press release
Published: 12 November 2019

Hyper-fast star spotted after ejection by our galaxy’s supermassive black hole

Astronomers have identified an ultrafast star traveling at a blistering 6 million km/h, after being ejected by the supermassive black hole at the heart at the Milky Way five million years ago. This star is moving at such speed it will leave the galaxy, and rocket into intergalactic space.

The discovery of the star, known as S5-HVS1, was led by Sergey Koposov from Carnegie Mellon University with a team of collaborators around the world, including Denis Erkal at the University of Surrey, as part of the Southern Stellar Stream Spectroscopic Survey (S5). Located in the constellation of Grus – ‘the Crane’ – S5-HVS1 was found to be moving ten times faster than most stars in the Milky Way. 

Astronomers have wondered about high velocity stars since their discovery only two decades ago. S5-HVS1 is unprecedented due to its high speed and close passage to the Earth, “only” 29,000 light years away. With this information, astronomers could track its journey back right into the centre of the Milky Way, where a four million solar mass black hole, known as Sagittarius A*, lurks. 

“This is very 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,” said Sergey Koposov, the lead author of this work. “We think the black hole ejected the star with a speed of thousands of kilometres per second at about the time when humanity’s ancestors were just learning to walk on two feet.”

Superfast stars can be ejected by black holes via the Hills Mechanism, proposed by astronomer Jack Hills thirty years ago. Originally, S5-HSV1 lived with a companion in a binary system, but they strayed too close to Sagittarius A*. In the gravitational tussle, the companion star was captured by the black hole, while S5-HVS1 was thrown out at extremely high speed.

“This is the first clear demonstration of the Hills Mechanism in action,” said Ting Li from Carnegie Observatories and Princeton University, and leader of the S5 Collaboration. “Seeing this star is really amazing”, she added, “as we know it must have formed in the Galactic Centre, a place very different to our local environment. It is a visitor from a strange land.”

The discovery of S5-HVS1 was made with the 3.9m Anglo-Australian Telescope near Coonabarabran, NSW, Australia, coupled with superb observations from the European Space Agency’s (ESA) Gaia satellite, which allowed the astronomers to reveal the full speed of the star and its journey from the centre of the Milky Way. 

“This result was especially cool since it not only informed us about the black hole in the centre of our galaxy, but it also allowed us to independently measure how fast our sun is moving relative to the Milky Way,” said Denis Erkal, Dynamical Modelling Coordinator for the S5 collaboration.

These results were published in Monthly Notices of the Royal Astronomical Society. The S5 Collaboration unites astronomers from the United States, United Kingdom, Australia and Chile. 

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