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Fast-moving stars can help astronomers investigate the Milky Way’s central black hole and the Galaxy’s gravitational field—but “fast-moving” describes different kinds of stars. Some are candidates for ejection from the Galactic Centre, while others, such as S301, remain in tight orbits around Sagittarius A*. Their measured motions and reconstructed paths—not speed alone—show what each star can tell us.
What are hypervelocity stars?
Hypervelocity stars are stars moving fast enough to potentially escape the Milky Way. Some are thought to have been flung outward after an interaction involving the supermassive black hole Sagittarius A*, at the Galaxy’s centre. But a high speed on its own does not prove that a star came from there: astronomers need accurate motion measurements and a reconstructed trajectory that supports that origin.
Not every star moving unusually fast is a confirmed hypervelocity star, and not every fast star is heading out of the Galaxy. Some candidates may have other origins, while S301 is a star orbiting close to Sagittarius A*, not an escaping star.
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Gaia measures motion across the Galaxy
The European Space Agency’s Gaia mission measures stellar positions, parallaxes—which help estimate distance—and movement across the sky. For stars with radial-velocity measurements, the motion toward or away from Earth, astronomers can estimate full three-dimensional velocities. Researchers then look for unusually fast candidates and trace their paths to assess where they may have come from.
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The resulting counts depend on the Gaia data release and the researchers’ selection method. In a Gaia DR2-era study, researchers identified 20 stars among seven million with full 3D velocity measurements that could be moving fast enough to eventually escape the Milky Way, according to the ESA report. These were candidates, not a census of confirmed escapees.
A separate analysis of Gaia’s first data release used a neural-network search and narrowed a dataset of two million stars to 80 candidates. Researchers traced six of those stars back to the Galactic Centre; each had a reported velocity above 360 km/s. One appeared to exceed 500 km/s, according to ESA’s 2017 account. Those figures describe that study’s sample and method, not the total number of fast-moving stars in the Galaxy.
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Interferometry tracks S301 near Sagittarius A*
S301 was found using a different approach. The European Southern Observatory (ESO) describes observations with the Very Large Telescope Interferometer and the GRAVITY instrument, later GRAVITY+. By combining light from four 8-metre telescopes, the facility creates a virtual telescope with 15 times the spatial resolution of one 8-metre telescope, according to ESO’s 19 August 2026 report. Researchers first glimpsed S301 in spring 2023, followed it to constrain its orbit, and traced its orbital history back to 2017.
What can these stars reveal about the Milky Way?
Some candidates may have been ejected from the Galactic Centre
When a star’s reconstructed path points back to the Galactic Centre, that supports an ejection scenario involving Sagittarius A*. It does not make every detail of the star’s history certain. ESA has also discussed possible alternatives for some fast-moving candidates, including stars originating in another galaxy, stars ejected after a companion’s supernova, and halo stars whose motion was altered by interactions as the Milky Way assembled. In its Gaia DR2-era report, co-author Tommaso Marchetti said, “Rather than flying away from the Galactic centre, most of the high velocity stars we spotted seem to be racing towards it.” The direction and reconstructed path matter as much as the measured speed.
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Escaping stars can probe the Galaxy’s gravitational field
If a star is moving out of the Milky Way, the shape of its trajectory can provide clues about the Galaxy’s gravitational pull, including the influence of dark matter. NASA’s account of HE 0437-5439, published in 2010 and updated in 2025, describes this as a way to study dark matter—not as proof that one star has settled its distribution. For that individual star, NASA reported a speed of 1.6 million miles (2.5 million kilometres) per hour and a location about 200,000 light-years from the Galactic Centre.
S301 may help test how a black hole’s spin affects an orbit
ESO reported that S301 completes an orbit around Sagittarius A* in 8.7 years. At its closest approach, it passes about 1.78 billion kilometres away—around 12 times the Earth–Sun distance—and reaches a reported speed of about 25,000 km/s, more than 8% of the speed of light. These figures describe its orbit around the black hole; they are not a measure of whether it can escape the Milky Way.
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Because S301 orbits so close to Sagittarius A*, relativistic effects make its motion sensitive to the black hole’s rotation. ESO says tracking at least two complete orbits could allow a direct measurement of the black hole’s spin. In the 19 August 2026 release, that measurement was a future goal, with a next close passage expected in 2031. Reinhard Genzel, a founding member of the observing collaboration, said S301 “opens a new window to the fundamental properties of spacetime in this extreme black-hole environment.”
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How to tell what a fast star’s speed does—and does not—mean
| Question | Potentially escaping hypervelocity candidate | S301 |
|---|---|---|
| What is it doing? | Moving fast enough that it could escape the Milky Way; escape and origin must be assessed from its measured motion and reconstructed path. | Orbiting Sagittarius A*; ESO reported an 8.7-year orbital period. |
| What does the speed help establish? | Whether the star could escape, considered alongside the relevant escape threshold, measurement uncertainty, and trajectory. | How quickly it moves along its close orbit; ESO reported about 25,000 km/s near closest approach. |
| What can its motion reveal? | A possible Galactic Centre origin and, for unbound trajectories, clues to the Galaxy’s gravitational field. | Relativistic orbital effects that could support a future measurement of Sagittarius A*’s spin. |
| What is not established by speed alone? | That the star definitely came from Sagittarius A* or will escape. | That it is an escaping hypervelocity star or that Sagittarius A*’s spin has already been measured. |
The historical Gaia studies and the newer S301 result concern different datasets, methods, and types of motion. The cited official reports do not provide one up-to-date total for all hypervelocity stars, so their candidate counts should be read in the context of each study.
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