Is there a Supermassive Black Hole at the Center of the Milky Way?

TL;DR

This review compiles observational evidence indicating that the Milky Way's center contains a supermassive black hole, based on stellar orbits, high orbital speeds, and a compact radio source near the Schwarzschild radius.

Contribution

It synthesizes multiple lines of astronomical observations to strongly support the existence of a supermassive black hole at the Galactic Center.

Findings

Stars orbit a common focal point with a mass of 4 million solar masses

Orbital speeds exceed 5,000 km/s near the center

Presence of a compact radio source near the Schwarzschild radius

Abstract

This review outlines the observations that now provide an overwhelming scientific case that the center of our Milky Way Galaxy harbors a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M) of 4 million solar masses within a radius of 100 Astronomical Units. Orbital speeds have been observed to exceed 5,000 km/s. At the focal position there is an extremely compact radio source (Sgr A*), whose apparent size is near the Schwarzschild radius (2GM/c^2). This radio source is motionless at the ~1 km/s level at the dynamical center of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon.