Astronomers using the Very Large Telescope Interferometer have identified S301, a star orbiting Sagittarius A* at record speed, offering a new opportunity to test general relativity near the Milky Way's supermassive black hole
Astronomers have identified the fastest known star in the Milky Way, a faint object designated S301, which races around the supermassive black hole Sagittarius A* at the galaxy's center. The discovery, based on high-precision observations with the European Southern Observatory's Very Large Telescope Interferometer (VLTI), provides a rare opportunity to study the effects of extreme gravity and test predictions of general relativity in the immediate environment of a massive black hole.
Observing S301's Extreme Orbit
S301 was detected using the GRAVITY instrument on the VLTI, which combines the light from multiple telescopes to achieve the angular resolution needed to track stars near Sagittarius A*. The star follows a highly elongated orbit, completing a full revolution around the black hole in just 8.7 years. At its closest approach, S301 comes within roughly 12 times the Earth-Sun distance of Sagittarius A*, a proximity unmatched by any previously tracked star in this region.
During its fastest passage, S301 reaches speeds of about 25,000 kilometers per second-over 8% the speed of light. This velocity is not only a record for stars in the Milky Way but also places S301 among the fastest-moving stars known anywhere. The star's orbit was reconstructed from VLTI data collected in 2023 and earlier archival observations from 2021 and 2017, allowing researchers to confirm its trajectory and speed with high confidence.
Testing Relativity Near a Black Hole
The close orbit of S301 offers a unique laboratory for testing Einstein's theory of general relativity. According to the theory, a rotating black hole should drag spacetime around with it-a phenomenon known as frame dragging or Lense-Thirring precession. Because S301 passes so near Sagittarius A*, its orbit is expected to experience measurable distortions over the next decade, potentially allowing astronomers to directly constrain the black hole's spin for the first time.
Previous studies of stars near Sagittarius A* have already provided evidence for relativistic effects, but S301's tighter orbit and higher speed make it especially sensitive to the predicted spacetime warping. The research team plans to continue monitoring S301 with upgraded instruments, including the forthcoming GRAVITY+ and the Extremely Large Telescope, to capture its next close approach in 2031 and refine measurements of the black hole's properties.
Possible Origins and Future Prospects
S301's highly eccentric orbit suggests it may have once been part of a binary star system that ventured too close to Sagittarius A*. In such scenarios, the black hole can capture one star while ejecting the other at high velocity-a process known as the Hills mechanism. The fate of S301's possible companion remains unknown, but the dynamics of this region are consistent with previous models of stellar interactions near supermassive black holes.
The discovery of S301 adds to a growing body of evidence about the complex environment at the heart of the Milky Way. As astronomers continue to track stars in this region, they are building a more detailed picture of how gravity operates under extreme conditions. Related research has also explored the impact of black hole activity on surrounding gas and stars, as seen in recent findings on energetic outflows from distant quasars, such as those described in a study of black hole-driven turbulence in intergalactic space.
The team's findings on S301 were published in the journal Nature on August 19, 2026, and represent a significant step toward direct measurements of black hole spin and further tests of fundamental physics in the strong-gravity regime.
Understanding Stellar Motions Near Black Holes
Tracking stars like S301 near Sagittarius A* relies on advanced techniques in infrared interferometry, which allow astronomers to resolve individual stellar orbits despite the dense and dusty environment at the galactic center. By combining light from multiple telescopes, the VLTI achieves the resolution necessary to distinguish stars separated by just a few milliarcseconds-equivalent to resolving a coin on the Moon from Earth. These observations not only reveal the dynamics of stars under the influence of a supermassive black hole but also provide critical tests of gravitational theory in a regime inaccessible elsewhere in the universe.
