S301 (Star)
S301 is a faint main-sequence star orbiting Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way. It was identified through near-infrared interferometric observations made with the GRAVITY instrument at the European Southern Observatory's Very Large Telescope Interferometer (VLTI). The discovery was reported by the GRAVITY collaboration in a paper published in Nature on 19 August 2026. S301 has an orbital period of approximately 8.7 years and follows a highly eccentric orbit that brings it substantially closer to Sagittarius A* than the previously known S-stars whose orbits have been measured. Its closest approach (pericenter) is approximately 140 Schwarzschild radii from the black hole, where its orbital velocity reaches AbOUT 25,000–25,600 kilometers per second, or approximately 8.3–8.5% of the speed of light.
The combination of S301's short orbital period, small pericenter and high orbital velocity makes it a highly sensitive probe of the relativistic gravitational field surrounding Sagittarius A*. In particular, its orbit may be sufficiently affected by the rotation of Sagittarius A* that continued measurements could provide a dynamical determination of the black hole's spin. This is due to a relativistic effect known as frame dragging, specifically the Lense–Thirring effect, in which the rotation of a massive rotating object causes the surrounding spacetime to be dragged relative to distant inertial frames.
Discovery and observations
S301 was first identified in observations obtained in the spring of 2023. At that time, the star appeared approximately 15 milliarcseconds northwest of Sagittarius A*. Over four observations during 2023, the star was observed to move outward with a very large proper motion. The measured motion was approximately 44 milliarcseconds per year in one coordinate direction and 27 milliarcseconds per year in the other, corresponding to an angular speed of approximately 2,000 km/s at the distance of the Galactic Centre. The trajectory was also slightly curved, indicating that the apparent motion was not adequately explained by uniform linear motion characteristic of a body not in orbit.
The rapid motion led the researchers to suspect that S301 might be on a tightly bound orbit around Sagittarius A*. Dedicated observations were subsequently obtained in 2024 and 2025. These observations provided eight additional astrometric measurements in 2024 and five in 2025. This allowed astronomers to create a mathematical representation of the star's orbit, which they used to predict where the star should have appeared in earlier observations. A likely precovery of S301 was subsequently found in data from 2021, while a less-likely one was found in data from 2017.
In total, the published orbital analysis used 19 astrometric positions covering more than eight years of observations. These measurements trace an elliptical path on the sky that is consistent with a bound orbit around Sagittarius A*. The use of multiple observing epochs was important because the orbital interpretation depends not simply on detecting the star but on measuring the change in its position over time.
S301 is extremely faint. Its measured K-band apparent magnitude is approximately 19.3 ± 0.3. The star has not yet been detected spectroscopically as a continuum source or through identifiable spectral features in the available Extremely Large Telescope spectrographic precursor data used by the researchers. Consequently, a direct radial-velocity measurement was not possible.
The lack of radial-velocity information means that two possible orbital orientations are consistent with the available astrometric measurements. They differ principally in the sign of the line-of-sight component, while other important orbital quantities, including the semi-major axis, eccentricity and pericenter epoch, remain very similar. In principle, light-travel-time effects, known as the Rømer delay, can help distinguish between The Two orientations, but the effect is too minor for current observations to distinguish between the two orbital possibilities.
Orbital properties
The published best-fit orbital solution gives S301 a semi-major axis of approximately 83 milliarcseconds. This is about 33% smaller than the semi-major axis of S2, the best-studied star orbiting Sagittarius A*. S301 has an orbital period of approximately 8.69 years, establishing it as the shortest-period star currently known to orbit Sagittarius A*. Before the discovery of S301, the record was held by S55, also designated S0-102, with an orbital period of approximately 12 years.
The orbit of S301 is exceptionally eccentric. The two viable orbital orientations have eccentricities of approximately 0.9832 and 0.9821 in the published solution. An eccentricity close to one indicates that most of the orbit's radial extent is concentrated far from its pericenter, with the star spending a relatively small proportion of its orbital period near its closest approach to the black hole. The corresponding pericenter distances are approximately 136 and 142 Schwarzschild radii. A Schwarzschild radius is the radius of a non-rotating black hole of a given mass. For S301, the pericentre is roughly an order of magnitude closer to Sagittarius A* than the pericenter of S2. It is of approximately the same order of magnitude as the distance from the Sun to Saturn.
