GARCHING, GERMANY — A star orbiting the supermassive black hole at the center of our galaxy is moving at about 25,000 kilometers per second, roughly 8% of the speed of light, making it the fastest star ever measured in the Milky Way. The European Southern Observatory announced the discovery of the star, designated S301, on August 19, with the results published in Nature. S301 completes an orbit around Sagittarius A* in 8.7 years, the shortest period recorded for any star there. At its closest approach it passes within roughly 1.7 billion kilometers of the black hole, about 20% farther out than Saturn orbits the Sun. The star itself is unremarkable, a Sun-like main-sequence star of about 1.1 to 1.5 solar masses. The observation was possible because of interferometry. The GRAVITY+ instrument on the Very Large Telescope Interferometer in Chile combines light from four 8-meter telescopes, producing roughly fifteen times the resolution of any single one. The galactic center is crowded and choked with dust, and stars this close to Sagittarius A* had been essentially impossible to isolate before. The team, led by doctoral student Felix Mang at the Max Planck Institute for Extraterrestrial Physics, first noticed S301 in 2023 imagery and pinned down its orbit using archival data reaching back to 2017. What the discovery does not yet deliver is the prize the team is after. The black hole's spin has not been measured. Spin should drag and twist S301's orbit in a way general relativity predicts, but confirming that requires tracking at least two complete orbits, another ten to fifteen years of observation.
Background
Spin matters because spin and mass together fully describe a black hole in general relativity. The mass of Sagittarius A* has been known for years, and the work establishing it earned Reinhard Genzel and Andrea Ghez a share of the 2020 Nobel Prize in Physics. Spin, by contrast, has only ever been inferred from models, never measured directly from a stellar orbit.
Why it matters: A single star, tracked patiently enough, can test general relativity in a gravitational field no laboratory can produce. That is an argument for funding instruments whose payoff arrives decades after the money is spent.
What to watch: S301's next close approach in 2031, and whether independent groups confirm the orbital solution.