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5h ago

Fastest Known Star S301 Found on Tight Orbit Around Milky Way’s Black Hole

Its 8.7-year, highly eccentric path plunges so close that combined astrometric plus future spectroscopic monitoring could measure Sagittarius A*’s spin within about a decade.

A wide-field visible-light view shows rich star clouds in the constellation Sagittarius in the direction of the center of the Milky Way galaxy, in this image released August 19, 2026. ESO and Digitized Sky Survey 2/Handout via REUTERS
Credit: Max Planck Institute for Extraterrestrial Physics
The Sagittarius A* black hole. Scientists hope to learn more about it by studying this speedy star's orbit
Image

Overview

  • The GRAVITY+ collaboration reported in Nature on Aug. 19, 2026 that it has identified S301, a very faint star on the closest and fastest orbit yet measured around the Galaxy’s central black hole, Sagittarius A*.
  • S301 completes an 8.7-year orbit with a periapse near 12 astronomical units and reaches peak speeds of roughly 25,000 km/s, about 8% of the speed of light, making it the fastest-known star in the Milky Way.
  • Detection relied on upgraded interferometry and new image-reconstruction methods with ESO’s VLTI/GRAVITY+, which pulled S301 out of crowded, much brighter stellar fields and traced positions back to 2017.
  • Current results use astrometry only, so the team lacks radial-velocity data and cannot yet distinguish two mirror-image orbital solutions; planned spectroscopic follow-up and continued monitoring are needed to resolve that ambiguity.
  • Because S301 passes far closer to Sgr A* than previous probes, its orbit is sensitive to frame-dragging from black-hole spin, and the team says combined observations over the next decade, plus ELT-era measurements at the 2031 pericenter, could deliver a direct spin measurement and tighter tests of general relativity.