S301's velocity increases dramatically as it approaches pericentre, with peak speeds of approximately 25,000 km/s, or about 8.5% of the speed of light. The high velocity is a consequence of the extremely deep well of gravitational potential in which the star moves during its closest approach to Sagittarius A*.
The orbital solution predicts that S301 passed pericentre around 2023, around the time it was discovered. Around this time, and after the curvature of the star's observed trajectory was at its highest.
Relativistic orbital effects
The orbit of S301 is strongly influenced by general-relativistic effects. A major effect is Schwarzschild precession, also called relativistic periapsis precession. In Newtonian physics, an object in orbit would follow the same ellipse on every revolution. General relativity instead predicts that the point of closest approach gradually advances (precesses), causing the orbital ellipse to rotate after each revolution.
For S301, the predicted Schwarzschild advance of the pericentre is approximately 2.0° per orbit for one potential orientation and 1.9° per orbit for the other. At those rates, the orientation of the orbital ellipse within its own plane would complete a full 360° rotation on a timescale of approximately 1,560–1,630 years.
The Schwarzschild effect happens regardless whether the central object is spinning. However, Sagittarius A* has been observed to rotate. A rotating black hole produces additional relativistic effects described by the Kerr metric. Among these is frame dragging, in which the rotation of the black hole modifies the geometry of spacetime around it. The resulting Lense–Thirring precession depends on the angular momentum of the black hole as well as on the orbit's distance and orientation relative to the black hole's spin axis. S301's close orbit is useful because the effect of spin on a body's orbit becomes much stronger at small distances from the black hole. According to the published analysis, the Lense–Thirring precession could reach approximately 0.11° per revolution if Sagittarius A* is rotating at the maximum rate. This is small compared with the Schwarzschild precession, but large enough that continued high-precision astrometric observations may be capable of detecting the difference.
The significance of this distinction is that the Schwarzschild contribution depends primarily on the central mass, whereas the Lense–Thirring contribution contains information about the black hole's angular momentum. Measuring the latter would provide a direct dynamical measurement of Sagittarius A*'s spin, which has historically proven difficult to measur.
Sensitivity to the spin of Sagittarius A*
The spin of a black hole is conventionally described by a dimensionless spin parameter, commonly denoted χ, defined as cJ/(GM²), where J is the black hole's angular momentum, M its mass, G the gravitational constant and c the speed of light. The parameter ranges from zero for a non-rotating black hole to one for an extremal Kerr black hole.
The spin of Sagittarius A* is difficult to measure dynamically because frame-dragging effects become weaker with increasing distance from the black hole. Most of the previously known S-stars have larger pericentre distances and therefore experience comparatively small spin-dependent perturbations over observationally practical timescales. S301's pericentre of approximately 140 Schwarzschild radii means it experiences larger relativistic effects over shorter timescales.
The GRAVITY collaboration describes S301 as one of the first, and currently the only practical, stellar-dynamical opportunities for measuring the spin of Sagittarius A* within roughly a decade. Continued measurements with GRAVITY, combined with future spectroscopic observations from the Extremely Large Telescope, are expected to make the effect increasingly distinguishable from the non-spinning Schwarzschild prediction.
The researchers performed simulations in which future observations were added to the existing data. In a representative mock dataset extending through 2035, the resulting constraints could determine the dimensionless spin magnitude with an uncertainty smaller than approximately 0.2 and could constrain the orientation of the spin axis to roughly ±30°. These values are predictions from simulated observations rather than measurements already obtained from S301, and their success depends on observational performance and on the actual orientation of Sagittarius A*'s spin relative to the orbit.
The effect of Lense–Thirring precession is extremely small. After several years, the predicted difference between the trajectories of S301 around a non-rotating black hole versus a rotating black hole would only differ by about an astronomical unit, or the distance between the Earth and the Sun. Despite this small physical difference, it is potentially within reach of the precision of interferometric astrometry combined with future Extremely Large Telescope spectroscopy.
Astrometric measurement
S301 was measured primarily through interferometric astrometry. GRAVITY combines the light collected by multiple telescopes of the VLTI to achieve substantially greater angular resolution than a single telescope of comparable aperture. This allows individual stars extremely close to Sagittarius A* to be spatially separated and their positions to be tracked over time.
