What did astronomers discover about the faint star S301’s record-breaking orbit around the Milky Way’s central supermassive black hole Sagittarius A*—including its speed, 8.7-year needle-thin elliptical orbit, closest distance and comparison with S2, how it was detected and confi

S301 is the closest and fastest-known star orbiting Sagittarius A*: it reaches about 25,000 km/s (over 8% of light speed) while following an exceptionally eccentric, 8.7-year orbit. Its next close passage, expected around 2031, offers a realistic chance to measure the black hole’s spin—but S301 alone is unlikely to measure its quadrupole moment or decisively test the no-hair theorem.

What was discovered

• S301 is a faint main-sequence star with a K-band magnitude of about 19.3, on an orbit with eccentricity about 0.982–0.983 and a period of 8.7 years.

• At pericentre it comes to roughly 280 gravitational radii from Sagittarius A*, or just over 12 AU—approximately the Sun–Saturn distance—and is about ten times closer to the black hole than S2 at closest approach.

• It reaches a peak speed of about 25,000 km/s, making it the fastest known star in the Milky Way.

• Its “needle-thin” orbit means it spends most of its time far away but races through the strong-gravity region briefly at pericentre, where relativistic effects are concentrated.

How astronomers found and checked it

• The team detected S301 with the GRAVITY near-infrared interferometer at ESO’s Very Large Telescope Interferometer, despite its extreme faintness and the crowded, bright Galactic-centre field.

• They tracked its astrometric position in GRAVITY data from 2023–2025, fitted a preliminary orbit, then predicted and recovered it in archival observations from 2021 and 2017. That successful backward prediction provided an important confirmation that the faint source follows the proposed orbit.

• Spectroscopy has not yet yielded a direct radial velocity for S301, so its present orbital solution is based primarily on precision sky-position measurements; future extremely large telescope spectroscopy is expected to improve the full three-dimensional orbit.

Why it can reveal Sagittarius A*’s spin

• General relativity predicts that a spinning black hole produces Lense–Thirring frame dragging: its angular momentum slightly twists surrounding spacetime and causes the orbital plane and node of a nearby inclined orbit to precess.

• S301 comes close enough for this spin-driven nodal precession to become detectable with present high-precision infrared interferometry and future spectroscopy. Measuring it would constrain both the magnitude and direction of Sagittarius A*’s spin.

• This would extend Galactic-centre tests beyond the mass-driven Schwarzschild precession measured with S2, testing the rotating-black-hole, or Kerr, description predicted by Einstein’s general relativity.

What 2031 observations could—and could not—establish

• Dense astrometric and spectroscopic monitoring around the expected 2031 pericentre should provide the strongest leverage on S301’s relativistic orbital changes and could enable a direct spin measurement from Lense–Thirring precession.

• Measuring the black hole’s quadrupole moment is a harder goal. The spin-quadrupole signal predicted for S301 is estimated to be only about 4% of the Lense–Thirring effect and is regarded as negligible for this orbit.

• Consequently, there is insufficient evidence to claim that S301’s 2031 passage alone can test the no-hair theorem. That theorem requires checking whether the black hole’s quadrupole moment has the value fixed by its mass and spin; independently measuring that small quadrupole contribution would likely require substantially closer stars, multiple suitable stellar orbits, or another observational method.