Astronomers have identified S301, the fastest known star in the Milky Way, orbiting Sagittarius A* at 25,000 km/s (15,500 miles/s) and completing an 8.7-year elliptical loop just 12 times Earth’s distance from the Sun.
The star S301, detected by the European Southern Observatory’s Very Large Telescope Interferometer (ESO’s VLTI), travels at 25,000 kilometers per second—100,000 times faster than a commercial jet—and orbits Sagittarius A* on a highly eccentric path. This proximity allows astronomers to study the black hole’s extreme gravitational effects, including its spin, which could validate Einstein’s general theory of relativity.
A Star That Tests Relativity
S301’s orbit, tracked since 2017, offers a unique opportunity to measure Sagittarius A*’s spin. For the first time, we would actually be able to measure very directly the spin of a massive black hole,
said Dr. Stefan Gillessen, a senior scientist at MPE and co-author of the study. The star’s velocity—peaking at 8% the speed of light—means it experiences extreme spacetime distortion, a phenomenon predicted by Einstein’s theory but rarely observed at such scales.
This is incredibly important as a discovery,
said Dr. Ziri Younsi, an astrophysicist at University College London not involved in the observations. It’s very exciting because this thing comes insanely close to Sagittarius A* and it’s travelling, at its peak, at about 8% of the speed of light, which is crazy.
How the Discovery Was Made
The findings, published in Nature, rely on data from the VLTI, which combines light from four 8-meter telescopes to achieve high-resolution imaging. Astronomers traced S301’s orbit by analyzing its motion, with projections suggesting its next closest approach to Sagittarius A* will occur in 2031. With this star we hope to measure, within the next 10 years, the spin of the black hole,
said Felix Mang, a PhD student at MPE and an author of the study published on the findings.
The star’s extreme speed and proximity challenge previous assumptions about black hole environments. Black holes are perceived as these cosmic vacuum cleaners engulfing everything they can get a handle on,
Mang said. But it’s just a massive object. It’s not really much different from something orbiting the sun and it’s going to remain on this orbit.
S301’s survival near the event horizon—though it will not cross it—demonstrates the complex interplay between gravity and orbital mechanics.
What This Means for Astronomy
The discovery could redefine how scientists study black holes. By using S301 as a “probe,” researchers aim to map the gravitational field around Sagittarius A* with unprecedented precision. The star’s orbit may also shed light on the Milky Way’s evolution, as supermassive black holes are believed to shape their host galaxies through gravitational interactions.
Experts predict more such stars could be found as telescopes improve. This is just the tip of the iceberg,
said Tuan Do, an astronomer at the University of California, Los Angeles, who had no role in the study. As we get fainter, there should be many more stars like this.
A New Window on the Milky Way’s Core
Studying S301 could answer fundamental questions about the universe. By observing how the star’s motion is affected by the black hole’s spin, scientists may confirm or refine theories about how gravity behaves in extreme conditions. Such insights could also inform models of other galaxies, where supermassive black holes are thought to play a critical role in star formation and galactic structure.
The research underscores the importance of long-term astronomical projects. As telescopes like the VLTI continue to push observational boundaries, the study of S301 may become a cornerstone of black hole research—offering a rare, direct view of one of the cosmos’ most mysterious phenomena.
