT), astronomers have discovered the absence of an unstable massive star in a dwarf galaxy. Scientists think this could indicate that the star became less bright and partially obscured by dust. An alternative explanation is that the star collapsed into a black hole without producing a supernova. “If true,” says team leader and PhD student Andrew Allan of Trinity College Dublin, Ireland, “this would be the first direct detection of such a monster star ending its life in this manner.” 54)
Figure 33: Artist’s impression of the disappearing star. This illustration shows what the luminous blue variable star in the Kinman Dwarf galaxy could have looked like before its mysterious disappearance (image credit: ESO/L. Calçada)
- Between 2001 and 2011, various teams of astronomers studied the mysterious massive star, located in the Kinman Dwarf galaxy, and their observations indicated it was in a late stage of its evolution. Allan and his collaborators in Ireland, Chile and the US wanted to find out more about how very massive stars end their lives, and the object in the Kinman Dwarf seemed like the perfect target. But when they pointed ESO’s VLT to the distant galaxy in 2019, they could no longer find the telltale signatures of the star. “Instead, we were surprised to find out that the star had disappeared!” says Allan, who led a study of the star published today in Monthly Notices of the Royal Astronomical Society. 55)
- Located some 75 million light-years away in the constellation of Aquarius, the Kinman Dwarf galaxy is too far away for astronomers to see its individual stars, but they can detect the signatures of some of them. From 2001 to 2011, the light from the galaxy consistently showed evidence that it hosted a ‘luminous blue variable’ star some 2.5 million times brighter than the Sun. Stars of this type are unstable, showing occasional dramatic shifts in their spectra and brightness. Even with those shifts, luminous blue variables leave specific traces scientists can identify, but they were absent from the data the team collected in 2019, leaving them to wonder what had happened to the star. “It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion,” says Allan.
- The group first turned the ESPRESSO instrument toward the star in August 2019, using the VLT’s four 8-meter telescopes simultaneously. But they were unable to find the signs that previously pointed to the presence of the luminous star. A few months later, the group tried the X-shooter instrument, also on ESO’s VLT, and again found no traces of the star.
- “We may have detected one of the most massive stars of the local Universe going gently into the night,” says team-member Jose Groh, also of Trinity College Dublin. “Our discovery would not have been made without using the powerful ESO 8-meter telescopes, their unique instrumentation, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO.” Ireland became an ESO member state in September 2018.
- The team then turned to older data collected using X-shooter and the UVES instrument on ESO’s VLT, located in the Chilean Atacama Desert, and telescopes elsewhere.“The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009,” says Andrea Mehner, a staff astronomer at ESO in Chile who participated in the study. “The comparison of the 2002 high-resolution UVES spectra with our observations obtained in 2019 with ESO's newest high-resolution spectrograph ESPRESSO was especially revealing, from both an astronomical and an instrumentation point of view.”
- The old data indicated that the star in the Kinman Dwarf could have been undergoing a strong outburst period that likely ended sometime after 2011. Luminous blue variable stars such as this one are prone to experiencing giant outbursts over the course of their life, causing the stars’ rate of mass loss to spike and their luminosity to increase dramatically.
- Based on their observations and models, the astronomers have suggested two explanations for the star’s disappearance and lack of a supernova, related to this possible outburst. The outburst may have resulted in the luminous blue variable being transformed into a less luminous star, which could also be partly hidden by dust. Alternatively, the team says the star may have collapsed into a black hole, without producing a supernova explosion. This would be a rare event: our current understanding of how massive stars die points to most of them ending their lives in a supernova.
- Future studies are needed to confirm what fate befell this star. Planned to begin operations in 2025, ESO’s Extremely Large Telescope (ELT) will be capable of resolving stars in distant galaxies such as the Kinman Dwarf, helping to solve cosmic mysteries such as this one.
• May 28, 2020: Researchers from the University of Geneva, have confirmed the existence of the Proxima b extrasolar planet using measurements from the Swiss-built ESPRESSO spectrograph on ESO's VLT (Very Large Telescope) in Chile. 56)
- The existence of a planet the size of Earth around the closest star in the solar system, Proxima Centauri, has been confirmed by an international team of scientists including researchers from the University of Geneva (UNIGE). The results, which you can read all about in the journal Astronomy & Astrophysics, reveal that the planet in question, Proxima b, has a mass of 1.17 earth masses and is located in the habitable zone of its star, which it orbits in 11.2 days. This breakthrough has been possible thanks to radial velocity measurements of unprecedented precision using ESPRESSO, the Swiss-manufactured spectrograph – the most accurate currently in operation – which is installed on the Very Large Telescope in Chile. Proxima b was first detected four years ago by means of an older spectrograph, HARPS – also developed by the Geneva-based team – which measured a low disturbance in the star’s speed, suggesting the presence of a companion.
- The ESPRESSO spectrograph has performed radial velocity measurements on the star Proxima Centauri, which is only 4.2 light-years from the Sun, with an accuracy of 30 cm/s or about three times more precise than that obtained with HARPS, the same type of instrument but from the previous generation.
- “We were already very happy with the performance of HARPS (High Accuracy Radial Velocity Planet Searcher), which has been responsible for discovering hundreds of exoplanets over the last 17 years”, begins Francesco Pepe, a professor in the Astronomy Department in UNIGE’s Faculty of Science and the man in charge of ESPRESSO. “We’re really pleased that ESPRESSO can produce even better measurements, and it’s gratifying and just reward for the teamwork lasting nearly 10 years.”
- Alejandro Suarez Mascareño, the article’s main author, adds: “Confirming the existence of Proxima b was an important task, and it’s one of the most interesting planets known in the solar neighborhood.”
- The measurements performed by ESPRESSO have clarified that the minimum mass of Proxima b is 1.17 earth masses (the previous estimate was 1.3) and that it orbits around its star in only 11.2 days.
- “ESPRESSO has made it possible to measure the mass of the planet with a precision of over one-tenth of the mass of Earth”, says Michel Mayor, winner of the Nobel Prize for Physics in 2019, honorary professor in the Faculty of Science and the ‘architect’ of all ESPRESSO-type instruments. “It’s completely unheard of.”
And what about life in all this?
- Although Proxima b is about 20 times closer to its star than the Earth is to the Sun, it receives comparable energy, so that its surface temperature could mean that water (if there is any) is in liquid form in places and might, therefore, harbor life.
- Having said that, although Proxima b is an ideal candidate for biomarker research, there is still a long way to go before we can suggest that life has been able to develop on its surface. In fact, the Proxima star is an active red dwarf that bombards its planet with X rays, receiving about 400 times more than the Earth.
- “Is there an atmosphere that protects the planet from these deadly rays?” asks Christophe Lovis, a researcher in UNIGE’s Astronomy Department and responsible for ESPRESSO’s scientific performance and data processing. “And if this atmosphere exists, does it contain the chemical elements that promote the development of life (oxygen, for example)? How long have these favorable conditions existed? We’re going to tackle all these questions, especially with the help of future instruments like the RISTRETTO spectrometer, which we’re going to build specially to detect the light emitted by Proxima b, and HIRES, which will be installed on the future ELT 39 m giant telescope that the European Southern Observatory (ESO) is building in Chile.”
Surprise: is there a second planet?
- In the meantime, the precision of the measurements made by ESPRESSO could result in another surprise. The team has found evidence of a second signal in the data, without being able to establish the definitive cause behind it. “If the signal was planetary in origin, this potential other planet accompanying Proxima b would have a mass less than one third of the mass of the Earth. It would then be the smallest planet ever measured using the radial velocity method”, adds Professor Pepe.
Figure 34: This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System (image credit: ESO, M. Kornmesser)
- It should be noted that ESPRESSO, which became operational in 2017, is in its infancy and these initial results are already opening up undreamt of opportunities. The road has been travelled at breakneck pace since the first extrasolar planet was discovered by Michel Mayor and Didier Queloz, both from UNIGE’s Astronomy Department. In 1995, the 51Peg b gas giant planet was detected using the ELODIE spectrograph with an accuracy of 10 m/s. Today ESPRESSO, with its 30 cm/s (and soon 10 after the latest adjustments) will perhaps make it possible to explore worlds that remind us of the Earth. 57)
• May 20, 2020: Observations made with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) have revealed the telltale signs of a star system being born. Around the young star AB Aurigae lies a dense disc of dust and gas in which astronomers have spotted a prominent spiral structure with a ‘twist’ that marks the site where a planet may be forming. The observed feature could be the first direct evidence of a baby planet coming into existence. 58) 59)
- “Thousands of exoplanets have been identified so far, but little is known about how they form,” says Anthony Boccaletti who led the study from the Observatoire de Paris, PSL University, France. Astronomers know planets are born in dusty discs surrounding young stars, like AB Aurigae, as cold gas and dust clump together. The new observations with ESO’s VLT, published in Astronomy & Astrophysics, provide crucial clues to help scientists better understand this process.
- “We need to observe very young systems to really capture the moment when planets form,” says Boccaletti. But until now astronomers had been unable to take sufficiently sharp and deep images of these young discs to find the ‘twist’ that marks the spot where a baby planet may be coming to existence.
- The new images feature a stunning spiral of dust and gas around AB Aurigae, located 520 light-years away from Earth in the constellation of Auriga (The Charioteer). Spirals of this type signal the presence of baby planets, which ‘kick’ the gas, creating “disturbances in the disc in the form of a wave, somewhat like the wake of a boat on a lake,” explains Emmanuel Di Folco of the Astrophysics Laboratory of Bordeaux (LAB), France, who also participated in the study. As the planet rotates around the central star, this wave gets shaped into a spiral arm. The very bright yellow ‘twist’ region close to the center of the new AB Aurigae image, which lies at about the same distance from the star as Neptune from the Sun, is one of these disturbance sites where the team believe a planet is being made.
- Observations of the AB Aurigae system made a few years ago with the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, provided the first hints of ongoing planet formation around the star. In the ALMA images, scientists spotted two spiral arms of gas close to the star, lying within the disc’s inner region. Then, in 2019 and early 2020, Boccaletti and a team of astronomers from France, Taiwan, the US and Belgium set out to capture a clearer picture by turning the SPHERE instrument on ESO’s VLT in Chile toward the star. The SPHERE images are the deepest images of the AB Aurigae system obtained to date.
- With SPHERE's powerful imaging system, astronomers could see the fainter light from small dust grains and emissions coming from the inner disc. They confirmed the presence of the spiral arms first detected by ALMA and also spotted another remarkable feature, a ‘twist’, that points to the presence of ongoing planet formation in the disc. "The twist is expected from some theoretical models of planet formation,” says co-author Anne Dutrey, also at LAB. “It corresponds to the connection of two spirals — one winding inwards of the planet’s orbit, the other expanding outwards — which join at the planet location. They allow gas and dust from the disc to accrete onto the forming planet and make it grow."
- ESO is constructing the 39-meter ELT (Extremely Large Telescope), which will draw on the cutting-edge work of ALMA and SPHERE to study extrasolar worlds. As Boccaletti explains, this powerful telescope will allow astronomers to get even more detailed views of planets in the making. “We should be able to see directly and more precisely how the dynamics of the gas contributes to the formation of planets,” he concludes.
Figure 35: This image shows the disc around the young AB Aurigae star, where ESO’s Very Large Telescope (VLT) has spotted signs of planet birth. Close to the center of the image, in the inner region of the disc, we see the ‘twist’ (in very bright yellow) that scientists believe marks the spot where a planet is forming. This twist lies at about the same distance from the AB Aurigae star as Neptune from the Sun. The image was obtained with the VLT’s SPHERE instrument in polarized light (image credit: ESO/Boccaletti et al.)
• May 01, 2020: An international team of astronomers has captured fifteen images of the inner rims of planet-forming disks located hundreds of light years away. These disks of dust and gas, similar in shape to a music record, form around young stars. The images shed new light on how planetary systems are formed. They were published in the journal Astronomy and Astrophysics. 60) 61)
- To understand how planetary systems, including our own, take shape, you have to study their origins. Planet-forming or protoplanetary disks are formed in unison with the star they surround. The dust grains in the disks can grow into larger bodies, which eventually leads to the formation of planets. Rocky planets like the Earth are believed to form in the inner regions of protoplanetary disks, less than five astronomical units (five times the Earth-Sun distance) from the star around which the disk has formed.
- Before this new study, several pictures of these disks had been taken with the largest single-mirror telescopes, but these cannot capture their finest details. “In these pictures, the regions close to the star, where rocky planets form, are covered by only few pixels,” says lead author Jacques Kluska from KU Leuven in Belgium. “We needed to visualize these details to be able to identify patterns that might betray planet formation and to characterize the properties of the disks." This required a completely different observation technique. “I’m thrilled that we now for the first time have fifteen of these images,” Kluska continues.
- Kluska and his colleagues created the images at the European Southern Observatory (ESO) in Chile by using a technique called infrared interferometry. Using ESO’s PIONIER instrument, they combined the light collected by four telescopes at the VLT (Very Large Telescope) observatory to capture the disks in detail. However, this technique does not deliver an image of the observed source. The details of the disks needed to be recovered with a mathematical reconstruction technique. This technique is similar to how the first image of a black hole was captured. “We had to remove the light of the star, as it hindered the level of detail we could see in the disks,” Kluska explains.
- “Distinguishing details at the scale of the orbits of rocky planets like Earth or Jupiter (as you can see in the images) – a fraction of the Earth-Sun distance – is equivalent to being able to see a human on the Moon, or to distinguish a hair at a 10 km distance,” notes Jean-Philippe Berger of the Université Grenoble-Alpes, who as principal investigator was in charge of the work with the PIONIER instrument. “Infrared interferometry is becoming routinely used to uncover the tiniest details of astronomical objects. Combining this technique with advanced mathematics finally allows us to turn the results of these observations into images.”
- Some findings immediately stand out from the images. “You can see that some spots are brighter or less bright, like in the images above: this hints at processes that can lead to planet formation. For example: there could be instabilities in the disk that can lead to vortices where the disk accumulates grains of space dust that can grow and evolve into a planet.”
- The team will do additional research to identify what might lie behind these irregularities. Kluska will also do new observations to get even more detail and to directly witness planet formation in the regions within the disks that lie close to the star. Additionally, Kluska is heading a team that has started to study 11 disks around other, older types of stars also surrounded by disks of dust, since it is thought these might also sprout planets.
Figure 36: The protoplanetary disks around the R CrA (left) and HD45677 (right) stars, captured with ESO’s VLT (Very Large Telescope) Interferometer. The orbits are added for reference. The star serves the same purpose, since its light was filtered out to get a more detailed image of the disk (image credit: Jacques Kluska et al.)
Figure 37: The fifteen images of protoplanetary disks, captured with ESO's Very Large Telescope Interferometer (image credit: KU Leuven, ESO)
• April 16, 2020: Observations made with ESO's Very Large Telescope (VLT) have revealed for the first time that a star orbiting the supermassive black hole at the center of the Milky Way moves just as predicted by Einstein's general theory of relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This long-sought-after result was made possible by increasingly precise measurements over nearly 30 years, which have enabled scientists to unlock the mysteries of the behemoth lurking at the heart of our galaxy. 62)
- “Einstein’s General Relativity predicts that bound orbits of one object around another are not closed, as in Newtonian Gravity, but precess forwards in the plane of motion. This famous effect — first seen in the orbit of the planet Mercury around the Sun — was the first evidence in favor of General Relativity. One hundred years later we have now detected the same effect in the motion of a star orbiting the compact radio source Sagittarius A* at the center of the Milky Way. This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun,” says Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany and the architect of the 30-year-long program that led to this result.
- Located 26,000 light-years from the Sun, Sagittarius A* and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. One of these stars, S2, sweeps in towards the supermassive black hole to a closest distance less than 20 billion kilometers (one hundred and twenty times the distance between the Sun and Earth), making it one of the closest stars ever found in orbit around the massive giant. At its closest approach to the black hole, S2 is hurtling through space at almost three percent of the speed of light, completing an orbit once every 16 years. “After following the star in its orbit for over two and a half decades, our exquisite measurements robustly detect S2’s Schwarzschild precession in its path around Sagittarius A*,” says Stefan Gillessen of the MPE, who led the analysis of the measurements published today in the journal Astronomy & Astrophysics. 63)
- Most stars and planets have a non-circular orbit and therefore move closer to and further away from the object they are rotating around. S2’s orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape. General Relativity provides a precise prediction of how much its orbit changes and the latest measurements from this research exactly match the theory. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.
Figure 38: This artist’s rendition illustrates the Schwarzschild precession of the star’s orbit, with the effect exaggerated for easier visualization (image credit: ESO/L. Calçada)
- The study with ESO’s VLT also helps scientists learn more about the vicinity of the supermassive black hole at the center of our galaxy. “Because the S2 measurements follow General Relativity so well, we can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*. This is of great interest for understanding the formation and evolution of supermassive black holes,” say Guy Perrin and Karine Perraut, the French lead scientists of the project.
- This result is the culmination of 27 years of observations of the S2 star using, for the best part of this time, a fleet of instruments at ESO’s VLT, located in the Atacama Desert in Chile. The number of data points marking the star’s position and velocity attests to the thoroughness and accuracy of the new research: the team made over 330 measurements in total, using the GRAVITY, SINFONI and NACO instruments. Because S2 takes years to orbit the supermassive black hole, it was crucial to follow the star for close to three decades, to unravel the intricacies of its orbital movement.
- The research was conducted by an international team led by Frank Eisenhauer of the MPE with collaborators from France, Portugal, Germany and ESO. The team make up the GRAVITY collaboration, named after the instrument they developed for the VLT Interferometer, which combines the light of all four 8-meter VLT telescopes into a super-telescope (with a resolution equivalent to that of a telescope 130 meters in diameter). The same team reported in 2018 another effect predicted by General Relativity: they saw the light received from S2 being stretched to longer wavelengths as the star passed close to Sagittarius A*. “Our previous result has shown that the light emitted from the star experiences General Relativity. Now we have shown that the star itself senses the effects of General Relativity,” says Paulo Garcia, a researcher at Portugal’s Center for Astrophysics and Gravitation and one of the lead scientists of the GRAVITY project.
- With ESO’s upcoming Extremely Large Telescope, the team believes that they would be able to see much fainter stars orbiting even closer to the supermassive black hole. “If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole,” says Andreas Eckart from Cologne University, another of the lead scientists of the project. This would mean astronomers would be able to measure the two quantities, spin and mass, that characterize Sagittarius A* and define space and time around it. “That would be again a completely different level of testing relativity," says Eckart.
Figure 39: This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018. This is a perfect laboratory to test gravitational physics and specifically Einstein's general theory of relativity (image credit: ESO/L. Calçada/spaceengine.org)
• March 11, 2020: Researchers using ESO's Very Large Telescope (VLT) have observed an extreme planet where they suspect it rains iron. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapor to the cooler night side where it condenses into iron droplets. 64)
Figure 40: This illustration shows a night-side view of the exoplanet WASP-76b. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400ºC, high enough to vaporise metals. Strong winds carry iron to the cooler night side where it condenses into iron droplets. To the left of the image, we see the evening border of the exoplanet, where it transitions from day to night ( image credit: ESO/M. Kornmesser)
- “One could say that this planet gets rainy in the evening, except it rains iron,” says David Ehrenreich, a professor at the University of Geneva in Switzerland. He led a study, published today in the journal Nature, of this exotic exoplanet. Known as WASP-76b, it is located some 640 light-years away in the constellation of Pisces.
- This strange phenomenon happens because the 'iron rain' planet only ever shows one face, its day side, to its parent star, its cooler night side remaining in perpetual darkness. Like the Moon on its orbit around the Earth, WASP-76b is ‘tidally locked’: it takes as long to rotate around its axis as it does to go around the star.
- On its day side, it receives thousands of times more radiation from its parent star than the Earth does from the Sun. It’s so hot that molecules separate into atoms, and metals like iron evaporate into the atmosphere. The extreme temperature difference between the day and night sides results in vigorous winds that bring the iron vapor from the ultra-hot day side to the cooler night side, where temperatures decrease to around 1500ºC.
- Not only does WASP-76b have different day-night temperatures, it also has distinct day-night chemistry, according to the new study. Using the new ESPRESSO instrument on ESO’s VLT in the Chilean Atacama Desert, the astronomers identified for the first time chemical variations on an ultra-hot gas giant planet. They detected a strong signature of iron vapor at the evening border that separates the planet’s day side from its night side. “Surprisingly, however, we do not see the iron vapor in the morning,” says Ehrenreich. The reason, he says, is that “it is raining iron on the night side of this extreme exoplanet.”
- “The observations show that iron vapor is abundant in the atmosphere of the hot day side of WASP-76b," adds María Rosa Zapatero Osorio, an astrophysicist at the Center for Astrobiology in Madrid, Spain, and the chair of the ESPRESSO science team. "A fraction of this iron is injected into the night side owing to the planet's rotation and atmospheric winds. There, the iron encounters much cooler environments, condenses and rains down."
- This result was obtained from the very first science observations done with ESPRESSO, in September 2018, by the scientific consortium who built the instrument: a team from Portugal, Italy, Switzerland, Spain and ESO.
- ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) was originally designed to hunt for Earth-like planets around Sun-like stars. However, it has proven to be much more versatile. “We soon realized that the remarkable collecting power of the VLT and the extreme stability of ESPRESSO made it a prime machine to study exoplanet atmospheres,” says Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile.
• February 14, 2020: Betelgeuse has been a beacon in the night sky for stellar observers but it began to dim late last year. At the time of writing Betelgeuse is at about 36% of its normal brightness, a change noticeable even to the naked eye. Astronomy enthusiasts and scientists alike were excitedly hoping to find out more about this unprecedented dimming. 67)
- A team led by Miguel Montargès, an astronomer at KU Leuven (Katholieke Universiteit Leuven — Catholic University of Leuven) in Belgium, has been observing the star with ESO's VLT (Very Large Telescope) since December, aiming to understand why it’s becoming fainter. Among the first observations to come out of their campaign is a stunning new image of Betelgeuse’s surface, taken late last year with the SPHERE instrument.
- The team also happened to observe the star with SPHERE in January 2019, before it began to dim, giving us a before-and-after picture of Betelgeuse. Taken in visible light, the images highlight the changes occurring to the star both in brightness and in apparent shape.
- Many astronomy enthusiasts
wondered if Betelgeuse’s dimming meant it was about to explode.
Like all red supergiants, Betelgeuse will one day go supernova, but
astronomers don’t think this is happening now. They have other
hypotheses to explain what exactly is causing the shift in shape and
brightness seen in the SPHERE images. “The two scenarios we are
working on are a cooling of the surface due to exceptional stellar
activity or dust ejection towards us,” says Montargès.
“Of course, our knowledge of red supergiants remains incomplete,
and this is still a work in progress, so a surprise can still
- Montargès and his team needed the VLT at Cerro Paranal in Chile to study the star, which is over 700 light-years away, and gather clues on its dimming. “ESO's Paranal Observatory is one of few facilities capable of imaging the surface of Betelgeuse,” he says. Instruments on ESO’s VLT allow observations from the visible to the mid-infrared, meaning astronomers can see both the surface of Betelgeuse and the material around it. “This is the only way we can understand what is happening to the star.”
- Another new image, obtained with the VISIR instrument on the VLT, shows the infrared light being emitted by the dust surrounding Betelgeuse in December 2019. These observations were made by a team led by Pierre Kervella from the Observatory of Paris in France who explained that the wavelength of the image is similar to that detected by heat cameras. The clouds of dust, which resemble flames in the VISIR image, are formed when the star sheds its material back into space.
- “The phrase ‘we are all made of stardust’ is one we hear a lot in popular astronomy, but where exactly does this dust come from?” says Emily Cannon, a PhD student at KU Leuven working with SPHERE images of red supergiants. “Over their lifetimes, red supergiants like Betelgeuse create and eject vast amounts of material even before they explode as supernovae. Modern technology has enabled us to study these objects, hundreds of light-years away, in unprecedented detail giving us the opportunity to unravel the mystery of what triggers their mass loss.”
