In the arms of a black hole: the fastest star in the Galaxy has been found
Astronomers have discovered the fastest known star in the Milky Way. An object called S301 orbits the supermassive black hole Sagittarius A* in the center of the Galaxy and accelerates to about 25,000 km/s at maximum approach. The observations may allow scientists to directly measure the rotation of a massive black hole for the first time. This was reported on August 19 in the journal Nature.
The fastest star in the Milky Way
S301 moves in an extremely elongated orbit around Sagittarius A*, whose mass is about 4.3 million times the mass of the Sun. The star completes a complete revolution in 8.7 years, which is the shortest known period of a star's orbit around the central black hole of the Milky Way. The previous record was held by the star S55, also known as S0-102, with a period of about 12 years.
When passing the point of the orbit closest to the black hole, the speed of S301 reaches 25-25.6 thousand km/s, or about 8.3–8.5% of the speed of light. This is about 90 million km/h and about 100 thousand km/h. times the speed of a regular passenger plane.
At this point, the distance between the star and the black hole is reduced to about 1.78 billion km. This corresponds to 12 distances from the Earth to the Sun and is only about 20% greater than the distance between the Sun and Saturn. None of the known stars near the center of the Milky Way comes this close to Sagittarius A*.
"This is unprecedented," said Felix Mang, a graduate student at the Max Planck Institute for Extraterrestrial Physics and one of the study's authors.
The orbit of S301 is characterized by an extremely high elongation: its eccentricity is more than 0.98. At its closest point, the star appears to be only 136-142 Schwarzschild radii away from the black hole— about ten times closer than the well-studied star S2.
It was S2 that for decades served as the main natural tool for testing the general theory of relativity in the center of the Galaxy. In 2018, astronomers detected a gravitational redshift during its approach to Sagittarius A*, and later discovered an orbital rotation consistent with Einstein's predictions.
How will S301 help measure the rotation of a black hole
According to the general theory of relativity, a rotating massive object drags the surrounding space-time with it. This phenomenon is known as the Lenze—Thirring effect, or frame of reference entrainment. It can be roughly compared to how a rotating spoon makes the water in a cup move.
The effect should gradually change the orientation of the orbit of a nearby object. However, the impact weakens rapidly as it moves away from the black hole, so it has been extremely difficult to detect it by the movement of known stars until now. The orbit of S301 passes close enough to Sagittarius A* so that the changes caused by rotation can potentially be measured in the time available to scientists.
The main relativistic effect acting on S301 is related to the curvature of space-time by the mass of the black hole. Because of it, the point of maximum convergence of the star shifts by about 1.9–2 degrees after each revolution. As a result, the trajectory does not close into an ordinary ellipse, but gradually forms a shape resembling a rosette.
The additional displacement caused by the rotation of Sagittarius A* will be significantly less. Its magnitude depends on the speed of rotation of the black hole and the relative position of the axis of rotation and the orbit of the star. The changes accumulated over several passes can nevertheless become quite noticeable for modern and under construction telescopes.
According to the researchers, over the next decade it will be possible to directly determine the rotation parameters of Sagittarius A* based on the motion of S301. This will allow us to verify the description of rotating black holes within the Kerr metric and obtain a new test of the general theory of relativity in conditions of strong gravity.
"This would be a key test of Einstein's theory," said Stefan Gilessen, one of the lead authors of the work, an employee of the Max Planck Institute for Extraterrestrial Physics.
At the same time, the rotation of the black hole has not yet been measured. The authors of the work only talk about the sensitivity of the S301 orbit to the corresponding effect and the possibility of registering it in the future. For a reliable result, astronomers need to continue observations, obtain data on the radial velocity of the star and clarify the spatial orientation of its orbit.
Currently, scientists have two possible options for the location of the S301 trajectory. The uncertainty is due to the fact that the existing spectroscopic data do not yet allow us to measure the speed of the star's movement along the line of sight. However, both solutions give almost the same period of rotation, eccentricity, and maximum approach distance.
How did you manage to view the S301
S301 is extremely difficult to observe: in the Earth's sky, it looks about 2 billion times dimmer than Betelgeuse, one of the brightest stars in the constellation Orion. Its apparent magnitude in the infrared range K is 19.3.
To detect the object, the researchers used the very large interferometer telescope of the European Southern Observatory (VLTI), located at the Paranal Observatory in Chile, and the GRAVITY instrument mounted on it. The system combines the light from four telescopes with mirrors with a diameter of 8 m, effectively creating a single virtual telescope. Its spatial resolution is about 15 times higher than the capabilities of one such telescope.
Researchers first noticed S301 in the spring of 2023. Then the star was purposefully observed in 2024 and 2025. Knowing the preliminary parameters of the orbit, scientists were able to find traces of it in earlier data, including observations from 2017 and 2021.
In total, the researchers obtained 19 astrometric positions of S301, from which they reconstructed its trajectory. Scientists have determined that the star's last maximum approach to Sagittarius A* occurred in early 2023. Based on the brightness and behavior of the object, the researchers assumed that S301 is a main sequence star of late spectral class A or early class F. The most likely type is F1.5. Its mass can range from 1.1 to 1.5 times the mass of the Sun, and its radius is about 1.4 to 1.6 solar radii.
The compact size allows the S301 to withstand close proximity to a black hole without tidal disruption. A larger giant star in such an orbit could begin to lose its outer layers, but no signs of such a process have been found in observations.
Where did the star get its unusual orbit from?
Stars cannot form at such a small distance from a supermassive black hole. Therefore, the authors of the study suggest that in the past S301 was part of a binary star system. As the pair approached Sagittarius A*, the tidal forces of the black hole could tear it apart. As a result, S301 was captured by gravity and moved into an elongated orbit. The second star could have been ejected at such a high speed that it was able to completely leave the Milky Way. This scenario is known as the Hills mechanism.
The next closest approach of S301 to a black hole is expected in 2031. By this time, scientists expect to continue observations using the GRAVITY+ instrument, and later connect the MICADO instrument to the ESO extremely large telescope under construction. MICADO should help to obtain the spectrum of S301 and measure its radial velocity. Combining these data with accurate position information will restore the three-dimensional motion of the star and eliminate the existing uncertainty in the orientation of the orbit.
Simulations of observations up to 2035 have shown that with a favorable orientation and high rotation speed of Sagittarius A*, scientists will be able to determine the dimensionless rotation parameter with an error of less than 0.2. To do this, it will be necessary to track at least two complete rotations of S301 and especially study in detail its passage near the black hole in 2031.
In the longer term, observations can help verify the so-called no-hair theorem. According to her, an astrophysical black hole can be fully described using mass, angular momentum and electric charge, which in real space conditions, as a rule, should be close to zero.
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