Astronomers have identified the fastest-known star in the Milky Way, a modest main-sequence star called S301 that whips around Sagittarius A*, the supermassive black hole at the galactic center, at roughly 25,000 kilometers per second. The discovery, published in Nature, provides a new tool for probing one of the most elusive properties of a black hole: its spin.
S301 is a bit heavier than the Sun, with a radius about 1.4 to 1.6 times solar, and it is too faint to be one of the giant stars in the galactic center. But its orbit is extraordinary. With an orbital period of just 8.7 years, it breaks the previous record held by the star S55, which takes about 12 years to complete a circuit. S301's path is also highly eccentric, with an eccentricity of about 0.98, bringing it to a pericenter distance of approximately 1.78 billion kilometers from the black hole — about ten times closer than the well-studied star S2 ever gets.
That close approach makes S301 an ideal probe of the black hole's spin. As a black hole spins, it drags the surrounding space-time around with it, a phenomenon known as frame dragging or the Lense-Thirring effect. This cosmic whirlpool twists the orbits of nearby stars, but the effect is subtle and difficult to measure unless a star ventures very close. S2, despite its 16-year orbit and meticulous tracking with the GRAVITY instrument on the European Southern Observatory's Very Large Telescope Interferometer, remains too far away to detect the spin without prohibitively long observations. S301, by contrast, reaches deep enough into the gravitational potential that its trajectory should be measurably altered by the black hole's rotation.
“Our group has determined the mass of the central black hole down to sub-percent precision,” said Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics and an author of the study. “A black hole is characterized exclusively by its mass, spin, or angular momentum and electric charge; now that we've measured the mass really, really accurately, the next step is to measure the spin.”
The team first spotted S301 in spring 2023 but could not immediately tell whether it was a foreground or background star. Follow-up observations in 2024 revealed a pronounced acceleration in its proper motion — the star was curving away from a linear trajectory, hinting that it was bound to the black hole. Continued observations in 2025 allowed the researchers to build a robust initial model of its orbit, confirming it was indeed a very eccentric, tightly orbiting star.
To lock down the orbit, the astronomers combed through archival data, searching for traces of S301 before its close approach. They found two pre-pericenter positions, in 2021 and 2017, though the 2017 signature was weak. In total, 19 distinct positions across nearly a decade of data outlined a clear elliptical path, showing that the star had zoomed past the black hole at its closest approach in early 2023.
Because S301 gets so close, general relativity strongly affects its orbit. The star experiences Schwarzschild precession, which shifts its orbit by about 2 degrees per revolution. Over roughly 1,500 to 1,600 years, those slight shifts accumulate into a full 360-degree rotation of the orbit. While this dramatic shift is driven by the colossal mass of Sagittarius A*, S301's extreme orbit is also subject to the subtler spin-induced Lense-Thirring precession. Measuring that effect could finally reveal how fast the Milky Way's central black hole is spinning, a key test of Einstein's general theory of relativity in the most extreme environment available to astronomers.
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