A star is racing close enough to test a black hole's spin

ESO's GRAVITY+ team found S301, the fastest known star in the Milky Way, orbiting Sagittarius A*, the roughly four-million-Sun black hole at the centre of our galaxy. S301 reaches about 25,000 km/s and completes one orbit in 8.7 years.1

The Nature paper fits 19 position measurements collected from 2017 to 2025. The orbit is highly elongated, with an eccentricity of about 0.983. At its closest point, S301 comes within 136–142 Schwarzschild radii of Sagittarius A* — roughly ten times closer than the well-studied star S2.2

A rotating black hole drags nearby spacetime, which can nudge the orbit of a star passing close enough. Continued observations may measure Sagittarius A*'s spin within about a decade, giving astronomers a new stellar test of strong-field relativity.13