Researchers discovered a new category of "hypervelocity star." The objects move so fast that the Milky Way's gravity can't rope them in.
"These new hypervelocity stars are very different from the ones that have been discovered previously," Vanderbilt University graduate student Lauren Palladino, lead author on the study, said in a statement. "The original hypervelocity stars are large blue stars and appear to have originated from the galactic center. Our new stars are relatively small - about the size of the sun - and the surprising part is that none of them appear to come from the galactic core."
The researchers made the discovery while making a map of the Milky Way using data of Sun-like stars' orbits taken from the Sloan Digital Sky Survey.
"It's very hard to kick a star out of the galaxy," Kelly Holley-Bockelmann, assistant professor of astronomy at Vanderbilt said. "The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy. None of these hypervelocity stars come from the center, which implies that there is an unexpected new class of hypervelocity star, one with a different ejection mechanism."
Researchers have estimated one of these runaway stars would need a "million-plus mile-per-hour kick relative to the motion of the galaxy" in order to break free. In order for the star to reach this velocity it would need help from a black hole with a mass equivalent to four million suns.
A "typical scenario" would include a pair of binary stars that are influenced by the black hole. The hungry black hole would cause one star to move in towards it while the other gets "flung" out into the great beyond.
In the past, researchers have pinpointed 18 giant blue hypervelocity stars that may have gone through this process; there are 20 other hypervelocity stars who do not have as clear of an origin.
"One caveat concerns the known errors in measuring stellar motions," Palladino said. "To get the speed of a star, you have to measure the position really accurately over decades. If the position is measured badly a few times over that long time interval, it can seem to move a lot faster than it really does. We did several statistical tests to increase the accuracy of our estimates. So we think that, although some of our candidates may be flukes, the majority are real."
The objects in question seem to have a similar composition to disk stars; which means it would be unusual for them to have originated in the galaxy's central bulge, it's surrounding halo, or outside of the galaxy.
"The big question is: what boosted these stars up to such extreme velocities? We are working on that now," Holley-Bockelmann said.
