Astronomers have discovered S301, the closest and fastest star ever seen orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way. Completing an 8.7-year orbit at roughly 55 million mph, the faint star provides researchers with an unprecedented laboratory to test Einstein’s theory of general relativity.
The center of the Milky Way sits some 27,000 light-years away, serving as one of the most extreme natural laboratories available to physicists. A small cluster of stars swarms the central supermassive black hole on tight, gravitational orbits. Because gravity there operates far more intensely than anywhere else astronomers can closely observe, those stars move in ways that classical physics never anticipated.
A Record-Breaking Orbit Around Sagittarius A*
The newly identified star, designated S301, sweeps past Sagittarius A* at a peak speed of roughly 55 million mph—about 8% the speed of light—after accelerating heavily on its approach. It completes a full lap every 8.7 years, making it both the fastest-moving stellar object ever recorded by science and the holder of the tightest known orbit around the 4.3 million-solar-mass black hole.
At its closest point, S301 skims within about 12 astronomical units of the black hole, roughly ten times closer than the previous record-holder, S2, ever approaches. That proximity places S301 roughly on par with the distance between the sun and Saturn. Despite those extreme forces, the star remains safe. Researchers describe it as an ordinary main-sequence star of about 1.5 solar masses, compact enough that the black hole’s tidal forces cannot tear it apart.
The star first appeared in images captured in spring 2023 as a faint smudge northwest of Sagittarius A*. Once researchers established a rough trajectory, they recovered the star in archival observations dating back to 2021 and 2017. In total, 19 measured positions spanning eight years trace out a complete, closed ellipse.
Filtering Out the Cosmic Noise
Pinpointing S301 required immense technical precision because the object is exceptionally dim—roughly two billion times dimmer than Betelgeuse, the 10th brightest star in the sky. Researchers compared detecting its light to trying to hear the buzz of a fly while a symphony orchestra is playing.
To overcome that challenge, the GRAVITY instrument on the European Southern Observatory’s Very Large Telescope in Chile, alongside the MICADO instrument on the Extremely Large Telescope, scanned the crowded region of the Galactic Center.
Probing Frame-Dragging and General Relativity
For decades, astronomers relied heavily on S2, a bright star on a 16-year orbit, to test Einstein’s theory of general relativity. Tracking S2 since 1992 revealed both gravitational redshift—light losing energy as it climbs out of a gravity well—and Schwarzschild precession, the slow rotation of an elliptical orbit. However, those effects depend solely on the mass of the black hole.
Black holes possess a second fundamental property: rotation. A spinning black hole drags the fabric of space around with it, much like a spoon stirring honey. Because that frame-dragging effect, known as the Lense-Thirring effect, fades rapidly with distance, measuring it requires a star that plunges far closer to the singularity than S2 ever reaches.
What Comes Next for Black Hole Physics
The discovery was formally reported in the journal Nature. While S301 alone cannot immediately confirm the exact spin rate of Sagittarius A*, researchers note it provides one of the first practical methods to determine spin using stellar dynamics.
Scientists point out that tracking the star’s rapid Schwarzschild precession—which swings the orbital ellipse around by about 2 degrees every lap—allows them to continuously tighten tests on general relativity and related theoretical models. Researchers project that within the next ten years, ongoing monitoring with instruments in Chile will bring a direct measurement of the black hole’s spin within reach.
