Astronomers are very confident that the Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, in a region called Sagittarius A*[2] because:
The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light-hours (1.8×1013m or 120 AU) from the center of the central object.[3]
From the motion of star S2, the object's mass can be estimated as 4.1 million M☉,[4][5] or about 8.2×1036kg.
The radius of the central object must be less than 17 light-hours, because otherwise, S2 would collide with it. In fact, recent observations from the star S14[6] indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit. However, applying the formula for the Schwarzschild radius yields just about 41 light-seconds, making it consistent with the escape velocity being the speed of light.
No known astronomical object other than a black hole can contain 4.1 million M☉ in this volume of space.
The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group[7] have provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole,[2] based on data from ESO's Very Large Telescope[8] and the Keck telescope.[9]
On 5 January 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers.[10]
Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy[10]
Ghez, A. M.(December 2008).“Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits”.Astrophysical Journal689(2): 1044–1062.arXiv:0808.2870.Bibcode:2008ApJ...689.1044G.doi:10.1086/592738.
Ghez,A. M.;Salim, S.;Hornstein, S. D.;Tanner, A.;Lu, J. R.;Morris, M.;Becklin, E. E.;Duchêne, G.(May 2005).“Stellar Orbits around the Galactic Center Black Hole”.The Astrophysical Journal620(2): 744–757.arXiv:astro-ph/0306130.Bibcode:2005ApJ...620..744G.doi:10.1086/427175.