Deepest Ever Images of Our Galaxy Show Stars Dancing Around Supermassive Black Hole

Astronomers have made the most precise measurements so far of the stars that orbit the supermassive black hole at the heart of our galaxy.

Observing the stellar dance at the heart of the Milky Way has shown astronomers that 99.9 percent of the mass located there is possessed by the central black hole, named Sagittarius A* (Sgr A*), with just 0.1 percent attributed to stars, gas, and dust, dark matter and smaller black holes.

The team used cutting-edge astronomical equipment to measure the movement of four stars in the immediate vicinity of Sgr A*—S2, S29, S38 and S55—revealing details of the mass distribution at the center of the Milky Way.

The galactic center of the Milky Way, around 27,000 light-years from the solar system, contains a mass of at least 4.3 million times that of the sun, but until now astronomers have struggled to determine how much of this mass belonged to Sgr A*. That's because the galactic center is packed with a wealth of other cosmic objects and dense clouds of gas and dust.

Stars at the Milky Way's Center (Labelled)
An illustration showing labelled stars close orbit around the supermassive black hole that dwells at the heart of the Milky Way, known as Sagittarius A* (Sgr A*). Astronomers have observed the dance of these stars in closer detail than ever before. ESO/International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine) Acknowledgment: M. Zamani (NSF's NOIRLab

"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," Stefan Gillessen from the Max-Planck-Institute for Extraterrestrial Physics said.

"The most straightforward way to answer that question is to closely follow the orbits of stars passing close to Sgr A*."

Gillessen, who is one of the authors of a paper about the team's work set to be published in the journal Astronomy & Astrophysics, told Newsweek: "There are many exciting scientific topics, which relate to the progress we have presented in the new paper. Perhaps the most important one: we might be able to measure the spin of the massive black hole in the Milky Way center from suitably short-period stellar orbits.

"A black hole is characterized by two numbers only, mass and spin. So knowing these two numbers is all we can know of it. It also means we should be able to predict any further dynamical effects the black hole induces."

The researchers took advantage of a phenomenon predicted by Einstein's theory of general relativity, the most precise theory we have to describe gravity and therefore the orbits of moons, planets and stars.

According to general relativity, orbits change their orientation over time tracing out a rosette-like pattern, a process called Schwarzschild precession. The astronomers traced out the rosette created by S2, S29, S38 and S55 by mapping their position and velocity.

To measure the velocities of the stars, the astronomers used spectroscopy from the Gemini Near-Infrared Spectrograph (GNIRS) at Gemini North near the summit of Maunakea in Hawai'i, and the SINFONI instrument on the European Southern Observatory's Very Large Telescope. The positions of the stars were measured with the GRAVITY instrument at the VLT Interferometer.

Milky Way
A diagram of stars dancing around the supermassive black hole at the heart of the Milky Way. GRAVITY collaboration/ESO

Detailing how these stars moved around Sgr A* and measuring the tiny variations in their orbits allowed the researchers to determine how mass was distributed in this region.

They found that mass within the orbit of the star S2 contributes only 0.1 percent of the total mass at the center of the Milky Way, leaving the other 99.9 percent owing to Sgr A*.

Measuring such tiny changes in the orbits of distant stars is no easy feat and a deeper investigation may have to wait until telescope technology improves.

"We will improve our sensitivity even further in future, allowing us to track even fainter objects," Gillessen said. "We hope to detect more than we see now, giving us a unique and unambiguous way to measure the rotation of the black hole."

He later told Newsweek: "Black holes should not rotate faster than a certain limit—we might be able to check that. In the future, we hope that we can detect many more stars which might be on tight enough orbits to test this. We have found a black hole in the galactic center—now we want to use it for testing physics."

Update 12/16/21, 11:50 a.m. ET: This article was updated to add comments from Stefan Gillessen.

Milky Way
An illustration showing stars close orbit around the supermassive black hole that dwells at the heart of the Milky Way, known as Sagittarius A* (Sgr A*). Astronomers have observed the dance of these stars in closer detail than ever before. International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine) Acknowledgement: M. Zamani (NSF's NOIRLab/NSF