Astronomers Find S301 — The Fastest Known Star in the Milky Way, Racing at ~15,500 mi/s Around Sagittarius A*

A faint star named S301 has been identified as the fastest-known star in the Milky Way. Observations show it streaks around Sagittarius A*, the supermassive black hole at our galaxy's center, reaching a top speed of roughly 15,500 miles per second (≈25,000 km/s) — about 8.3% of light speed.

Credit: ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin and S. Guisard

S301 travels on an exceptionally tight, highly eccentric orbit that takes just 8.7 years to complete. At closest approach it comes to within about 12 astronomical units (≈12 times the Earth–Sun distance) of Sagittarius A*, a proximity unprecedented among known stars near the galactic center.

This visible light wide-field view shows the rich star clouds in the constellation of Sagittarius (the Archer) in the direction of the center of our Milky Way galaxy. The entire image is filled with vast numbers of stars — but far more remain hidden behind clouds of dust and are only revealed in infrared images. This view was created from photographs in red and blue light and form part of the Digitized Sky Survey 2. | Credit: ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin and S. Guisard

Why This Matters

The star's extreme speed and close approach give astronomers a powerful new probe of the roughly 4.3-million-solar-mass black hole. Under Einstein's general relativity, a rotating black hole should drag spacetime around it — an effect called frame dragging or Lense–Thirring precession. Because S301 ventures so near Sagittarius A*, those relativistic distortions are expected to nudge its orbit in measurable ways within about a decade, potentially allowing a direct measurement of the black hole's spin.

This sequence of images, taken with the GRAVITY instrument at ESO's Very Large Telescope Interferometer (VLTI), show several stars orbiting Sagittarius A*, the supermassive black hole at the centre of our galaxy. One of these stars, known as S301, was recently found to pass much closer to the black hole than any other known star. | Credit: ESO/GRAVITY collaboration

"What makes this star exceptional is that it orbits Sagittarius A* on an extremely tight path, completing one circuit in just 8.7 years and approaching the black hole at only about 12 times the Earth–Sun distance. That is unprecedented," said Felix Mang, a Ph.D. student at the Max Planck Institute for Extraterrestrial Physics.

How S301 Was Found

Researchers first detected S301 in 2023 using the GRAVITY instrument on the European Southern Observatory's (ESO) Very Large Telescope Interferometer (VLTI) in Chile. The team then traced the star's motion backward in archival observations from 2021 and 2017 to reconstruct its orbit. The orbit's elongated shape suggests S301 may be the surviving member of a binary system that wandered too close to the black hole; the companion could have been ejected at high speed while S301 was captured.

Ongoing monitoring with the upgraded GRAVITY+ instrument and, later, the ESO Extremely Large Telescope will track S301's motion in detail. Its next close pass is expected in 2031; observations spanning two full orbits could permit the first direct measurement of Sagittarius A*'s spin.

"For the first time, we would actually be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein's theory," said co-author Stefan Gillessen of the Max Planck Institute.

Credit: ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin and S. Guisard. The team's results were published in Nature on August 19.