Astronomers say S5-HVs1 ventured close to supermassive black hole before being ejected
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.
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An illustration by the Royal Astronomical Society of the
location of the hyper-fast star and the direction of its motion Photograph:
Sergey Koposov/PA |
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 MonthlyNotices of the Royal Astronomical Society.