The researchers used several independent analysis approaches. S301 was identified in reconstructed GRAVITY images and was also recovered using separate point-source data fitting techniques. They also separately used the CLEAN imaging procedure to extracting the faint source from noise in the interferometric data. The agreement between their results served as a cross-check of the source's inferred position.
The observational problem is complicated by the strong and variable infrared emission of Sagittarius A* and by the high stellar density of the Galactic Centre. The published analysis therefore used dedicated data processing and cleaning procedures designed to distinguish faint sources from the brighter and variable central emission.
Physical nature
The available photometry indicates that S301 is a compact main-sequence star rather than a giant. Using its observed K-band magnitude, an assumed extinction toward the Galactic Centre and an adopted Galactic-Centre distance of approximately 8.3 kiloparsecs, the researchers derive an absolute K-band magnitude of about 2.28. This is consistent with a relatively faint main-sequence star and inconsistent with the luminosity expected for a giant at the same distance.
The photometric analysis places S301 in the late-A to early-F spectral range. Under the adopted stellar models, its properties are consistent with an approximately F1.5 star with a luminosity of about 5.5 times that of the Sun and a mass of somewhat less than 1.5 solar masses in the representative model. More generally, the researchers estimate a mass of approximately 1.1–1.5 solar masses depending on its age, with younger stellar ages corresponding to higher inferred masses.
The compact nature of S301 is important to its survival in its orbit. A large evolved star would have a substantially larger physical radius and would therefore encounter much stronger tidal forces at its pericentre, possibly tidally disrupting and destroying the star. The observed continued existence of S301 after its close passage provides evidence against it being a giant. A main-sequence interpretation is also consistent with photometric data.
The researchers' models indicate that S301's radius is expected to be approximately 1.4–1.6 solar radii, depending on its adopted stellar parameters. If true, this would mean that S301's tidal-disruption radius is closer to Sagittarius A* than its pericentre, and therefore, the star would not be destroyed as it makes its closest approach to the black hole. The study also finds that tidal heating is not expected to dominate the star's evolution or orbital dynamics.
Direct spectroscopy would provide a more definitive determination of S301's physical properties. In particular, a measurement of its radial velocity would be able to determine which of the two potential orbital solutions is correct. The GRAVITY collaboration expects that future observations with the Extremely Large Telescope and its planned MICADO instrument will be capable of detecting the star spectroscopically.
Possible origin
The unusually high eccentricity of S301 has important implications for its possible origin. The GRAVITY collaboration proposes that S301 may be the captured remnant of a binary system that passed sufficiently close to Sagittarius A* for the black hole's tidal field to disrupt the binary. This process is generally known as the Hills mechanism.
In the Hills mechanism, a stellar binary approaches a massive black hole. If the binary passes close enough, tidal forces can overcome the gravitational binding between its two members. One star can become tightly bound to the black hole while the other receives sufficient energy to escape the Galactic Centre at very high velocity as a hypervelocity star. The captured star remains on a highly eccentric orbit around the black hole.
The orbital properties of S301 are consistent with such a history. The study argues that the star's long-term orbital evolution would not have significantly altered its semi-major axis through two-body relaxation or gravitational-wave inspiral over its lifetime. Consequently, the present orbit can still provide information about the binary from which S301 originated.
Using the measured orbit, the researchers infer that the progenitor binary could have had a separation of approximately 0.1 astronomical units. Compact binaries with such separations are known among F-type stars, and the authors argue that the properties of S301 are compatible with a binary-disruption origin. Under this interpretation, its companion would have been ejected from the Galactic Centre as a hypervelocity star while S301 remained gravitationally bound to Sagittarius A*.
A separate 2026 study has proposed that S301 may specifically be related to the known hypervelocity star S5-HVS1, which has a measured Galactic-Centre origin. That work argues that the mass and orbital properties of S301 are expected of the captured component of a Hills-disrupted binary. However, it is not confirmed that the two stars were originally in a binary.
Relationship to the S-star cluster
S301 belongs observationally to the population of so-called S-stars: young or relatively young stars observed in the central parsec of the Milky Way and gravitationally bound to Sagittarius A*. These stars are particularly useful in Galactic-Centre astronomy because their motions are heavily affected by the black hole.
