What is the approximate mass of Sagittarius A*, the Milky Way’s central black hole?

Sagittarius A*, the Milky Way’s central black hole, has an approximate mass of 4 million solar masses.

Astronomers determined this mass by tracking stars orbiting the compact object at the center of our galaxy. In particular, observations of stars such as S2 revealed extremely fast orbits around an invisible, highly concentrated source. Applying orbital physics showed that roughly four million times the Sun’s mass is packed into a very small region.

Sagittarius A* is a supermassive black hole, but it is relatively quiet compared with the engines of bright active galaxies. It lies about 26,000 light-years from Earth in the direction of the constellation Sagittarius. The asterisk in its name distinguishes the compact radio source from the broader central region of the Milky Way.

This mass is an estimate, not the black hole’s physical diameter. A nonrotating black hole with four million solar masses would have a Schwarzschild radius of roughly 12 million kilometres; its event-horizon diameter would be about 24 million kilometres.