S2 was the first star known to pass close enough to Sagittarius A* for astronomers to measure relativistic effects.
S2, also called S0-2, is a young, hot star orbiting the supermassive black hole at the center of the Milky Way. It completes an elongated orbit in about 16 years and approaches within roughly 120 astronomical units of Sagittarius A*. Long-term infrared observations tracked its position and velocity through the crowded Galactic Center.
During its 2018 closest approach, astronomers measured gravitational redshift and later detected the predicted Schwarzschild precession of its orbit. These observations provided strong tests of general relativity in a region dominated by a supermassive black hole.
S2 is not the same thing as Sagittarius A*. The star is a luminous object orbiting the black hole, while Sagittarius A* is the compact radio source associated with the black hole itself. Other stars, including S4716, have been reported with shorter orbital periods, but S2 remains the best-established and most extensively observed reference star.