S2 has historically been the most important member of this population for relativistic tests. Its repeated close approaches have allowed astronomers to measure the gravitational redshift, transverse Doppler effect and Schwarzschild precession associated with the central black hole. S301 allows the possibility of measuring even more extreme relativistic effects because it passes significantly closer to Sagittarius A*.
Gravitational interactions with these other stars or with unseen masses could potentially perturb S301's orbit, meaning that the orbit would be slightly different than if those stars were not there. However, the researchers estimate that such perturbations are minor for S301 in the context of the spin measurement because the star orbits close enough to the black hole that relativistic precession dominates over the perturbations expected from nearby stars. Nonetheless, continued monitoring of the Galactic Centre population remains important for modelling these possible sources of orbital disturbance.
Comparison with S2 and other short-period stars
S301 differs from S2 in several significant respects. Although S2 remains the better-observed star and has played the central role in previous tests of general relativity around Sagittarius A*, S301 has a smaller semi-major axis and a substantially smaller pericentre distance. Its orbital period is approximately 8.7 years compared with approximately 16 years for S2, while S55/S0-102 previously represented the shortest-period orbit known at approximately 12 years.
The principal advantage of S301 is therefore not simply its shorter period but its combination of short period and extreme eccentricity. These properties cause the star to enter a much stronger gravitational field during each close passage. As a consequence, the relativistic changes accumulate faster than they would for a star remaining at larger distances.
The published analysis estimates that S301's Schwarzschild precession is approximately 1.9–2.0° per orbit. This is substantially larger than the relativistic shift associated with the orbit of S2. The spin-dependent Lense–Thirring term is also correspondingly larger, which is why S301 is considered a promising target for a direct stellar-dynamical spin measurement.
Future observations
The scientific value of S301 depends heavily on continued monitoring. Each additional astrometric measurement refines the orbital solution and increases the ability to distinguish the effects produced by different gravitational models. In particular, observations during and after subsequent orbital phases can separate the slowly accumulating effects of Schwarzschild precession from the smaller spin-dependent perturbations.
Future near-infrared interferometry with the VLTI is expected to provide continued high-precision positional measurements. The Extremely Large Telescope, currently under construction, is expected to add high-angular-resolution spectroscopy. The combination of astrometry and spectroscopy would provide both the orbital plane of the star relative to the sky as well as its velocity relative to the Earth's line of sight, allowing a single orbital solution to be narrowed down.
Spectroscopic observations would also provide an independent test of the star's physical classification and could yield information about its rotational velocity. The GRAVITY collaboration notes that the expected properties of a captured F-type binary component imply a stellar equatorial rotation velocity of approximately 20–70 km/s under the proposed Hills scenario.
Scientific significance
Individual S-stars have already demonstrated that relativistic effects predicted by general relativity can be measured through their motion around Sagittarius A*. S301 occupies a more extreme orbit in which higher-order relativistic terms become observationally relevant on humanly accessible timescales.
A successful measurement of the Lense–Thirring effect using S301 would be significant because it would provide an orbital measurement of the angular momentum of the Milky Way's central black hole. The result would complement other observational approaches to studying Sagittarius A*, including measurements of its electromagnetic emission and the structure of the material immediately surrounding it. Stellar dynamics has the advantage that the measurement would arise from the motion of an external test body in the black hole's gravitational field rather than from measurement of the black hole itself.
S301 is also significant as a potential record-holder within the population of known Galactic-Centre stars. Its approximately 8.7-year period is the shortest currently reported around Sagittarius A*, while its approximately 140-Schwarzschild-radius pericentre is roughly ten times smaller than that of S2. Its peak speed of approximately 25,000 km/s makes it one of the most relativistic stellar orbits currently known around a supermassive black hole.
Current limitations
Despite the strength of the orbital evidence, several important properties of S301 remain uncertain. The star has not yet been detected with sufficient spectroscopic signal to obtain a direct radial velocity. Consequently, its orbit could potentially be oriented in two directions. A future spectroscopic detection should allow the orientation to be narrowed down.
Its exact stellar mass, age and spectral type are also inferred primarily from photometry and stellar-evolution models rather than from direct spectroscopy, which would allow a more precise determination based on the star's blackbody spectrum.
Likewise, the Hills mechanism is presently a proposed explanation for S301's high eccentricity rather than an observationally confirmed origin. The possible association with S5-HVS1 is even more tentative. Establishing either scenario would require additional observational evidence.