Fastest Star In The Galaxy

Quite recently, a pair of stars approached the giant black hole at the centre of our Milky Way galaxy.

One star was swallowed and lost to creation entirely – adding its paltry mass to Sagittarius A, the black hole weighing in at four million suns that lurks in our galaxy’s core.

Its companion received a massive gravitational slingshot and is in the process of being spat out of the galaxy entirely – at the highest speed ever recorded in our neighbourhood.

It’s big: 2 1/2 times as massive as our Sun; and it’s fast – travelling at 1755km per second, 0.0006 per cent of the speed of light or Sydney to Melbourne and back in a single heartbeat.

But relax, it’s not headed our way.

The star, dubbed HVS-1 (for high velocity star 1) is now headed south, out of the galactic plane, and into the lonely deep where the stars are few – in racing parlance: weather cold, track fast.

“It’s been given enough energy that it’s leaving the galaxy and in 100 million years it will have escaped the Milky Way’s gravitational pull,” says the rogue star’s co-discoverer, Australian National University astronomer, Emeritus Professor Gary Da Costa.

The discovery also confirms a 30-plus year old theory of astronomer Jack Hills, who predicted that if a binary star system ever encountered Sagittarius A, one would be swallowed and its partner would achieve galactic escape velocity.

Professor Da Costa and his ANU colleagues Dr Dougal Mackey and Dr Thomas Nordlander were involved with the study, which was led by Dr Sergey Koposov from Carnegie Mellon University as part of the Southern Stellar Streams Spectroscopic Survey.

“When Sergey got the velocity, he thought there had been a mistake. The team at Siding Springs confirmed the velocity and we got very excited and stated looking at its orbit to find out where it had come from.”

When the team tracked HVS-1’s orbit back to Sagittarius A, they had their culprit.

Mackey says the team spotted the fast-moving star serendipitously while searching for the shredded remains of small galaxies orbiting the Milky Way.

“The star is only 29,000 light years away, quite close by galactic standards, which means the team could measure its trajectory very precisely,” says Mackey.

Nordlander says supermassive black holes can slingshot stars by interacting with a binary stellar system, whereby two stars orbit around each other.

“If such a binary system approaches a black hole too closely, the black hole can capture one of the stars into a close orbit and kick out the other at very high speed.”

ANU Siding Spring Observatory’s 2dF instrument is the best in the world for studying the sparsely distributed stars in the outskirts of the Milky Way, thanks to its ability to obtain simultaneous measurements of up to 400 targets at a time.

Follow-up observations made with the ANU’s 2.3 metre telescope played an important role in confirming the star’s extreme speed.

The results of the study are published in the Monthly Notices of the Royal Astronomical Society.

And HVS-1 will keep travelling until it reaches the end of its days, says Da Costa.

“It will probably have reached the end of its evolutionary life before it encounters any other galaxies.”

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