Astronomers discover S301, fastest star in Milky Way orbiting black hole

At the center of our galaxy, a star is moving faster than anything else we know of in the Milky Way. Astronomers have named it S301, and it is traveling at 25,000 kilometers per second as it loops around Sagittarius A*, the supermassive black hole anchoring our cosmic neighborhood. The discovery, made using the Very Large Telescope Interferometer at Chile's Paranal Observatory and published in Nature, opens a door that has been locked for a century: the chance to measure, directly and for the first time, how fast a black hole actually spins.

The star itself is impossibly faint—two billion times dimmer than Betelgeuse, the orange giant visible in Earth's night sky. Finding it required combining light from four eight-meter telescopes into a single virtual instrument with resolution fifteen times sharper than any one telescope alone. When astronomers first spotted S301 in spring 2023, they began the patient work of tracking its path. What they found was extraordinary. The star completes a full orbit around Sagittarius A* in just 8.7 years, approaching within twelve times the distance between Earth and the Sun—closer to a supermassive black hole than any star previously observed. At its nearest point, S301 reaches speeds equivalent to roughly 8 percent of light's velocity, making it 100,000 times faster than a commercial airliner.

The star's existence poses a puzzle that reveals its origin story. Stars cannot form so close to a black hole; the gravitational forces would tear apart any protoplanetary disk before it could coalesce. S301 must have arrived there some other way. The leading explanation is that it once belonged to a binary pair—two stars orbiting each other—until Sagittarius A*'s gravity tore the system apart. One star was ejected at high speed and likely fled the galaxy entirely. The other, S301, became trapped in the black hole's grip, condemned to loop endlessly around it.

Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics and a 2020 Nobel laureate for his earlier work proving Sagittarius A* exists, called the discovery groundbreaking. "Because it orbits so close to Sagittarius A*, S301 opens a new window for studying the fundamental properties of space-time in this extreme black hole environment," he said. Felix Mang, a doctoral student at the institute and co-author of the study, emphasized what makes S301 unique: its tight, rapid orbit and proximity to the black hole had never been seen before.

The real prize lies a decade ahead. Einstein's theory of relativity predicts that a rotating black hole drags space-time around itself, warping the orbits of nearby objects. By observing S301 complete at least two full circuits around Sagittarius A*, astronomers believe they can measure these warping effects precisely enough to calculate the black hole's rotation rate. Juan Osorno, an astronomer at the Paris Observatory–PSL, noted that without S301, researchers would need several more decades of observations of other, slower stars to approach such a measurement. The star's next closest approach to the black hole comes in 2031. What happens in the years that follow could finally answer a question that has haunted astrophysics since Einstein first wrote his equations: how fast does the universe's most extreme object spin?