S301 is the fastest known star orbiting the Milky Way’s black hole

The faint star S301 is moving so fast that its speed is easier to state than to picture: 25,000 km/s, or more than 8 percent of the speed of light. It is now the fastest known star in the Milky Way, racing around Sagittarius A, the supermassive black hole at the galaxy’s center, on an orbit that takes 8.7 years*.

Its closest approach came in 2023, when S301 passed nearer to Sagittarius A than any star previously identified there. At that point, it came within 12 times the distance between Earth and the Sun*, according to Interesting Engineering. The orbit is also highly elongated, with an eccentricity of 0.9832, suggesting that S301 is close to escaping the black hole’s gravitational hold.

Astronomers first spotted S301 with GRAVITY, an instrument on the European Southern Observatory’s Very Large Telescope Interferometer in Chile. GRAVITY combines light from four individual telescopes, giving it the resolution of a single telescope with a 130-meter diameter. The team used observations from 2023 to estimate the orbit, then traced it through earlier data from 2021 and 2017.

The orbit also offers a clue about S301’s origin. Researchers believe it could once have belonged to a binary star system that wandered too close to Sagittarius A*. The black hole may have captured S301 while throwing its companion outward, leaving the surviving star on its unusually stretched, high-speed path.

And then? S301 gives astronomers a way to test the black hole itself. General relativity predicts that a spinning black hole drags spacetime around with it, gradually changing the orbit of a nearby star. The researchers say those changes could become measurable within roughly a decade; observations across two full orbits could reveal Sagittarius A’s spin. The next close pass is expected in 2031*, but these conclusions still depend on tracking a faint object over time and reproducing the predicted orbital changes.