Milky Way’s fastest star discovered spinning around black hole

• Astronomers discovered the fastest star in the Milky Way, S301, orbiting the central black hole Sagittarius A* at 25,000 km per second
• S301 was likely part of a binary system torn apart by the black hole, with its partner star ejected from the galaxy.
• Scientists aim to use S301's orbit to measure the black hole’s spin and test Einstein’s theory of relativity within the next decade.

PARIS – Astronomers have discovered the fastest star in the Milky Way zooming at 25,000 km a second around the supermassive black hole at the centre of our galaxy.

The star, named S301, orbits so dangerously close to the black hole that scientists believe it can shed light on this unimaginably powerful cosmic monster – and test out Albert Einstein’s theory of relativity.

“Decades carefully tracking stars orbiting our galaxy’s central black hole, Sagittarius A*, have led to this breakthrough discovery,” Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany said in a statement.

“Because it orbits so close to Sagittarius A*, S301 opens a new window to the fundamental properties of space-time in this extreme black-hole environment,” added Genzel, who shared a 2020 physics Nobel prize for discovering the black hole.

To spot S301 – which is two billion times fainter than the star Betelgeuse in our sky – the team of astronomers used the European Southern Observatory’s Very Large Telescope in the Chilean desert.

They first noticed the star in 2023, and have been tracking its orbit since.

“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,” explained Felix Mang, a co-author of a new study describing the discovery.

The star gets roughly as close to the black hole as Saturn gets to our Sun, which is “unprecedented”, he emphasised.

Torn from its partner

When the star gets closest to the immense suck of the black hole, it is travelling around 25,000 km per second.

That is 100,000 times faster than a commercial airplane, or more than 8 per cent of the speed of light, making it the fastest star ever detected in our galaxy.

Scientists believe that S301 was once part of a binary star system that was torn apart by the almighty pull of the black hole.

While S301 became trapped in the black hole’s gravity, its partner star was blasted out with such force it likely left the galaxy entirely.

The astronomers will get another good look at the star when it makes its next closest approach to the black hole in 2031.

The team hopes that data from two complete orbits will allow them to determine the star’s trajectory with enough precision that they can also directly measure the black hole’s rotation.

Like most objects in the universe, black holes are thought to spin.

Einstein’s theory of general relativity predicted that a spinning black hole would drag space-time along with it, twisting it in a way that changes the orbits of the surrounding stars.

Within the next 10 years, the astronomers hope to use S301 to measure the black hole’s spin for the first time.

“Without this star, we would need to measure the motion of other stars for several more decades to get anywhere close to measuring the spin of the black hole,” explained French astronomer and study co-author Juan Osorno.

The study was published in Nature on Aug 19. AFP