Astrophysicists Make Most Precise Measurement Yet of Milky Way’s Central Black Hole Mass

Sagittarius A*, supermassive black hole at the center of our Milky Way Galaxy, has a mass of 4.3 million times that of the Sun, according to an analysis of observations by the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI).

In this illustration of the center of the Milky Way, stars are seen to be in close orbit around Sagittarius A*. Image credit: Gemini Observatory / NOIRLab / NSF / AURA / J. da Silva, Spaceengine / M. Zamani, NOIRLab.

“We want to learn more about the black hole at the center of the Milky Way, Sagittarius A*,” said Dr. Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics who was awarded a Nobel Prize in 2020 for the Sagittarius A* research.”

“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.”

“With the 2020 Nobel prize in physics awarded for the confirmation that Sagittarius A* is indeed a black hole, we now want to go further,” added Dr. Stefan Gillessen, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics.

“We would like to understand whether there is anything else hidden at the center of the Milky Way, and whether general relativity is indeed the correct theory of gravity in this extreme laboratory.”

“VLTI gives us this incredible spatial resolution and with the new images we reach deeper than ever before,” said Dr. Julia Stadler, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics.

“We are stunned by their amount of detail, and by the action and number of stars they reveal around the black hole.”

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 Sagittarius 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. Image credit: ESO / GRAVITY Collaboration.

On a quest to find stars close to Sagittarius A*, Dr. Genzel, Dr. Gillessen, Dr. Stadler and their colleagues from the GRAVITY Collaboration used a machine-learning technique called Information Field Theory.

They made a model of how the real sources may look, simulated how the GRAVITY instrument would see them, and compared this simulation with GRAVITY observations.

This allowed them to find and track stars — called S2, S29, S38, and S55 — around Sagittarius A* with unparalleled depth and accuracy.

Remarkably, they found a star, named S300, which had not been seen previously.

They also found that the S29 star made its closest approach to the black hole in May 2021.

This record-holder star passed Sagittarius A* at a distance of just 13 billion km (8.1 billion miles), about 90 times the Sun-Earth distance, at the stunning speed of 8,740 km/sec (5,431 miles/sec). No other star has ever been observed to pass that close to, or travel that fast around, the black hole.

“Interferometry is a complex technique, 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 center,” said GRAVITY principal investigator Dr. Frank Eisenhauer, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics.

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 4.3 million times that of the Sun. This is the most precise estimate of the mass of Sagittarius A* to date.

The scientists also managed to fine-tune the distance to Sagittarius A*, finding it to be 27,000 light-years away.

“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,” Dr. Genzel said.

The new results appear in two papers in the journal Astronomy & Astrophysics.

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GRAVITY Collaboration. 2021. Mass distribution in the Galactic center based on interferometric astrometry of multiple stellar orbits. A&A, in press; doi: 10.1051/0004-6361/202142465

GRAVITY Collaboration. 2021. Deep images of the Galactic center with GRAVITY. A&A, in press; doi: 10.1051/0004-6361/202142459