Astronomers have discovered what could be the fastest known star in the Milky Way. Named S301, the star is racing around Sagittarius A*, the supermassive black hole at the center of our galaxy, at an astonishing speed of about 25,000 kilometers per second.
That is more than 8% of the speed of light and roughly 100,000 times faster than a commercial aircraft. But S301 is remarkable for another reason: it passes closer to Sagittarius A* than any other star observed so far. Its extreme orbit could give scientists a rare opportunity to measure the rotation, or spin, of the black hole and test Einstein’s theory of general relativity in one of the most extreme environments in the Universe.
A Star on an Extraordinary Orbit
Sagittarius A* contains the mass of roughly four million Suns and lies at the center of the Milky Way. For decades, astronomers have carefully tracked stars moving around it to understand how gravity behaves near such an enormous object.
S301 has now emerged as an especially valuable target.
The star completes one orbit around Sagittarius A* in only 8.7 years. During its closest approach, it comes within approximately 12 times the distance between Earth and the Sun. This puts it extraordinarily close to the black hole.
According to Felix Mang, a Ph.D. student at the Max Planck Institute for Extraterrestrial Physics (MPE) and an author of the study, the orbit of S301 is unprecedented among the stars observed around Sagittarius A*.
At its closest point, S301 reaches approximately 25,000 km/s, making it the fastest star currently known in our galaxy.
Why S301 Is So Important
Speed alone does not make S301 scientifically special. Its close approach to Sagittarius A* is what makes the star particularly valuable.
According to Einstein's general theory of relativity, massive objects do not simply pull on objects through space. They also curve spacetime around them. A rotating black hole takes this effect even further.
A spinning black hole can drag the surrounding spacetime along with its rotation. This phenomenon is known as frame dragging.
Imagine placing a heavy ball on a stretched sheet and spinning it. The sheet around the ball would be twisted and dragged. A rotating black hole produces a much more extreme version of this effect in spacetime.
For a star orbiting far away from a black hole, this effect is extremely difficult to detect. But S301 travels so close and so quickly that the influence could become measurable.
Astronomers therefore hope that S301 could provide the first direct measurement of the spin of a massive black hole.
A New Test of Einstein's Theory
Einstein's general theory of relativity has successfully passed many tests, but scientists continue searching for situations where its predictions can be examined under the most extreme conditions.
Sagittarius A* provides exactly such an environment.
If astronomers can accurately track S301's orbit over multiple years, they may be able to identify tiny changes caused by the black hole's rotation. Measuring these changes would allow researchers to determine how fast Sagittarius A* spins.
Stefan Gillessen of MPE, who also played a leading role in the research, says that directly measuring the spin of a massive black hole would provide an important test of Einstein's theory.
Researchers expect that the next decade will be particularly important. S301 is predicted to make its next closest approach to Sagittarius A* in 2031.
By comparing its motion during multiple passages, astronomers can gradually build an extremely precise picture of its orbit.
How Did Astronomers Find Such a Faint Star?
Detecting S301 was far from easy.
The star appears approximately 2 billion times fainter than Betelgeuse, one of the brightest stars visible in the night sky. It is located in the crowded central region of the Milky Way, where countless stars and the bright surroundings of Sagittarius A* make observations extremely challenging.
To overcome this problem, the research team used the Very Large Telescope Interferometer (VLTI) at the European Southern Observatory's Paranal Observatory in Chile.
The VLTI combines light collected by four 8-meter telescopes. Instead of functioning as four separate telescopes, they can work together as an interferometer, effectively creating a much more powerful "virtual telescope."
The system provides approximately 15 times higher spatial resolution than a single 8-meter telescope.
The team used the GRAVITY instrument, and later its upgraded version, GRAVITY+, to observe the region surrounding Sagittarius A*.
The researchers first detected S301 in 2023 and continued observing it to determine its orbit. They were also able to trace its orbital history back to observations from 2017.
These observations revealed that the star had passed closest to Sagittarius A* in early 2023.
A Star That May Have Been Thrown Into the Black Hole's Grip
Another mystery surrounds the origin of S301.
Stars cannot easily form extremely close to a massive black hole because the black hole's powerful gravitational environment makes star formation difficult.
So how did S301 get there?
One possibility is that S301 once belonged to a binary star system—two stars gravitationally bound to each other.
When the binary system came close to Sagittarius A*, the black hole's enormous tidal forces could have torn the two stars apart.
One star may have become trapped in an orbit around the black hole, becoming S301, while its companion was violently ejected at enormous speed.
The escaping companion may have been accelerated enough to leave the Milky Way entirely. Such objects are known as hypervelocity stars.
This possible origin story could help astronomers understand how stars move through the chaotic environment surrounding the Milky Way's central black hole.
The Next Big Opportunity Comes in 2031
The discovery of S301 is only the beginning.
Astronomers will continue observing the star using GRAVITY+ and, in the future, the MICADO instrument on the European Southern Observatory's Extremely Large Telescope (ELT).
The ELT is expected to provide unprecedented observing capabilities, allowing researchers to track S301's position and movement with extraordinary precision.
The crucial moment will come in 2031, when S301 makes its next close passage around Sagittarius A*.
By observing at least two complete orbits, scientists hope to constrain the star's trajectory accurately enough to detect the subtle influence of the black hole's spin.
If successful, astronomers could determine the rotation of Sagittarius A* directly for the first time.
A New Window Into Extreme Gravity
S301 is much more than a record-breaking speedster. It could become a natural laboratory for studying gravity under conditions that cannot be reproduced on Earth.
Its extraordinary speed, extremely close orbit and repeated passages around Sagittarius A* give scientists a rare opportunity to observe how a star behaves in strongly curved and rotating spacetime.
As Felix Mang put it, successfully measuring the black hole's spin would be a "dream come true."
The discovery demonstrates how decades of patient observations, combined with increasingly powerful instruments, can reveal new details about the most mysterious objects in the Universe.
If future observations succeed, S301 may do more than earn the title of the fastest known star in the Milky Way. It could also help humanity measure the spin of the black hole at the heart of our galaxy—and provide one of the most precise tests yet of Einstein's description of gravity.
Reference: Abd El Dayem, K., Abuter, R., Aimar, N. et al. Discovery of a star sensitive to the spin of Sagittarius A*. Nature (2026). https://doi.org/10.1038/s41586-026-10894-w
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