From ESOblog (EU): “The VLT Interferometer-20 years of scientific discoveries” 

From ESOblog (EU)


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European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) (CL)

26 November 2021


Twenty years ago, on 29 October 2001, two of the 8.2-m telescopes at ESO’s Very Large Telescope (VLT)[below] were linked for the first time into a huge virtual telescope: the VLT Interferometer (VLTI)[below]. In the previous two articles in this series we heard from some of the people who made this feat possible. In this final post we will tell you about some of the amazing scientific results obtained with the VLTI, and what the future holds for this unique facility.

Despite their huge sizes for human standards, astronomical objects are so far away that studying them in detail requires incredibly large telescopes. The largest optical telescopes have mirrors 8-10 m wide, and ESO’s Extremely Large Telescope [below] currently under construction in Chile will have a 39-m mirror, the largest of its kind. But discerning the smallest details on cosmic objects often requires even bigger telescopes, well over 100 m wide.

Fortunately, a technique called interferometry allows us to circumvent this problem by linking several telescopes into a single virtual one as large as the separation between them. ESO’s Very Large Telescope Interferometer does precisely that: it combines the light of up to four 8.2-m Unit Telescopes (UT) or four 1.8-m Auxiliary Telescopes, the equivalent of a 130-m or 200-m wide telescope, respectively.

The VLTI has enabled a wealth of astronomical discoveries, from directly studying the atmosphere of exoplanets to testing General Relativity around the supermassive black hole at the centre of the Milky Way. In this article we will learn about some other fantastic results as told by their protagonists, and how ESO’s VLTI is currently being improved even further.


Sebastian Hoenig
Credit: University of Southampton Communications & Marketing

Sebastian Hoenig is a Professor of Observational and Computational Astrophysics at The University of Southampton (UK), where he also serves as Head of the Astronomy Group.

All the big galaxies in the Universe harbour a supermassive black hole in their centre with masses of millions to billions times that out our Sun. How did they grow so big? We don’t quite understand this yet, but we do know that these black holes undergo episodes of growth when they gobble dust and gas from their host galaxies. This material gets very hot and shines as bright or brighter than all the stars in the galaxy around it. This is what we call Active Galactic Nuclei (AGN) and we observe them to understand how supermassive black holes grow.

To see gas and dust falling onto supermassive black holes we need to discern tiny scales, smaller than the distance between the Sun and the nearest star, in galaxies that are tens or hundreds of millions of light years away. The VLTI is the only way we can test our hypotheses of how supermassive black holes grow.


Artist’s impression of the surroundings of the supermassive black hole in NGC 3783, a spiral galaxy 135 million lightyears away. This image is based on VLTI data collected by Hoenig and his team, which showed that, in addition to the dusty doughnut around the black hole, there is also dust being blown out above and below it.
Credit: M. Kornmesser/ ESO.

The VLTI observes at infrared wavelengths, and can detect warm dust that is mixed together with gas. We thought that this dusty gas forms a thick doughnut around the AGN. But in 2012 and 2013 we observed some AGN with the VLTI in great detail, and found that a lot of dust was present in a region where we didn’t expect it: above and below the doughnut. We now think that a significant amount of the dust is blown away by the strong radiation from the AGN. This dusty wind can carry material from very small scales away into the galaxy or even out of it. Without the VLTI, we wouldn’t have been able to see these dusty winds.

Observing AGN with the VLTI is often challenging as we are pushing the system to the faintest limit of what can be detected. It’s always great to work together with the telescope/instrument operators and the support astronomers to find the best settings that make everything work. Always worth the chocolate I bring along!

I am one of the co-investigators of GRAVITY+ [no image], an upgrade to the current GRAVITY instrument at the VLTI.


Among other results, GRAVITY tested Einstein’s General Relativity close to the supermassive black hole at the centre of our galaxy, a result that contributed to the 2020 Nobel Prize in Physics.

Sgr A* from ESO [Observatoire européen austral][Europäische Südsternwarte] (EU) (CL) VLT.

We are now improving the instrument to make it orders of magnitude more sensitive and open up interferometry to many new science topics. I am excited to see what new scientific ideas the ESO community can come up with once GRAVITY+ is available.

Rebeca García López. Credit: R. García López.

Rebeca García López

Rebeca García López is a Lecturer/Assistant Professor at The University College Dublin (IE), where she holds an Ad Astra Fellowship.

Stars are not eternal; like humans, they are born, evolve and die. Stars form within clouds of gas and dust, when a dense region within one of these clouds starts to collapse, and a disc of matter forms around the infant star. In this early phase, matter from this disc falls onto the star. With time, and as the material within the disc falls and disperses, planets will form.

The closest young stellar objects are more than 400 light years away, which makes it really hard to distinguish fine details in them. To observe an analogue of our Solar System 400 light years away, we would need a telescope around 50 m in diameter to distinguish planet Earth from the parent star. Building such a telescope poses many technological challenges, but interferometry allows us to circumvent this. With the VLTI we can achieve a resolving power equivalent to that of a 130-m telescope, which allows us to obtain incredible details of the inner regions of protoplanetary discs at orbits equivalent to that of the Earth or even closer to the parent star.