Figure 41: The red supergiant star Betelgeuse, in the constellation of Orion, has been undergoing unprecedented dimming. This stunning image of the star’s surface, taken with the SPHERE instrument on ESO’s Very Large Telescope late last year, is among the first observations to come out of an observing campaign aimed at understanding why the star is becoming fainter. When compared with the image taken in January 2019, it shows how much the star has faded and how its apparent shape has changed ( image credit: ESO/M. Montargès et al.)
• December 16, 2019: New observations of the center of our home galaxy have allowed astronomers to reconstruct, for the first time, the history of star formation in the center of the Milky Way. Previously, it had been assumed that stars in the so-called nuclear stellar disk had formed continuously over the past billions of years. Instead, the new results imply a burst of star formation activity more than 8 billion years followed by a quiet period, and then another burst of activity about one billion years ago. The re-written evolutionary history has consequences for the formation of the bar-shaped feature of our galaxy’s disk. The results have been published in the journal Nature Astronomy. 68) 69) 70)
Figure 42: Taken with the HAWK-I instrument on ESO’s VLT (Very Large Telescope) in the Chilean Atacama Desert, this stunning image shows the Milky Way’s central region with an angular resolution of 0.2 arcseconds. This means the level of detail picked up by HAWK-I is roughly equivalent to seeing a football (soccer ball) in Zurich from Munich, where ESO’s headquarters are located. - The image combines observations in three different wavelength bands. The team used the broadband filters J (centered at 1250 nm, in blue), H (centered at 1635 nm, in green), and Ks (centered at 2150 nm, in red), to cover the near infrared region of the electromagnetic spectrum. By observing in this range of wavelengths, HAWK-I can peer through the dust, allowing it to see certain stars in the central region of our galaxy that would otherwise be hidden (image credit: ESO/Nogueras-Lara et al.)
ESO’s Very Large Telescope (VLT) has observed the central part of the Milky Way with spectacular resolution and uncovered new details about the history of star birth in our galaxy. Thanks to the new observations, astronomers have found evidence for a dramatic event in the life of the Milky Way: a burst of star formation so intense that it resulted in over a hundred thousand supernova explosions.
Figure 43: This beautiful image of the Milky Way’s central region, taken with the HAWK-I instrument on ESO’s Very Large Telescope, shows interesting features of this part of our galaxy. This image highlights the Nuclear Star Cluster (NSC) right in the center and the Arches Cluster, the densest cluster of stars in the Milky Way. Other features include the Quintuplet cluster, which contains five prominent stars, and a region of ionized hydrogen gas (HII), image credit: ESO/Nogueras-Lara et al.
- "Our unprecedented survey of a large part of the Galactic center has given us detailed insights into the formation process of stars in this region of the Milky Way," says Rainer Schödel from the Institute of Astrophysics of Andalusia in Granada, Spain, who led the observations. "Contrary to what had been accepted up to now, we found that the formation of stars has not been continuous,” adds Francisco Nogueras-Lara, who led two new studies of the Milky Way central region while at the same institute in Granada.
- In the study, published today in Nature Astronomy, the team found that about 80% of the stars in the Milky Way central region formed in the earliest years of our galaxy, between eight and 13.5 billion years ago. This initial period of star formation was followed by about six billion years during which very few stars were born. This was brought to an end by an intense burst of star formation around one billion years ago when, over a period of less than 100 million years, stars with a combined mass possibly as high as a few tens of million Suns formed in this central region.
- “The conditions in the studied region during this burst of activity must have resembled those in ‘starburst’ galaxies, which form stars at rates of more than 100 solar masses per year,” says Nogueras-Lara, who is now based at the Max Planck Institute for Astronomy in Heidelberg, Germany. At present, the whole Milky Way is forming stars at a rate of about one or two solar masses per year.
- “This burst of activity, which must have resulted in the explosion of more than a hundred thousand supernovae, was probably one of the most energetic events in the whole history of the Milky Way,” he adds. During a starburst, many massive stars are created; since they have shorter lifespans than lower-mass stars, they reach the end of their lives much faster, dying in violent supernova explosions.
- This research was possible thanks to observations of the Galactic central region done with ESO’s HAWK-I instrument on the VLT in the Chilean Atacama Desert. This infrared-sensitive camera peered through the dust to give us a remarkably detailed image of the Milky Way’s central region, published in October in Astronomy & Astrophysics by Nogueras-Lara and a team of astronomers from Spain, the US, Japan and Germany. The stunning image shows the galaxy’s densest region of stars, gas and dust, which also hosts a supermassive black hole, with an angular resolution of 0.2 arcseconds. This means the level of detail picked up by HAWK-I is roughly equivalent to seeing a football (soccer ball) in Zurich from Munich, where ESO’s headquarters are located.
- This image is the first release of the GALACTICNUCLEUS survey. This program relied on the large field of view and high angular resolution of HAWK-I on ESO’s VLT to produce a beautifully sharp image of the central region of our galaxy. The survey studied over three million stars, covering an area corresponding to more than 60,000 square light-years at the distance of the Galactic center (one light-year is about 9.5 trillion kilometers).
Bursts of star formation activity (Ref. 68)
- With their result of two intense episodes of star formation, the researchers have rewritten part of our home galaxy’s history: Previously, it had been assumed that the stars in the central region of the Milky Way had formed gradually over the past billions of years. The new timeline has consequences for a number of other astronomical phenomena.
- Notably, it constrains the growth history of our galaxy’s central black hole. Gas flowing into the central regions of our galaxy drive both star formation and the increase in central black hole mass. The newly reconstructed star formation history indicates that our central black hole is likely to have reached most of its present mass earlier than eight billion years ago.
- The brief, but intense burst of star formation activity one billion years ago is likely to be one of the most energetic events in the history of our galaxy. Hundreds of thousands of newly formed massive stars would have exploded as supernovae within millions of years.
- The results also force astronomers to rethink another fundamental feature of our galaxy. The Milky Way is a barred spiral galaxy, with an elongated region estimated between 3,000 and 15,000 light-years long linking the inner ends of its two major spiral arms. Such bar structures are thought to be very efficient at funneling gas into a galaxy’s central region, which would lead to the formation of new stars.
- The billions of years without star formation in the nuclear galactic disk forces astronomers to rethink this scenario. During those quiet years, gas was evidently not funneled into the Galactic center in sufficient amounts. Francisco Nogueras Lara (then Instituto de Astrofísica de Andalucía, now a post-doctoral researcher at MPIA), lead author of the article, says: “Either the galactic bar has come into existence only recently, or such bars are not as efficient in funneling gas as is commonly assumed. In the latter case, some event – like a close encounter with a dwarf galaxy – must have triggered the gas flow towards the Galactic center about one billion years ago.”
Reconstructing the formation history of the Galactic center
- The reconstruction of the history of the nuclear Galactic disk makes use of some of the fundamental insights of astronomers into star formation. Stars only live for a certain span of time, which depends on their mass and chemical composition.
- Whenever many stars have been born at the same time, which is a common occurrence, astronomers can look at the ensemble, plot the star’s brightness against the reddishness of their color (“color-magnitude diagram”) and deduce how long ago the ensemble was formed. One among several age indicators is the “red clump” of stars that have already begun to fuse helium in their core regions. From the average brightness of stars in that clump, one can deduce the age of that group of stars.
Challenges of observing the Galactic center
- But there is a catch: All those techniques require astronomers to study separate stars. For the Milky Way’s central regions, that is a highly challenging task. As seen from Earth, the Galactic center is hidden behind gigantic clouds of dust, requiring infrared observations to “look through” the clouds.
- But then, such observations are bound to see too many stars in the Milky Way’s center! The Galactic center is very dense, with between a thousand and a hundred thousand stars in a cube with a side-length of one light-year. When astronomers observe very dense star fields of this kind, those stellar disks will overlap in the telescope image. Separating such fields into separate stars is difficult – but necessary if you want to reconstruct the formation history of the Galactic center.
The right instrument for the job
- Given those challenges, when Rainer Schödel (Instituto de Astrofísica de Andalucía, PI of the GALACTICNUCLEUS survey), MPIA’s Nadine Neumayer and their colleagues began planning to tackle the history of our Milky Way’s central region in late 2014, they knew they would have to find the right instrument for the job. As Neumayer explains: “We needed a near-infrared instrument with a large field of view, able to observe the Milky Way’s central region which is in the Southern Sky. ESO’s HAWK-I instrument was ideal for our survey.” HAWK-I is an infrared camera at the 8 meter Very Large Telescope at the Paranal Observatory of the European Southern Observatory (ESO) in Chile.
- For their GALACTICNUCLEUS survey, the astronomers observed the Galactic center region with HAWK-I for 16 nights managing to obtain accurate photometry of more than three million stars. Using a special technique known as holographic imaging, the astronomers were able to distinguish between stars as little as 0.2 arc seconds apart. With this accuracy, you could distinguish two one-cent coins viewed from a distance of more than 8 kilometers. Two clearly visible “red clumps” in the resulting color-magnitude diagram allowed for the reconstruction of the formation history of the Galactic nuclear disk.
- As a next step, the astronomers are now studying the influence of dust on their observations (extinction and reddening). Taking into accounts the effect of dust should allow for even more precise reconstructions of the history of our galaxy’s central regions in the future.
• November 21, 2019: An international research team led by scientists from Göttingen and Potsdam proved for the first time that the galaxy NGC 6240 contains three supermassive black holes. The unique observations, published in the journal Astronomy & Astrophysics, show the black holes close to each other in the core of the galaxy. The study points to simultaneous merging processes during the formation of the largest galaxies in the universe. 71)
- Massive Galaxies like the Milky Way typically consist of hundreds of billions of stars and host a black hole with a mass of several million up to several 100 million solar masses at their centers. The galaxy known as NGC 6240 is known as an irregular galaxy due to its particular shape. Until now, astronomers have assumed that it was formed by the collision of two smaller galaxies and therefore contains two black holes in its core. These galactic ancestors moved towards each other at velocities of several 100 km/s and are still in the process of merging. The galaxy system which is around 300 million light years away from us – close by cosmic standards – has been studied in detail at all wavelengths, and has so far been regarded as a prototype for the interaction of galaxies.
- "Through our observations with extremely high spatial resolution we were able to show that the interacting galaxy system NGC 6240 hosts not two – as previously assumed – but three supermassive black holes in its center," reports Professor Wolfram Kollatschny from the University of Göttingen, the lead author of the study. Each of the three heavyweights has a mass of more than 90 million Suns. They are located in a region of space less than 3000 light-years across, i.e. in less than one hundredth of the total size of the galaxy. "Up until now, such a concentration of three supermassive black holes had never been discovered in the universe," adds Dr Peter Weilbacher of the Leibniz Institute for Astrophysics Potsdam (AIP). "The present case provides evidence of a simultaneous merging process of three galaxies along with their central black holes.”
- The discovery of this triple system is of fundamental importance for understanding the evolution of galaxies over time. Until now it has not been possible to explain how the largest and most massive galaxies, which we know from our cosmic environment in the "present time", were formed just by normal galaxy interaction and merging processes over the course of the previous 14 billion years approximately, ie the age of our universe. "If, however, simultaneous merging processes of several galaxies took place, then the largest galaxies with their central supermassive black holes were able to evolve much faster,” Peter Weilbacher summarizes. "Our observations provide the first indication of this scenario.”
- For the unique high-precision observations of the galaxy NGC 6240 using the 8 meter VLT, a telescope operated by ESO (European Southern Observatory)in Chile, the 3D MUSE ((Multi Unit Spectroscopic Explorer) instrument was used in spatial high-resolution mode together with four artificially generated laser stars and an adaptive optics system. Thanks to the sophisticated technology, images are obtained with a sharpness similar to that of the Hubble Space Telescope but additionally contain a spectrum for each image pixel. These spectra were decisive in determining the motion and masses of the supermassive black holes in NGC 6240.
- The scientists assume that the observed, imminent merging of the supermassive black holes in a few million years will also generate very strong gravitational waves. In the foreseeable future, signals of similar objects can be measured with the planned satellite-based gravitational wave detector LISA and further merging systems can be discovered. 72)
Figure 44: The irregular galaxy NGC 6240. New observations show that it harbors not two but three supermassive black holes at its core. The northern black hole (N) is active and was known before. The zoomed-in new high-spatial resolution image shows that the southern component consists of two supermassive black holes (S1 and S2). The green color indicates the distribution of gas ionized by radiation surrounding the black holes. The red lines show the contours of the starlight from the galaxy and the length of the white bar corresponds to 1000 light years [image credit: P Weilbacher (AIP), NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)]
• October 28, 2019: Astronomers using ESO’s SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument at the VLT (Very Large Telescope) have revealed that the asteroid Hygiea could be classified as a dwarf planet. The object is the fourth largest in the asteroid belt after Ceres, Vesta and Pallas. For the first time, astronomers have observed Hygiea in sufficiently high resolution to study its surface and determine its shape and size. They found that Hygiea is spherical, potentially taking the crown from Ceres as the smallest dwarf planet in the Solar System. 73) 74)
- As an object in the main asteroid belt of our solar system, Hygiea satisfies right away three of the four requirements to be classified as a dwarf planet: it orbits around the Sun, it is not a moon and, unlike a planet, it has not cleared the neighborhood around its orbit. The final requirement is that it has enough mass for its own gravity to pull it into a roughly spherical shape. This is what VLT observations have now revealed about Hygiea.
- “Thanks to the unique capability of the SPHERE instrument on the VLT, which is one of the most powerful imaging systems in the world, we could resolve Hygiea’s shape, which turns out to be nearly spherical,” says lead researcher Pierre Vernazza from the Laboratoire d'Astrophysique de Marseille in France. “Thanks to these images, Hygiea may be reclassified as a dwarf planet, so far the smallest in the Solar System.”
- The team also used the SPHERE observations to constrain Hygiea’s size, putting its diameter at just over 430 km. Pluto, the most famous of dwarf planets, has a diameter close to 2400 km, while Ceres is close to 950 km in size.
- Surprisingly, the observations also revealed that Hygiea lacks the very large impact crater that scientists expected to see on its surface, the team report in the study published today in Nature Astronomy. Hygiea is the main member of one of the largest asteroid families, with close to 7000 members that all originated from the same parent body. Astronomers expected the event that led to the formation of this numerous family to have left a large, deep mark on Hygiea.
- “This result came as a real surprise as we were expecting the presence of a large impact basin, as is the case on Vesta,” says Vernazza. Although the astronomers observed Hygiea’s surface with a 95% coverage, they could only identify two unambiguous craters. “Neither of these two craters could have been caused by the impact that originated the Hygiea family of asteroids whose volume is comparable to that of a 100 km-sized object. They are too small,” explains study co-author Miroslav Brož of the Astronomical Institute of Charles University in Prague, Czech Republic.
- The team decided to investigate further. Using numerical simulations, they deduced that Hygiea’s spherical shape and large family of asteroids are likely the result of a major head-on collision with a large projectile of diameter between 75 and 150 km. Their simulations show this violent impact, thought to have occurred about 2 billion years ago, completely shattered the parent body. Once the left-over pieces reassembled, they gave Hygiea its round shape and thousands of companion asteroids. “Such a collision between two large bodies in the asteroid belt is unique in the last 3–4 billion years,” says Pavel Ševeček, a PhD student at the Astronomical Institute of Charles University who also participated in the study.
- Studying asteroids in detail has been possible thanks not only to advances in numerical computation, but also to more powerful telescopes. “Thanks to the VLT and the new generation adaptive-optics instrument SPHERE, we are now imaging main belt asteroids with unprecedented resolution, closing the gap between Earth-based and interplanetary mission observations,” Vernazza concludes.
Figure 45: A new SPHERE/VLT image of Hygiea, which could be the Solar System’s smallest dwarf planet yet. As an object in the main asteroid belt, Hygiea satisfies right away three of the four requirements to be classified as a dwarf planet: it orbits around the Sun, it is not a moon and, unlike a planet, it has not cleared the neighborhood around its orbit. The final requirement is that it have enough mass that its own gravity pulls it into a roughly spherical shape. This is what VLT observations have now revealed about Hygiea [image credit: ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)]
Figure 46: New observations with ESO’s SPHERE instrument on the Very Large Telescope have revealed that the surface of Hygiea lacks the very large impact crater that scientists expected to see on its surface. Since it was formed from one of the largest impacts in the history of the asteroid belt, they were expecting to find at least one large, deep impact basin, similar to the one on Vesta (bottom right in the central panel). The new study also found that Hygiea is spherical, potentially taking the crown from Ceres as the smallest dwarf planet in the Solar System. The team used the SPHERE observations to constrain Hygiea’s size, putting its diameter at just over 430 km, while Ceres is close to 950 km in size (image credit: ESO/P. Vernazza et al., L. Jorda et al./MISTRAL algorithm (ONERA/CNRS)
• August 7, 2019: Colorful and wispy, this intriguing collection of objects is known as the Seagull Nebula, named for its resemblance to a gull in flight. Made up of dust, hydrogen, helium and traces of heavier elements, this region is the hot and energetic birthplace of new stars. The remarkable detail captured here by ESO’s VLT Survey Telescope (VST) reveals the individual astronomical objects that make up the celestial bird, as well as the finer features within them. The VST is one of the largest survey telescopes in the world observing the sky in visible light. 75)
Figure 47: The Rosy Glow of a Cosmic Seagull (image credit: ESO/VPHAS+ team/N.J. Wright (Keele University)
- The main components of the Seagull are three large clouds of gas, the most distinctive being Sharpless 2-296, which forms the “wings”. Spanning about 100 light-years from one wingtip to the other, Sh2-296 displays glowing material and dark dust lanes weaving amid bright stars. It is a beautiful example of an emission nebula, in this case an HII region, indicating active formation of new stars, which can be seen peppering this image.
- It is the radiation emanating from these young stars that gives the clouds their fantastical colors and makes them so eye-catching, by ionizing the surrounding gas and causing it to glow. This radiation is also the main factor that determines the clouds’ shapes, by exerting pressure on the surrounding material and sculpting it into the whimsical morphologies we see. Since each nebula has a unique distribution of stars and may, like this one, be a composite of multiple clouds, they come in a variety of shapes, firing astronomers’ imaginations and evoking comparisons to animals or familiar objects.
- This diversity of shapes is exemplified by the contrast between Sh2-296 and Sh2-292. The latter, seen here just below the “wings”, is a more compact cloud that forms the seagull’s “head”. Its most prominent feature is a huge, extremely luminous star called HD 53367 that is 20 times more massive than the Sun, and which we see as the seagull’s piercing “eye”. Sh2-292 is both an emission nebula and a reflection nebula; much of its light is emitted by ionized gas surrounding its nascent stars, but a significant amount is also reflected from stars outside it.
- The dark swathes that interrupt the clouds’ homogeneity and give them texture are dust lanes – paths of much denser material that hide some of the luminous gas behind them. Nebulae like this one have densities of a few hundred atoms/cm3, much less than the best artificial vacuums on Earth. Nonetheless, nebulae are still much denser than the gas outside them, which has an average density of about 1 atom/cm3.
- The Seagull lies along the border between the constellations of Canis Major (The Great Dog) and Monoceros (The Unicorn), at a distance of about 3700 light-years in one arm of the Milky Way. Spiral galaxies can contain thousands of these clouds, almost all of which are concentrated along their whirling arms.
- Several smaller clouds are also counted as part of the Seagull Nebula, including Sh2-297, which is a small, knotty addition to the tip of the gull’s upper “wing”, Sh2-292 and Sh2-295. These objects are all included in the Sharpless Catalogue, a list of over 300 clouds of glowing gas compiled by American astronomer Stewart Sharpless.
- This image was taken using the VLT Survey Telescope (VST), one of the largest survey telescopes in the world observing the sky in visible light. The VST is designed to photograph large areas of the sky quickly and deeply.
• June 10, 2019: Breakthrough Watch, the global astronomical program looking for Earth-like planets around nearby stars, and the European Southern Observatory (ESO), Europe’s foremost intergovernmental astronomical organization, today announced “first light” on a newly-built planet-finding instrument at ESO’s VLT (Very Large Telescope) in the Atacama Desert, Chile. 76)
- The instrument, called NEAR (Near Earths in the AlphaCen Region), is designed to hunt for exoplanets in our neighboring star system, Alpha Centauri, within the “habitable zones” of its two Sun-like stars, where water could potentially exist in liquid form. It has been developed over the last three years and was built in collaboration with the University of Uppsala in Sweden, the University of Liège in Belgium, the California Institute of Technology in the US, and Kampf Telescope Optics in Munich, Germany. 77)
- Since 23 May ESO’s astronomers at ESO’s VLT have been conducting a ten-day observing run to establish the presence or absence of one or more planets in the star system. Observations will conclude tomorrow, 11 June. Planets in the system (twice the size of Earth or bigger), would be detectable with the upgraded instrumentation. The near- to thermal-infrared range is significant as it corresponds to the heat emitted by a candidate planet, and so enables astronomers to determine whether the planet’s temperature allows liquid water.
- Alpha Centauri is the closest star system to our Solar System, at 4.37 light-years (about 25 trillion miles) away. It consists of two Sun-like stars, Alpha Centauri A and B, plus the red dwarf star, Proxima Centauri. Current knowledge of Alpha Centauri’s planetary systems is sparse. In 2016, a team using ESO instruments discovered one Earth-like planet orbiting Proxima Centauri. But Alpha Centauri A and B remain unknown quantities; it is not clear how stable such binary star systems are for Earth-like planets, and the most promising way to establish whether they exist around these nearby stars is to attempt to observe them.
- Imaging such planets, however, is a major technical challenge, since the starlight that reflects off them is generally billions of times dimmer than the light coming to us directly from their host stars; resolving a small planet close to its star at a distance of several light-years has been compared to spotting a moth circling a street lamp dozens of miles away. To solve this problem, in 2016 Breakthrough Watch and ESO launched a collaboration to build a special instrument called a thermal infrared coronagraph, designed to block out most of the light coming from the star and optimized to capture the infrared light emitted by the warm surface of an orbiting planet, rather than the small amount of starlight it reflects. Just as objects near to the Sun (normally hidden by its glare) can be seen during a total eclipse, so the coronagraph creates a kind of artificial eclipse of its target star, blocking its light and allowing much dimmer objects in its vicinity to be detected. This marks a significant advance in observational capabilities.
- The coronagraph has been installed on one of the VLT’s four 8-meter-aperture telescopes, upgrading and modifying an existing instrument, called VISIR, to optimize its sensitivity to infrared wavelengths associated with potentially habitable exoplanets. It will therefore be able to search for heat signatures similar to that of the Earth, which absorbs energy from the Sun and emits it in the thermal infrared wavelength range. NEAR modifies the existing VISIR instrument in three ways, combining several cutting-edge astronomical engineering achievements. First, it adapts the instrument for coronagraphy, enabling it to drastically reduce the light of the target star and thereby reveal the signatures of potential terrestrial planets. Second, it uses a technique called adaptive optics to strategically deform the telescope’s secondary mirror, compensating for the blur produced by the Earth’s atmosphere. Third, it employs novel chopping strategies that also reduce noise, as well as potentially allowing the instrument to switch rapidly between target stars — as fast as every 100 milliseconds — maximizing the available telescope time.
- Pete Worden, Executive Director of the Breakthrough Initiatives, said: “We’re delighted to collaborate with the ESO in designing, building, installing and now using this innovative new instrument. If there are Earth-like planets around Alpha Centauri A and B, that’s huge news for everyone on our planet.”
- “ESO is glad to bring its expertise, existing infrastructure, and observing time on the Very Large Telescope to the NEAR project,” commented ESO project manager Robin Arsenault.
