Black hole at Milky Way core powers galaxy's fastest stars

The black hole at the centre of the galaxy is to blame for sling-shotting "hypervelocity stars" out of the Milky Way at up to 1.8 million miles per hour, according to new evidence from research involving a University of Michigan astronomer.

In this illustration, the hot, blue star HE 0437-5439 has been tossed out of the center of our Milky Way galaxy with enough speed to escape the galaxy's gravitational clutches. The stellar outcast is rocketing through the Milky Way's distant outskirts at 1.6 million miles an hour, high above the galaxy's disk, about 200,000 light-years from the center. The star is destined to roam intergalactic space. Credit: NASA, ESA, and G. Bacon.

Hypervelocity stars, discovered about five years ago, are the fastest stars astronomers have ever observed. They are escaping the galaxy at beyond what was thought to be its stellar speed limit.

The researchers used the Hubble Space Telescope to trace the trajectory and origin of HE 0437-5439, a speeding star in the southern hemisphere too faint to be seen with the naked eye. A paper about their findings has been accepted to Astrophysical Journal Letters.

"This is the first objective evidence that these hypervelocity stars do come from the center of the galaxy," said Oleg Gnedin, an assistant professor in the U-M Department of Astronomy. "It's an example of a very violent interaction that happens as a direct consequence of the black hole there.

"We had theorised that you could only get such high velocity if you kick a star from very close to a black hole in a special way that involves another star or object. It's a three-body interaction. The black hole rips apart a binary or tertiary star system, captures one of the companions and jettisons the others."