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# Astronomers find a star circling our galaxy's black hole at 25,000 kilometres a second
- URL: https://www.bushletter.com/astronomers-find-a-star-circling-our-galaxy-s-black-hole-at-25-000-kilometres-a-second/
- Published: 2026-08-21T00:00:00.000Z
- Updated: 2026-08-20T23:59:59.000Z
- Description: The numbers are hard to hold in the mind. S301 is a newly discovered star orbiting Sagittarius A* at roughly 25,000 kilometres per second, making it the fastest known star in the Milky Way, moving at about 8 per cent of the speed of light, fast enough to cross Australia's entire east coast in a
- Author: Editor
- Tags: Science, Europe, European Southern Observatory

![Alex Mercer](https://res.cloudinary.com/dz77sb7j1/image/upload/v1774262566/bushletter/authors/alex-mercer.png)

By **Alex Mercer** · 2026-08-20

TLDR

Astronomers have confirmed S301 as the fastest known star in the Milky Way, circling the galactic centre's black hole at 25,000 kilometres a second. Its 8.7-year orbit brings it closer to Sagittarius A\* than any tracked star before, and the distortion of its path could finally reveal the black hole's spin.

KEY TAKEAWAYS

01S301 travels at roughly 25,000 km/s, making it the fastest known star in the Milky Way.

02The star completes a full orbit around Sagittarius A\* in just 8.7 years.

03At closest approach, S301 passes within about 1.78 billion km of the black hole.

04Frame-dragging from the spinning black hole measurably twists S301's orbital path.

05A confirmed spin measurement would constitute a direct test of Einstein's general relativity.

## A star moving at 8 per cent of the speed of light

The numbers are hard to hold in the mind. S301 is a newly discovered star orbiting Sagittarius A\* at roughly 25,000 kilometres per second, making it the fastest known star in the Milky Way, moving at about 8 per cent of the speed of light, fast enough to cross Australia's entire east coast in a fraction of a millisecond.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com) That velocity is not a quirk of measurement. It is what happens when a star is caught in the gravitational machinery of a four-million-solar-mass black hole at close range.

The GRAVITY+ Collaboration made the discovery using ESO's Very Large Telescope Interferometer at the Paranal Observatory in Chile. The findings are set to appear in *Nature* in a paper titled "Discovery of a star sensitive to the spin of Sgr A\*".[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com) The instrument combines light from four 8.2-metre telescopes to resolve faint stars near the galactic centre, stars roughly two billion times fainter than Betelgeuse, with a precision that earlier generations of astronomers could not approach.

## An orbit tighter than anything tracked before

What separates S301 from other stars previously mapped near Sagittarius A\* is not just speed but geometry. At its closest approach, S301 passes within about 1.78 billion kilometres of the black hole, roughly 12 times the Earth-Sun distance, a tighter pericentre than any previously observed star.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com) That gap is smaller than Saturn's orbit around our own Sun.

Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics and a study author, put the geometry plainly. "What is special about this star is that it's orbiting Sagittarius A\* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the Sun," Mang said. "That is unprecedented."[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com)

S301 completes that full circuit in 8.7 years.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com) Stars such as S2, among the best-tracked near Sagittarius A\*, take about 16 years and stay considerably further out. The tighter the orbit, the stronger the relativistic effects, and the more useful the star becomes as a precision instrument pointed at the black hole itself.

## Frame-dragging: how a spinning black hole bends space

The physics here is genuinely strange, and the engineering challenge of measuring it equally demanding. According to Einstein's general relativity, a rotating massive object does not merely curve spacetime around it; it drags spacetime along with it, the way a spinning ball bearing drags oil. This is frame-dragging, also called the Lense-Thirring effect, and around a supermassive black hole it is not negligible.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com)

The practical consequence is that a star's orbital plane does not stay fixed. It precesses, rotating slowly, in a way that depends directly on how fast the black hole is spinning and in which direction. Because S301's orbit brings it so close to the black hole, it experiences the frame-dragging effect strongly enough to offer a direct, measurable probe of the black hole's spin for the first time.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com)

Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics and a Nobel Prize laureate who has spent decades leading efforts to track stars at the galactic centre, described what S301 represents. "Decades carefully tracking stars orbiting our galaxy's central black hole, Sagittarius A\*, have led to this breakthrough discovery of a very promising star. Because it orbits so close to Sagittarius A\*, S301 opens a new window to the fundamental properties of spacetime in this extreme black-hole environment," Genzel said.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com)

## A direct test of general relativity

The spin of Sagittarius A\* has never been directly measured. Estimates and indirect inferences exist, but they carry large uncertainties. Measuring it through the precession of S301's orbit would change that, providing a number derived from observable mechanics rather than modelling assumptions.

General relativity has passed every test put to it so far, but nearly all those tests have been conducted in comparatively weak gravitational fields. The environment immediately around a four-million-solar-mass black hole is as extreme as anything accessible to observation from Earth, and deviations from Einstein's predictions are most likely to show up there.

Stefan Gillessen, a researcher at MPE and co-author of the study, was direct about what a confirmed spin measurement would mean. "For the first time, we would actually be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein's theory," Gillessen said.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com)

## The instrument that made it possible

Detecting S301 required the GRAVITY+ upgrades to ESO's Very Large Telescope Interferometer. Combining the light of four large telescopes through interferometry, the system can resolve structure at the galactic centre with enough angular precision to separate and track individual faint stars in one of the most crowded regions of sky observable from Earth.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com) The upgrade extended the instrument's sensitivity, moving the detectable threshold deeper into the stellar population near Sagittarius A\*.

Astronomers have been mapping the galactic centre systematically since the 1990s, first with infrared cameras on the VLT such as NACO and SINFONI, and from 2017 with the original GRAVITY interferometer.[\[1\]](https://www.eso.org/public/news/eso2612/?ref=bushletter.com) That decades-long baseline of positional data allowed the team to reconstruct S301's orbit with enough confidence to characterise its pericentre and velocity.

What comes next is continued tracking. Confirming the orbital precession caused by frame-dragging and extracting a spin value will require observing S301 through multiple 8.7-year cycles with the precision GRAVITY+ provides. The paper is scheduled to appear in *Nature*.

SOURCES & CITATIONS

1. [ESO Press Release eso2612: Discovery of a star sensitive to the spin of Sgr A\*](https://www.eso.org/public/news/eso2612/?ref=bushletter.com)

FREQUENTLY ASKED QUESTIONS

What is S301 and why is it significant?

S301 is a star recently discovered orbiting Sagittarius A\*, the supermassive black hole at the centre of the Milky Way. It is the fastest known star in the galaxy, moving at about 25,000 km/s, and its tight 8.7-year orbit brings it closer to the black hole than any previously tracked star.

What is frame-dragging and how does S301 help measure it?

Frame-dragging is the effect predicted by Einstein's general relativity in which a rotating massive object drags spacetime around with it. A spinning black hole causes nearby orbiting stars to precess, meaning their orbital planes slowly rotate. Because S301 orbits so close to Sagittarius A\*, this precession is strong enough to detect, giving astronomers a direct way to measure the black hole's spin for the first time.

How close does S301 get to Sagittarius A\* at its nearest point?

At pericentre, S301 approaches to within about 1.78 billion kilometres of the black hole, which is roughly 12 times the Earth-Sun distance, a tighter approach than any previously observed star.

What instrument detected S301?

S301 was detected using the GRAVITY+ instrument at ESO's Very Large Telescope Interferometer in Chile, which combines light from four 8.2-metre telescopes to achieve the resolution needed to track faint stars near the galactic centre.

![Alex Mercer](https://res.cloudinary.com/dz77sb7j1/image/upload/v1774262566/bushletter/authors/alex-mercer.png)

[Alex Mercer](https://bushletter.com/author/alex-mercer/?ref=bushletter.com)

Alex Mercer writes about technology, energy and infrastructure. He likes the physical end of the story: the plants, the grids and the machines that everything else depends on.