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

This review outlines the observations that now provide an overwhelming scientific case that the center of the Milky Way 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 × 10⁶ M_⊙ within a radius of 100 AU. Orbital speeds have been observed to exceed 5,000 km s^-1. At the focal position there is an extremely compact radio source (Sgr A*), whose apparent size is near the Schwarzschild radius (2GM/c²). This radio source is motionless at the ~ 1 km s^-1 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.