- “This is a valuable opportunity, as — in addition to its own science goals — the NEAR experiment is also a pathfinder for future planet-hunting instruments for the upcoming Extremely Large Telescope,” says Markus Kasper, ESO’s lead scientist for NEAR.
- “NEAR is the first and (currently) only project that could directly image a habitable exoplanet. It marks an important milestone. Fingers crossed — we are hoping a large habitable planet is orbiting Alpha Cen A or B” commented Olivier Guyon, lead scientist for Breakthrough Watch.
- “Human beings are natural explorers,” said Yuri Milner, founder of the Breakthrough Initiatives, “It is time we found out what lies beyond the next valley. This telescope will let us gaze across.”
Figure 48: NEAR experiment sees first light. Newly-built planet-finding instrument installed on Very Large Telescope, Chile, begins 100-hour observation of nearby stars Alpha Centauri A and B, aiming to be first to directly image a habitable exoplanet (image credit: ESO)
The data from the NEAR experiment are publicly available from the ESO archive under program ID 2102.C-5011. A pre-processed and condensed package of all the data will be made available shortly after the campaign concludes. In addition, the Python-based high-contrast imaging data reduction tool PynPoint has been adapted to process NEAR data, and will be provided to members of the astronomical community who would like to use the data but do not have their own data reduction tools. https://pynpoint.readthedocs.io/en/latest/near.html
Breakthrough Watch is a global astronomical program aiming to identify and characterize planets around nearby stars. The program is run by an international team of experts in exoplanet detection and imaging. https://breakthroughinitiatives.org/initiative/4
The Breakthrough Initiatives are a suite of scientific and technological programs, founded by Yuri Milner, investigating life in the Universe. Along with Breakthrough Watch, they include Breakthrough Listen, the largest ever astronomical search for signs of intelligent life beyond Earth, and Breakthrough Starshot, the first significant attempt to design and develop a space probe capable of reaching another star. https://breakthroughinitiatives.org/
• June 3, 2019: The unique capabilities of the SPHERE instrument on ESO’s Very Large Telescope have enabled it to obtain the sharpest images of a double asteroid as it flew by Earth on 25 May. While this double asteroid was not itself a threatening object, scientists used the opportunity to rehearse the response to a hazardous Near-Earth Object (NEO), proving that ESO’s front-line technology could be critical in planetary defence. 78)
- The International Asteroid Warning Network (IAWN) coordinated a cross-organizational observing campaign of the asteroid 1999 KW4 as it flew by Earth, reaching a minimum distance of 5.2 million km on 25 May 2019. 1999 KW4 is about 1.3 km wide, and does not pose any risk to Earth. Since its orbit is well known, scientists were able to predict this fly-by and prepare the observing campaign.
Figure 49: Side by side observation and artist's impression of Asteroid 1999 KW4
- ESO joined the campaign with its flagship facility, the Very Large Telescope (VLT). The VLT is equipped with SPHERE — one of the very few instruments in the world capable of obtaining images sharp enough to distinguish the two components of the asteroid, which are separated by around 2.6 km.
- SPHERE was designed to observe exoplanets; its state-of-the-art adaptive optics (AO) system corrects for the turbulence of the atmosphere, delivering images as sharp as if the telescope were in space. It is also equipped with coronagraphs to dim the glare of bright stars, exposing faint orbiting exoplanets.
- Taking a break from its usual night job hunting exoplanets, SPHERE data helped astronomers characterize the double asteroid. In particular, it is now possible to measure whether the smaller satellite has the same composition as the larger object.
- “These data, combined with all those that are obtained on other telescopes through the IAWN campaign, will be essential for evaluating effective deflection strategies in the event that an asteroid was found to be on a collision course with Earth,” explained ESO astronomer Olivier Hainaut. “In the worst possible case, this knowledge is also essential to predict how an asteroid could interact with the atmosphere and Earth’s surface, allowing us to mitigate damage in the event of a collision.”
- “The double asteroid was hurtling by the Earth at more than 70,000 km/h, making observing it with the VLT challenging,” said Diego Parraguez, who was piloting the telescope. He had to use all his expertise to lock on to the fast asteroid and capture it with SPHERE.
- Bin Yang, VLT astronomer, declared “When we saw the satellite in the AO-corrected images, we were extremely thrilled. At that moment, we felt that all the pain, all the efforts were worth it.” Mathias Jones, another VLT astronomer involved in these observations, elaborated on the difficulties. “During the observations the atmospheric conditions were a bit unstable. In addition, the asteroid was relatively faint and moving very fast in the sky, making these observations particularly challenging, and causing the AO system to crash several times. It was great to see our hard work pay off despite the difficulties!”
- While 1999 KW4 is not an impact threat, it bears a striking resemblance to another binary asteroid system called Didymos which could pose a threat to Earth sometime in the distant future.
- Didymos and its companion called “Didymoon” are the target of a future pioneering planetary defence experiment. NASA’s DART spacecraft will impact Didymoon in an attempt to change its orbit around its larger twin, in a test of the feasibility of deflecting asteroids. After the impact, ESA’s Hera mission will survey the Didymos asteroids in 2026 to gather key information, including Didymoon’s mass, its surface properties and the shape of the DART crater.
- The success of such missions depends on collaborations between organizations, and tracking Near-Earth Objects is a major focus for the collaboration between ESO and ESA. This cooperative effort has been ongoing since their first successful tracking of a potentially hazardous NEO in early 2014.
- “We are delighted to be playing a role in keeping Earth safe from asteroids,” said Xavier Barcons, ESO’s Director General. “As well as employing the sophisticated capabilities of the VLT, we are working with ESA to create prototypes for a large network to take asteroid detection, tracking and characterization to the next level.”
- “We are delighted to be playing a role in keeping Earth safe from asteroids,” said Xavier Barcons, ESO’s Director General. “As well as employing the sophisticated capabilities of the VLT, we are working with ESA to create prototypes for a large network to take asteroid detection, tracking and characterization to the next level.”
- This recent close encounter with 1999 KW4 comes just a month before Asteroid Day, an official United Nations day of education and awareness about asteroids, to be celebrated on 30 June. Events will be held on five continents, and ESO will be among the major astronomical organizations taking part. The ESO Supernova Planetarium & Visitor Centre will host a range of activities on the theme of asteroids on the day, and members of the public are invited to join in the celebrations.
• May 2, 2019: Gaia, operated by the European Space Agency (ESA), surveys the sky from orbit to create the largest, most precise, three-dimensional map of our Galaxy. One year ago, the Gaia mission produced its much-awaited second data release, which included high-precision measurements — positions, distance and proper motions — of more than one billion stars in our Milky Way galaxy. This catalog has enabled transformational studies in many fields of astronomy, addressing the structure, origin and evolution the Milky Way and generating more than 1700 scientific publications since its launch in 2013. 79)
- In order to reach the accuracy
necessary for Gaia’s sky maps, it is crucial to pinpoint the
position of the spacecraft from Earth. Therefore, while Gaia scans the
sky, gathering data for its stellar census, astronomers regularly
monitor its position using a global network of optical telescopes,
including the VST at ESO’s Paranal Observatory.
The VST is currently the largest survey telescope observing the sky in
visible light, and records Gaia’s position in the sky every
second night throughout the year.
- “Gaia observations require a special observing procedure,” explained Monika Petr-Gotzens, who has coordinated the execution of ESO’s observations of Gaia since 2013. “The spacecraft is what we call a ‘moving target’, as it is moving quickly relative to background stars — tracking Gaia is quite the challenge!”
- “The VST is the perfect tool for picking out the motion of Gaia,” elaborated Ferdinando Patat, head of the ESO’s Observing Programs Office. “Using one of ESO’s first-rate ground-based facilities to bolster cutting-edge space observations is a fine example of scientific cooperation.”
- “This is an exciting ground-space collaboration, using one of ESO’s world-class telescopes to anchor the trailblazing observations of ESA’s billion star surveyor,” commented Timo Prusti, Gaia project scientist at ESA.
- The VST observations are used by ESA’s flight dynamics experts to track Gaia and refine the knowledge of the spacecraft’s orbit. Painstaking calibration is required to transform the observations, in which Gaia is just a speck of light among the bright stars, into meaningful orbital information. Data from Gaia’s second release was used to identify each of the stars in the field of view, and allowed the position of the spacecraft to be calculated with astonishing precision — up to 20 marcsec (milliarcseconds).
- “This is a challenging process: we are using Gaia’s measurements of the stars to calibrate the position of the Gaia spacecraft and ultimately improve its measurements of the stars,” explains Timo Prusti.
- “After careful and lengthy data processing, we have now achieved the accuracy required for the ground-based observations of Gaia to be implemented as part of the orbit determination,” says Martin Altmann, lead of the Ground Based Optical Tracking (GBOT) campaign at the Center for Astronomy of Heidelberg University, Germany.
- The GBOT information will be used to improve our knowledge of Gaia’s orbit not only in observations to come, but also for all the data that have been gathered from Earth in the previous years, leading to improvements in the data products that will be included in future releases.
Figure 50: This image, a composite of several observations captured by ESO’s VLT Survey Telescope (VST), shows the ESA spacecraft Gaia as a faint trail of dots across the lower half of the star-filled field of view. These observations were taken as part of an ongoing collaborative effort to measure Gaia’s orbit and improve the accuracy of its unprecedented star map (image credit: ESO)
• March 27, 2019: For the first time, astronomers have succeeded in investigating an exoplanet using optical interferometry. The new method allowed astronomers to measure the position of the exoplanet HR 8799e with unprecedented accuracy. Also, the planet's spectrum was recorded as precisely as never before, paving the way for future searches for life on other planets. The measurements, which were obtained with the participation of astronomers from the Max Planck Institutes for Astronomy and for Extraterrestrial Physics, were performed with the GRAVITY instrument at ESO's Paranal Observatory. 80)
- Investigating exoplanets in detail and without confounding noise is difficult. In general, with increasing distance, it becomes more and more difficult to image fine details of an astronomical object. Furthermore, exoplanets are typically buried in the glare of their much brighter host stars. Now, a group of researchers led by Sylvestre Lacour of the Observatoire de Paris and the Max Planck Institute for Extraterrestrial Physics, also including MPIA researchers, has been able to demonstrate a new method of investigation that mitigates these problems and thereby provides a new perspective on exoplanets.
- Key to the new technique is the GRAVITY instrument, which has been in operation at the European Southern Observatory's Very Large Telescope Interferometer (VLTI) at Paranal Observatory in Chile since 2016. Using a technique known as interferometry, which exploits the wave nature of light, GRAVITY is able to combine the light of several telescopes to form a common image. Combined, the four 8-metre-telescopes of the Very Large Telescope (VLT) can make images so detailed that a single telescope would need to have a mirror diameter of approximately 100 meters to provide the same level of detail.
- The study of the exoplanet HR 8799e that has now been published is the first to demonstrate the potential of interferometric observations for the investigation of exoplanets in practice. The planet is one of only a few (about 120 out of 4000) for which direct images exist; so far, most exoplanets have only been detected indirectly. HR 8977e is part of a young five-body-system, a mere 130 light-years away from us, which consists of the star HR 8799 and four planets (as far as we know, at least). All of the planets are gas giants with between 5 and 10 times the mass of Jupiter.
- This result was announced today in a letter in the journal Astronomy and Astrophysics by the GRAVITY Collaboration, in which they present observations of the exoplanet HR8799e using optical interferometry. The exoplanet was discovered in 2010 orbiting the young main-sequence star HR8799, which lies around 129 light-years from Earth in the constellation of Pegasus. 81) 82)
- Today’s result, which
reveals new characteristics of HR8799e, required an instrument with
very high resolution and sensitivity. GRAVITY can use ESO’s VLT’s four unit telescopes to work together to mimic a single larger telescope using a technique known as interferometry. This creates a super-telescope — the VLTI — that collects and precisely disentangles the light from HR8799e’s atmosphere and the light from its parent star.
- HR8799e is a ‘super-Jupiter’, a world unlike any found in our Solar System, that is both more massive and much younger than any planet orbiting the Sun. At only 30 million years old, this baby exoplanet is young enough to give scientists a window onto the formation of planets and planetary systems. The exoplanet is thoroughly inhospitable — leftover energy from its formation and a powerful greenhouse effect heat HR8799e to a hostile temperature of roughly 1000 °C.
- This is the first time that optical interferometry has been used to reveal details of an exoplanet, and the new technique furnished an exquisitely detailed spectrum of unprecedented quality — ten times more detailed than earlier observations. The team’s measurements were able to reveal the composition of HR8799e’s atmosphere — which contained some surprises.
Figure 51: This artist’s impression shows the observed exoplanet, which goes by the name HR8799e. The GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) has made the first direct observation of an exoplanet using optical interferometry. This method revealed a complex exoplanetary atmosphere with clouds of iron and silicates swirling in a planet-wide storm. The technique presents unique possibilities for characterizing many of the exoplanets known today. (image credit: ESO, Luis Calçada)
- “Our analysis showed that HR8799e has an atmosphere containing far more carbon monoxide than methane — something not expected from equilibrium chemistry,” explains team leader Sylvestre Lacour researcher CNRS at the Observatoire de Paris - PSL and the Max Planck Institute for Extraterrestrial Physics. “We can best explain this surprising result with high vertical winds within the atmosphere preventing the carbon monoxide from reacting with hydrogen to form methane.”
- The team found that the atmosphere also contains clouds of iron and silicate dust. When combined with the excess of carbon monoxide, this suggests that HR8799e’s atmosphere is engaged in an enormous and violent storm.
- “Our observations suggest a ball of gas illuminated from the interior, with rays of warm light swirling through stormy patches of dark clouds,” elaborates Lacour. “Convection moves around the clouds of silicate and iron particles, which disaggregate and rain down into the interior. This paints a picture of a dynamic atmosphere of a giant exoplanet at birth, undergoing complex physical and chemical processes.”
- This result
builds on GRAVITY’s string of impressive discoveries, which have
included breakthroughs such as last year’s observation
of gas swirling at 30% of the speed of light just outside the event
horizon of the massive Black Hole in the Galactic Centre. It also adds a new way of observing exoplanets to the already extensive arsenal of methods available to ESO’s telescopes and instruments — paving the way to many more impressive discoveries.
• March 14, 2019: ESO's Very Large Telescope (VLT) has caught a glimpse of an ethereal nebula hidden away in the darkest corners of the constellation of Orion (The Hunter) — NGC 1788, nicknamed the Cosmic Bat. This bat-shaped reflection nebula doesn’t emit light — instead it is illuminated by a cluster of young stars in its core, only dimly visible through the clouds of dust. Scientific instruments have come a long way since NGC 1788 was first described, and this image taken by the VLT is the most detailed portrait of this nebula ever taken. 83)
- Even though this ghostly nebula in Orion appears to be isolated from other cosmic objects, astronomers believe that it was shaped by powerful stellar winds from the massive stars beyond it. These streams of scorching plasma are thrown from a star’s upper atmosphere at incredible speeds, shaping the clouds secluding the Cosmic Bat’s nascent stars.
- NGC 1788 was first described by the German–British astronomer William Herschel,
who included it in a catalogue that later served as the basis for one
of the most significant collections of deep-sky objects, the New General Catalogue (NGC).
A nice image of this small and dim nebula had already been captured by
the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory, but this
newly observed scene leaves it in the proverbial dust. Frozen in
flight, the minute details of this Cosmic Bat's dusty wings were imaged
for the twentieth anniversary of one of ESO's most versatile
instruments, the FOcal Reducer and low dispersion Spectrograph
- FORS2 is an instrument mounted on Antu, one of the VLT's 8.2-meter Unit Telescopes at the Paranal Observatory, and its ability to image large areas of the sky in exceptional detail has made it a coveted member of ESO's fleet of cutting-edge scientific instruments.
Since its first light 20 years ago, FORS2 has become known as
“the Swiss army knife of instruments”. This moniker
originates from its uniquely broad set of functions . FORS2’s
versatility extends beyond purely scientific uses — its ability
to capture beautiful high-quality images like this makes it a
particularly useful tool for public outreach.
- This image was taken as part of ESO’s Cosmic Gems program, an outreach initiative that uses ESO telescopes to produce images of interesting, intriguing or visually attractive objects for the purposes of education and public outreach. The program makes use of telescope time that cannot be used for science observations, and — with the help of FORS2 — produces breathtaking images of some of the most striking objects in the night sky, such as this intricate reflection nebula. In case the data collected could be useful for future scientific purposes, these observations are saved and made available to astronomers through the ESO Science Archive.
Figure 52: Hidden in one of the darkest corners of the Orion constellation, this Cosmic Bat is spreading its hazy wings through interstellar space two thousand light-years away. It is illuminated by the young stars nestled in its core — despite being shrouded by opaque clouds of dust, their bright rays still illuminate the nebula. Too dim to be discerned by the naked eye, NGC 1788 reveals its soft colors to ESO's Very Large Telescope in this image — the most detailed to date (image credit: ESO)
Figure 53: Hidden in one of the darkest corners of the Orion constellation, this Cosmic Bat is spreading its hazy wings through interstellar space two thousand light-years away. It is illuminated by the young stars nestled in its core — despite being shrouded by opaque clouds of dust, their bright rays still illuminate the nebula. Too dim to be discerned by the naked eye, NGC 1788 reveals its soft colors to ESO's Very Large Telescope in this image — the most detailed to date (video credit: ESO)
• February 6, 2019: This region of the Large Magellanic Cloud (LMC) glows in striking colors in this image (Figure 54) captured by the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO's Very Large Telescope (VLT). The region, known as LHA 120-N 180B - N180 B for short - is a type of nebula known as an H II region (pronounced "H two"), and is a fertile source of new stars. 84) 85)
- The LMC is a satellite galaxy of the Milky Way, visible mainly from the Southern Hemisphere. At only around 160,000 light-years away from the Earth, it is practically on our doorstep. As well as being close to home, the LMC’s single spiral arm appears nearly face-on, allowing us to inspect regions such as N180 B with ease.
- H II regions are interstellar clouds of ionized hydrogen — the bare nuclei of hydrogen atoms. These regions are stellar nurseries — and the newly formed massive stars are responsible for the ionization of the surrounding gas, which makes for a spectacular sight. N180 B’s distinctive shape is made up of a gargantuan bubble of ionized hydrogen surrounded by four smaller bubbles.
- Deep within this glowing cloud, MUSE has spotted a jet emitted by a fledgling star — a massive young stellar object with a mass 12 times greater than our Sun. The jet — named Herbig–Haro 1177, or HH 1177 for short — is shown in detail in this accompanying image. This is the first time such a jet has been observed in visible light outside the Milky Way, as they are usually obscured by their dusty surroundings. However, the relatively dust-free environment of the LMC allows HH 1177 to be observed at visible wavelengths. At nearly 33 light-years in length, it is one of the longest such jets ever observed.
- HH 1177 tells us about the early lives of stars. The beam is highly collimated; it barely spreads out as it travels. Jets like this are associated with the accretion discs of their star, and can shed light on how fledgling stars gather matter. Astronomers have found that both high- and low-mass stars launch collimated jets like HH 1177 via similar mechanisms — hinting that massive stars can form in the same way as their low-mass counterparts.
- MUSE has recently been vastly improved by the addition of the Adaptive Optics Facility, the Wide Field Mode of which saw first light in 2017. Adaptive optics is the process by which ESO’s telescopes compensate for the blurring effects of the atmosphere — turning twinkling stars into sharp, high-resolution images. Since obtaining these data, the addition of the Narrow Field Mode, has given MUSE vision nearly as sharp as that of the NASA/ESA Hubble Space Telescope — giving it the potential to explore the Universe in greater detail than ever before.
Figure 54: This dazzling region of newly-forming stars in the Large Magellanic Cloud (LMC) was captured by the MUSE (Multi Unit Spectroscopic Explorer) instrument on ESO’s Very Large Telescope. The relatively small amount of dust in the LMC and MUSE’s acute vision allowed intricate details of the region to be picked out in visible light (image credit: ESO, A McLeod, et al.)
• November 29, 2018: Detailed observations of the quasar 3C 273 with the GRAVITY instrument reveal the structure of rapidly moving gas around the central super-massive black hole, the first time that the so-called “broad line region” could be resolved. The international team of astronomers was thus able to measure the mass of the black hole with unprecedented precision. This measurement confirms the fundamental assumptions of the most commonly used method to measure the mass of central black holes in distant quasars. Studying these black holes and determining their masses is an essential ingredient to understanding galaxy evolution in general. 86)
- An international team of astronomers has now used the GRAVITY instrument to look deep into the heart of the quasar and was able to actually observe the structure of rapidly moving gas around the central black hole. So far, such observations had not been possible due to the small angular size of this inner region, which is about the size of our Solar system but at a distance of some 2.5 billion light years. The GRAVITY instrument combines all four ESO VLT telescopes in a technique called interferometry, which allows a huge gain in angular resolution, equivalent to a telescope with 130 meters in diameter. Thus the astronomers can reveal structures at the level of 10 µas (micro-arcsec), which corresponds to about 0.1 light years at the distance of the quasar (or an object the size of a 1-Euro-coin on the Moon).
- “GRAVITY allowed us to resolve the so-called ‘broad line region’ for the first time ever, and to observe the motion of gas clouds around the central black hole”, explains Eckhard Sturm, lead author from the Max Planck Institute for Extraterrestrial Physics (MPE). “Our observations reveal that the gas clouds do whirl around the central black hole.” 87)
- The broad atomic emission lines are an observational hallmark of quasars, clearly indicating the extra-galactic origin of the source. So far, the size of the broad line region is measured mainly by a method called “reverberation mapping”. Brightness variations of the quasar’s central engine cause a light echo once the radiation hits clouds further out – the larger the size of the system, the later the echo. In the best cases, the motions of the gas can also be identified, often implying a disk in rotation. This result, derived from timing information, can now be confronted with spatially resolved observations with GRAVITY.
- “Our results support the fundamental assumptions of reverberation mapping,” confirms Jason Dexter, co-lead author from MPE. “Information about the motion and size of the region immediately around the black hole are crucial to measure its mass,” he adds. For the first time, the method was now tested experimentally and passed its test with flying colors, confirming previous mass estimates of about 300 million solar masses for the black hole. Thus, GRAVITY provides both a confirmation of the main method used previously to determine black hole masses in quasars and a new and highly accurate, independent method to measure such masses. It thereby promises to provide a benchmark for measuring black hole masses in thousands of other quasars.
- Quasars play a fundamental role in the history of the Universe, as their evolution is intricately tied to galaxy growth. While astronomers assume that basically all large galaxies harbor a massive black hole at their center, so far only the one in our Milky Way has been accessible for detailed studies.
- “This is the first time that we can spatially resolve and study the immediate environs of a massive black hole outside our home galaxy, the Milky Way,” emphasizes Reinhard Genzel, head of the infrared research group at MPE. “Black holes are intriguing objects, allowing us to probe physics under extreme conditions – and with GRAVITY we can now probe them both near and far.”
Figure 55: This animation shows the zoom from an optical image of the quasar to an artistic representation of the surroundings of a supermassive black hole. There is a dusty ring of very hot material collapsing onto the gravity trap and often a jet in which material is ejected at high velocities at the poles. Astronomers have now succeeded in spatially depicting the so-called broad line region, in which gas clouds swirl around the central black hole (video credit: L. Calcada/ESO)
Figure 56: Powerhouse in space: The quasar 3C273 resides in a giant elliptical galaxy in the constellation of Virgo at a distance of about 2.5 billion light years. It was the first quasar ever to be identified (image credit: ESA/Hubble & NASA)
• November 19, 2018: University of Sydney astronomers, working with international colleagues, have found a star system like none seen before in our galaxy. The scientists believe one of the stars—about 8000 light years from Earth—is the first known candidate in the Milky Way to produce a dangerous gamma-ray burst, among the most energetic events in the universe, when it explodes and dies. 88)
Figure 57: This is an image of Apep captured at 8 µm in the thermal infrared with the VISIR camera on the European Southern Observatory's VLT telescope, Mt Paranal, Chile. The system can be seen to be a binary, with a much fainter companion to the North of the heart of the system. This companion is not believed to play a role in the sculping of the extended dust plume, about 12 arcseconds across. The origin of this structure comes from the central region, believed itself to contain a binary (the whole thing being a triple star), image credit: Peter Tuthill/University of Sydney/ESO
- The system, comprising a pair of scorchingly luminous stars, was nicknamed Apep by the team after the serpentine Egyptian god of chaos. One star is on the brink of a massive supernova explosion.
- The findings, published today in Nature Astronomy, are controversial as no gamma-ray burst has ever been detected within our own galaxy, the Milky Way. 89)
- Yet in the southern constellation of Norma, nestled just beneath Scorpio's tail, astronomers have discovered this uniquely beautiful star system.
- At its heart, wrapped in an elegantly sculpted plume of dust and gas, lies a powerful binary pair.
- The two hot, luminous stars—known to astronomers as Wolf-Rayets - orbit each other every hundred years or so, according to the research conducted at the Sydney Institute for Astronomy.
- This orbital dance is embossed on a fast wind streaming off the stars. Using spectroscopy, the astronomers have measured the velocity of the stellar winds as fast as 12 million km/hr, about 1 percent the speed of light.
Figure 58: This animated gif is intended to illustrate the geometry of the structure that we have witnessed in the Apep system. From a single image, it is harder to understand the 3-D structure. The central binary (only: not the wider Northern companion in the triple) is illustrated as the blue star at the center. The geometry given is that believed typical for a Wolf-Rayet colliding pinwheel system: that is an optically thin dust plume distributed over the surface of a cone that is dictated by the colliding winds. The whole outflow structure is wrapped into a spiral by the orbital motion of the presumed central binary. Further the dust formation has a specific onset and cessation, which truncate the spiral at the outer and inner limits (for example, giving rise to the notable elliptical hole). Note this is a toy animation to illustrate a fly-around of the structure, and not a model fitted to the data that describes the dust flow process. The looping animation proceeds for about half an orbit (say roughly 60 years) with a pause at about the present epoch. Note that the motion we actually recorded with VISIR in the real data only spans 3 years (image credit: Peter Tuthill/University of Sydney/ESO)
- Dr. Joe Callingham, lead author of the study, said: "We discovered this star as an outlier in a survey with a radio telescope operated by the University of Sydney. We knew immediately we had found something quite exceptional: the luminosity across the spectrum from the radio to the infrared was off the charts." Dr. Callingham is now at the Netherlands Institute for Radio Astronomy. — "When we saw the stunning dust plume coiled around the these incandescent stars, we decided to name it 'Apep' - the monstrous serpent deity and mortal enemy of Sun god Ra from Egyptian mythology."
- That sculpted plume is what makes the system so important, said Professor Peter Tuthill, research group leader at the University of Sydney. "When we saw the spiral dust tail we immediately knew we were dealing with a rare and special kind of nebula called a pinwheel," Professor Tuthill said. "The curved tail is formed by the orbiting binary stars at the center, which inject dust into the expanding wind creating a pattern like a rotating lawn sprinkler. Because the wind expands so much, it inflates the tiny coils of dust revealing the physics of the stars at the heart of the system."
- However, the data on the plume presented a conundrum: the stellar winds were expanding 10 times faster than the dust.
- "It was just astonishing," Professor Tuthill said. "It was like finding a feather caught in a hurricane just drifting along at walking pace."
- Dr. Benjamin Pope, a co-author from New York University, said: "The key to understanding the bizarre behavior of the wind lies in the rotation of the central stars. - What we have found in the Apep system is a supernova precursor that seems to be very rapidly rotating, so fast it might be near break-up."
- Wolf-Rayet stars, like those driving Apep's plume, are known to be very massive stars at the ends of their lives; they could explode as supernovae at any time.
- "The rapid rotation puts Apep into a whole new class. Normal supernovae are already extreme events but adding rotation to the mix can really throw gasoline on the fire."
- The researchers think this might be the recipe for a perfect stellar storm to produce a gamma-ray burst, which are the most extreme events in the Universe after the Big Bang itself. Fortunately, Apep appears not to be aimed at Earth, because a strike by a gamma-ray burst from this proximity could strip ozone from the atmosphere, drastically increasing our exposure to UV light from the Sun.
- "Ultimately, we can't be certain what the future has in store for Apep," Professor Tuthill said. "The system might slow down enough so it explodes as a normal supernova rather than a gamma-ray burst. However, in the meantime, it is providing astronomers a ringside seat into beautiful and dangerous physics that we have not seen before in our galaxy."
• October 31, 2018: ESO’s exquisitely sensitive GRAVITY instrument has added further evidence to the long-standing assumption that a supermassive black hole lurks in the center of the Milky Way. New observations show clumps of gas swirling around at about 30% of the speed of light on a circular orbit just outside its event horizon — the first time material has been observed orbiting close to the point of no return, and the most detailed observations yet of material orbiting this close to a black hole. 90)
Figure 59: ESO’s GRAVITY instrument on the VLT Interferometer has been used by scientists from a consortium of European institutions, including ESO, to observe flares of infrared radiation coming from the accretion disc around Sagittarius A*, the massive object at the heart of the Milky Way. The observed flares provide long-awaited confirmation that the object in the center of our galaxy is, as has long been assumed, a supermassive black hole. The flares originate from material orbiting very close to the black hole’s event horizon — making these the most detailed observations yet of material orbiting this close to a black hole (image credit: MPE Garching, Observatoire de Paris, Université Grenoble Alpes, CNRS, Max Planck Institute for Astronomy, University of Cologne, Portuguese CENTRA – Centro de Astroﬁsica e Gravitação and ESO)
- While some matter in the
accretion disc — the belt of gas orbiting Sagittarius A* at
relativistic speeds — can orbit the black hole safely, anything
that gets too close is doomed to be pulled beyond the event horizon.
The closest point to a black hole that material can orbit without being
irresistibly drawn inwards by the immense mass is known as the
innermost stable orbit, and it is from here that the observed flares
- "It’s mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light," marvelled Oliver Pfuhl, a scientist at the MPE. "GRAVITY’s tremendous sensitivity has allowed us to observe the accretion processes in realtime in unprecedented detail."
- These measurements were only
possible thanks to international collaboration and state-of-the-art
instrumentation. The GRAVITY instrument which made this work possible
combines the light from four telescopes of ESO’s VLT to create a
virtual super-telescope 130 meters in diameter, and has already been
used to probe the nature of Sagittarius A*.
- Earlier this year, GRAVITY and SINFONI, another instrument on the VLT, allowed the same team to accurately measure the close fly-by of the star S2 as it passed through the extreme gravitational field near Sagittarius A*, and for the first time revealed the effects predicted by Einstein’s general relativity in such an extreme environment. During S2’s close fly-by, strong infrared emission was also observed.
- "We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*," explained Pfuhl. "During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!"
- This emission,
from highly energetic electrons very close to the black hole, was
visible as three prominent bright flares, and exactly matches
theoretical predictions for hot spots orbiting close to a black hole of
four million solar masses. The flares are thought to originate from
magnetic interactions in the very hot gas orbiting very close to
- Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, who led the study, explained: "This always was one of our dream projects but we did not dare to hope that it would become possible so soon." Referring to the long-standing assumption that Sagittarius A* is a supermassive black hole, Genzel concluded that "the result is a resounding confirmation of the massive black hole paradigm." 91)
• October 01, 2018: An unexpected abundance of Lyman-alpha emission in the HUDF (Hubble Ultra Deep Field) region was discovered by an international team of astronomers using the MUSE (Multi Unit Spectroscopic Explorer) instrument on ESO’s VLT (Very Large Telescope). The discovered emission covers nearly the entire field of view — leading the team to extrapolate that almost all of the sky is invisibly glowing with Lyman-alpha emission from the early Universe. 92) 93)
- Astronomers have long been accustomed to the sky looking wildly different at different wavelengths, but the extent of the observed Lyman-alpha emission was still surprising. “Realizing that the whole sky glows in optical when observing the Lyman-alpha emission from distant clouds of hydrogen was a literally eye-opening surprise,” explained Kasper Borello Schmidt, a member of the team of astronomers behind this result.
- “This is a great discovery!” added team member Themiya Nanayakkara. “Next time you look at the moonless night sky and see the stars, imagine the unseen glow of hydrogen: the first building block of the universe, illuminating the whole night sky.”
- The HUDF region the team observed is an otherwise unremarkable area in the constellation of Fornax (the Furnace), which was famously mapped by the NASA/ESA Hubble Space Telescope in 2004, when Hubble spent more than 270 hours of precious observing time looking deeper than ever before into this region of space.
- The HUDF observations revealed thousands of galaxies scattered across what appeared to be a dark patch of sky, giving us a humbling view of the scale of the Universe. Now, the outstanding capabilities of MUSE have allowed us to peer even deeper. The detection of Lyman-alpha emission in the HUDF is the first time astronomers have been able to see this faint emission from the gaseous envelopes of the earliest galaxies. This composite image shows the Lyman-alpha radiation in blue superimposed on the iconic HUDF image.
- MUSE, the instrument behind these latest observations, is a state-of-the-art integral field spectrograph installed on Unit Telescope 4 of the VLT at ESO’s Paranal Observatory.
When MUSE observes the sky, it sees
the distribution of wavelengths in the light striking every pixel in
its detector. Looking at the full spectrum of light from astronomical
objects provides us with deep insights into the astrophysical processes
occurring in the Universe.
Figure 60: Deep observations made with the MUSE spectrograph on ESO’s VLT have uncovered vast cosmic reservoirs of atomic hydrogen surrounding distant galaxies. The exquisite sensitivity of MUSE allowed for direct observations of dim clouds of hydrogen glowing with Lyman-alpha emission in the early Universe — revealing that almost the whole night sky is invisibly aglow (image credit: eso 1832)
• September 12, 2018: This wonderful image shows the resplendent spiral galaxy NGC 3981 suspended in the inky blackness of space. This galaxy, which lies in the constellation of Crater (the Cup), was imaged in May 2018 using the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) instrument on ESO’s Very Large Telescope (VLT). 94)
- FORS2 is mounted on Unit Telescope 1 (Antu) of the VLT at ESO’s Paranal Observatory in Chile. Amongst the host of cutting-edge instruments mounted on the four Unit Telescopes of the VLT, FORS2 stands apart due to its extreme versatility. This ”Swiss Army knife” of an instrument is able to study a variety of astronomical objects in many different ways — as well as being capable of producing beautiful images like this one.
- The sensitive gaze of FORS2 revealed NGC 3981’s spiral arms, strewn with vast streams of dust and star-forming regions, and a prominent disc of hot young stars. The galaxy is inclined towards Earth, allowing astronomers to peer right into the heart of this galaxy and observe its bright center, a highly energetic region containing a supermassive black hole. Also shown is NGC 3981’s outlying spiral structure, some of which appears to have been stretched outwards from the galaxy, presumably due to the gravitational influence of a past galactic encounter.
- NGC 3981 certainly has many galactic neighbors. Lying approximately 65 million light years from Earth, the galaxy is part of the NGC 4038 group, which also contains the well-known interacting Antennae Galaxies. This group is part of the larger Crater Cloud, which is itself a smaller component of the Virgo Supercluster, the titanic collection of galaxies that hosts our own Milky Way galaxy.
- NGC 3981 is not the only interesting feature captured in this image. As well as several foreground stars from our own galaxy, the Milky Way, FORS2 also captured a rogue asteroid streaking across the sky, visible as the faint line towards the top of the image. This particular asteroid has unwittingly illustrated the process used to create astronomical images, with the three different exposures making up this image displayed in the blue, green and red sections of the asteroid’s path.
- This image was taken as part of ESO’s Cosmic Gems program, an outreach initiative to produce images of interesting, intriguing or visually attractive objects using ESO telescopes, for the purposes of education and public outreach. The program makes use of telescope time that cannot be used for science observations. In case the data collected could be useful for future scientific purposes, these observations are saved and made available to astronomers through ESO’s science archive.
Figure 61: FORS2, an instrument mounted on ESO’s Very Large Telescope, has observed the spiral galaxy NGC 3981 in all its glory. The image was captured as part of the ESO Cosmic Gems Program, which makes use of the rare occasions when observing conditions are not suitable for gathering scientific data. Instead of sitting idle, the ESO Cosmic Gems Program allows ESO’s telescopes to be used to capture visually stunning images of the southern skies (image credit: ESO)
• August 8, 2018: Whereas ESO’s VLT (Very Large Telescope) can observe very faint astronomical objects in great detail, when astronomers want to understand how the huge variety of galaxies come into being they must turn to a different sort of telescope with a much bigger field of view. The VST (VLT Survey Telescope) is such a telescope. It was designed to explore vast swathes of the pristine Chilean night skies, offering astronomers detailed astronomical surveys of the southern hemisphere. 95) 96)
- The powerful surveying properties
of the VST led an international team of astronomers to conduct the VST
Early-type GAlaxy Survey (VEGAS)  to examine a collection of
elliptical galaxies in the southern hemisphere . Using the sensitive
detector at the heart of the VST , a team led by Marilena Spavone
from INAF-Astronomical Observatory of Capodimonte in Naples, Italy,
captured images of a wide variety of such galaxies in different
- One of these galaxies is NGC 5018, the milky-white galaxy near the center of this image. It lies in the constellation of Virgo (The Virgin) and may at first resemble nothing but a diffuse blob. But, on closer inspection, a tenuous stream of stars and gas — a tidal tail — can be seen stretching outwards from this elliptical galaxy. Delicate galactic features such as tidal tails and stellar streams are hallmarks of galactic interactions, and provide vital clues to the structure and dynamics of galaxies.
- As well as the many elliptical (and a few spiral) galaxies in this remarkable 400-megapixel image, a colorful variety of bright foreground stars in our own Milky Way Galaxy also pepper the image. These stellar interlopers, such as the vividly blue HD 114746 near the center of the image, are not the intended subjects of this astronomical portrait, but happen to lie between the Earth and the distant galaxies under study. Less prominent, but no less fascinating, are the faint tracks left by asteroids in our own Solar System. Just below NGC 5018, the faint streak left by the asteroid 2001 TJ21 (110423) — captured over several successive observations — can be seen stretching across the image. Further to the right, another asteroid — 2000 WU69 (98603) — left its trace in this spectacular image.
- While astronomers set out to investigate the delicate features of distant galaxies millions of light-years from Earth, in the process they also captured images of nearby stars hundreds of light-years away, and even the faint trails of asteroids only light-minutes away in our own Solar System. Even when studying the furthest reaches of the cosmos, the sensitivity of ESO telescopes and dark Chilean skies can offer entrancing observations much closer to home.
Figure 62: A glittering host of galaxies populate this rich image taken with ESO’s VST (VLT Survey Telescope), a state-of-the-art 2.6-m telescope designed for surveying the sky in visible light. The features of the multitude of galaxies strewn across the image allow astronomers to uncover the most delicate details of galactic structure (image credit: eso1827 — Photo Release)
• July 26, 2018: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole. Obscured by thick clouds of absorbing dust, the closest supermassive black hole to the Earth lies 26 000 light-years away at the center of the Milky Way. This gravitational monster, which has a mass four million times that of the Sun, is surrounded by a small group of stars orbiting around it at high speed. This extreme environment — the strongest gravitational field in our galaxy — makes it the perfect place to explore gravitational physics, and particularly to test Einstein’s general theory of relativity. 97)
Figure 63: Artist’s impression of S2 passing supermassive black hole at the center of the Milky Way. Observations made with ESO’s VLT (Very Large Telescope) have for the first time revealed the effects predicted by Einstein’s general relativity on the motion of a star passing through the extreme gravitational field near the supermassive black hole in the center of the Milky Way. This long-sought result represents the climax of a 26-year-long observation campaign using ESO’s telescopes in Chile (image credit: ESO, M. Kommesser)
- New infrared observations from the exquisitely sensitive GRAVITY, SINFONI and NACO
instruments on ESO’s VLT have now allowed astronomers to follow
one of these stars, called S2, as it passed very close to the black
hole during May 2018. At the closest point this star was at a distance
of less than 20 billion kilometers from the black hole and moving at a
speed in excess of 25 million kilometers per hour — almost three
percent of the speed of light. 98)
Figure 64: Orbit diagram of S2 around the supermassive black hole at the center of the Milky Way. It was compiled from observations with ESO telescopes and instruments over a period of more than 25 years. The star takes 16 years to complete one orbit and was very close to the black hole in May 2018 (image credit: ESO/MPE/GRAVITY Collaboration)
- The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity. The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity.
- These extremely precise measurements were made by an international team led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, in conjunction with collaborators around the world, at the Paris Observatory–PSL, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA
– Centro de Astroﬁsica e Gravitação and ESO. The
observations are the culmination of a 26-year series of
ever-more-precise observations of the center of the Milky Way using ESO
Figure 65: Cosmic swarm of bees: This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018 (image credit: ESO/L. Calçada/spaceengine.org)
- “This is the second time that we have observed the close passage of S2 around the black hole in our galactic center. But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution,” explains Genzel. “We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects.”
- The new measurements clearly reveal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole. And the change in the wavelength of light from S2 agrees precisely with that predicted by Einstein’s theory of general relativity. This is the first time that this deviation from the predictions of the simpler Newtonian theory of gravity has been observed in the motion of a star around a supermassive black hole.
- The team used SINFONI to measure the velocity of S2 towards and away from Earth and the GRAVITY instrument in the VLT Interferometer (VLTI) to make extraordinarily precise measurements of the changing position of S2 in order to define the shape of its orbit. GRAVITY creates such sharp images that it can reveal the motion of the star from night to night as it passes close to the black hole — 26 000 light-years from Earth.
- “Our first observations of S2 with GRAVITY, about two years ago, already showed that we would have the ideal black hole laboratory,” adds Frank Eisenhauer (MPE), Principal Investigator of GRAVITY and the SINFONI spectrograph. “During the close passage, we could even detect the faint glow around the black hole on most of the images, which allowed us to precisely follow the star on its orbit, ultimately leading to the detection of the gravitational redshift in the spectrum of S2.”
- More than one hundred years after he published his paper setting out the equations of general relativity, Einstein has been proved right once more — in a much more extreme laboratory than he could have possibly imagined!
- Françoise Delplancke, head of the System Engineering Department at ESO, explains the significance of the observations: “Here in the Solar System we can only test the laws of physics now and under certain circumstances. So it’s very important in astronomy to also check that those laws are still valid where the gravitational fields are very much stronger.”
- Continuing observations are expected to reveal another relativistic effect very soon — a small rotation of the star’s orbit, known as Schwarzschild precession — as S2 moves away from the black hole.
- Xavier Barcons, ESO’s Director General, concludes: “ESO has worked with Reinhard Genzel and his team and collaborators in the ESO Member States for over a quarter of a century. It was a huge challenge to develop the uniquely powerful instruments needed to make these very delicate measurements and to deploy them at the VLT in Paranal. The discovery announced today is the very exciting result of a remarkable partnership.”
Figure 66: This diagram shows the motion of the star S2 as it passes close to the supermassive black hole at the center of the Milky Way. It was compiled from observations with the GRAVITY instrument in the VLT interferometer. At this point the star was travelling at nearly 3% of the speed of light and its shift in position can be seen from night to night. The sizes of the star and the black hole are not to scale (image credit: ESO/MPE/GRAVITY Collaboration) 99)
• July 18,2018: ESO's VLT has achieved first light with a new adaptive optics mode called laser tomography — and has captured remarkably sharp test images of the planet Neptune, star clusters and other objects. The pioneering MUSE instrument in Narrow-Field Mode, working with the GALACSI adaptive optics module, can now use this new technique to correct for turbulence at different altitudes in the atmosphere. It is now possible to capture images from the ground at visible wavelengths that are sharper than those from the NASA/ESA Hubble Space Telescope. The combination of exquisite image sharpness and the spectroscopic capabilities of MUSE will enable astronomers to study the properties of astronomical objects in much greater detail than was possible before. 100)
Figure 67: This image of the planet Neptune was obtained during the testing of the Narrow-Field adaptive optics mode of the MUSE/GALACSI instrument on ESO’s Very Large Telescope. The corrected image is sharper than a comparable image from the NASA/ESA Hubble Space Telescope (image credit: ESO/P. Weilbacher (AIP))
Figure 68: Neptune from the VLT with and without adaptive optics (image credit: ESO)
- The MUSE (Multi Unit Spectroscopic Explorer) instrument on ESO’s Very Large Telescope (VLT) works with an adaptive optics unit called GALACSI (Ground Atmospheric Layer Adaptive Optics for Spectroscopic Imaging). This makes use of the Laser Guide Star Facility, 4LGSF,
a subsystem of the AOF (Adaptive Optics Facility). The AOF provides
adaptive optics for instruments on the VLT's Unit Telescope 4 (UT4).
MUSE was the first instrument to benefit from this new facility and it
now has two adaptive optics modes — the Wide Field Mode and the
Narrow Field Mode.
- The MUSE Wide Field Mode coupled to GALACSI in ground-layer mode
corrects for the effects of atmospheric turbulence up to 1 km above the
telescope over a comparatively wide field of view. But the new Narrow
Field Mode using laser tomography corrects for almost all of the
atmospheric turbulence above the telescope to create much sharper
images, but over a smaller region of the sky.
- With this new capability, the 8 m UT4 reaches the theoretical limit of image sharpness and is no longer limited by atmospheric blur. This is extremely difficult to attain in the visible and gives images comparable in sharpness to those from the NASA/ESA Hubble Space Telescope. It will enable astronomers to study in unprecedented detail fascinating objects such as supermassive black holes at the centers of distant galaxies, jets from young stars, globular clusters, supernovae, planets and their satellites in the Solar System and much more.
- Adaptive optics is a technique to compensate for the blurring effect of the Earth’s atmosphere, also known as astronomical seeing, which is a big problem faced by all ground-based telescopes. The same turbulence in the atmosphere that causes stars to twinkle to the naked eye results in blurred images of the Universe for large telescopes. Light from stars and galaxies becomes distorted as it passes through our atmosphere, and astronomers must use clever technology to improve image quality artificially.
- To achieve this, four brilliant lasers are fixed to UT4 that project columns of intense orange light 30 cm in diameter into the sky, stimulating sodium atoms high in the atmosphere and creating artificial Laser Guide Stars. Adaptive optics systems use the light from these “stars” to determine the turbulence in the atmosphere and calculate corrections one thousand times per second, commanding the thin, deformable secondary mirror of UT4 to constantly alter its shape, correcting for the distorted light.
- MUSE is not the only instrument to benefit from the Adaptive Optics Facility. Another adaptive optics system, GRAAL (GRound layer Adaptive optics Assisted by Lasers), is already in use with the infrared camera HAWK-I. This will be followed in a few years by the powerful new instrument ERIS (Enhanced Resolution Imager and Spectrograph). Together these major developments in adaptive optics are enhancing the already powerful fleet of ESO telescopes, bringing the Universe into focus.
- This new mode also constitutes a major step forward for the ESO’s Extremely Large Telescope, which will need Laser Tomography to reach its science goals. These results on UT4 with the AOF will help to bring ELT’s engineers and scientists closer to implementing similar adaptive optics technology on the 39 meter giant.
• July 11, 2018: New observations with ESO’s Very Large Telescope show the star cluster RCW 38 in all its glory (Figure 69). This image was taken during testing of the HAWK-I (High Acuity Wide field K-band Imager) camera with the GRAAL [(Ground-layer AOM (Adaptive Optics Module) Assisted by Lasers)] adaptive optics system. It shows RCW 38 and its surrounding clouds of brightly glowing gas in exquisite detail, with dark tendrils of dust threading through the bright core of this young gathering of stars. 101)
- The central area of RCW 38 is visible here as a bright, blue-tinted region, an area inhabited by numerous very young stars and protostars that are still in the process of forming. The intense radiation pouring out from these newly born stars causes the surrounding gas to glow brightly. This is in stark contrast to the streams of cooler cosmic dust winding through the region, which glow gently in dark shades of red and orange. The contrast creates this spectacular scene — a piece of celestial artwork.
