Supermassive Black Hole Ejected a Star That Will 'Leave the Galaxy and Never Return'

Astronomers have discovered a runaway star travelling through the Milky Way at roughly 3.7 million miles per hour, accelerated to such high speeds by the supermassive black hole at the center of the galaxy.

A team led by Sergey Koposov from Carnegie Mellon University spotted the ultrafast star—dubbed S5-HVS1—in the constellation Grus using the 3.9-meter Anglo-Australian Telescope (AAT) near Coonabarabran, Australia, according to a study published in the journal Monthly Notices of the Royal Astronomical Society.

Further observations with the European Space Agency's Gaia satellite revealed the star is moving around ten times faster than most stars in the galaxy and is on course to exit the Milky Way forever.

"The velocity of the discovered star is so high that it will inevitably leave the Galaxy and never return," Douglas Boubert, a co-author on the study from the University of Oxford in the U.K., said in a statement.

These kinds of ultrafast stars were only discovered around two decades ago and astronomers know of only a few examples. So-called "hyper-velocity stars" travel at sufficient speeds to escape the galaxy by overcoming its strong gravitational pull. In order for this to be possible, these stars need to be accelerated to extremely high velocities by incredibly massive objects.

According to a hypothesis proposed around 30 years ago, supermassive black holes—such as the one which lies at the center of the Milky Way—would be capable of such a feat if a binary star system came too close to one. In this case, the black hole would swallow one of the stars and eject the other at high velocities. This process is known as the Hills Mechanism.

S5-HVS1, Sagittarius A*
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

By analyzing the star's speed and distance from Earth—a relatively "close" 29,000 light-years away—the astronomers were able to calculate that S5-HVS1's journey began at the center of the Milky Way, which hosts a supermassive black hole known as Sagittarius A*.

"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 Center," Koposov said in a statement.

"We think the black hole ejected the star with a speed of thousands of kilometers 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," he said.

The researchers say that this study provides the first clear demonstration of the Hills Mechanism in action at the center of the galaxy.

"Seeing this star is really amazing as we know it must have formed in the Galactic Center, a place very different to our local environment. It is a visitor from a strange land," Ting Li, another author of the study from Carnegie Observatories and Princeton University, said in the statement.

In comments provided to Newsweek, Koposov added: "It is quite unique. In my opinion, a comparable result was the discovery in 2005 by Warren Brown of the very first hyper-velocity star with the speed of around 850 km/s [528 mps]—i.e. two times slower than S5HVS1. Since then there were some other discoveries of fast stars, but none of those discoveries ever could be certain that the star was ejected by a black hole in the center of the Milky Way. This is what makes our discovery unique."

This article was updated to include additional comments from Sergey Koposov.