S301: The Fastest Star Near Sagittarius A* Could Reveal the Black Hole’s Spin

S301 is also exceptionally faint, with a reported K-band magnitude of about 19.3. Researchers classify it as a likely main-sequence star, although its faintness makes precise characterization difficult.

How astronomers found it

The GRAVITY team detected S301 in spring 2023 with the GRAVITY instrument on the European Southern Observatory’s Very Large Telescope Interferometer. The star appeared about 15 milliarcseconds northwest of Sgr A* and then moved outward, showing rapid, slightly curved motion across the 2023 observations.

Once researchers used those measurements to estimate S301’s orbit, they searched earlier GRAVITY observations for where the star should have appeared. They recovered it in archival data from 2021 and then from 2017, extending the observational baseline and strengthening the orbital interpretation.

The orbit is still preliminary. In particular, the researchers do not yet have a radial-velocity measurement for S301, so future observations will be important for refining its three-dimensional motion and orbital parameters.

Why S301 could measure black-hole spin

A rotating black hole does more than attract nearby objects. In general relativity, its angular momentum can drag local spacetime around with it, an effect known as frame-dragging or Lense–Thirring precession. For an orbiting star, this can gradually change the orientation of the orbital plane.

The strength of that effect falls rapidly with distance. S301’s unusually small pericenter therefore makes it much more sensitive to Sgr A*’s spin than stars on wider orbits. By repeatedly measuring the star’s position with high precision, astronomers could look for the small, cumulative changes expected from a rotating black hole and compare them with the orbital shifts expected if the black hole were nonrotating.

That is a future opportunity, not a result already achieved. Continued astrometric monitoring before S301’s anticipated close approach in late 2031—and for roughly a decade afterward—could determine whether the accumulated motion is consistent with a rapidly rotating or stationary Sgr A*.

A longer-term test of the no-hair theorem

S301 may eventually support an even more demanding test: measuring the black hole’s quadrupole moment. This describes how rotation affects the shape of the black hole’s gravitational field beyond the simpler effects of mass and spin.

In the Kerr solution of general relativity, an isolated black hole’s externally measurable properties are determined by its mass and spin, with electric charge expected to be negligible for an astrophysical black hole. Testing the predicted relationship between spin and quadrupole moment would therefore probe the so-called no-hair theorem.

That measurement is considerably harder than detecting spin. The quadrupole signal is smaller and could be confused with gravitational perturbations from other stars or unseen compact objects near the Galactic center. Researchers therefore describe it as a possible longer-term goal rather than a guaranteed outcome.

What happens next

The immediate priority is to keep tracking S301 and improve the orbital fit. More measurements can establish the star’s radial motion, sharpen the timing and distance of pericenter, and distinguish relativistic effects from uncertainties in the surrounding stellar environment.

If the predicted orbit holds, S301 will offer an unusually sensitive test of how a supermassive black hole rotates and how that rotation influences nearby spacetime. Its record speed is the headline, but its real scientific value lies in turning a faint, fast-moving star into a precision probe of Sagittarius A*.