- Previous images of this region taken in optical wavelengths are strikingly different — optical images appear emptier of stars due to dust and gas blocking our view of the cluster. Observations in the infrared, however, allow us to peer through the dust that obscures the view in the optical and delve into the heart of this star cluster.
- HAWK-I is installed on Unit Telescope 4 (Yepun) of the VLT, and operates at near-infrared wavelengths. It has many scientific roles, including obtaining images of nearby galaxies or large nebulae as well as individual stars and exoplanets. GRAAL is an adaptive optics module which helps HAWK-I to produce these spectacular images. It makes use of four laser beams projected into the night sky, which act as artificial reference stars, used to correct for the effects of atmospheric turbulence — providing a sharper image.
- This image was captured as part of a series of test observations — a process known as science verification — for HAWK-I and GRAAL. These tests are an integral part of the commissioning of a new instrument on the VLT, and include a set of typical scientific observations that verify and demonstrate the capabilities of the new instrument.
- The Science Verification of HAWK-I with the GRAAL adaptive optics module was presented in an article in ESO’s quarterly journal ”The Messenger” entitled HAWK-I GRAAL Science Verification. 102)
- The Principal Investigator of the observing proposal which led this spectacular image was Koraljka Muzic (CENTRA, University of Lisbon, Portugal). Her collaborators were Joana Ascenso (CENTRA, University of Porto, Portugal), Amelia Bayo (University of Valparaiso, Chile), Arjan Bik (Stockholm University, Sweden), Hervé Bouy (Laboratoire d’astrophysique de Bordeaux, France), Lucas Cieza (University Diego Portales, Chile), Vincent Geers (UKATC, UK), Ray Jayawardhana (York University, Canada), Karla Peña Ramírez (University of Antofagasta, Chile), Rainer Schoedel (Instituto de Astrofísica de Andalucía, Spain), and Aleks Scholz (University of St Andrews, UK).
- The science verification team was composed of Bruno Leibundgut, Pascale Hibon, Harald Kuntschner, Cyrielle Opitom, Jerome Paufique, Monika Petr-Gotzens, Ralf Siebenmorgen, Elena Valenti and Anita Zanella, all from ESO.
Figure 69: This image shows the star cluster RCW 38, as captured by the HAWK-I infrared imager mounted on ESO’s VLT (Very Large Telescope) in Chile. By gazing into infrared wavelengths, HAWK-I can examine dust-shrouded star clusters like RCW 38, providing an unparalleled view of the stars forming within. This cluster contains hundreds of young, hot, massive stars, and lies some 5500 light-years away in the constellation of Vela (The Sails), image credit:
• June 20, 2018: Each exoplanet revolves around a star, like the Earth around the Sun. This is why it is generally impossible to obtain images of an exoplanet, so dazzling is the light of its star. However, a team of astronomers, led by a researcher from the University of Geneva (UNIGE) and member of NCCR PlanetS, had the idea of detecting certain molecules that are present in the planet’s atmosphere in order to make it visible, provided that these same molecules are absent from its star. Thanks to this innovative technique, the device is only sensitive to the selected molecules, making the star invisible and allowing the astronomers to observe the planet directly. 103)
Figure 70: The planet becomes visible when looking for H2O or CO molecules. However, as there is no CH4 nor NH3 in its atmosphere, it remains invisible when looking for these molecules, just as its host star which contains none of those four elements (image credit: UNIGE)
- Until now, astronomers could only very rarely directly observe the exoplanets they discovered, as they are masked by the enormous luminous intensity of their stars. Only a few planets located very far from their host stars could be distinguished on a picture, in particular thanks to the SPHERE instrument installed on the VLT (Very Large Telescope) in Chile, and similar instruments elsewhere.
- Jens Hoeijmakers, researcher at the Astronomy Department of the Observatory of the Faculty of Science of the UNIGE and member of NCCR PlanetS, wondered if it would be possible to trace the molecular composition of the planets. “By focusing on molecules present only on the studied exoplanet that are absent from its host star, our technique would effectively “erase” the star,leaving only the exoplanet,” he explains.
- Erasing the star thanks to molecular spectra: To test this new technique, Jens Hoeijmakers and an international team of astronomers used archival images taken by the SINFONI(Spectrograph for INtegral Field Observations in the Near Infrared) instrument of the star beta pictoris, which is known to be orbited by a giant planet, beta pictoris b. Each pixel in these images contains the spectrum of light received by that pixel. The astronomers then compared the spectrum contained in the pixel with a spectrum corresponding to a given molecule, for example water vapor, to see if there is a correlation. If there is a correlation, it means that the molecule is present in the atmosphere of the planet.
- By applying this technique to beta pictoris b, Jens Hoeijmakers notices that the planet becomes perfectly visible when he looks for water (H2O) or carbon monoxide (CO). However, when he applies his technique to methane (CH4) and ammonia (NH3), the planet remains invisible, suggesting the absence of these molecules in the atmosphere of beta pictoris b.
- Molecules, new planetary thermometer: The host star beta pictoris remains invisible in all four situations. Indeed, this star is extremely hot and at this high temperature, these four molecules are destroyed. “This is why this technique allows us not only to detect elements on the surface of the planet, but also to sense the temperature which reigns there”, explains the astronomer of UNIGE. The fact that astronomers cannot find beta pictoris b using the spectra of methane and ammonia is therefore consistent with a temperature estimated at 1700 degrees for this planet, which is too high for these molecules to exist.
- “This technique is only in its infancy”, enthuses Jens Hoeijmakers. “It should change the way planets and their atmospheres are characterized. We are very excited to see what it will give on future spectrographs like ERIS (Enhanced Resolution Imager and Spectrograph) on the VLT in Chile or HARMONI (High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph) on the ELT (Extremely Large Telescope) which will be inaugurated in 2025, also in Chile,” he concludes.
• April 11, 2018: The SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument on ESO’s VLT (Very Large Telescope) in Chile allows astronomers to suppress the brilliant light of nearby stars in order to obtain a better view of the regions surrounding them. This collection of new SPHERE images is just a sample of the wide variety of dusty discs being found around young stars (Figure 71). 104)
- These discs are wildly different in size and shape — some contain bright rings, some dark rings, and some even resemble hamburgers. They also differ dramatically in appearance depending on their orientation in the sky — from circular face-on discs to narrow discs seen almost edge-on.
- SPHERE’s primary task is to discover and study giant exoplanets orbiting nearby stars using direct imaging. But the instrument is also one of the best tools in existence to obtain images of the discs around young stars — regions where planets may be forming. Studying such discs is critical to investigating the link between disc properties and the formation and presence of planets.
- Many of the young stars shown here come from a new study of T Tauri stars, a class of stars that are very young (less than 10 million years old) and vary in brightness. The discs around these stars contain gas, dust, and planetesimals — the building blocks of planets and the progenitors of planetary systems.
- These images also show what our own Solar System may have looked like in the early stages of its formation, more than four billion years ago.
- Most of the images presented were obtained as part of the DARTTS-S (Discs ARound T Tauri Stars with SPHERE) survey. The distances of the targets ranged from 230 to 550 light-years away from Earth. For comparison, the Milky Way is roughly 100 000 light-years across, so these stars are, relatively speaking, very close to Earth. But even at this distance, it is very challenging to obtain good images of the faint reflected light from discs, since they are outshone by the dazzling light of their parent stars.
- Another new SPHERE observation is the discovery of an edge-on disc around the star GSC 07396-00759, found by the SHINE (SpHere INfrared survey for Exoplanets) survey. This red star is a member of a multiple star system also included in the DARTTS-S sample but, oddly, this new disc appears to be more evolved than the gas-rich disc around the T Tauri star in the same system, although they are the same age. This puzzling difference in the evolutionary timescales of discs around two stars of the same age is another reason why astronomers are keen to find out more about discs and their characteristics.
- Astronomers have used SPHERE to obtain many other impressive images, as well as for other studies including the interaction of a planet with a disc, the orbital motions within a system, and the time evolution of a disc.
- The new results from SPHERE, along with data from other telescopes such as ALMA, are revolutionizing astronomers’ understanding of the environments around young stars and the complex mechanisms of planetary formation.
Figure 71: New images from the SPHERE instrument on ESO’s Very Large Telescope are revealing the dusty discs surrounding nearby young stars in greater detail than previously achieved. They show a bizarre variety of shapes, sizes and structures, including the likely effects of planets still in the process of forming (image credit: ESO/H. Avenhaus et al./E. Sissa et al./DARTT-S and SHINE collaborations) 105)
Figure 72: This spectacular image from the SPHERE instrument on ESO's Very Large Telescope shows the dusty disc around the young star IM Lupi in finer detail than ever before (image credit: ESO/H. Avenhaus et al./DARTT-S collaboration) 106)
• April 5, 2018: New images from ESO’s Very Large Telescope in Chile and other telescopes reveal a rich landscape of stars and glowing clouds of gas in one of our closest neighbouring galaxies, the Small Magellanic Cloud. The pictures have allowed astronomers to identify an elusive stellar corpse buried among filaments of gas left behind by a 2000-year-old supernova explosion. The MUSE instrument was used to establish where this elusive object is hiding, and existing Chandra X-ray Observatory data confirmed its identity as an isolated neutron star. 107) 108)
- Spectacular new pictures, created from images from both ground- and space-based telescopes (Figure 73), tell the story of the hunt for an elusive missing object hidden amid a complex tangle of gaseous filaments in the Small Magellanic Cloud, about 200 000 light-years from Earth.
- New data from the MUSE instrument on ESO’s Very Large Telescope in Chile has revealed a remarkable ring of gas in a system called 1E 0102.2-7219, expanding slowly within the depths of numerous other fast-moving filaments of gas and dust left behind after a supernova explosion. This discovery allowed a team led by Frédéric Vogt, an ESO Fellow in Chile, to track down the first ever isolated neutron star with low magnetic field located beyond our own Milky Way galaxy.
- The team noticed that the ring was centered on an X-ray source that had been noted years before and designated p1. The nature of this source had remained a mystery. In particular, it was not clear whether p1 actually lies inside the remnant or behind it. It was only when the ring of gas — which includes both neon and oxygen — was observed with MUSE that the science team noticed it perfectly circled p1. The coincidence was too great, and they realized that p1 must lie within the supernova remnant itself. Once p1’s location was known, the team used existing X-ray observations of this target from the Chandra X-ray Observatory to determine that it must be an isolated neutron star, with a low magnetic field.
- In the words of Frédéric Vogt: “If you look for a point source, it doesn’t get much better than when the Universe quite literally draws a circle around it to show you where to look.”
- When massive stars explode as supernovae, they leave behind a curdled web of hot gas and dust, known as a supernova remnant. These turbulent structures are key to the redistribution of the heavier elements — which are cooked up by massive stars as they live and die — into the interstellar medium, where they eventually form new stars and planets.
- Typically barely ten kilometers across, yet weighing more than our Sun, isolated neutron stars with low magnetic fields are thought to be abundant across the Universe, but they are very hard to find because they only shine at X-ray wavelengths [Highly-magnetic spinning neutron stars are called pulsars. They emit strongly at radio and other wavelengths and are easier to find, but they are only a small fraction of all the neutron stars predicted to exist]. The fact that the confirmation of p1 as an isolated neutron star was enabled by optical observations is thus particularly exciting.
- Co-author Liz Bartlett, another ESO Fellow in Chile, sums up this discovery: “This is the first object of its kind to be confirmed beyond the Milky Way, made possible using MUSE as a guidance tool. We think that this could open up new channels of discovery and study for these elusive stellar remains.”
Figure 73: An isolated neutron star in the Small Magellanic Cloud. The image combines data from the MUSE instrument on ESO’s Very Large Telescope in Chile and the orbiting the NASA/ESA Hubble Space Telescope and NASA Chandra X-Ray Observatory (image credit: ESO)
Legend to Figure 73: The reddish background image comes from the NASA/ESA Hubble Space Telescope and reveals the wisps of gas forming the supernova remnant 1E 0102.2-7219 in green. The red ring with a dark center is from the MUSE instrument on ESO's Very Large Telescope and the blue and purple images are from the NASA Chandra X-Ray Observatory. The blue spot at the center of the red ring is an isolated neutron star with a weak magnetic field, the first identified outside the Milky Way.
• March 5, 2018: First Light of the MATISSE Interferometer at ESO’s Very Large Telescope in Chile. After 12 years of design and development in Europe, the MATISSE interferometry instrument has been installed during the last 3 months at ESO’s VLT (Very Large Telescope). MATISSE combines four of the VLT telescopes to obtain an interferometer with an extremely high spatial resolution. This instrument allows astronomers to study the environment of young stars, the surfaces of stars and Active Galactic Nuclei in the mid-infrared wavelength range. In February 2018, this new, powerful and technically challenging instrument successfully achieved ‘First Light’. This achievement consummates the decade-long efforts of a large number of engineers and astronomers in France, Germany and in the Netherlands, including the infrared interferometry research group at the Max Planck Institute for Radio Astronomy in Bonn, Germany. 109) 110)
- The initial MATISSE observations of the red supergiant star Betelgeuse, which is expected to explode as a supernova in a few hundred thousand years, showed that it still has secrets to reveal. The new observations show evidence that the star appears to have a different size when seen at different wavelengths. Such data will allow astronomers to further study the huge star’s surroundings and how it is shedding material into space.
- The principal investigator of MATISSE, Bruno Lopez (Observatoire de la Côte d’Azur (OCA), Nice, France), explains its unique power: “Single telescopes can achieve image sharpness that is limited by the size of their mirrors. To obtain even higher resolution, we combine — or interfere — the light from four different VLT telescopes. Doing this enables MATISSE to deliver the sharpest images of any telescope ever in the 3–13 µm wavelength range, where it will complement the James Webb Space Telescope’s future observations from space.”
- Thomas Henning, director at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and MATISSE co-principal investigator, comments: "By looking at the inner regions of protoplanetary discs with MATISSE, we hope to learn the origin of the various minerals contained in these discs — minerals that will later go on to form the solid cores of planets like the Earth.”
- Walter Jaffe, the project scientist and co-principal investigator from University of Leiden in the Netherlands, and Gerd Weigelt, co-principal investigator from the Max Planck Institute for Radio Astronomy (MPIfR), Bonn, Germany, add: “MATISSE will give us dramatic images of planet-forming regions, multiple stars, and, when working with the VLT Unit Telescopes, also the dusty discs feeding supermassive black holes. We hope also to observe details of exotic objects in our Solar System, such as volcanoes on Io, and the atmospheres of giant exoplanets.”
- MATISSE’s first light marks a big step forward in the scope of current optical/infrared interferometers and will allow astronomers to obtain interferometric images with finer detail over a wider wavelength range than currently possible. MATISSE will also complement the instruments planned for ESO’s upcoming ELT (Extremely Large Telescope), in particular METIS (Mid-infrared ELT Imager and Spectrograph). MATISSE will observe brighter objects than METIS, but with higher spatial resolution.
- Andreas Glindemann, MATISSE project manager at ESO, concludes: “Making MATISSE a reality has involved the work of many people over many years and it is wonderful to see the instrument working so well. We are looking forward to the exciting science to come!”
Figure 74: Four-telescope interferogram of the star Sirius recorded at “First light” observations on 18 February 2018 with VLTI-MATISSE. This image is a colorized version of the interferogram recorded at infrared wavelengths. Blue color corresponds to short infrared wavelengths, red corresponds to long wavelengths. The colors illustrate the changing wavelengths of the data. Interferograms are the raw data required for reconstructing high-resolution images of astronomical objects (image credit: ESO/MATISSE Consortium)
• February 17, 2018: It seems nothing can escape the inexorable spread of light pollution — not even the giant telescopes probing the heavens above northern Chile, a region whose pristine dark skies, long considered a paradise for astronomers, are under increasing threat. 111)
- The Atacama desert, 1,200 km north of the capital Santiago, provides ideal conditions where astronomers study the stars in darkness so profound they appear like diamonds on velvet. - Scientists estimate that by 2020, Chile — a critically important country for optical and radio astronomy — will host 70 percent of the globe's astronomical infrastructure.
- But the ever-expanding use of cheap LED (Light-Emitting Diode) lighting in the booming South American country is starting to concern astronomers desperately trying to safeguard some of the world's darkest skies.
- "Unfortunately, as we have more and more white lights, the deterioration of the skies has increased by up to 30 percent compared to the end of the last decade," said scientist Pedro Sanhueza.
- Chile takes the problem of light pollution so seriously that Sanhueza heads up an organization called the Office for the Protection of Quality of the Sky (OPCC).
Figure 75: Moonshine photo of the VLT (Very Large Telescope) observatory on the Cerro Paranal mountain in the Atacama Desert of northern Chile (image credit: ESO)
- Its main task is to make the people of northern Chile aware of the particularly high night-sky quality and the negative impacts of light pollution.
- Sanhueza says that though the quality is good, the sky over northern Chile is becoming "an area of risk," threatening the profound nocturnal darkness required for the study of phenomena such as solar flares, planetary nebulae, black holes and supernovas.
- Fueling the threat, he adds, are communities such as Antofagasta, Coquimbo and La Serena, where LED lights are increasingly used in homes, streetlights, store signs and billboards.
- A study published in December in the journal Science Advances has shown that global lighting has increased in both quantity and intensity by about 2 percent per year from 2012 to 2016.
- Urban boom: At the Paranal Observatory deep in the Atacama desert, which houses the European Southern Observatory's Very Large Telescope array — consisting of four telescopes — staff are doing all they can to limit light leaking out into the atmosphere.
- After sunset, vehicles travelling around the observatory are prohibited anything but their parking lights. Flashlights, if needed, are turned to the ground.
- Astronomers' residences at the base — 2,635 meters above sea level — are dimly-lit, to avoid interfering with observation of the galaxies above.
- But the urban boom has been an unstoppable reality for 20 years in northern Chile, where cities have felt the economic effects of the boom in copper, of which the South American country is the world's largest producer.
- Halos of light above towns are easily visible from observatories within a 150 km radius.
- "We have measured the impact of this and we have already experienced difficulties making observations at 20 degrees above the horizon. That's going to increase a lot and will prevent us from studying the furthest stars," said Chris Smith, head of the observatory at Tololo, some 80 km from the town of La Serena.
- Hazardous to health: Urban growth has gone hand in hand with huge infrastructure projects to extract copper and even the construction of a brightly-lit highway through the Atacama itself.
- Smith is calling for more education in schools about the sustainable use of light, the need to use "warmer" sources of lighting that are less polluting, and to avoid turning them towards the sky. "We are already seeing a big level of impact and we need to control it now," Smith, an American astronomer, says — adding there can be "no question of shutting observatories down."
- However, this is what many fear could ultimately happen to the Mount Palomar observatory in California, which has had to drastically cut down its activities because of the light pollution from Los Angeles.
- The Chilean government in 2012 approved a new lighting standard designed to cut down on pollution, but nevertheless, scientists acknowledge that it's an uphill battle.
• February 13, 2018: This ghostly image features a distant and pulsating red giant star known as R Sculptoris. Situated 1,200 light-years away in the constellation of Sculptor, R Sculptoris is something known as a carbon-rich AGB (Asymptotic Giant Branch) star, meaning that it is nearing the end of its life. At this stage, low- and intermediate-mass stars cool off, create extended atmospheres, and lose a lot of their mass — they are on their way to becoming spectacular planetary nebulae. 112) 113) 114)
- While the basics of this mass-loss process are understood, astronomers are still investigating how it begins near the surface of the star. The amount of mass lost by a star actually has huge implications for its stellar evolution, altering its future, and leading to different types of planetary nebulae. As AGB stars end their lives as planetary nebulae, they produce a vast range of elements — including 50% of elements heavier than iron — which are then released into the Universe and used to make new stars, planets, moons, and eventually the building blocks of life.
- One particularly intriguing feature of R Sculptoris is its dominant bright spot, which looks to be two or three times brighter than the other regions. The astronomers that captured this wonderful image, using ESO’s VLTI (Very Large Telescope Interferometer ), have concluded that R Sculptoris is surrounded by giant “clumps” of stellar dust that are peeling away from the shedding star. This bright spot is, in fact, a region around the star with little to no dust, allowing us to look deeper into the stellar surface.
Figure 76: Image of the pulsating red giant star R Sculptoris, based on observations made with the VLTI Paranal Observatory under program IDs 090.D-0136, 093.D-0015, 096.D-0720 (image credit, ESO, Research Team)
• January 30, 2018: ESO’s VLT (Very Large Telescope) now has a second instrument working with the powerful AOF (Adaptive Optics Facility). The infrared instrument HAWK-I (High Acuity Wide-field K-band Imager) is now also benefiting from sharper images and shorter exposure times. This follows the successful integration of the AOF with MUSE (Multi Unit Spectroscopic Explorer). 115)
- The AOF is a long-term project that is nearing completion on ESO’s VLT. It provides adaptive optics correction for all the instruments attached to one of the VLT Unit Telescopes (UT4, also known as Yepun).
- Adaptive optics works to compensate for the blurring effect of the Earth’s atmosphere. This upgrade now enables HAWK-I to obtain sharper images, needing shorter exposure times than before to obtain similar results. By using the AOF, astronomers can now get good image quality with HAWK-I, even when the weather conditions are not perfect.
- Following a series of tests of the new system, the commissioning team of astronomers and engineers were rewarded with a series of spectacular images, including one of the Tarantula Nebula star-forming region in the Large Magellanic Cloud.
- The AOF, which made these observations possible, is composed of many parts working together. These include the Four Laser Guide Star Facility (4LGSF) and the UT4’s very thin deformable secondary mirror, which is able to change its shape. The 4LGSF shines four 22 W laser beams into the sky to make sodium atoms in the upper atmosphere glow as bright points of light, forming artificial guide stars.
- Sensors in the adaptive optics module GRAAL (GRound layer Adaptive optics Assisted by Lasers) use these artificial guide stars to determine the atmospheric conditions. One thousand times per second, the AOF system calculates the correction that must be applied to the telescope’s deformable secondary mirror to compensate for the atmospheric disturbance.
- GRAAL corrects for the turbulence in the layer of atmosphere up to about 500 m above the telescope — the “ground layer”. Depending on the conditions, atmospheric turbulence occurs at all altitudes, but studies have shown that the largest fraction of the disturbance occurs in the ground layer of the atmosphere.
- The corrections applied by the AOF rapidly and continuously improve the image quality by concentrating the light to form sharper images, allowing HAWK-I to resolve finer details and detect fainter stars than previously possible.
- MUSE and HAWK-I are not the only instruments that will benefit from the AOF; in the future, the new instrument ERIS (Enhanced Resolution Imager and Spectrograph) will be installed on the VLT. The AOF is also a pathfinder for adaptive optics on ESO’s ELT (Extremely Large Telescope).
Figure 77: This image of the dramatic star formation region 30 Doradus, also known as the Tarantula Nebula, was created from a mosaic of images taken using the HAWK-I instrument working with the Adaptive optics Facility of ESO’s Very Large Telescope in Chile. The stars are significantly sharper than the same image without adaptive optics being used, and fainter stars can be seen (image credit: ESO) 116)
• January 17, 2018: Globular star clusters are huge spheres of tens of thousands of stars that orbit most galaxies. They are among the oldest known stellar systems in the Universe and date back to near the beginning of galaxy growth and evolution. More than 150 are currently known to belong to the Milky Way. 117)
- One particular cluster, called NGC 3201 and situated in the southern constellation of Vela (The Sails), has now been studied using the MUSE instrument on ESO’s Very Large Telescope in Chile. An international team of astronomers has found that one of the stars in NGC 3201 is behaving very oddly — it is being flung backwards and forwards at speeds of several hundred thousand kilometers per hour, with the pattern repeating every 167 days.
Figure 78: Artist’s impression of the black hole binary system in NGC 3201. Astronomers using ESO’s MUSE instrument on the Very Large Telescope in Chile have discovered a star in the cluster NGC 3201 that is behaving very strangely. It appears to be orbiting an invisible black hole with about four times the mass of the Sun — the first such inactive stellar-mass black hole found in a globular cluster. This important discovery impacts on our understanding of the formation of these star clusters, black holes, and the origins of gravitational wave events. This artist’s impression shows how the star and its massive but invisible black hole companion may look, in the rich heart of the globular star cluster (image credit: ESO/L. Calçada/spaceengine.org)
- Lead author Benjamin Giesers (Georg-August-Universität Göttingen, Germany) was intrigued by the star’s behavior: “It was orbiting something that was completely invisible, which had a mass more than four times the Sun — this could only be a black hole! The first one found in a globular cluster by directly observing its gravitational pull.” 118)
- The relationship between black holes and globular clusters is an important but mysterious one. Because of their large masses and great ages, these clusters are thought to have produced a large number of stellar-mass black holes — created as massive stars within them exploded and collapsed over the long lifetime of the cluster.
- ESO’s MUSE instrument provides astronomers with a unique ability to measure the motions of thousands of far away stars at the same time. With this new finding, the team have for the first time been able to detect an inactive black hole at the heart of a globular cluster — one that is not currently swallowing matter and is not surrounded by a glowing disc of gas. They could estimate the black hole’s mass through the movements of a star caught up in its enormous gravitational pull.
- From its observed properties the star was determined to be about 0.8 times the mass of our Sun, and the mass of its mysterious counterpart was calculated at around 4.36 times the Sun’s mass — almost certainly a black hole.
- Recent detections of radio and X-ray sources in globular clusters, as well as the 2016 detection of gravitational-wave signals produced by the merging of two stellar-mass black holes, suggest that these relatively small black holes may be more common in globular clusters than previously thought.
- Giesers concludes: “Until recently, it was assumed that almost all black holes would disappear from globular clusters after a short time and that systems like this should not even exist! But clearly this is not the case — our discovery is the first direct detection of the gravitational effects of a stellar-mass black hole in a globular cluster. This finding helps in understanding the formation of globular clusters and the evolution of black holes and binary systems — vital in the context of understanding gravitational wave sources.”
• December 13, 2017: The OmegaCAM camera on ESO’s VLT Survey Telescope has captured this glittering view of the stellar nursery called Sharpless 29 (Figure 79). Many astronomical phenomena can be seen in this giant image, including cosmic dust and gas clouds that reflect, absorb, and re-emit the light of hot young stars within the nebula. 119)
- The region of sky pictured is listed in the Sparpless Catalog of H II regions: interstellar clouds of ionized gas, rife with star formation. Also known as Sh 2-29, Sharpless 29 is located about 5500 light-years away in the constellation of Sagittarius (The Archer), next door to the larger Lagoon Nebula. It contains many astronomical wonders, including the highly active star formation site of NGC 6559, the nebula at the center of the image.
- This central nebula is Sharpless 29’s most striking feature. Though just a few light-years across, it showcases the havoc that stars can wreak when they form within an interstellar cloud. The hot young stars in this image are no more than two million years old and are blasting out streams of high-energy radiation. This energy heats up the surrounding dust and gas, while their stellar winds dramatically erode and sculpt their birthplace. In fact, the nebula contains a prominent cavity that was carved out by an energetic binary star system. This cavity is expanding, causing the interstellar material to pile up and create the reddish arc-shaped border.
- When interstellar dust and gas are bombarded with ultraviolet light from hot young stars, the energy causes them to shine brilliantly. The diffuse red glow permeating this image comes from the emission of hydrogen gas, while the shimmering blue light is caused by reflection and scattering off small dust particles. As well as emission and reflection, absorption takes place in this region. Patches of dust block out the light as it travels towards us, preventing us from seeing the stars behind it, and smaller tendrils of dust create the dark filamentary structures within the clouds.
- The rich and diverse environment of Sharpless 29 offers astronomers a smorgasbord of physical properties to study. The triggered formation of stars, the influence of the young stars upon dust and gas, and the disturbance of magnetic fields can all be observed and examined in this single area.
- But young, massive stars live fast and die young. They will eventually explosively end their lives in a supernova, leaving behind rich debris of gas and dust. In tens of millions of years, this will be swept away and only an open cluster of stars will remain.
- Sharpless 29 was observed with ESO’s OmegaCAM on the VLT Survey Telescope (VST) at Cerro Paranal in Chile. OmegaCAM produces images that cover an area of sky more than 300 times greater than the largest field of view imager of the NASA/ESA Hubble Space Telescope, and can observe over a wide range of wavelengths from the ultraviolet to the infrared. Its hallmark feature is its ability to capture the very red spectral line H-alpha, created when the electron inside a hydrogen atom loses energy, a prominent occurrence in a nebula like Sharpless 29.
Figure 79: The OmegaCAM camera on ESO’s VLT Survey Telescope has captured this glittering view of the stellar nursery called Sharpless 29. Many astronomical phenomena can be seen in this giant image, including cosmic dust and gas clouds that reflect, absorb, and re-emit the light of hot young stars within the nebula (image credit: ESO)
• August 23, 2017: To the unaided eye the famous, bright star Antares shines with a strong red tint in the heart of the constellation of Scorpius (The Scorpion). It is a huge and comparatively cool red supergiant star in the late stages of its life, on the way to becoming a supernova. 120)
- A team of astronomers, led by Keiichi Ohnaka, of the Universidad Católica del Norte in Chile, has now used ESO’s Very Large Telescope Interferometer (VLTI) at the Paranal Observatory in Chile to map Antares’s surface and to measure the motions of the surface material. This is the best image of the surface and atmosphere of any star other than the Sun.
- The VLTI is a unique facility that can combine the light from up to four telescopes, either the 8.2 meter Unit Telescopes, or the smaller Auxiliary Telescopes, to create a virtual telescope equivalent to a single mirror up to 200 m across. This allows it to resolve fine details far beyond what can be seen with a single telescope alone.
- “How stars like Antares lose mass so quickly in the final phase of their evolution has been a problem for over half a century,” said Keiichi Ohnaka, who is also the lead author of the paper. “The VLTI is the only facility that can directly measure the gas motions in the extended atmosphere of Antares — a crucial step towards clarifying this problem. The next challenge is to identify what’s driving the turbulent motions.”
- Using the new results the team has created the first two-dimensional velocity map of the atmosphere of a star other than the Sun. They did this using the VLTI with three of the Auxiliary Telescopes and an instrument called AMBER to make separate images of the surface of Antares over a small range of infrared wavelengths. The team then used these data to calculate the difference between the speed of the atmospheric gas at different positions on the star and the average speed over the entire star. This resulted in a map of the relative speed of the atmospheric gas across the entire disc of Antares — the first ever created for a star other than the Sun.
- The astronomers found turbulent, low-density gas much further from the star than predicted, and concluded that the movement could not result from convection, that is, from large-scale movement of matter which transfers energy from the core to the outer atmosphere of many stars. They reason that a new, currently unknown, process may be needed to explain these movements in the extended atmospheres of red supergiants like Antares.
- “In the future, this observing technique can be applied to different types of stars to study their surfaces and atmospheres in unprecedented detail. This has been limited to just the Sun up to now,” concludes Ohnaka. “Our work brings stellar astrophysics to a new dimension and opens an entirely new window to observe stars.”
Figure 80: Using ESO’s VLTI astronomers have constructed the most detailed image ever of a star — the red supergiant star Antares. They have also made the first map of the velocities of material in the atmosphere of a star other than the Sun, revealing unexpected turbulence in Antares’s huge extended atmosphere. The results were published in the journal Nature. 121)
• May 23, 2013: This new picture celebrates an important anniversary for the VLT (Very Large Telescope) – it is fifteen years since the first light on the first of its four Unit Telescopes, on 25 May 1998. Since then the four original giant telescopes have been joined by the four small Auxiliary Telescopes that form part of the VLT Interferometer (VLTI). The VLT is one of the most powerful and productive ground-based astronomical facilities in existence. In 2012 more than 600 refereed scientific papers based on data from the VLT and VLTI were published (ann13009). 122)
- Interstellar clouds of dust and gas are the nurseries where new stars are born and grow. The new picture shows one of them, IC 2944, which appears as the softly glowing pink background . This image is the sharpest view of the object ever taken from the ground. The cloud lies about 6500 light-years away in the southern constellation of Centaurus (The Centaur). This part of the sky is home to many other similar nebulae that are scrutinized by astronomers to study the mechanisms of star formation.
- Emission nebulae like IC 2944 are composed mostly of hydrogen gas that glows in a distinctive shade of red, due to the intense radiation from the many brilliant newborn stars. Clearly revealed against this bright backdrop are mysterious dark clots of opaque dust, cold clouds known as Bok globules. They are named after the Dutch-American astronomer Bart Bok, who first drew attention to them in the 1940s as possible sites of star formation. This particular set is nicknamed the Thackeray Globules (they were discovered from South Africa by the English astronomer A. David Thackeray in 1950).
- Larger Bok globules in quieter locations often collapse to form new stars but the ones in this picture are under fierce bombardment from the ultraviolet radiation from nearby hot young stars. They are both being eroded away and also fragmenting, rather like lumps of butter dropped into a hot frying pan. It is likely that Thackeray’s Globules will be destroyed before they can collapse and form stars.
- Bok globules are not easy to study. As they are opaque to visible light it is difficult for astronomers to observe their inner workings, and so other tools are needed to unveil their secrets — observations in the infrared or in the submillimeter parts of the spectrum, for example, where the dust clouds, only a few degrees over absolute zero, appear bright. Such studies of the Thackeray globules have confirmed that there is no current star formation within them.
Figure 81: With this new view of a spectacular stellar nursery ESO is celebrating 15 years of the Very Large Telescope — the world's most advanced optical instrument. This picture reveals thick clumps of dust silhouetted against the pink glowing gas cloud known to astronomers as IC 2944. These opaque blobs resemble drops of ink floating in a strawberry cocktail, their whimsical shapes sculpted by powerful radiation coming from the nearby brilliant young stars (image credit: ESO)
• August 2011: The Eyes are about 50 million light-years away in the constellation of Virgo (The Virgin) and are some 100,000 light-years apart. The nickname comes from the apparent similarity between the cores of this pair of galaxies — two white ovals that resemble a pair of eyes glowing in the dark when seen in a moderate-sized telescope. 123)
- But although the centers of these two galaxies look similar, their outskirts could not be more different. The galaxy in the lower right, known as NGC 4435, is compact and seems to be almost devoid of gas and dust. In contrast, in the large galaxy in the upper left (NGC 4438) a lane of obscuring dust is visible just below its nucleus, young stars can be seen left of its center, and gas extends at least up to the edges of the image.
- The contents of NGC 4438 have been stripped out by a violent process: a collision with another galaxy. This clash has distorted the galaxy’s spiral shape, much as could happen to the Milky Way when it collides with its neighboring galaxy Andromeda in three or four billion years.
- NGC 4435 could be the culprit. Some astronomers believe that the damage caused to NGC 4438 resulted from an approach between the two galaxies to within about 16,000 light-years that happened some 100 million years ago. But while the larger galaxy was damaged, the smaller one was significantly more affected by the collision. Gravitational tides from this clash are probably responsible for ripping away the contents of NGC 4438, and for reducing NGC 4435’s mass and removing most of its gas and dust.
- Another possibility is that the giant elliptical galaxy Messier 86, further away from The Eyes and not visible in this image, was responsible for the damage caused to NGC 4438. Recent observations have found filaments of ionized hydrogen gas connecting the two large galaxies, indicating that they may have collided in the past.
- The elliptical galaxy Messier 86 and The Eyes belong to the Virgo Cluster, a very rich grouping of galaxies. In such close quarters, galaxy collisions are fairly frequent, so perhaps NGC 4438 suffered from encounters with both NGC 4435 and Messier 86.
- This picture is the first to be produced as part of the ESO Cosmic Gems program (Figure 82). This is a new initiative to produce astronomical images for educational and public outreach purposes. The program mainly makes use of time when the sky conditions are not suitable for science observations to take pictures of interesting, intriguing or visually attractive objects. The data are also made available to professional astronomers through ESO’s science archive.
- In this case, although there were some clouds, the atmosphere was exceptionally stable, which allowed very sharp details to be revealed in this image taken using the VLT’s FORS2 (FOcal Reducer and low dispersion Spectrograph) instrument (installed on the VLT’s Unit Telescope 1). Light passing through two different filters was used: red (colored red) and green-yellow (colored blue), and the exposure times were 1800 seconds and 1980 seconds, respectively.
Figure 82: ESO’s Very Large Telescope has taken a striking image of a beautiful yet peculiar pair of galaxies nicknamed The Eyes. The larger of these, NGC 4438, was once a spiral galaxy but has become badly deformed by collisions with other galaxies in the last few hundred million years. This picture is the first to come out of ESO’s Cosmic Gems program, an initiative in which ESO has granted dedicated observing time for outreach purposes (image credit: ESO)
• March 13, 2002: One of the most fundamental tasks of modern astrophysics is the study of the evolution of the Universe . This is a daunting undertaking that requires extensive observations of large samples of objects in order to produce reasonably detailed maps of the distribution of galaxies in the Universe and to perform statistical analysis. Much effort is now being put into mapping the relatively nearby space and thereby to learn how the Universe looks today . But to study its evolution, we must compare this with how it looked when it still was young . This is possible, because astronomers can "look back in time" by studying remote objects - the larger their distance, the longer the light we now observe has been underway to us, and the longer is thus the corresponding "look-back time." This may sound easy, but it is not. Very distant objects are very dim and can only be observed with large telescopes. Looking at one object at a time would make such a study extremely time-consuming and, in practical terms, impossible. To do it anyhow, we need the largest possible telescope with a highly specialized, exceedingly sensitive instrument that is able to observe a very large number of (faint) objects in the remote universe simultaneously. 124) 125)
- VIMOS (VLT VIsible Multi-Object Spectrograph) is such an instrument. It can obtain many hundreds of spectra of individual galaxies in the shortest possible time; in fact, in one special observing mode, up to 6400 spectra of the galaxies in a remote cluster during a single exposure, augmenting the data gathering power of the telescope by the same proportion. This marvelous science machine has just been installed at the 8.2-m MELIPAL telescope, the third unit of the VLT (Very Large Telescope) at the ESO Paranal Observatory. A main task will be to carry out 3-dimensional mapping of the distant Universe from which we can learn its large-scale structure.
- "First light" was achieved on February 26, 2002, and a first series of test observations has successfully demonstrated the huge potential of this amazing facility. Much work on VIMOS is still ahead during the coming months in order to put into full operation and fine-tune the most efficient "galaxy cruncher" in the world. VIMOS is the outcome of a fruitful collaboration between ESO and several research institutes in France and Italy, under the responsibility of the Laboratoire d'Astrophysique de Marseille (CNRS, France). The other partners in the "VIRMOS Consortium" are the Laboratoire d'Astrophysique de Toulouse, Observatoire Midi-Pyrénées, and Observatoire de Haute-Provence in France, and Istituto di Radioastronomia (Bologna), Istituto di Fisica Cosmica e Tecnologie Relative (Milano), Osservatorio Astronomico di Bologna, Osservatorio Astronomico di Brera (Milano) and Osservatorio Astronomico di Capodimonte (Naples) in Italy.
Figure 83: The Crab Nebula (Messier 1), as observed by VIMOS. This well-known object is the remnant of a stellar explosion in the year 1054 (image credit: ESO)
Legend to Figure 83: The image is a composite VRI image obtained on March 4, 2002. The individual exposures lasted 180 seconds; image quality 0.7 arcsec FWHM; field 7 x 7 arcmin2; North is up and East is left.
Some VLT development states and instruments
• On 17 March 2011 light collected by all four of the 8-meter Unit Telescopes of ESO’s Very Large Telescope was successfully combined for the first time using PIONIER (a visiting instrument at the Paranal Observatory, developed at LAOG/IPAG in Grenoble, France), a new generation instrument in the VLT Interferometer. 126)
- To have all four Unit Telescopes (UTs) finally working together as a single telescope is a major step in the development of the VLT — the original design always anticipated that the four 8 m telescopes would be able to work either independently or together as part of the giant VLT Interferometer (VLTI). Coincidentally, the new observations took place on the 10th anniversary of the first successful combination of two beams within the VLTI.
- Among the main science goals for the four UTs, working together with PIONIER, are to try to reveal the signatures of planets in the making, to explore the natures and fates of stars by providing images of their surfaces and their environments and to understand better the powerful engines associated with black holes at the centers of galaxies.
- When combined, the UTs can potentially provide image sharpness that equals that of a telescope with a diameter of up to 130 meters. Three telescopes have been combined regularly since the VLT/VLTI began observing, offering three unique baselines; the ability to combine four telescopes bumps this number up to six and allows the very fine structure in astronomical objects to be studied much more easily.
- Individually the 8meter telescopes can spy objects four billion times fainter than the naked eye can see and working together the four large telescopes can pick up details about 16 times finer than can be seen with one UT.
- An earlier important step towards unleashing the full potential of the VLTI was when light from all four of the 1.8 meter VLT Auxiliary Telescopes (ATs) was combined using PIONIER.
Figure 84: Photo of the VLT platform in 2011 (image credit: ESO)
• November 4, 2010: Light coming from the four 1.8 meter Auxiliary Telescopes at the European Southern Observatory’s VLTI (Very Large Telescope Interferometer) based in Paranal, Chile, has been successfully combined for the first time using a new visiting instrument called PIONIER (France). This is an important step towards unleashing the full potential of the VLTI to use multiple telescopes together to reveal fine detail in distant objects. A joint team from Grenoble LAOG (Laboratoire d’Astrophysique de Grenoble) and ESO achieved this very challenging feat of engineering only five days after unpacking the equipment on the mountain. 127)
- The VLTI engineers had to control the distance traversed by the light from the widely separated telescopes with an accuracy of about one hundredth of the thickness of a strand of human hair. Once the light reached PIONIER, it was then channelled into the heart of the instrument: a remarkable optical circuit, smaller than a credit card, that finally brought the light waves from the different telescopes together in a very precise way so that they could create interference. The resulting resolving power of the telescope array has the sharpness not of the individual 1.8 meter Auxiliary Telescopes, but that of a much bigger “virtual telescope” about 100 m across, limited only by how far apart the telescopes can be positioned.
- PIONIER, developed at LAOG in Grenoble, France, is a visiting instrument at the Paranal Observatory, complementing ESO’s existing AMBER and MIDI instruments. AMBER has previously combined the light from three of the telescopes at the VLTI to study many objects, including the surface of the variable star T Leporis (eso0906). PIONIER, however, will eventually allow the VLTI to go one stage further; with the additional information that a fourth telescope brings to the table, it should be possible to use complex mathematical processing techniques to create more detailed images. The PIONIER team hopes to produce its first images by early 2011.
Figure 85: Photo of the PIONIER instrument, shown here in the VLTI laboratory at ESO’s Paranal Observatory in Chile, has been used to combine the light from the four 1.8 meter Auxiliary Telescopes for the first time (image credit: ESO)
ESPRESSO (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations)
ESPRESSO is a super-stable Optical High Resolution Spectrograph for the combined coude' focus of the VLT. It can be operated by either one of the UTs or collecting the light from up to 4 UTs simultaneously.
The main scientific drivers for ESPRESSO are:
• the measurement of high precision radial velocities of solar type stars for search for rocky planets
• the measurement of the variation of the physical constants
• the analysis of the chemical composition of stars in nearby galaxies
These science cases require an efficient, high-resolution, extremely stable and accurate spectrograph.
ESPRESSO, installed at the VLT facility, will combine unprecedented radial velocity precision with the large collecting area of the UTs. Moreover it will be capable of collecting the light simultaneously from the 4UTs, to measure precisely faint or high redshift objects.
The understanding of the formation and evolution of planetary systems is one of the most exciting science cases of these days. The radial velocity technique has been so far the most productive in terms of extra-solar planet detections. Low mass planets (one to few Earth masses) are especially interesting because according to formation models they could represent the bulk of the planet population. However they are more elusive and require extremely stable instruments. The HARPS instrument, with a precision better than 1m/s, has discovered up to now the vast majority of planets with masses smaller than Neptune, giving an invaluable experience in view of the realization of more precise instruments. With a radial velocity precision better than 10 cm/s, an Earth mass planet in the habitable zone of a low mass star can be detected.
The instrument will also have the capability to acquire the most accurate measurements of the fundamental constants α (the fine structure constant) and µ (the proton to electron mass ratio) as a function of redshift, therefore addressing the question of whether the constants of the Standard Model of Physics vary with the age of the Universe. Its capability to collect the light from all the 4 UTs simultaneously will enlarge the number of accessible QSOs (Quasi-Stellar Objects) with great benefit for this science case.
The combination of high resolution and high efficiency opens the possibility of measuring the chemical composition of stars in galaxies other than the Milky Way with unprecedented accuracy.
Given the large light collecting power and its efficiency, its high spectral resolution, and its extreme radial velocity precision and accuracy, we expect that ESPRESSO will not only fulfill its main scientific objectives, but also open new opportunities in observational Astronomy with hopefully new and unexpected results.
ESPRESSO was designed and built by a consortium consisting of: the Astronomical Observatory of the University of Geneva and University of Bern, Switzerland; INAF (Osservatorio Astronomico di Trieste) and INAF (Osservatorio Astronomico di Brera), Italy; Instituto de Astrofísica de Canarias, Spain; Instituto de Astrofisica e Ciências do Espaço, Universidade do Porto and Universidade de Lisboa, Portugal; and ESO. The co-principal investigators are Francesco Pepe (University of Geneva, Switzerland), Stefano Cristiani (INAF (Osservatorio Astronomico di Trieste), Italy, Rafael Rebolo (IAC, Tenerife, Spain) and Nuno Santos (Instituto de Astrofisica e Ciencias do Espaco, Universidade do Porto, Portugal).
ESPRESSO is a fiber-fed, cross-dispersed, high-resolution, Echelle spectrograph. The telescope light is fed to the instrument via a Coude-Train optical system and fibers. ESPRESSO is located in the VLT CCL (Combined-Coude Laboratory) at the incoherent focus, where a front-end unit can combine the light from up to 4 Unit Telescopes (UT) of the VLT. The telescope light is fed to the instrument via a so-called Coudé Train optical system and within optical fibers. The target and sky light enter the instrument simultaneously through two separate fibers, which together form the slit of the spectrograph. 128) 129)
The incoherent combined focus: a new facility for a larger telescope: Although foreseen since 1977 in the original VLT plan, the incoherent combined focus of the VLT has never been implemented. Only provision for it, in terms of space left in the UTs structures and ducts in the rock of the mountain, is what is actually available at VLT. As part of the project agreement, the ESPRESSO Consortium has been asked to materialize such a focus providing the necessary hardware and software as part of the deliverables. The implementation of the Coudé Train is requiring substantial changes in the Paranal Observatory infrastructure yielding to an elaborated interfaces management. This new facility will allow to use the four telescopes as a large 16 meter equivalent telescope.
Enhanced flexibility and power: ESPRESSO will be located in the VLT’s CCL and, unlike any other instrument built so far, will receive light from any of the four UTs, allowing for a more flexible usage of the observation time. The light of the single UT scheduled to work with ESPRESSO is then fed into the spectrograph (single-UT modes). Alternatively, the combined light of all the UTs can be fed into ESPRESSO simultaneously (multi-UT mode).
