Is Something Else Hidden at the Center of the Milky Way?

Astronomers have measured more precisely than ever before the position and velocity of four stars in the immediate vicinity of Sagittarius A* (Sgr A*),^[1] the supermassive black hole that lurks at the center of the Milky Way. The motions of these stars — called S2, S29, S38, and S55 — were found to follow paths that shows that the mass in the center of the Milky Way is almost entirely due to the Sgr A* black hole, leaving very little room for anything else.

The research team used a variety of cutting-edge astronomical facilities in this research. To measure the velocities of the stars, they used spectroscopy from the Gemini Near Infrared Spectrograph (GNIRS) at Gemini North near the summit of Maunakea in Hawai‘i, part of the international Gemini Observatory, a program of NSF’s NOIRLab, and the SINFONI instrument on the European Southern Observatory’s

Illustration of the black hole Sagittarius A* at the center of the Milky Way. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/(Spaceengine), Acknowledgment: M. Zamani (NSF’s NOIRLab)

“We are very grateful to Gemini Observatory, whose GNIRS instrument gave us the critical information we needed,” said Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics and co-recipient of the 2020 Nobel Prize in physics. “This research shows worldwide collaboration at its best.” 

The Galactic Center of the Milky Way, located roughly 27,000 light-years from the Sun, contains the compact radio source Sgr A* that astronomers have identified as a supermassive black hole 4.3 million times as massive as the Sun. Despite decades of painstaking observations — and the Nobel Prize awarded for discovering the identity of Sgr A*^[3] — it has been difficult to conclusively prove that the majority of this mass belongs only to the supermassive black hole and does not also include a vast amount of matter such as stars, smaller black holes, interstellar dust and gas, or dark matter.

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 kilometers, just 90 times the distance between the Sun and Earth. Another star, named S300, was detected for the first time in new VLTI observations reported by ESO.
Using Gemini North of the international Gemini Observatory, a Program of NSF’s NOIRLab and ESO’s VLT, astronomers have measured more precisely than ever before the position and velocity of these stars S29 and S55 (as well as stars S2 and S38), and found them to be moving in a way that shows that the mass in the center of the Milky Way is almost entirely due to the Sagittarius A* black hole, leaving very little room for anything else. Credit: ESO/GRAVITY collaboration

“With the 2020 Nobel prize in physics awarded for the confirmation that Sgr A* is indeed a black hole, we now want to go further. 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,” explained Stefan Gillessen, one of the astronomers involved in this work. “The most straightforward way to answer that question is to closely follow the orbits of stars passing close to Sgr A*.”

Einstein’s general theory of relativity predicts that the orbits of stars around a supermassive compact object are subtly different from those predicted by classical Newtonian physics. In particular, general relativity predicts that the orbits of the stars will trace out an elegant rosette shape — an effect known as Schwarzschild precession. To actually see stars tracing out this rosette, the team tracked the position and velocity of four stars in the immediate vicinity of Sgr A* — called S2, S29, S38, and S55. The team’s observations of the extent to which these stars precessed allowed them to infer the distribution of mass within Sgr A*. They discovered that any extended mass within the orbit of the S2 star contributes at most the equivalent of 0.1% of the mass of the supermassive black hole.