Astronomers Discover the Fastest Star in the Milky Way, Racing Around Our Black Hole at 8% the Speed of Light

The faint star S301 orbits Sagittarius A* in just 8.7 years and could let astronomers directly measure a black hole's spin for the first time — a key test of Einstein's relativity.

Astronomers have found the fastest star in the Milky Way: a faint object called S301 that tears around Sagittarius A*, the supermassive black hole at our galaxy's center, at roughly 25,000 kilometers per second — more than 8% of the speed of light. The discovery, made with the European Southern Observatory's Very Large Telescope Interferometer and its GRAVITY+ instrument in Chile, is published in Nature.

S301 is remarkable for its orbit, not just its speed. The star completes a full loop of the four-million-solar-mass black hole in only 8.7 years, and at closest approach swings within 1.78 billion kilometers of it — about 12 times the Earth–Sun distance and nearer than any star observed before. Because it dives so deep into the black hole's gravitational grip, S301 is the first known star that could allow astronomers to directly measure a black hole's spin, a key test of Einstein's general theory of relativity.

Relativity predicts that a spinning black hole drags and twists the spacetime around it, subtly shifting the orbits of nearby stars — and S301 orbits closely enough to feel that effect. The team, led by Felix Mang at the Max Planck Institute for Extraterrestrial Physics and including Nobel laureate Reinhard Genzel, thinks S301 was once half of a binary pair torn apart by the black hole's tides, leaving the survivor locked into its blistering, tight orbit. The star's next closest approach comes in 2031, when follow-ups with GRAVITY+ and the upcoming Extremely Large Telescope could pin down Sagittarius A*'s spin for the first time.

Sources: ESO | Space.com | Nature