Galactic Bulge (4) ...

The Mass Inside the Orbit of S2
The most remarkable thing about the star S2 is that, by Kepler's laws, the details of its orbit tell us how much mass is contained inside the orbit. The extremely high velocity of S2 on its orbit is already an indication that this mass is very large, and the extremely close approach of S2 to Sgr A* means that this mass must be contained in a very small region.

The adjacent right figure illustrates using data on star motion in the vicinity of Sgr A* to estimate the mass contained within successively smaller radii. The data point at the smallest separation (furthest to the left) is the one inferred from the orbit of S2 and the red curve is the best fit to the available data. The flattening of this curve to a constant mass at small radii with a value of about 2.6 million solar masses (intersection of the red curve with the left axis) indicates that there is a mass of this size with a radius of less than 17 light hours near the position of Sgr A*. Many astronomers believe that this result can only be explained by a black hole of 2.6 million solar masses at the center of the Milky Way, since any other conceivable explanation would either generate much more light than is visible, or would give a different behavior of the enclosed mass with radius than that observed in the above right figure. Thus, the orbit of the star S2 may be the most conclusive evidence yet that our galaxy has a supermassive black hole at its center.