A newly discovered faint star racing around Sagittarius A* could give astronomers a new way to measure the spin of the supermassive black hole at the centre of the Milky Way. Named S301, the star completes an orbit in about 8.7 years and approaches Sagittarius A* far more closely than the famous star S2, placing it in a region where subtle effects produced by the black hole's rotation may eventually become measurable.
The discovery was made by the GRAVITY+ Collaboration using the GRAVITY instrument at the European Southern Observatory's Very Large Telescope Interferometer. Researchers report that S301 has the shortest known orbital period among stars established around Sagittarius A* and follows an extremely elongated orbit that brings it to only about 136–142 Schwarzschild radii from the black hole.
A Star on an Extreme Orbit Around Sagittarius A*
Sagittarius A* has a mass of approximately 4.3 million Suns and lies about 8.3 kiloparsecs from Earth. Stars orbiting close to it provide astronomers with natural test particles for studying gravity in one of the most extreme environments accessible to direct astronomical observations.
For years, S2 has been the most important of these stars. It completes an orbit in roughly 16 years and passes within about 1,400 Schwarzschild radii of Sagittarius A*. Observations of S2 have already revealed gravitational redshift and the relativistic advance of its orbit predicted by the Schwarzschild description of a non-rotating black hole.
S301 goes considerably deeper into the gravitational field. The researchers find a semi-major axis of about 83 milliarcseconds and an eccentricity close to 0.983. Its closest approach is therefore around ten times smaller than that of S2.
At pericentre, S301 is calculated to travel at approximately 25,000–25,600 kilometres per second, corresponding to about 8.3–8.5% of the speed of light. At such velocities and distances, relativistic corrections to its motion become substantial.
How GRAVITY Found S301
The team has regularly observed the central region surrounding Sagittarius A* with GRAVITY since 2017. The instrument combines light from multiple telescopes of the Very Large Telescope Interferometer, allowing extremely precise infrared measurements of stars packed into the crowded Galactic Centre.
S301 was first recognised in observations from spring 2023, when researchers identified a faint source about 15 milliarcseconds northwest of Sagittarius A*. Its rapid movement suggested that the object could be following a compact orbit around the black hole.
Dedicated observations followed during 2024 and 2025. Once a preliminary orbit was established, the researchers worked backwards and searched earlier observations for the star. They found a strong inference of S301 in 2021 data and a weaker detection in observations from 2017.
In total, the study uses 19 astrometric positions spanning more than eight years. Together, those measurements trace an ellipse on the sky that can be fitted with a relativistic orbital model.
S301 is extremely faint, with a measured K-band magnitude of 19.3 ± 0.3. The researchers have not yet obtained a radial-velocity measurement because existing ERIS spectroscopy did not detect a sufficiently strong continuum or spectral feature. As a result, two possible three-dimensional orientations of the orbit remain consistent with the current data.
Relativity Is Already Reshaping the Orbit
S301's orbit is so compact and eccentric that it does not return to precisely the same orientation after each revolution. General relativity predicts that the point of closest approach advances as the star moves through the curved spacetime surrounding Sagittarius A*.
The study calculates a relativistic pericentre advance of approximately 1.9–2.0 degrees during each 8.7-year orbit. This Schwarzschild precession is substantially larger than the relativistic orbital shift previously measured for S2.
But the researchers are particularly interested in a still subtler effect produced by a rotating black hole: Lense–Thirring precession, commonly described as frame dragging.
In general relativity, a spinning massive object does not merely curve spacetime. Its rotation also drags the surrounding spacetime with it. An object moving sufficiently close to a rotating black hole should therefore experience small changes in the orientation of its orbit that depend on both the magnitude and direction of the black hole's spin.
S301 Could Become a Probe of Black Hole Spin
For S301, the predicted in-plane Lense–Thirring contribution is about 0.11 degrees per orbit multiplied by factors representing the magnitude and orientation of Sagittarius A*'s spin. Although small, this effect is potentially within reach of high-precision astronomical measurements.
The researchers simulated observations extending from 2026 through 2035 to estimate how well future measurements could recover the black hole's spin. In one favourable scenario involving a maximally rotating black hole with its spin approximately aligned with S301's orbital angular momentum, simulated observations could constrain the dimensionless spin parameter to an uncertainty below 0.2.
