The center of our galaxy is an extreme environment. There, in the vicinity of the supermassive black hole Sgr A* weighing about 4 million Suns, dozens of S-stars crowd on fast elliptical orbits. Some of them likely got there via the Hills mechanism: a passing binary star was torn apart by tidal forces, leaving one star forever bound to the black hole while the other was flung out at a speed exceeding the galactic escape velocity. Until now, these two populations—hypervelocity stars and S-stars—have been studied separately, and no pair has ever been linked. Identifying such a pair would allow us to reconstruct the details of the disruption and test dynamical models near black holes.
The researchers capitalized on the uniqueness of S5-HVS1—the only hypervelocity star whose trajectory reliably points back to the Galactic center. Its mass (2.35 M⊙) and velocity (1799 km/s) are known with high precision. According to Law of Universal Gravitation, the specific orbital energy of the captured star is linked to its semi-major axis, and the law of conservation of energy provides a direct relationship between the companion's mass and its orbital size: a ≈ 500 AU (m/M⊙). The authors built a probabilistic model of the pair's origin within a Bayesian inference framework: by randomly varying the original binary's parameters (mass, mass ratio, phase) and simulating the subsequent diffusion of orbital angular momentum via two-body relaxation, they computed how plausible the observed configuration of S301 is under the hypothesis that it is related to S5-HVS1. This probability was compared against the null hypothesis of a random distribution of S-star orbits.
The key parameters of S301, measured by the GRAVITY interferometer, land squarely on the theoretical prediction. Its semi-major axis of 687±6 AU, combined with a photometric mass of 1.1–1.5 M⊙ estimated from stellar evolution models, calls for a mass of about 1.37 M⊙—right in the middle of the allowed range. The orbital eccentricity e=0.9832 means that at periastron the star comes within 11.5 AU of the black hole—closer than any other known S-star. Such an elongated orbit is natural for a recent tidal capture, although scattering of angular momentum over 4.8 million years might have slightly reduced the eccentricity. The individual Bayes factor for S301 reaches 90 (or 42 depending on the density profile), indicating strong evidence against a chance coincidence. However, when all 18 stars in the catalog with similar orbits are taken into account—with corrections for the survival and detection probability of the companion—the overall Bayes factor remains of order unity. This still supports the pairing hypothesis, but a final conclusion awaits more precise data.
If the relationship between S301 and S5-HVS1 is confirmed, astronomers will have the first documented “ejected–captured” pair resulting from the Hills mechanism. This would allow a detailed reconstruction of the original binary system and the tidal disruption itself, including the flyby distance and orbital phase at the moment of catastrophe. Moreover, such a link would serve as a calibration point for stellar dynamics models in the nuclei of other galaxies, where direct observations of individual stars are not yet feasible.
Future observations with the Extremely Large Telescope (ELT) will provide high-resolution spectra to measure the precise mass of S301 and its chemical composition. Stars born in the same binary system should have a nearly identical set of elements, which would be far more convincing than a mere match of orbital parameters.
The results will impact our understanding of processes near supermassive black holes, the mechanisms that populate the S-star cluster and the origin of hypervelocity stars, as well as statistical methods for cross-identification in multi-messenger astronomy.
Obtain the spectroscopic mass and chemical composition of S301, and compute the full GRAVITY selection function by injecting and recovering artificial stars in the data.
The study directly connects to unresolved questions: how does the observed distribution of S-stars around Sgr A* form; why is there a “zone of avoidance” for moderate eccentricities; what fraction of hypervelocity stars are produced by the Hills mechanism versus other processes—dynamical interactions in clusters or supernova explosions.
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What the work does
The researchers tested the hypothesis that the hypervelocity star S5-HVS1 and the S-star S301 were once components of a binary system that was torn apart by the tidal forces of the supermassive black hole Sgr A*. Using Bayesian modeling, they assessed the likelihood of the observed configuration of S301, relying on its orbital parameters from GRAVITY and its photometric mass, and compared it with the null hypothesis of a random distribution of S-star orbits.
Where its strength is
The work offers a concrete observational test of the Hills mechanism by comparing independently measured orbits of hypervelocity and S-stars, grounded in the conservation of energy and dynamical modeling. The link between S5-HVS1 and S301 provides a rare opportunity to reconstruct the history of tidal disruption and orbital evolution near the black hole.
Where it could go next
Worth trying
Why this may matter
Confirming the connection between hypervelocity and S-stars would turn them into a unified laboratory for studying the dynamics and population of binaries in the Galactic center, and would also allow calibrating models of tidal disruptions.
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The periastron of S301—11.5 astronomical units — is comparable to the distance from the Sun to Saturn. At that distance, a black hole roughly 20 times larger than our star would span a disk visible to the naked eye in the sky, if one could see the event horizon through the accretion flow.