The Tool Desk
Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Fast-moving stars give astronomers clues about the Milky Way’s centre and gravitational structure—but they are not all doing the same thing. Some are candidates for having been flung out of the Galactic Centre and may eventually escape the Galaxy. Others have uncertain origins. S301, reported by the European Southern Observatory (ESO) in August 2026, is different again: it orbits Sagittarius A*, the Milky Way’s central black hole, rather than flying away.
What counts as a hypervelocity star?
“Hypervelocity star” generally describes a star moving fast enough to escape its galaxy, or considered a candidate to do so. The label does not, by itself, establish where the star came from. Astronomers need reliable measurements of its motion and distance, then reconstruct its path to assess whether it could have originated near the Milky Way’s centre.
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That distinction matters because a high speed alone is not proof that a star was ejected by Sagittarius A*. Some high-speed candidates may have other origins, while stars close to the black hole can be extraordinarily fast yet remain bound in orbit.
How Gaia helps find fast-moving stars
The European Space Agency’s Gaia mission measures stellar positions and parallaxes, which help estimate distances, as well as motion across the sky. For a subset of stars, Gaia also measures radial velocity—their motion toward or away from Earth. Combining these measurements gives astronomers a star’s three-dimensional velocity, a more useful basis for assessing its path than sky motion alone.
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Researchers search these data for unusually fast candidates, account for measurement precision, and reconstruct likely trajectories. The resulting counts depend on the Gaia data release and the study’s selection method; they are not a census of every fast-moving star.
Two Gaia studies, two different candidate samples
| Study context | Reported result | What it means |
|---|---|---|
| Gaia DR2-era analysis, reported by ESA | Twenty candidates among seven million stars with full 3D velocity measurements | The study found stars that could be travelling fast enough eventually to escape the Milky Way; these were candidates, not a confirmed total population. ESA’s account |
| Gaia first-release analysis, reported by ESA in 2017 | Six stars traced back to the Galactic Centre, each reported above 360 km/s; one appeared to exceed 500 km/s | The team first narrowed a two-million-star dataset to 80 candidates, then identified six whose reconstructed paths pointed back to the centre. ESA’s 2017 account |
These figures describe different studies and datasets, so they should not be combined into a single current count. The cited reports do not give one up-to-date total for all hypervelocity stars.
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Where might fast stars come from?
A reconstructed path that leads back to the Galactic Centre can support an ejection scenario involving the central black hole. But the origin remains an inference from observations and trajectory calculations, not something established by speed alone.
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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteIn its Gaia DR2-era discussion, ESA noted that some candidates appeared to be moving toward the centre rather than away from it. Possible explanations included stars arriving from another galaxy, a star flung away when its companion exploded as a supernova, or a Milky Way halo star whose motion was affected by interactions during the Galaxy’s assembly. Those possibilities show why “fast-moving” and “ejected by Sagittarius A*” are not interchangeable descriptions.
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What the stars can reveal about the Milky Way
The central black hole and stellar ejections
Stars whose trajectories point back toward the Galactic Centre can help test explanations for how stars are accelerated there. The evidence is strongest when the measured motion is precise enough for the reconstructed path to meaningfully support a central origin.
The Galaxy’s gravitational field and dark matter
An unbound star’s trajectory is shaped by the Milky Way’s gravity as it travels outward. NASA’s 2010 account of the star HE 0437-5439 described how the shape of a hyperfast star’s path could help measure the gravitational pull of dark matter. That is a potential research application of trajectory data—not evidence that this one star has settled the Milky Way’s dark-matter distribution.
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NASA reported that HE 0437-5439 was moving at 1.6 million miles (2.5 million kilometres) per hour and lay about 200,000 light-years from the Galactic Centre in that account. Those figures refer to this individual star. NASA’s Hubble Mission Team account, published in 2010 and updated in 2025, discusses the reported inference about its origin.
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S301: fast, but orbiting Sagittarius A*
S301 should not be confused with an escaping hypervelocity star. ESO’s 19 August 2026 report describes it as a star in a close orbit around Sagittarius A*. It completes an orbit in 8.7 years, reaches about 25,000 km/s near its closest approach, and passes about 1.78 billion kilometres—around 12 times the Earth–Sun distance—from the black hole. ESO says that speed is over 8% of the speed of light.
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Those numbers describe S301’s orbit around Sagittarius A*, not its speed relative to the Milky Way’s escape threshold. An orbiting star can move at immense speed near a black hole without thereby being an escaping star.
How astronomers followed its orbit
ESO reports that researchers used the Very Large Telescope Interferometer (VLTI) and the GRAVITY instrument, later GRAVITY+, to detect and follow S301 near Sagittarius A*. The VLTI combines light from four 8-metre telescopes to create a virtual telescope with 15 times the spatial resolution of one 8-metre telescope. The team first glimpsed S301 in spring 2023, gathered observations to constrain its orbit, and traced its orbital history back to 2017. ESO’s report on S301 describes the observations and results.
Why the orbit could help measure the black hole’s spin
S301’s close orbit is sensitive to relativistic effects associated with Sagittarius A*’s rotation. ESO says that tracking at least two complete orbits could make a direct measurement of the black hole’s spin possible. The August 2026 release presented that as a future goal, not a measurement already completed; it described further observations and a next close passage in 2031.
Quick Recap
How to interpret a claim about a “fast star”
- Check what was measured: a full 3D velocity combines motion across the sky with radial velocity where available.
- Check the uncertainty: a candidate’s speed and path are only as useful as the precision of the underlying measurements.
- Ask what “fast” means in context: an escape candidate is being assessed against the Galaxy’s gravitational hold; S301’s reported peak speed describes motion in a close orbit around Sagittarius A*.
- Separate observation from inference: a measured position and motion can support a reconstructed origin, but they do not prove it by themselves.
- Keep counts tied to their study: Gaia candidate numbers depend on the release, dataset and selection method, rather than giving a universal population total.
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