Hyper-fast star ejected by supermassive black hole


Astronomers see first demonstration of the ‘Hills mechanism’.


An artist's impression of the Milky Way's big black hole flinging the star from the galaxy's centre. 

James Josephides (Swinburne Astronomy Productions)

Astronomers have discovered a star travelling at more than six million kilometres an hour through the Milky Way after being flung from our galaxy’s centre by a supermassive black hole.

The eviction occurred about five million years ago, an international team reports in the Monthly Notices of the Royal Astronomical Society, and the star is moving so quickly that it will leave the Milky Way in about 100 million years, never to return.

Known as S5-HVS1, it is located in the constellation of Grus (the Crane). It was discovered using the 3.9-metre Anglo-Australian Telescope near Coonabarabran, in Australia, coupled with observations from the European Space Agency’s Gaia satellite.

“We traced this star’s journey back to the centre of our galaxy, which is pretty exciting,” says co-author Gary Da Costa from the Australian National University. “[It] is travelling at record-breaking speed: 10 times faster than most stars in the Milky Way, including our Sun.”

The work was led by Sergey Koposov from Carnegie Mellon University, US, as part of the Southern Stellar Streams Spectroscopic Survey (S5), which involves astronomers from the US, the UK, Australia and Chile.

Astronomers have wondered about high-velocity stars since their discovery two decades ago. S5-HVS1 is unprecedented, due to its high speed and close passage to the Earth, only 29 thousand light-years away.

With this information, Koposov and his team could track its journey back into the centre of the Milky Way, where a four million solar mass black hole, known as Sagittarius A*, lurks.

“This is super exciting, as we have long suspected that black holes can eject stars with very high velocities. However, we never before had a clear association of such a fast star with the galactic centre,” Koposov says.

It is also, the researchers say, the first clear demonstration of the “Hills mechanism” – a method by which superfast stars can be ejected by black holes that was proposed by astronomer Jack Hills 30 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.

  1. https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stz3081/5612212
  2. https://s5collab.github.io/
  3. https://s5collab.github.io/
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