Figure 86: The ESPRESSO optical design (image credit: ESPRESSO Consortium)
Several optical ‘tricks’ have been used to obtain high spectral resolution and efficiency despite the large size of the telescope and the 1 arcsec sky aperture of the instrument:
• At the spectrograph entrance the APSU (Anamorphic Pupil Slicing Unit) shapes the beam in order to compress it in cross-dispersion and splits in two smaller beams, while superimposing them on the echelle grating to minimize its size. The rectangular white pupil is then re-imaged and compressed.
• Given the wide spectral range, a dichroic beam splitter separates the beam in a blue and a red arm. Each arm is optimized for image quality and optical efficiency.
• The cross-disperser has the function of separating the dispersed spectrum in all its spectral orders. In addition, an anamorphism is re-introduced to make the pupil square and to compress the order height such that the inter-order space and the SNR per pixel are both maximized. Both functions are accomplished using VPHGs (Volume Phase Holographic Gratings) mounted on prisms.
• Finally, two optimized camera lens systems image the full spectrum from 380 nm to 780 nm on two large 92 mm x 92 mm CCDs with 10 µm pixels.
Figure 87: Spectral format and shape of the pseudo slit (image credit: ESPRESSO Consortium)
ESPRESSO development status:
• February 13, 2018: The ESPRESSO instrument on ESO’s Very Large Telescope in Chile has used the combined light of all four of the 8.2 m Unit Telescopes for the first time. Combining light from the Unit Telescopes in this way makes the VLT the largest optical telescope in existence in terms of collecting area. This picture shows in highly simplified form how the light collected by all four VLT Unit Telescopes is combined in the ESPRESSO instrument, located under the VLT platform. 130) 131)
- One of the original design goals of ESO's VLT was for its four UTs (Unit Telescopes) to work together to create a single giant telescope. With the first light of the ESPRESSO spectrograph using the four-Unit-Telescope mode of the VLT, this milestone has now been reached.
- After extensive preparations by the ESPRESSO consortium (led by the Astronomical Observatory of the University of Geneva, with the participation of research centers from Italy, Portugal, Spain and Switzerland) and ESO staff, ESO's Director General Xavier Barcons initiated this historic astronomical observation with the push of a button in the control room.
Figure 88: The ESPRESSO instrument achieves first light with all four Unit Telescopes (image credit: ESO, L. Calçada)
• December 6, 2017: ESPRESSO has achieved first light on ESO’s Very Large Telescope at the Paranal Observatory in northern Chile. This new, third-generation echelle spectrograph is the successor to ESO’s hugely successful HARPS instrument at the La Silla Observatory. HARPS can attain a precision of around 1 m/s in velocity measurements, whereas ESPRESSO aims to achieve a precision of just a few cm/s, due to advances in technology and its placement on a much bigger telescope. 132)
- The lead scientist for ESPRESSO, Francesco Pepe from the University of Geneva in Switzerland, explains its significance: “This success is the result of the work of many people over 10 years. ESPRESSO isn’t just the evolution of our previous instruments like HARPS, but it will be transformational, with its higher resolution and higher precision. And unlike earlier instruments it can exploit the VLT’s full collecting power — it can be used with all four of the VLT Unit Telescopes at the same time to simulate a 16 m telescope. ESPRESSO will be unsurpassed for at least a decade — now I am just impatient to find our first rocky planet!”
Figure 89: This colorful image shows spectral data from the First Light of the ESPRESSO instrument on ESO's Very Large Telescope in Chile. The light from a star has been dispersed into its component colors. This view has been colorized to indicate how the wavelengths change across the image, but these are not exactly the colors that would be seen visually. Close inspection shows many dark spectral lines in the stellar spectra and also the regular double spots from a calibration light source. The dark gaps are features of how the data is taken, and are not real (image credit: ESO/ESPRESSO team)
- ESPRESSO can detect tiny changes in the spectra of stars as a planet orbits. This radial velocity method works because a planet’s gravitational pull influences its host star, causing it to “wobble” slightly. The less massive the planet, the smaller the wobble, and so for rocky and possibly life-bearing exoplanets to be detected, an instrument with very high precision is required. With this method, ESPRESSO will be able to detect some of the lightest planets ever found.
- The test observations included observations of stars and known planetary systems. Comparisons with existing HARPS data showed that ESPRESSO can obtain similar quality data with dramatically less exposure time.
- Although the main goal of ESPRESSO is to push planet hunting to the next level, finding and characterizing less massive planets and their atmospheres, it also has many other applications. ESPRESSO will also be the world’s most powerful tool to test whether the physical constants of nature have changed since the Universe was young. Such tiny changes are predicted by some theories of fundamental physics, but have never been convincingly observed.
• September 14, 2015: Engineers at ESO have recently completed the difficult process of aligning the grating. The production and alignment of this component is one of the key ESO contributions to the ESPRESSO project. The grating is the largest ever assembled at ESO, and its length matches the largest echelle grating ever made — the 1.2 x 0.3 m grating for the HIRES spectrograph at the Keck 10 m telescope. 133)
- After its final alignment, the grating is fixed in a permanent mount. All its components are made of Zerodur (the same material that is used for the mirrors of the VLT) and will require no further adjustments, ever. This mounting technique was pioneered at ESO, and demonstrated to work on earlier instruments.
- When installed at ESO’s Paranal Observatory in Chile in 2016, ESPRESSO will combine the light from all four Unit Telescopes of the Very Large Telescope to create a virtual 16-meter aperture telescope. Its diffraction grating will split up the light into its component colors for analysis — spreading the light as a prism does, although relying on a different physical mechanism.
Figure 90: The huge diffraction grating at the heart of the ultra-precise ESPRESSO spectrograph — the next generation in exoplanet detection technology — is pictured undergoing testing in the cleanroom at ESO Headquarters in Garching, Germany (image credit: ESO, M. Zamani)
• August 7, 2013: ESO has signed contracts with Winlight Systems (France) for the construction of two cameras for the powerful new exoplanet-finding instrument, ESPRESSO. 134)
- ESPRESSO is an ultra-stable spectrograph that will be installed at ESO’s Paranal Observatory in Chile in 2016. It will be capable of combining light from all four Unit Telescopes of the Very Large Telescope (VLT) to create a virtual 16 meter aperture telescope.
- ESPRESSO is expected to allow astronomers to detect Earth-like planets around nearby stars using the radial velocity method . It will also have many other science applications, including the search for possible variations of the constants of nature at different times and in different directions through the study of the light from very distant quasars.
- The new contract is for the provision of the two refractive cameras, one for the red and one for the blue parts of the spectrum. These are vital components of the instrument. Utilizing highly non-spherical surfaces and novel design principles, they achieve excellent image quality over a large field with only three optical elements.
Figure 91: Engineering rendering of the ESPRESSO instrument (image credit: ESO)
HAWK-I (High Acuity Wide field K-band Imager)
HAWK-I is a cryogenic instrument, a wide field K-band imager, which was installed on the adapter/rotator of one of the Nasmyth foci of the VLT Unit Telescope 4 (UT4) as shown in Figures 92 and 93. HAWK-I is equipped with a mosaic of four 2 k x 2 k arrays and operates from 0.85 -2.5 µm over 7.5 arcmin x 7.5 arcmin with 0.1 arcsec pixels. A novel feature is the use of all reflective optics that, together with filters of excellent throughput and detectors of high quantum efficiency, has yielded an extremely high throughput. 135) 136)
Table 3: An overview of the HAWK-I characteristics
With the exception of the entrance window, the optical design is based on an all reflective configuration. The purpose of the optical configuration is to adapt the F-number of the input beam to the pixel field-of-view requirement (0.1 arcsec/pixel) and to limit the stray light reaching the detector by means of a cold field stop located at the entrance of the instrument and the third mirror acting as a cold pupil stop.
Just before the light reaches the detectors, two filter wheels allow the insertion of Broad Band (Y, J, H, Ks) and NarrowBand (interstellar lines and cosmological) filters.
The HAWK-I focal plane is equipped with a mosaic of four 2 k x 2 k Rockwell HgCdTe MBE (Molecular Beam Epitaxy) HAWAII 2 RG arrays. The packaging of this 2 x 2 mosaic detector is provided by GL Scientific re-using the design developed for the JWST program. The acquisition system is based on the IRACE (Infrared Array Control Electronics) system developed at ESO.
Figure 92: The HAWK-I instrument mounted on the telescope's Nasmyth (side) port. HAWK-I is attached on Yepun, Unit Telescope number 4 of ESO's Very Large Telescope and saw First Light on the night of 31 July 2007. HAWK-I covers about 1/10th the area of the Full Moon in a single exposure. It is uniquely suited to the discovery and study of faint objects, such as distant galaxies or small stars and planets (image credit: ESO)
Figure 93: Cut through HAWK-I for an optical and mechanical overview. Blue: optical components; black: cold assembly, filter wheels, detector assembly; green: radiation shield; red: vessel structure, cryogenic components, electronic rack (image credit: ESO)
HAWK-I performance: The 7.5' x 7.5'
FOV of HAWK-I is covered by four Hawaii-2RG chips of 2048 x 2028 pixels
each (1 pixel corresponds
Finally, due to necessary baffling in the all-reflective optical design of HAWK-I, some vignetting at the edges of the field has turned out to be inevitable due to positioning tolerances of the light baffles. The vignetting measured on sky is summarized in Table 4. Note that although the +Y edge vignetting is small in amplitude, it extends to around 40 pixels at <10%.
Figure 94: HAWK-I field-of-view coverage by the detector mosaic. Left: the layout of the field of view on the sky – note the small gap of ~15'' between the four detectors. Right: the relative orientation of the four detectors with the average gap size given in pixels (image credit: ESO)
Table 4: HAWK-I field-of-view vignetting. Note: The last column represents the maximum extinction of a vignetted pixel, i.e. the percentage of light absorbed in the pixel row or column, with respect to the mean of the field.
GRAAL (Ground-layer AOM (Adaptive Optics Module) Assisted by Lasers):
HAWK-I is also designed to work in the future with a GRAAL as part of the AOF (Adaptive Optics Facility) for VLT. GRAAL) will improve the quality of HAWK-I images reducing by 12 % in Y and 21% in Ks band the diameter collecting 50% of the energy for 1 arcsec visible seeing conditions over the entire 7.5 arcmin x 7.5 arcmin FOV. 137)
The commissioning instrument for the DSM is a submodule of GRAAL labeled MCM (Maintenance & Commissioning Mode) and will be implemented inside the AO module. One 40 x 40 SH (Shack-Hartmann) retractable WFS (Wave Front Sensor) and its associated optical system will be mounted on the AO structure and will use NGS (Natural Guide-Star) on-axis for wavefront sensing. The AO module consists of the following units (Figure 95):
Figure 95: Mechanical design of GRAAL. Left: view from the Hawk-I side; Right: view from the Nasmyth flange side (image credit: ESO)
As of 2017, GRAAL is part of AOF and associated with the DSM (Deformable Secondary Mirror) and the 4 Laser Guide Star Facility (4LGSF). It defines an AO system developed to increase the performance of the HAWK-I instrument. Commissioning is proceeding and Science Verification is planned for January 2018, for which applications are invited. 138)
GALACSI (Ground Atmospheric Layer Adaptive Optics for Spectroscopic Imaging):
GALACSI is part of the AOF (Adaptive Optics Facility), and associated with the DSM and the 4LGSF it defines an AO system developed to increase the performance of the MUSE (Multi Unit Spectroscopic Explorer) instrument, a panoramic integral-field spectrograph working at visible wavelength built by a consortium led by CRAL (Centre de Recherche Astrophysique de Lyon).The system GALACSI combined with the MUSE instrument is dubbed MUSE facility.
GALACSI and GRAAL benefit from several common developments for the AOF and other applications. Specifically these are the DSM and the ASSIST (Adaptive Secondary Setup and Instrument Simulator (AOF test bench)), 4-LGSF, NGC (New General detector Controller) WFS camera and the SPARTA (Real Time Computer platform for the AOF).
FLAMES (Fiber Large Array Multi Element Spectrograph)
LAMES is the multi-object, intermediate and high resolution spectrograph of the VLT. Mounted at the Nasmyth A platform of UT2,FLAMES can access targets over a large corrected field of view (25 arcmin diameter). It consists of three main components: 139)
• A Fiber Positioner (OzPoz) hosting two plates: while one plate is observing the other positions the fibers for the subsequent observations, therefore limiting the dead time between one observation and the next to less than 15 minutes, including the telescope preset and the acquisition of the next field.
• A medium-high resolution optical spectrograph, GIRAFFE (Fiber-fed multi-object spectrograph and part of the VLT) FLAMES facility), with three types of feeding fiber systems : MEDUSA, IFU, ARGUS.
• A link to the UVES (V-Visual Echelle Spectrograph) instrument (Red Arm) via 8 single fibers of 1 arcsec entrance aperture.
Special observing software (FLAMES OS) coordinates the operation of the different subsystems, also allowing simultaneous acquisition of UVES and GIRAFFE observations. For combined observations, the exposure times for UVES and GIRAFFE do not need to be the same. Note that it is not possible to observe simultaneously in two GIRAFFE modes, or to observe the same target simultaneously with the two spectrographs.
• GIRAFFE is a medium-high (R=5500-65100) resolution spectrograph for the entire visible range, 370-950 nm. It is equipped with two gratings and several filters are available to select the required spectral range. Five additional fibers allow simultaneous wavelength calibration of every exposure. Each object can be only observed in one, or a fraction of a single echelle order at once. The fiber system feeding GIRAFFE consists of the following components:
- The MEDUSA fibers, which allow up to 132 separate objects(including sky fibers) to be observed in one go. Two separate sets of MEDUSA fibers exists, one per positioner plate. Each fiber has an aperture of 1.2 arcsec on the sky.
- The IFU (Integral Field Unit): each deployable IFU consists of a rectangular array of 20 microlenses of 0.52 arcsec each,giving an aperture of 2 x 3 arcsecs. For each plate there are 15 IFU units dedicated to objects and another 15 dedicated to sky measurements. In the latter, only the central fiber is present.
- ARGUS: the large integral field unit ARGUS is mounted at the center of one plate of the fiber positioner and consists of a rectangular array of 22 x 14 microlenses. Two magnification scales are available: ``1:1'' with a sampling of 0.52 arcsec/microlens and a total aperture of 11.5 x 7.3 arcsec, and ``1:1.67'' with 0.3 arcsec/microlens and a total aperture of 6.6 x 4.2 arcsec. In addition, 15 ARGUS sky fibers can be positioned in the 25 arcmin field.
GIRAFFE is equipped with one 2 x 4K EEV CCD (15 µm pixels),with a scale of 0.3 arcsec/pixel in MEDUSA, IFUs and ARGUS direct mode,and a scale of 0.15 arcsec/pixel in the enlarged ARGUS mode. GIRAFFE is operated with 39 fixed setups (31 high resolution + 8 low resolution modes).
• UVES is the high resolution spectrograph at UT2 of the VLT (see Section6.4). It was designed to work in long slit mode but it has been possible to add a fiber mode (6 to 8 fibers, depending on setup and/or mode) fed by the FLAMES positioner to its Red Arm only. Only the three standard UVES Red setups are offered, with central wavelength of 520,580 and 860 nm, respectively.
The standard readout mode of FLAMES-UVES is 225 kHz (unbinned) which ensures low readout noise. As of P76 a high-speed readout mode (625 kHz, unbinned, low gain) with increased readout noise but less overheads is offered in visitor mode only.
With an aperture on the sky of 1 arcsec, the fibers project onto 5 UVES pixels giving a resolving power of 47000. For faint objects and depending on the spectral region, one or more fibers can be devoted to recording the sky contribution. In addition, for the 580 nm setup only, a separate calibration fiber is available to acquire simultaneous ThAr (Thorium Argon) calibration spectra. This allows very accurate radial velocity determinations. In this configuration, 7 fibers remain available for targets on sky.
FORS2 (FOcal Reducer/low dispersion Spectrograph 2)
FORS2 is a multi mode (imaging, polarimetry, long slit and multi-object spectroscopy) optical instrument mounted on the UT1 Cassegrain focus of VLT. FORS2 works in the wavelength range 330-1100 nm. Two different magnifications can be used with pixel scales of 0.25''/pixel (with the Standard Resolution collimator) and 0.125''/pixel (with the High Resolution collimator). The corresponding field sizes are 6.8' x 6.8' and 4.25' x 4.25', respectively. The two different magnifications are chosen by selecting one of two different collimators, hence each magnification has to be calibrated independently. An unbinned CCD readout mode is only offered for applications that specifically require it and must be explicitly requested in the proposal. 140)
Imaging: FORS2 offers imaging with a wide range of broad- and narrow-band filters. The narrow-band filters are exchangeable and chosen from a large range available filters depending on the user-request. It is also possible to use the jaws of the MOS unit as occulting bars to avoid saturation by unwanted bright objects.
Spectroscopy: FORS2 has a number of grisms available with different resolutions, including a number of high-throughput Volume-Phased Holographic (VPH) grisms.
Long-Slit (LSS) mode: FORS2 has 9 long-slits with fixed widths of between 0.3'' and 2.5''.
Moveable Slitlets (MOS) mode: FORS2 has a set of 19 pairs of arms that can be moved into the focal plane to form slitlets with user-defined widths.
Spectroscopic Mask (MXU) mode: In addition, FORS2 offers the possibility to insert in the focal plane a mask where slits of different length, width and shape can be cut with a dedicated laser cutting machine. Up to 10 masks can be mounted in a mask unit inside the instrument and each mask can have up to 470 slits, depending on the grism and filter used. The FIMS tool must be used for Phase 2 preparation of the mask cutting files. Performance in this mode is equivalent to that of the standard MOS mode.
Polarimetry: The polarimetric modes allow the measurement of linear and circular polarization, both for direct imaging (IPOL) and spectroscopy (PMOS). The position angle and degree of the linear polarization or of the circular polarization of an object are determined by using a remotely controlled rotatable lambda/2- or lambda/4-plate in front of the Wollaston prism.
Note: a field-dependent instrumental polarization pattern was discovered in the FORS1 linear polarization mode. This spurious polarization field shows a high degree of axial symmetry and smoothly increases from less than 3x10e-4 on the optical axis to 7x10e-3 at a distance of 3 arcmin from it (V band). The problem is yet to be characterized on FORS2, but it is likely it will have the same characteristics.
Detector: Two detector systems are available for FORS2. The first is a detector consisting of a mosaic of two 2k x 4k MIT CCD (15 µm pixels), which provides excellent red sensitivity (> 750 nm) and very low fringing. The second is a mosaic of two 2k x 4k E2V CCDs (15 µm pixels), which was formerly the FORS1 (post-upgrade) detector. This is very sensitive in the blue range (< 500 nm) but shows a lot of fringes above 650 nm. The E2V detector is currently only available in Visitor Mode and must be requested at phase I.
Table 5: FORS2 imaging modes
The direct imaging "Mag-limit" is the broad band magnitude calculated for a point source of zero color (A0V star) which would give a S/N of 5 in one hour with dark sky, clear conditions, a seeing FWHM of 0.8'' and an airmass of 1.2. The U, B, V magnitudes are calculated using the broadband filters of the standard instrument configuration.
Table 6: FORS2 spectroscopic modes
Legend to Table 6:  In longslit spectroscopy the slit is chosen out of a set of 9 slits with fixed width between 0.3'' and 2.5''.  In multiobject spectroscopy one may have 19 slitlets of length alternating between 20'' and 22''.
The spectroscopic Mag-limit given in the table above are the R-band magnitudes of a point source of zero color which would give a S/N of5 per pixel at 650 nm (grisms 150 I and 600 R) in the continuum in one hour with dark sky, clear conditions, a seeing FWHM of 0.8'', an airmass of 1.2, and using a 1.0" slit and the SR-collimator. The two limits given are for the two representative resolutions. The limits on spectropolarimetry are those which for linear/circular polarization allow a 1% accuracy in determination of degree of polarization for one hour of total integration time.
FORS Instrumental Mask Simulator (FIMS): To prepare precise target acquisitions at Phase 2, ESO provides the FIMS software tool. FIMS is required when using FORS2 in several spectroscopic modes, and is also used to prepare occulting bar imaging and spectropolarimetry observations. Phase 1 proposers who wish to justify their time request by optimizing movable or MXU slitlet positions during Phase 1, may find it useful to download and install FIMS. Please refer to the FIMS page for instructions on how to install FIMS and to the FIMS User's Manual on how to use FIMS.
Accurate Astrometry or Pre-imaging Required: Highly accurate relative astrometry is required for any observing mode which in Phase 2 will make use of FIMS or blind offset acquisitions. The mask preparation with FIMS requires input images which are astrometrically corrected within the definitions and precision given below. DSS images will, in almost all cases, not be suitable for the task.
In general the relative astrometry must be known to better than 1/6 of the slit widths all over the field of view. Relative astrometry here means that the slit positions must be known relative to those of reference stars in the field of view with the given precision.
If images of adequate quality are not available, Phase 1 proposers must apply for pre-imaging defined as a separate run in the Phase 1 proposal and should be clearly marked as pre-imaging in "instrument configuration" section of the proposal. Failure to do so will, in case the program is approved for execution, result in the deduction of the time necessary for pre-imaging from the allocation destined to the main part of the project. As a rule, pre-imaging runs are carried out in Service Mode, even for programs whose main (spectroscopic) runs are conducted in Visitor Mode.
FOcal Reducer and low dispersion Spectrograph
"Of all instruments at Paranal, this one is the Swiss Army knife". This is the way Henri Boffin, the instrument scientist behind the FOcal Reducer and low dispersion Spectrograph 2 or FORS2, describes the instrument that is most in demand at ESO's Paranal Observatory. The key to success is that FORS2, installed on UT1 (Antu) of the Very Large Telescope (VLT), is able to study many different astronomical objects in many different ways. 141)
For example, FORS2 can take images of relatively large areas of the sky with very high sensitivity. No wonder that some of the most iconic photos taken with the VLT used this instrument (see eso9845d, eso9948f, eso0202a, eso0338a, eso0338c, eso0617a, and more recently eso1244a and eso1348a).
But FORS2 can also take spectra of one (eso9920r), two or even several tens of objects in the sky simultaneously (eso0223b). "When used as a spectrograph, FORS2 disperses the light into very sophisticated rainbows that help astronomers study chemical composition or estimate the distances of remote objects," says Boffin.
And this is not all. FORS2 can also measure the polarization of light and is therefore used at the VLT to determine whether some astronomical objects have strong magnetic fields.
Observations with FORS2 and its twin brother FORS1 (decommissioned in 2009) have together led to almost 1800 papers to be published in scientific journals as of 2014, with an average of about 100 scientific papers per year. "Basically, whatever you can think of, you can do it with FORS2. Apart from making the coffee the astronomers need at night!"
