Superfast star found leaving Milky Way at 1,700km per second

Astronomers have spotted a star heading out of the Milky Way at more than 6m km/h (3.7m mph), or 1,700km per second, after an encounter with the supermassive black hole at the centre of the galaxy.

The star is moving so fast that in about 100m years it will exit the Milky Way and spend the rest of its life sailing alone through intergalactic space. Although it was predicted 30 years ago that black holes could fling stars out of the galaxy at phenomenal speeds, it is the first time that such an event has been recorded.

Gary Da Costa, an astronomer and emeritus professor at the Australian National University (ANU) in Canberra, said: “We traced this star’s journey back to the centre of our galaxy, which is pretty exciting.”

Da Costa and colleagues have deduced that 5m years ago, the star was part of a binary star system that had ventured extremely close to Sagittarius A*, the location of the Milky Way’s central black hole, which has a mass equivalent to more than 4m suns.

As the twin stars spiralled inwards, at some point the closer of the two switched into a binary partnership with the black hole that would ultimately end in it being gobbled up and disappearing into oblivion. The dynamics of this interaction resulted in the original partner being ejected at extremely high speed.

The process is known as the Hills mechanism, after the astronomer Jack Hills who proposed the scenario more than 30 years ago.

“This star is travelling at record-breaking speed, 10 times faster than most stars in the Milky Way, including our sun,” said Da Costa. “In astronomical terms, the star will be leaving our galaxy fairly soon and it will likely travel through the emptiness of intergalactic space for eternity.”

The star, known as S5-HVS1, is the third-fastest star ever measured. The other two were ones that had been boosted to high speeds in supernovae explosions.

“Excluding these somewhat special cases, this star is far and away the fastest ever spotted,” said Dougal Mackey, a co-author also at ANU College of Science.

The team made the discovery of the star using the 3.9-metre Anglo-Australian Telescope at the ANU Siding Spring Observatory. The team were looking at star streams in the Milky Way halo that are coasting towards the galactic disk but had some spare capacity to look at other stars, and through these observations made the serendipitous discovery of S5-HVS1.

After the star exits the Milky Way it will continue its journey through the intergalactic space. “It will keep going and eventually end up as a white dwarf like our sun; it just won’t have any neighbours,” said Da Costa.

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

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