For a very long time astronomers suspected that young stars collect matter via their magnetic fields, and that this material falls towards the surface of the star at supersonic velocities. However, how matter from a planet-forming disc is channeled onto the stellar surface had never been observed before, because the nearest young star is so far away that it requires some of the biggest telescopes in the world, and very sophisticated instrumentation. In 2019 we used GRAVITY to observe hot gas around the star TW Hydrae, and found that its size and velocity matched what theoretical models predicted.

Paranal is like heaven for astronomers, much better than any 5 star hotel you could be in. There everything revolves around science, excellence, and state-of-the-art instrumentation. Every time I go there I feel like a child on Christmas morning. My fondest memory is the first time I observed with the VLTI using three of the four 8-m UT telescopes with the AMBER instrument. I felt like the master of the Universe. All those people just working for you: engineers, telescope operators, support astronomers, almost the full control room working for you! A lot of responsibility, but a lot of fun as well. Later on, I had the opportunity to use all four UT telescopes at the same time with GRAVITY, this time the full control room for you!

Jaques Kluska. Credit: J. Kluska.

Jacques Kluska

Jacques Kluska is a FWO senior postdoctoral fellow at Katholieke Universiteit Leuven [Katholieke Universiteit te Leuven](BE), as well as lead scientist of the Belgian VLTI expertise centre.

Exoplanets are planets that are orbiting one or several stars other than the Sun. Studying how exoplanets are born is also important to understand how planets in our Solar system formed, including Earth. This is key to understanding our origins, to estimate how common exoplanets like the Earth are, and which ones would be able to host life.

Planets form around young stars (less than 10 million years old), which are surrounded by a disc of dust and gas that looks like a pancake with the star in a little hole at its centre. Dust grains in this disc will grow and will come together to form planets. Earth-like planets are thought to form in the inner regions of such discs, close to the star. The VLTI has been able to observe not only the structure of discs where Earth-like planets form, but also determine the kind of dust grains we can find there, which are the building blocks of these future planets.

We recently obtained 15 images of such planet-forming discs using the VLTI. These images reveal the disc regions within 5 astronomical units –– 5 times the Sun-Earth distance –– from the central young star, with details as small as a tenth of an astronomical unit! In other words: we can see the environment in which future Earth-like planets may form. This achievement is the result of a long-term technological development that led to the construction of the first instrument that can combine four telescopes at the VLTI: PIONIER.

I started my PhD at the end of this process and was lucky to be in position to produce these images.

Images of planet-forming discs around 15 young stars obtained with PIONIER at the VLTI.
Credit: Kluska et al.

I’ve been to Paranal several times during my PhD. It was always super exciting and it was never difficult to switch to a night schedule because of the excitement to observe with the VLTI! Sometimes I encountered unexpected challenging situations. I remember one occasion when an optical fibre of PIONIER failed. Together with Jean-Philippe Berger, who was my PhD supervisor and who initiated the development of PIONIER, we had to replace the whole integrated optic component with a spare one, and realign the whole instrument before the night started. It took us the whole day, and it was actually a lot of fun to manipulate the instrument and discover all its details.

Claudia Paladini. Credit: C. Paladini.

I am lucky enough to be part of the generation of “interferometrists” able to produce such images. My favourite result in this regard is our PIONIER image of π1 Gruis, a red giant star 530 light-years away. We were able to see the granulation on its surface caused by convective cells that transport material up and down the star. We could even measure the size of these cells, proving the prediction made for this class of stars in 1975 by the German physicist and astronomer Karl Schwarzschild.

One of my fondest memories of the VLTI was bringing the MATISSE instrument back into operations after the COVID break.

ESO CNRS VLT Matisse Multi-AperTure mid-Infrared SpectroScopic Experiment.

After one week of testing, on the night of Dec 13th 2020 we observed the red supergiant star Betelgeuse.

Betelgeuse-a superluminous red giant star 650 light-years away in the infrared from the European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)Herschel Space Observatory (EU) Stars like Betelgeuse, end their lives as supernovae. Credit: Decin et al.
Exactly one hundred years earlier that day, the American physicist Albert Michelson and astronomer Francis Pease observed the same star with interferometry, marking the first ever measurement of the diameter of a star other than the Sun. Our team was very excited!

Antoine Mérand. Credit: J. Girard

Antoine Mérand is a staff astronomer at ESO in Germany. As the VLTI Programme Scientist, he oversees the strategic development of the VLTI.

The original vision for VLTI was ambitious in terms of sensitivity and image resolution. The VLTI has not yet reached its full potential, and amazingly the path laid out more than 30 years ago is still up to date. Improvements to the VLTI are a strong component of the currently ongoing developments to maintain the scientific performance of the VLT and VLTI in the 2030’s, started in 2019 by a conference held at ESO.