Science highlights with FORS
• Constraining size, shape and color of first-observed interstellar asteroid (eso1737)
• Observations of first light from gravitational wave source (eso1733)
• First detection of titanium oxide in an exoplanet (eso1729)
• Observations of neutron star that possibly confirm 80-year-old prediction about the vacuum (eso1641)
• Observations of galaxy clusters (eso1548)
• Alignments between supermassive black hole axes and large-scale structure revealed (eso1438)
• FORS helps explain shape of planetary nebula (eso1244)
• FORS was used to spot "Dark Galaxies", an early phase of galaxy formation, which are essentially gas-rich galaxies without stars (eso1228)
• VLT "rediscovered" life on Earth (eso1210)
• Comet Halley in the cold – the most distant view of a regular visitor (eso0328)
• Cosmological gamma-ray bursts and hypernovae linked by FORS1 and FORS2 observations (eso0318)
Figure 96: ESOcast 190: Chile Chill 12 — Fire in the Heavens: In Chile Chill 12 the evocative tunes of ESO's Music Ambassador Dimitris Polychroniadis are set to stunning visuals from ESO's expansive video archive. Sit back, relax, and enjoy a stellar walk through the Universe on the border between science and art (ESO, Published on 11January 2019) 142)
Figure 97: This photo shows the twin instruments, FORS2 at KUEYEN (in the foreground) and FORS1 at ANTU, seen in the background through the open ventilation doors in the two telescope enclosures. Although they look alike, the two instruments have specific functions (image credit: ESO) 143)
Figure 98: This intriguing new view of a spectacular stellar nursery IC 2944 is being released to celebrate a milestone: 15 years of ESO's Very Large Telescope. This image also shows a group of thick clouds of dust known as the Thackeray globules silhouetted against the pale pink glowing gas of the nebula. These globules are under fierce bombardment from the ultraviolet radiation from nearby hot young stars. They are both being eroded away and also fragmenting, rather like lumps of butter dropped onto a hot frying pan. It is likely that Thackeray's globules will be destroyed before they can collapse and form new stars (image credit: ESO) 144)
MATISSE (Multi AperTure mid-Infrared SpectroScopic Experiment)
MATISSE is a second-generation interferometry instrument for the ESO VLTI (Very Large Telescope Interferometer). The interconnection of three or four telescopes makes it possible to capture visibilities and closure phases and allows pictures in the mid infrared range to be reconstructed. The maximum baseline length of 200 m (the distance between two telescopes) makes for a high resolution in the generated images. 145) 146) 147)
MATISSE operates in the wavelength range from 2.8 µm to 13 µm. The combined light of the telescopes is spectrally dispersed in order to measure the wavelength dependence of the visibility and closure phase. Two imaging sensors are used to cover the wide wavelength range. One imaging sensor (HAWAII/2RG) covers the wavelength range of the L and M bands (2.8 µm to 5.2 µm), and the other imaging sensor (Aquarius) covers the N band (8 µm to 13 µm). The HAWAII 2RG imaging sensor features low readout noise, while the Aquarius imaging sensor provides a high readout rate. The Aquarius imaging sensor provides precise measurements despite the high thermal background radiation.
- INSU: Observatoire de la Cote d'Azur and University of Sophia-Antipolis, Nice, France
- MPIA (Max-Planck-Institut für Astronomie), Heidelberg, Germany
- MPIfR (Max-Planck-Institut für Radioastronomie), Bonn, Germany
- NOVA (Netherlands Research School for Astronomy), Leiden, The Netherlands
- Institut für Theoretische Physik und Astrophysik, University of Kiel, Germany
- Institut für Astronomie, University of Vienna, Austria.
Scientific objectives: 148)
The objective of MATISSE is image reconstruction. It will extend the astrophysical potential of the VLTI by overcoming the ambiguities existing in the interpretation of simple visibility measurements. MATISSE will measure closure phase relations thus offering an efficient capability for image reconstruction.
The unique performance of MATISSE is partly related to the existence of the four large apertures of the VLT (UTs) that permits to push the sensitivity limits to values required by selected astrophysical programs such as the study of AGN (Active Galactic Nuclei) and protoplanetary discs.
Moreover, the evaluated performance of MATISSE is linked to the availability of ATs (Auxiliary Telescope for the VLTI) which are relocatable in position in about 30 different stations allowing the exploration of the Fourier plane with up to 200 meters baseline length. Key science programs using the ATs cover for example the formation and evolution of planetary systems, the birth of massive stars as well as the observation of the high-contrast environment of hot and evolved stars.
During Phase A, three constituents of the planetary systems were identified for which MATISSE will bring new insight:
• Protoplanetary disks (T Tauri, HAeBe) and planetary debris disks (beta Pic type),
• Minor bodies of our solar system: main belt asteroids and the comets,
• Young giant planets and so-called hot Jupiter-like planets.
The AMBER (Astronomical Multi-BEam combineR (VLTI Instrument)) and MIDI (Mid-infrared Interferometric Instrument (VLTI)) instruments have started to observe the brightest protoplanetay disks in the infrared sky, approximately a dozen objects. The current capabilities of other observatories are similar (see e.g. Keck Interferometer).
In addition, in our own solar system, a few asteroids have been observed by MIDI (Delbo et al. 2009, ApJ 694, 1228), demonstrating the feasibility to characterize solar system minor bodies with interferometry.
However, for extrasolar planet detections and/or characterization, interferometry has not reached enough dynamic range so far to successfully observe any of them (see e.g. Matter et al. 2010, A& A 515, 69 for MIDI, or Millour et al. 2008, SPIE 7013, 41 and Absil et al. 2010 in press for AMBER).
The second important science topic is active galactic nuclei. The nominal MATISSE sensitivity in blind mode at N-band is 0.6 Jy in 4-telescope mode, similar to the nominal MIDI sensitivity, although to date MIDI has been used to obtain correlated fluxes of AGNs down to 0.17 Jy. The nominal L-band sensitivity from the performance analysis is 0.1 Jy. For MIDI observations, various lists of AGNs have been assembled, often based on the list of Veron-City and Veron 2006 (A& A 455, 773).
MATISSE is a mid-infrared spectro-interferometer combining the beams of up to 4 UTs/ATs of the VLT Interferometer. The number of combined beams is 4. The instrument will be able to operate with 3 or 2 beams. The instrument sensitivity, sampling and throughput are optimized for L- and N-band. The L-band is specified from 3.2 to 3.9 µm and the N-band from 8.0 to 13.0 µm. MATISSE will operate also in M-band, from 4.5 to 5.0 µm. The L-, M- and N-bands can be observed simultaneously.
The instrument will be able to observe with different spectral resolutions. 2 spectral resolutions are possible in N-band (R ~ 30, R ~ 200) and 3 in L&M-bands (R ~ 30, R ~ 500 for L- and M-band, R ~1000 for L-band only). Due to readout time, the full simultaneous coverage of the L- & M- bands in low and medium resolutions, and the full coverage of the L- band in high spectral resolution require an external fringe tracker.
MATISSE will measure: coherent flux, visibilities, closure phases and differential phases. Differential visibilities can also be derived. These quantities will be measured as a function of the wavelength in the selected spectral bands and resolutions.
MATISSE will have an imaging mode (2D image observation without dispersion) for field acquisition and a non-interferometric imaging mode (photometric channels) for flux measurements. It will have also internal devices allowing detector calibration (flat-fielding, bad pixel map), relative flux calibration, wavelength calibration, and instrumental contrast measurement.
MATISSE is a four-beam experiment with a multi-axial global combination. The interferometric beam and the photometric beams receive, respectively, the I and P fraction of the incoming flux (observations without photometric channels are also possible). For an observation with 4 telescopes with photometric channels, 5 images are produced on the detector (4 photometric channels and the interferometric one).During observations with 4 telescopes, the interferogram (in each spectral channel) contains a pattern with 6 fringe periods and is dispersed in the spectral direction. The spatial size of this interferometric channel is larger than the photometric ones in order to optimize the sampling of the 6 fringe structures. The beam combination is made by the camera optics. At this level, the beam configuration is non redundant (separation B between beams equal to 3D, 9D and 6D where D is the spatial diameter of the beam) in order to avoid crosstalk between the fringe peaks in the Fourier space.
The Fourier transform of each spectral column of the interferometric image is thus composed by 6 fringe peaks centered at different frequencies Bij/λ (3 D/λ, 6 D/λ, 9 D/λ, 12 D/λ, 15 D/λ, 18 D/λ) and a low frequency peak that contains the object photometry and the thermal background coming from the 4 telescopes. In order to measure the coherent fluxes with a good accuracy, the design of MATISSE is based on the use of spatial filters, including image and pupil stops.
In order to measure closure and differential phases with a good accuracy, a beam commutation can be made in order to reduce the effect of the instrumental defects on the useful signal.
To measure the coherent fluxes and all the derived interferometric measures such as the differential visibility and phase and the closure phase, the key problem is to eliminate the cross talks between the low frequency peak and all the other peaks that introduce sensitivity of the fringe peaks to variations of the thermal background. Two methods are combined in MATISSE to ensure this result with a large margin: spatial modulation like in AMBER combined with temporal modulation like in MIDI. In addition, to measure the absolute visibility we also have to find the true source photometry. To do that, it is necessary to separate the stellar flux from the sky background, using chopping.
Some devices such as artificial sources, hot screen, optics for flat field or pupil visualization, special material for spectral calibration are implemented in the instrument in order to perform alignment, test, maintenance, calibration and acquisition operations. MATISSE will operate with 2 modes:
• High Sensitivity mode: this mode has no photometry and all photons are collected in the interferometric beam. This maximizes the sensitivity and also the SNR on the coherent flux and the differential and closure phases. Chopping is optional in this mode.
• Simultaneous Photometry mode: this mode uses photometry (2/3 of flux in the interferometric channel and 1/3 in the photometric ones) and chopping to measure the average source photometry and therefore extract the visibility from the coherent flux (the chopping period is longer than the coherence time and hence the chopping has no influence on the limiting magnitude). For an observation with 4 telescopes with photometric channels, 5 images are produced on the detector (4 photometric channels and the interferometric one).
MATISSE project development status (Ref. 148):
• Preliminary acceptance at Paranal Observatory, Chile: Scheduled for 2019
• First light on telescope: February 2018
• Preliminary acceptance Europe: September 2017
• Final Design Review, March 2012
• Optical and Cryogenics Final Design Review, September 2011
• Preliminary Design Review, December 2010.
Figure 99: The figure shows a 3D optical layout for the set of four beams inside the cryostat for the L-and M-bands (image credit: MATISSE Consortium)
Figure 100: The very complex instrument MATISSE during installation at Paranal Observatory (image credit: ESO, P. Horálek) 149)
Gravity is a four way beam combination second generation instrument for the VLTI (Very Large Telescope Interferometer ). Its main operation mode makes use of all four 8 m UTs (Unit Telescopes) to measure astrometric distances between objects located within the 2 arcsec FOV (Field of View) of the VLTI. With the sensitivity of the UTs and the ~10 µas (micro arcsec) astrometric precision, it will allow to measure orbital motions within the galactic center with unprecedented precision. Other modes of the instrument will allow imaging and the use of the 1.8 m Auxiliary Telescopes. 150)
GRAVITY was developed by a collaboration consisting of the Max Planck Institute for Extraterrestrial Physics (Germany), LESIA of Paris Observatory–PSL/CNRS/Sorbonne Université/Univ. Paris Diderot and IPAG of Université Grenoble Alpes/CNRS (France), the Max Planck Institute for Astronomy (Germany), the University of Cologne (Germany), the CENTRA–Centro de Astrofísica e Gravitação (Portugal) and ESO. The PI (Principal Investigator) of GRAVITY is Frank Eisenhauer.
GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular resolution and a collecting area of 200 m2. The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phase-tracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. 151)
Science Objectives: GRAVITY will carry out the ultimate empirical test to show whether or not the Galactic Center harbors a black hole (BH) of four million solar masses and will finally decide if the near-infrared flares from Sgr A* originate from individual hot spots close to the last stable orbit, from statistical fluctuations in the inner accretion zone or from a jet. If the current hot-spot interpretation of the near-infrared (NIR) flares is correct, GRAVITY has the potential to directly determine the spacetime metric around this BH. GRAVITY may even be able to test the theory of general relativity in the presently unexplored strong field limit. GRAVITY will also be able to unambiguously detect intermediate mass BHs, if they exist. It will dynamically measure the masses of supermassive BHs (SMBHs) in many active galactic nuclei, and probe the physics of their mass accretion, outflow and jets with unprecedented resolution. Furthermore, GRAVITY will explore young stellar objects, their circumstellar discs and jets, and measure the properties of binary stars and exoplanet systems. In short, GRAVITY will enable dynamical measurements in an unexplored regime.
GRAVITY provides high precision narrowangle astrometry and phase-referenced interferometric imaging in the astronomical K-band (2.2 µm). It combines the light from four Unit Telescopes (UTs) or Auxiliary Telescopes (ATs), measuring the interferograms from six baselines simultaneously. The instrument has three main components: the IR wavefront sensors; the beam-combiner instrument; and the laser metrology system.
The GRAVITY IR wavefront sensors will be mounted in the Coudé rooms of the UTs and will command the existing Multiple Application Curvature Adaptive Optics (MACAO) deformable mirrors. The system can work on either of the two beams (on-axis or off-axis) behind the PRIMA star separators. Any additional tip/tilt from the beam relay down to the VLTI laboratory will be corrected by a dedicated laser-guiding system. Low frequency drifts of the field and pupil will be corrected by GRAVITY's internal acquisition and guiding camera. The interplay of these systems will guarantee an unperturbed and seeing-corrected beam at the entrance of the beam-combiner instrument in the VLTI laboratory. The interferometric instrument will work on the 2 arsec (for UTs) or 4 arcsec (for ATs) VLTI field of view. Both the reference star and the science object have to lie within this field of view. The light of the two objects from the four telescopes is coupled into optical fibres for modal filtering, to compensate for the differential delay and to adjust the polarisation. The fibres feed two integrated optics beam combiners and the coherently combined light is dispersed in two spectrometers. A low resolution spectrometer provides internal phase- and group-delay tracking on the reference star, and thus enables long exposure times on the science target. Three spectral resolutions with up to R~4000 are implemented in the science spectrometer, and a Wollaston prism provides basic polarimetry.
GRAVITY will measure the visibility of the reference star and the science object simultaneously for all spectral channels, and the differential phase between the two objects. This information will be used for interferometric imaging exploring the complex visibilities, and for astrometry using the differential phase and group delay. All functions of the GRAVITY beam-combiner instrument are implemented in a single cryostat for optimum stability, cleanliness, and thermal background suppression. The internal path lengths of the VLTI and GRAVITY are monitored using dedicated laser metrology. The laser light is back-propagated from the beam combiner and covers the full beam up to the telescope spider above the primary mirror. GRAVITY will make use of high-speed IR photoncountingdetector arrays in both the adaptive optics systems and the fringe tracker. These devices do not suffer from high readoutnoise, which in current IR arrays is tenor more electrons per pixel at framerates of a few hundred Hz.
The GRAVITY system is not a monolithic instrument. 152) It is a collection of subsystems that aims to precisely control the wavefront of the incoming light and its path through the VLTI system before the actual combination of beams takes place and interference fringes are created. A unique aspect of GRAVITY, and the first time this is ever realized, is its ability to interfere the light coming from either a single astronomical source (single-field mode or on-axis) or from two sources simultaneously (dual-field mode or off-axis). In dual-field, the GRAVITY system can perform phase referenced observations supported by the accurate knowledge of the path length which is assessed by a laser system. In this mode of observation, the interferometric phase of the primary star is calibrated to that of the secondary against the detrimental influence of the atmosphere. This enables highly accurate angle measurements on sky and is the basis for GRAVITY's astrometric observing mode. In GRAVITY's imaging observing mode, dual-field observations allow to observe relatively faint targets and use a brighter star as the fringe-track star. The difference between the two modes of GRAVITY is the observation strategy and the use of the laser metrology to measure the light-path.
The subsystems of GRAVITY include:
• The IR wavefront sensing system CIAO (Coudé Infrared Adaptive Optics). CIAO is located in each of the UT coude room and it will operate with the deformable mirror of the MACAOs (Multiple Application Curvature Adaptive Optics).
• A polarization control system to counteract polarization effects in the VLTI. GRAVITY can work either in a split or a combined polarization mode.
• An active pupil guide system including LED sources mounted on each of the telescope spiders.
• A field-guide system to track the position of the source and ensure proper injection into mono-mode fibers.
The overall GRAVITY system interacts and works in harmony with the VLT-I system of telescopes and delay lines. The operation of various GRAVITY subsystems is transparent to the user.
The single and dual field modes. The primary unit of GRAVITY is the Beam Combining Instrument (BCI) that performs the acquisition process and provides the interferometric fringes. The GRAVITY BCI is cryogenically cooled and physically located in the VLT-I laboratory. Within the BCI cryostat, a field is separated and two stars find their way into either the science channel or the fringe-tracking channel. In single-field the FT channel and SC channel receive light from the same star which is split 50%-50%. The GRAVITY fringe-tracker (FT) forms an integral part of the observational approach, i.e. GRAVITY science observations are always done with active fringe-tracking. The FT fringe position is analyzed at a frequency of approximately a kHz in order to correct for the atmospheric and instrumental piston (i.e. a residual optical path difference between beams) by modulating piezo mounted mirrors within the instrument. The FT star thus allows longer detector integration times in the science channel (SC, up to 60 seconds) without compromising the contrast of the fringe pattern. In dual field mode (astrometric or imaging), the fringes of one star are formed in the science channel, and those of the other star in the fringe tracker channel. The two astronomical sources can have an angular separation up to 2 arcsec when observing with the UTs, or 4 arcsec with the ATs. For separations up to 0.4 arcsec and 1.6 arcsec on the UTs and ATs, respectively, the dual-field "on-axis" set-up us used. In this case, the light is split using a beam splitter so that only 50% of the light from each source are injected into the FT and SC channels. In the dual field "off-axis" setup, for larger separations, a roof-top mirror is used and 100% of the light of the FT and SC objects reach the respective fibers.
GRAVITY delivers spectrally dispersed interference fringes that allow stellar interferometry. The FT spectrometer always operates at low spectral resolution (R ~ 22). Taking advantage of the longer integration times, the science channel records the entire K-band at each of the three implemented spectral resolutions of R ~ 22, 500 and 4000. The fringes give access to interferometric quantities such as absolute and differential visibility, spectral differential phase and closure phase. These quantities provide information of physical phenomena at a spatial resolution that can reach 2 mas (depending on the VLT-I baseline), as well as time-resolved differential astrometry at the exquisite accuracy of a few tens of µas.
Figure 101: The Beam Combining Instrument (BCI) with its sub-components labelled (image credit: ESO)
Astrometry and phase-referenced imaging: Astrometry aims at measuring the separation between two targets, whereas phase-referenced imaging aims at measuring the phase of the SC target in reference to the FT target. Both techniques use the dual field mode of GRAVITY and rely on the laser metrology to make the connection between the SC and FT measurements. For astrometry, the metrology measures the evolution of the differential optical path difference as a function of time, which through the interferometer baselines can be converted into a separation between the targets. For phase-referenced imaging the metrology is used to relocate the FT target reference to a separation offset close to the SC target, which helps for producing images. As the laser metrology provides relative optical path measurements only, a metrology zero must be determined for both techniques to work. This determination is typically achieved by swapping a pair of targets (i.e. reversing the sign of the separation), which separates the sidereal metrology signal that changes sign, from the constant metrology zero. When swapping is not directly possible on the target pair of interest, due to a very faint SC target not observable with the FT, the metrology zero determination can be carried out on a more balanced nearby pair. Other than the requirement to calibrate the metrology zero-points, astrometry and phase-referenced imaging observations are similar to dual-field observations and have the same observing constraints.
Whereas the instrument itself is thus perfectly capable of carrying out astrometric and phase-referenced observations, the astrometric part of the GRAVITY pipeline is still under heavy development. The production of separations and referenced phases still rely on custom tools developed by a small group of astrometry-minded astronomers gravitating around the instrument. In addition, the performance of these modes is not fully characterized.
MUSE (Multi Unit Spectroscopic Explorer)
MUSE is the latest of the second-generation instruments to be installed on Yepun (UT4), the fourth Unit Telescope of the Very Large Telescope at the Paranal Observatory. The instrument was built under ESO contract by the MUSE consortium consisting of AIP Potsdam, CRAL Lyon, ESO, EHT Zürich, IRAP Toulouse, Leiden Observatory, and IAG Göttingen. 153)
Like SINFONI, MUSE is an integral field spectrograph (IFS). An IFS allows you to observe the entirety of an astronomical object in one go, and for each pixel measures the intensity of the light as a function of its color, or wavelength. The resulting data is a 3D set where each pixel of the image has a full spectrum of the light. MUSE splits the field of view into 24 individual image segments or channels which are each split further into 48 slices or "mini slits", giving a total of 1152 mini slits. Each set of 48 mini slits is injected into a spectrograph, which disperses the light into its constituent colors, and MUSE measures over 4000 of these colors! From this, the 3D image is created.
"MUSE has been built with the intention of studying the content and processes going on in the very early Universe, when the first stars and galaxies were forming," explains Fernando Selman, Instrument Scientist for MUSE. "Closer in time and space, MUSE will map the dark matter distribution in clusters of galaxies using the gravitational microlensing effect on background galaxies." MUSE will also provide detailed information about the internal dynamics of many classes of galaxies with unprecedented detail. "It has already been used to study the Sombrero Galaxy in Virgo, and, in the same cluster, a recently discovered new type of object — a galaxy being destroyed after falling into the cluster and encountering the cluster's hot gaseous corona," continues Fernando. An image of this galaxy is shown on this page. The stars themselves found within bigger objects will also be a focus of study — impressive vistas of the Tarantula Nebula and its huge collection of massive young stars have been obtained during the testing phase, and an enormous mosaic of the Orion Nebula has been produced.
With nearly 400 million pixels to be processed in realtime, MUSE has presented Paranal Observatory with new computation and communication challenges. During the first phase of commissioning alone, nearly half a billion spectra were produced!
MUSE and adaptive optics: MUSE entered a new era in 2017 with the advanced capabilities of the Adaptive Optics Facility (AOF). The AOF with Artificial laser stars (4LGSF); deformable active mirrors (ann16078); multiple wavefront sensors GALACSI will develop the full potential of MUSE and is comparable to moving the telescope 900 meters above the Paranal summit, a height free of the effects of the most turbulent layers of the atmosphere, giving much clearer images than before.
MUSE was will be the first of the VLT's second generation instruments to taste these new capabilities. The module GALACSI will routinely deliver images to MUSE at optical wavelengths of a quality that was previously possible only on the few clearest nights of the year if at all.
"MUSE with the AOF system will allow for the completion of surveys of the remote Universe with unique sensitivity, permitting studies of the earliest galaxies and large scale structures," says Selman. "During its lifetime we expect many great contributions from this instrument for a wide range of astronomical investigations".
Instrument: MUSE has a modular structure composed of 24 identical IFU modules that together sample, in Wide Field Mode (WFM), a near-contiguous 1 squared arcmin field of view. Spectrally the instrument samples almost the full optical domain with a mean resolution of 3000. Spatially, the instrument samples the sky with 0.2 arcseconds spatial pixels in the currently offered Wide Field Mode with natural seeing (WFM-noAO). 154) 155) 156)
MUSE offers the GLAO (Ground Layer Adaptive Optics) mode of the VLT Adaptive Optics Facility (AOF) via the GALACSI adaptive optics module, offering an AO corrected 1'x1' field of view with 0.2 arcseconds sampling (WFM-AO). Starting P103 offers a 7.5 x 7.5 arcsec2 LTAO corrected field of view sampled at 0.025"/pixel (NFM-AO).
• Wavelength range: 480-930 nm (nominal); 465-930 nm (extended)
• Detectors: 24 x 4 k x 4 k MIT/LL CCD
• AO type: Ground Layer, 4 x (5-10 W) lasers
• Throughput WFM (Wide Field Mode): 14 % (480 nm) 35 % (750 nm) 14 % (930 nm)
• Throughput NFM (Narrow Field Mode): 13 % (480 nm) 26 % (750 nm) 11 % (930 nm)
Figure 102: MUSE is a second-generation instrument in development for ESO's Very Large Telescope (VLT), due to begin operation in 2012. MUSE is an extremely powerful and innovative 3D spectrograph with a wide field of view, providing simultaneous spectra of numerous adjacent regions in the sky. The instrument is fed by a new multiple-laser adaptive optics system on the VLT. The development of MUSE has been a key experience for the next generation instruments, for both the VLT and the planned Extremely Large Telescope (ELT). The VLT instrumentation program is the most ambitious ever conceived for a single observatory (image credit: ESO)
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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).
Mission Status Instruments ESPRESSO Hawk-I FLAMES FORS2 MATISSE GRAVITY MUSE References Back to top