Astronomers have identified S301, the fastest-moving star yet found in the Milky Way, which orbits the supermassive black hole Sagittarius A* at the galaxy's centre. Its extreme speed offers researchers a rare opportunity to measure the black hole's spin, a property that has so far eluded precise determination and would help confirm predictions of general relativity about how rotating black holes drag spacetime around them.

At its fastest, S301 travels at 25,000 kilometres per second, around 8 percent of the speed of light and more than 100 times the Sun's orbital speed, completing a full orbit around Sagittarius A* every 8.7 years. Its closest approach to the black hole, first observed in early 2023 using the European Southern Observatory's GRAVITY instrument, brings it as near as the distance between Saturn and the Sun; astronomers believe it was likely torn from a binary companion by the black hole's tidal forces. The star will make its next close pass in 2031, and researchers expect that within a decade they could use tiny deviations in its orbit to pin down Sagittarius A*'s rotation, something that would take far longer using other known stars such as S2.