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This Runaway ‘Hypervelocity’ Star Was Catapulted from the Milky Way’s Disk

“This discovery dramatically changes our view on the origin of fast-moving stars.”

by Becky Ferreira
Mar 14 2019, 1:00pm

The Milky Way. Image: NASA/JPL-Caltech

Every so often, a star that is minding its own business gets gravitationally catapulted out of the galaxy at speeds of roughly one million miles per hour. Scientists have spotted more than 20 of these “hypervelocity stars,” but a new study reveals that one of them, named LAMOST-HVS1, has a very different back story from the rest.

In order to be booted from the galaxy, stars must encounter an unusually extreme force. That’s why scientists think hypervelocity stars are accelerated by the supermassive black hole at the center of the Milky Way. According to previous studies, binary star systems that drift too close to the black hole could be torn apart by strong tidal forces, resulting in one star being consumed while the other is drop-kicked out of the galaxy.

When LAMOST-HVS1 was first discovered in 2014, scientists presumed that it came from the galactic center like all the other fast-moving stars. But because the star is located about 42,000 light years from Earth—which makes it the closest known hypervelocity star to the solar system—researchers did not rule out that it may have originated elsewhere.

A team led by University of Michigan astronomers Kohei Hattori and Monica Valluri have now confirmed that LAMOST-HVS1 was likely launched from the Norma spiral arm in the Milky Way’s disk, thousands of light years from the supermassive black hole.

The results, published Tuesday in The Astrophysical Journal, represent the first time that a hypervelocity star has been tracked back to a location outside of the galactic center.

“This discovery dramatically changes our view on the origin of fast-moving stars,” said Valluri in a statement. “The fact that the trajectory of this massive fast-moving star originates in the disk rather that at the galactic center indicates that the very extreme environments needed to eject fast-moving stars can arise in places other than around supermassive black holes.”

Read More: Watch the Milky Way’s Black Hole Fling Stars at a Million Miles an Hour

LAMOST-HVS1 must have encountered an object, or objects, with an enormous mass to eject the star from the Norma arm some 33 million years ago, the team said. It could have been an intermediate-mass black hole, or perhaps a cluster of young giant stars.

“The natal star cluster of LAMOST-HVS1 may be an undiscovered young massive cluster near the Norma spiral arm,” the team wrote in the study. “It was probably ejected by a three-or four-body dynamical interaction with more massive objects in a high-density environment.”

Scientists have never observed these types of objects in this region of the galactic disk, but the runaway star has potentially laid out a roadmap to track down high-mass novelties that might be lurking there.

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