It all began with the discovery of Sagittarius A*, a persistent radio source located at the Galactic Center of the Milky Way that turned out to be a supermassive black hole (SMBH). This discovery was accompanied by the realization that SMBHs exist at the heart of most galaxies, which account for their energetic nature and the hypervelocity jets extending from their center. Since then, scientists have been trying to get a better look at Sag A* and its surroundings to learn more about the role SMBHs play in the formation and evolution of our galaxy.

This has been the goal of the GRAVITY collaboration, an international team of astronomers and astrophysicists that have been studying the core of the Milky Way for the past thirty years. Using the ESO’s Very Large Telescope Interferometer (VLTI), this team obtained the deepest and sharpest images to date of the region around Sag A*. These observations led to the most precise measurement yet of the black hole’s mass and revealed a never-before-seen star that orbits close to it.

The GRAVITY collaboration is made of scientists from the Max Planck Institute for Extraterrestrial Physics (MPE), the Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), the Centre national de la recherche scientifique (CNRS), the Centro de Astrofisica e Gravitação (CENTRA), the Max Planck Institute for Astronomy (MPIA), and the European Southern Observatory (ESO). The collaboration takes its name from the GRAVITY adaptive optics instrument that they developed for the VLTI.

This unique instrument combines the light of all four 8.2-meter (~27 ft) telescopes at the Very Large Telescope’s (VLT) located at the Paranal Observatory in Chile – a technique known as interferometry. MPE director Reinhard Genzel, a member of the GRAVITY collaboration, was awarded a Nobel Prize in 2020 for his extensive research of Sagittarius A*. As he said in an ESO press release, this latest research offers new insight into the many questions astronomers have had about the SMBH at the center of our galaxy:

“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.”

The collaboration team also employed a machine-learning technique called Information Field Theory. This consisted of modeling how the real light sources would appear, how GRAVITY would observe them, then comparing the simulated results to the actual observations. This allowed them to acquire highly-accurate measurements of Sag A* and images of Galactic Center that were 20 times sharper than any made by the individual VLT telescopes alone.

In addition to the GRAVITY observations, the team also used data from two former VLT instruments (NACO and SINFONI) and measurements from the Keck Observatory and NOIRLab’s Gemini Observatory in the US. During their observation period, which ran from March to July 2021, the team used these instruments to make precise measurements of the stars that orbit Sag A* as they made their closest approach.


Images obtained by the GRAVITY instrument on the VLTI between March and July 2021, showing stars orbiting very close to Sgr A*, the supermassive black hole at the heart of the Milky Way. Credit: ESO/GRAVITY collaboration

This included S29, which holds the record for making the closest and speediest approach around Sag A* ever observed. This star made its nearest pass in late May 2021, passing within 13 billion km (8 billion mi) – or 90 times the distance between the Earth and Sun (90 AU) – and achieving a velocity of 8,740 km per second (5430 mps). In addition, they found a new star (S300) that was previously undetected, demonstrating the power and effectiveness of their observations.

“The VLTI gives us this incredible spatial resolution, and with the new images, we reach deeper than ever before,” said Julia Stadler, a researcher at the MPA who led the team’s imaging efforts. “We are stunned by
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