The S301 star near Sagittarius A is the fastest known star in the Milky Way and may help astronomers directly measure the central black hole’s spin.
A record-setting orbit around the Milky Way’s central black hole
Astronomers working with the European Southern Observatory’s Very Large Telescope Interferometer have identified S301, a faint star orbiting Sagittarius A*, the Milky Way’s central supermassive black hole. Announced by ESO on August 19, 2026, the result matters because this exceptionally tight orbit could allow a direct measurement of the black hole’s rotation—a difficult but important test of gravity in an extreme environment.
According to ESO’s release, S301 reaches about 25,000 kilometers per second, or more than 8% of the speed of light, at its closest approach. The star completes one orbit in just 8.7 years and came closest to Sagittarius A* in early 2023. Its reported closest distance is about 1.78 billion kilometers, roughly 12 times the average Earth-Sun distance.
The S301 star near Sagittarius A is the fastest known star in the Milky Way and may provide a direct way to measure the spin of the galaxy’s central black hole.
Why can S301 help measure the spin of Sagittarius A*?
A black hole’s spin is not a visible solid-body rotation. In Einstein’s general relativity, a rotating massive object drags nearby spacetime slightly along with it, an effect called frame-dragging or the Lense-Thirring effect. The closer and faster an orbiting object is, the stronger this subtle influence should be.
ESO reports that S301 is the first known star whose orbit may be suitable for directly detecting this effect around Sagittarius A*. Repeated, very precise measurements of the star’s position and motion could reveal whether its orbit shifts in the way predicted for spacetime around a spinning black hole. This is a scientific goal, not yet a completed measurement: the collaboration says it hopes to measure the spin within the next decade.
How astronomers detected such a faint star
S301 is extraordinarily challenging to observe because it lies in the crowded, dusty region around the Milky Way’s center and is extremely faint from Earth’s perspective. The team used the GRAVITY instrument, now upgraded as GRAVITY+, at ESO’s VLTI in Chile.
The VLTI combines light collected by four 8-meter telescopes. By using interferometry—combining the light waves so they act like observations from a much larger virtual instrument—it can distinguish fine detail that a single telescope cannot resolve. ESO says the team first glimpsed S301 in spring 2023, then used observations reaching back to 2017 to constrain its orbit.
What S301’s path may reveal about its past
Stars are not expected to form so close to a supermassive black hole, and ESO notes that S301’s orbit points to a possible earlier encounter involving a binary star system. In that scenario, Sagittarius A*’s tidal gravity pulled the pair apart: one star became bound to the black hole, while the other was flung outward at high speed.
That is an interpretation based on the star’s orbital properties rather than a direct observation of the breakup. Continued monitoring will refine the trajectory and test the explanation. ESO says the next close approach is expected in 2031, and observations spanning at least two full orbits would provide the precision needed for a spin measurement.
What does the S301 star near Sagittarius A discovery show?
The discovery shows how a star on an unusually compact orbit can serve as a moving test particle in the gravitational field of a supermassive black hole. It does not show that astronomers have already measured Sagittarius A*’s spin, but it identifies a promising route to do so.
The result is also a reminder that the center of the Milky Way is an active laboratory for testing physics. S301 is not visible to the unaided eye or typical amateur equipment; its importance comes from high-resolution infrared interferometry and long-term tracking of its orbit.
