At the center of the Milky Way, resides a supermassive black hole known as Sagittarius A*. It weighs over 4 million times our Sun. There are a handful of stars that orbit pretty close to this behemoth and now we have the deepest, clearest images of them yet. Collected over several months, these images revealed new views of the motions of the stars around Sagittarius A* and even spotted an entirely one. 

These stars are called S-stars and they have wild, long orbits, taking between 14 and 18 years to complete a turn around Sgr A*. Published in two papers (here and here) in Astronomy & Astrophysics, the team’s observations revealed the closest and fastest a star has ever been observed going around a black hole. Observed between March and July 2021, star S29 was just 13 billion kilometers (8 billion miles) away from Sgr A*, about 90 times the Sun-Earth distance. It was moving at an incredible speed of 8,740 kilometers (5430 miles) per second.

They also identified a previously unknown star, now known as S300. Studying the motion of these stars is an excellent proxy for probing the properties of the supermassive black hole, allowing astronomers to refine estimates for mass and distance. 

“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?" co-author Reinhard Genzel, a director at the Max Planck Institute for Extraterrestrial Physics (MPE), said in a statement. 

"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.” 

In these observations, the team confirmed the motion of several previously known stars, such as S62 but also discovered new ones like S300. The orbits also allowed for the most precise measurements of the mass of the black hole, at 4.3 million times our Sun, and its distance from Earth, at 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,” explained Genzel, who won the Nobel Prize in 2020 for his work on Sagittarius A*. 

The team, known as the GRAVITY collaboration, used the European Southern Observatory’s Very Large Telescope Interferometer to obtain these deepest images of the galactic center, developing a new analysis technique that allowed them to zoom 20 times deeper than previously possible.

Incredible though that is, when the Extremely Large Telescope (ELT) based in Chile comes online in a few years, the collaboration will become GRAVITY+ and deliver even more detailed analyses.