The forecast assumes astrometric precision of about 100 microarcseconds, future radial-velocity measurements accurate to approximately 1 kilometre per second, regular observations during each year and additional measurements around pericentre. Under those assumptions, the simulated data distinguish the maximally spinning case from a non-rotating black hole at greater than five-sigma significance.
That result is a forecast rather than a measurement of Sagittarius A*'s spin. The actual sensitivity will depend on the black hole's true spin magnitude and orientation, observational precision, orbital coverage and the ability to model other gravitational effects near the Galactic Centre. The study therefore concludes that there is a reasonable prospect of directly measuring the spin within roughly a decade rather than claiming that such a measurement has already been made.
Future ELT Observations Could Be Critical
Obtaining S301's radial velocity will be an important next step. Based on its brightness, the researchers identify the star as probably a late A-type or early F-type main-sequence star, with an estimated mass of roughly 1.1–1.5 solar masses. Their preferred interpretation corresponds approximately to spectral type F1.5.
Future spectroscopy with the MICADO instrument on ESO's Extremely Large Telescope is expected to be capable of detecting the star's spectral features and measuring its radial velocity. Combining spectroscopy with continued GRAVITY+ astrometry would provide a much more complete reconstruction of the three-dimensional orbit.
The researchers also note that sufficiently precise future spin measurements will require more sophisticated relativistic modelling, including second post-Newtonian corrections, particularly when attempting to measure relatively low black-hole spins.
Other Objects Could Complicate the Measurement
Sagittarius A* is not completely isolated. Stars, stellar remnants and potentially a population of stellar-mass black holes occupy the region around it. Their gravitational influence can perturb S301's orbit and potentially imitate or obscure some of the precession caused by frame dragging.
The researchers tested this problem with simulations containing a population of stellar-mass black holes around Sagittarius A*. Their results suggest that such Newtonian perturbations will generally be smaller than the Lense–Thirring signal for moderate-to-high black-hole spins and favourable orientations, although some configurations could produce stronger interference.
There may also be a useful observational distinction between the effects. Frame dragging is strongest close to S301's pericentre, while perturbations from surrounding objects can produce their largest observable deviations elsewhere in the orbit. Tracking the star through multiple orbital phases may therefore help astronomers separate the signals.
S301 May Be the Survivor of a Destroyed Binary
The unusual orbit also provides clues to S301's history. Forming a star directly this close to Sagittarius A* is difficult because of the enormous tidal forces produced by the black hole. The researchers instead suggest that S301 may have arrived through the Hills mechanism.
In this process, a binary star system passes close enough to a massive black hole to be torn apart by its gravity. One component can be captured into a highly eccentric orbit around the black hole while the other is flung away at high velocity.
The extreme eccentricity of S301 is naturally compatible with this mechanism. From its present orbit, the researchers estimate that the original binary may have had a separation of approximately 0.05–0.2 astronomical units and an orbital period of roughly 5–20 days.
If this interpretation is correct, S301 would be the captured member of that former binary, while its companion would have escaped as a hypervelocity star. The study does not establish this history conclusively, however. Dynamical interactions over time can alter stellar eccentricities, so the present orbit does not uniquely preserve the conditions under which S301 arrived.
A New Laboratory for Strong-Gravity Physics
S301 provides astronomers with an unusually sensitive stellar probe of the spacetime surrounding the Milky Way's central black hole. Its combination of a short orbital period, extreme eccentricity and exceptionally small pericentre brings it into a regime where the rotation of Sagittarius A* may leave an observable signature within a practical observing timescale.
A successful spin measurement would add an independent dynamical test to other methods used to estimate black-hole rotation. Over longer periods and with still more precise measurements, the researchers suggest that stars such as S301 could eventually contribute to constraints on the black hole's quadrupole moment and tests of the Kerr black-hole description predicted by general relativity.
For now, Sagittarius A*'s spin remains unmeasured by S301. The discovery instead provides something potentially more valuable for the coming decade: a natural experimental probe moving repeatedly through one of the strongest gravitational fields that astronomers can track star by star.