S2 was the star that passed closest to Sagittarius A* in 2018 and helped test general relativity. Also called S0-2, it is a hot, massive star following a highly elongated orbit around the Milky Way’s central black hole.
S2 reached pericenter on May 19, 2018, passing within about 120 astronomical units of Sagittarius A*. At that point it was traveling at roughly 7,650 kilometers per second, nearly 3% of the speed of light. Its orbit takes about 16 years, making repeated close approaches valuable for long-term observations.
Astronomers measured effects including gravitational redshift and relativistic changes in the star’s motion. The observations agreed with Einstein’s general relativity. S2 is not the closest-known star on every measure—other faint stars have been reported on tighter orbits—but it remains the best-known and most extensively monitored test star near Sagittarius A*.