Closest star yet tests gravity around Milky Way’s monster black hole

Using ESO’s GRAVITY instrument at the VLT, astronomers have found S301, a faint star at the Milky Way’s center on the closest, fastest orbit around Sagittarius A*. With an 8.7-year period, it swings within about 12 astronomical units of the black hole at pericenter, racing at roughly 55 million mph (about 0.008c). Tracked across eight years in 19 positions, the star shows strong general-relativistic effects such as Schwarzschild precession, and within a decade could enable a direct measurement of the black hole’s spin. S301 likely originated from a disrupted binary, and its discovery boosts tests of gravity in extreme fields while hinting at many more such stars detectable with GRAVITY+.