The discovery of a supermassive black hole at the center of the Milky Way galaxy

Black holes are fascinating astrophysical objects, yet they are rarely encountered in undergraduate-level physics courses. Here, we use concepts accessible to upper division physics majors to explain the discovery of a supermassive black hole at the center of the Milky Way galaxy, which was recognized by the award of the 2020 Nobel Prize in Physics. Infrared observations of the innermost region of the Milky Way galaxy show that many stars orbit around the galactic center at distances comparable to the size of the solar system with decadal-scale periods. We show how, given the sizes and periods of these stellar orbits, Newton's version of Kepler's third law can be used to determine the mass and maximum radius of the concentration of mass at the galactic center. We consider various possible physical interpretations consistent with this mass and maximum radius, rejecting flat disks due to dynamical instabilities, clouds of gas and/or dust due to gravitational instabilities, and clusters of compact objects due to collisional instabilities. This demonstrates that a supermassive black hole is the only possible explanation for the observed orbits of stars close to the galactic center.