You might have heard about the supermassive black hole hidden at the center of our Milky Way, Sagittarius A* (Sgr A*). Astronomers have managed to measure the movement of stars around it – it is the most precise measurement of the kind we’ve ever had!

Using the Gemini North Telescope in Hawaii, scientists found that almost all mass (99.9%) right at the center of our galaxy belongs to the black hole – only 0.1% of this mass can have stars, interstellar dust and gas, dark matter and even smaller black holes. 

To find this out, astronomers made very precise measurements of the position and velocity of four stars close to Sagittarius A*. They found that these stars – S2, S29, S38, and S55 – move in a way that shows the mass in the center of our galaxy is almost completely due to this black hole.

In order to do such measurements, an international team of astronomers used many instruments, such as ESO’s Very Large Telescope in Chile and the Gemini North Telescope in Hawaii. 

Despite many years of observation, it was very difficult – until now – to prove that most of the mass in the center of our Milky Way belonged to the Sgr A* supermassive black hole alone. 

Astronomers now want to understand what else is hidden in the center of our galaxy, and whether the theories that we have now – such as Einstein’s general theory of relativity – are really the best ones to understand this mysterious place. What if we need a new theory to explain the way black holes work?

Image: In this illustration, stars are seen to be in close orbit around the supermassive black hole that lurks at the center of the Milky Way, known as Sagittarius A*.

Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/(Spaceengine) Acknowledgement: M. Zamani (NSF's NOIRLab)