Among the many projects proposed by the community is the ambitious GRAVITY+ project, aiming at dramatically improving the sensitivity of VLTI by installing a new adaptive optics system and laser guide stars in all four UTs, which will make it possible to observe fainter stars.

GRAVITY+ has successfully passed its preliminary design study in July 2021 and currently awaits decision to go ahead for construction. Among the unique science cases are: a better understanding of AGNs and their close environment in the role they play in the evolution of galaxies, an unparalleled precision in determining the orbits of exoplanets to constrain planet formation scenarios, and a unique depth of exploration of the Galactic Centre to potentially measure the spin of our Galaxy’s central black hole, Sgr A*.

Other smaller initiatives are also welcomed at VLTI, called visitor instruments, which are dedicated small instruments aimed at specific science cases. Many such projects exist in the community which, in addition to GRAVITY+, promise a dynamic future for VLTI, full of exciting and unique astrophysical results.

See the full article here .


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European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) is the foremost intergovernmental astronomy organisation 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. ESO carries out an ambitious programme 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 organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, 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 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-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

European Southern Observatory(EU) La Silla HELIOS (HARPS Experiment for Light Integrated Over the Sun).

ESO 3.6m telescope & HARPS atCerro LaSilla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

MPG Institute for Astronomy [Max-Planck-Institut für Astronomie](DE) 2.2 meter telescope at/European Southern Observatory(EU) Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

European Southern Observatory(EU)La Silla Observatory 600 km north of Santiago de Chile at an altitude of 2400 metres.

European Southern Observatory(EU) , Very Large Telescope at Cerro Paranal in the Atacama Desert •ANTU (UT1; The Sun ) •KUEYEN (UT2; The Moon ) •MELIPAL (UT3; The Southern Cross ), and •YEPUN (UT4; Venus – as evening star). Elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo.

European Southern Observatory(EU)VLTI Interferometer image, Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening.

ESO Very Large Telescope 4 lasers on Yepun (CL)

Glistening against the awesome backdrop of the night sky above ESO’s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT, a major asset of the Adaptive Optics system.

ESO/NTT NTT at Cerro La Silla , Chile, at an altitude of 2400 metres.

Part of ESO’s Paranal Observatory, the VLT Survey Telescope (VISTA) observes the brilliantly clear skies above the Atacama Desert of Chile. It is the largest survey telescope in the world in visible light, with an elevation of 2,635 metres (8,645 ft) above sea level.

European Southern Observatory/National Radio Astronomy Observatory(US)/National Astronomical Observatory of Japan(JP) ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres.

European Southern Observatory(EU) ELT 39 meter telescope to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

European Southern Observatory(EU)/MPG Institute for Radio Astronomy [MPG Institut für Radioastronomie](DE) ESO’s Atacama Pathfinder Experiment(CL) high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft).

Leiden MASCARA instrument cabinet at Cerro La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft).

ESO Next Generation Transit Survey telescopes, an array of twelve robotic 20-centimetre telescopes at Cerro Paranal,(CL) 2,635 metres (8,645 ft) above sea level.

ESO Speculoos telescopes four 1 meter robotic telescopes at ESO Paranal Observatory 2635 metres 8645 ft above sea level.

TAROT telescope at Cerro LaSilla, 2,635 metres (8,645 ft) above sea level.

European Southern Observatory(EU) ExTrA telescopes at erro LaSilla at an altitude of 2400 metres.

A novel gamma ray telescope under construction on Mount Hopkins, Arizona. A large project known as the Čerenkov Telescope Array composed of hundreds of similar telescopes to be situated in the Canary Islands and Chile at, ESO Cerro Paranal site The telescope on Mount Hopkins will be fitted with a prototype high-speed camera, assembled at the. University of Wisconsin–Madison and capable of taking pictures at a billion frames per second. Credit: Vladimir Vassiliev.

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU), The new Test-Bed Telescope 2is housed inside the shiny white dome shown in this picture, at ESO’s LaSilla Facility in Chile. The telescope has now started operations and will assist its northern-hemisphere twin in protecting us from potentially hazardous, near-Earth objects.The domes of ESO’s 0.5 m and the Danish 0.5 m telescopes are visible in the background of this image.Part of the world-wide effort to scan and identify near-Earth objects, the European Space Agency’s Test-Bed Telescope 2 (TBT2), a technology demonstrator hosted at ESO’s La Silla Observatory in Chile, has now started operating. Working alongside its northern-hemisphere partner telescope, TBT2 will keep a close eye on the sky for asteroids that could pose a risk to Earth, testing hardware and software for a future telescope network.

European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) The open dome of The black telescope structure of the‘s Test-Bed Telescope 2 peers out of its open dome in front of the rolling desert landscape. The telescope is located at ESO’s La Silla Observatory, which sits at a 2400 metre altitude in the Chilean Atacama desert.