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S301 is a star orbiting Sagittarius A*—the supermassive black hole at the Milky Way’s centre—at a reported peak speed of 25,000 km/s, more than 8% of the speed of light. Its tight, 8.7-year orbit could let astronomers measure the black hole’s spin by tracking how that spin subtly changes the star’s path. The spin has not yet been measured from S301: the result so far is the star’s orbit, and a future measurement depends on additional observations.
What makes S301 remarkable?
“Fastest” here means peak orbital speed, not a star travelling freely across the galaxy. S301 follows an orbit around Sagittarius A*, reaching its greatest speed near its closest approach. The 2026 Nature paper reports an 8.7-year orbital period and a peak speed of 25,000 km/s. The European Southern Observatory (ESO) gives the closest-approach distance as about 1.78 billion km, roughly 12 times the Earth–Sun distance. ESO’s announcement describes it as the fastest star known at the time of publication; that record could change with future observations.
The speed and close passage matter because gravity’s effects become more pronounced near the black hole. S301’s orbit is unusually sensitive to Sagittarius A*’s properties, especially its spin, compared with the better-known star S2.
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How did astronomers measure its orbit?
The team identified S301 in 2023 using GRAVITY, a near-infrared interferometric instrument on ESO’s Very Large Telescope Interferometer, then followed its motion in dedicated observations in 2024 and 2025. The paper reports 19 astrometric positions—measurements of the star’s apparent location relative to Sagittarius A*. Earlier observing epochs also provide a strong inference from 2021 and a weaker one from 2017. Fitting these positions yielded the reported 8.7-year orbit. The Nature paper details the observations and orbital analysis.
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The orbit is a measured result; the black hole’s spin is not. Available ERIS integral-field spectroscopy did not detect S301 either as a continuum source or through spectral features, so the study does not yet report radial-velocity information for the star. Radial velocity—the component of motion toward or away from Earth—would add a useful dimension to the orbital measurements.
How can a star’s orbit reveal black-hole spin?
A star close to Sagittarius A* acts as a tracer of the black hole’s gravitational field. Astronomers have already studied relativistic effects in stellar orbits, including gravitational redshift, the transverse Doppler effect and Schwarzschild precession. A rotating black hole adds another influence: in the Kerr description of spacetime, its spin causes frame dragging, also called Lense–Thirring precession. In broad terms, the rotating black hole subtly shifts the orientation of an orbit over time.
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Because S301 passes closer to Sagittarius A* than S2 does, its orbit is more responsive to this spin-related effect on timescales astronomers can observe. The challenge is to distinguish that subtle signal from the other influences shaping the star’s motion. That requires continued, precise tracking rather than a single close-approach measurement.
What would it take to measure the spin?
The study presents a future opportunity, not a completed spin measurement. More observations are needed to refine the orbit, including radial-velocity data. The authors discuss future spectroscopy, including observations with the Extremely Large Telescope, as part of the path toward a spin constraint.
The paper also gives a numerical forecast, but it is conditional: mock-data simulations suggest a projected uncertainty below 0.2 in the dimensionless spin parameter under stated assumptions, including a maximally spinning black hole and simulated observations through 2035. This is not a present-day measurement or constraint. The forecast depends on assumptions about spin and orientation and on expected astrometric and radial-velocity precision. The authors also note that robust future estimates require modelling to second post-Newtonian order, particularly to avoid systematic bias at low spins. The actual result will depend on the data obtained and the modelling applied.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How might S301 have reached this orbit?
S301’s high orbital eccentricity—the degree to which its path is elongated rather than circular—suggests a possible origin through the Hills mechanism. In that scenario, a binary star passes close enough to the black hole to be torn apart; one star can be captured into a tight orbit while the other is ejected. The orbit supports this as a proposed explanation, but astronomers did not observe such an event happening to S301.
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