Runaway star ejected from the centre of the Milky Way at incredible speed

An artist’s impression of S5-HVS1’s ejection by Sagittarius A*, the black hole at the centre of the galaxy. The black hole and the captured binary partner to S5-HVS1 are seen far away in the left corner of the picture, while S5-HVS1 is in the foreground, speeding away from them. ( James Josephides (Swinburne Astronomy Productions))

Astronomers have discovered a star travelling at an incredible 6 million km/h — ten times faster than the average star — after being ejected by the supermassive black hole at the centre of the Milky Way five million years ago.

Carnegie Mellon University Assistant Professor of Physics Sergey Koposov discovered the star — named S5-HVS1 — as part of the Southern Stellar Stream Spectroscopic Survey (S5).

“The velocity of the discovered star is so high that it will inevitably leave the galaxy and never return,” said Douglas Boubert from the University of Oxford, a co-author of the study.

An artist’s impression of the ejection mechanism of a star by a supermassive black hole. Credit: James Josephides (Swinburne Astronomy Productions)

S5-HVS1 — located in the constellation of Grus — is part of a population of objects known as ‘high-velocity stars’ (HVSs). These stars sparked curiosity amongst astronomers after the first example was discovered in 2005. In the next 14 years, many more examples of HVSs have been uncovered.

But, even amongst these aptly-named stars, S5-HVS1 is exceptional for its high speed. The star’s close passage to Earth at a mere (in astronomical terms) 2.9 x 10⁴ light-years away, also makes it somewhat unique.

Armed with information about the runaway star’s blazing speed coupled with its close proximity has allowed astronomers to track its trajectory back to the centre of the Milky Way and the supermassive black hole — Sagittarius A* (Sgr A*) — which dwells there.

“This is super exciting, as we have long suspected that black holes can eject stars with very high velocities. However, we never had an unambiguous association of such a fast star with the galactic centre,” says Koposov, the lead author of this work and member of Carnegie Mellon’s McWilliams Center for Cosmology. “We think the black hole ejected the star with a speed of thousands of kilometres per second about five million years ago.

“This ejection happened at the time when humanity’s ancestors were just learning to walk on two feet.”

A bad break-up?

So how on Earth did S5-HVS1 come to be travelling at such an extraordinary speed?

Astronomers believe that the star was once part of a binary system with a companion star. It was ejected from this partnership after both stars’ orbits strayed too close to Sgr A*. Whilst its partner was captured by the incredible gravitational attraction of the supermassive black hole, the gravitational struggle tore S5-HVS1 free and launched it on its rapid journey.

The location of S5-HVS1 on the sky and the direction of its motion. The star is flying away from the galactic centre, from which it was ejected 5 million years ago. ( Sergey Koposov)

This process is known as the ‘Hills mechanism’ and was first suggested by astronomer Jack Hills thirty years ago and has long been considered as a likely mechanism for the origins of high-velocity stars.

“This is the first clear demonstration of the Hills Mechanism in action,” points out Ting Li from Carnegie Observatories and Princeton University, and leader of the S5 Collaboration. “Seeing this star is really amazing as we know it must have formed in the galactic centre, a place very different from our local environment.

“It is a visitor from a strange land.”

An exceptional observation

The astronomers made the discovery of S5-HVS1 was made with 3.9-metre Anglo-Australian Telescope (AAT) near Coonabarabran, NSW, Australia. The team was only able to assess the true speed of the star and details of its incredible journey when these observations were coupled with further data from the European Space Agency’s Gaia satellite.

“The observations would not be possible without the unique capabilities of the 2dF instrument on the AAT,” adds Daniel Zucker, an astronomer at Macquarie University in Sydney, Australia, and a member of the S5 executive committee. “It’s been conducting cutting-edge research for over two decades and still is the best facility in the world for our project.”

The team’s results are published in the journal Monthly Notices of the Royal Astronomical Society.

“I am so excited this fast-moving star was discovered by S5,” says Kyler Kuehn, at Lowell Observatory and a member of the S5 executive committee. “While the main science goal of S5 is to probe the stellar streams — disrupting dwarf galaxies and globular clusters — we dedicated spare resources of the instrument to searching for interesting targets in the Milky Way, and voila, we found something amazing for ‘free.’

“With our future observations, hopefully, we will find even more!”

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