How do the orbits of stars near Sagittarius A* provide evidence for a black hole?

The presence of a supermassive black hole at the center of the Milky Way Galaxy, known as Sagittarius A*, is confirmed by the observed orbital motions of stars in its immediate vicinity.12

Because the galactic center is obscured by thick dust, astronomers utilize infrared observations to track these stars. By monitoring their paths over time, researchers have determined that these stars move in elliptical orbits around an invisible, extremely massive point.231

These observations allow scientists to apply the principles of celestial mechanics, specifically Kepler’s laws of planetary motion, to calculate the mass of the central object. The high velocities and tight orbital paths of these stars indicate that the central mass is equivalent to approximately four million times that of the Sun. The combination of this immense mass and the extremely small dimensions of the region provides conclusive evidence that the object is a supermassive black hole. For this groundbreaking work, astronomers Andrea Ghez and Reinhard Genzel were awarded the 2020 Nobel Prize for Physics.241