New class of hypervelocity stars escaping gravitational grasp of Milky Way

A team of astronomers has discovered a surprising new class of "hypervelocity stars", which are solitary stars, moving fast enough to escape the gravitational grasp of the Milky Way galaxy.

"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.

"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," Palladino said.

The discovery came as Palladino, working under the supervision of Kelly Holley-Bockelmann, assistant professor of astronomy at Vanderbilt, was mapping the Milky Way by calculating the orbits of Sun-like stars in the Sloan Digital Sky Survey, a massive census of the stars and galaxies in a region covering nearly one quarter of the sky.

"It's very hard to kick a star out of the galaxy," Holley-Bockelmann said.

Astrophysicists calculate that a star must get a million-plus mile-per-hour kick relative to the motion of the galaxy to reach escape velocity.

They also estimate that the Milky Way's central black hole has a mass equivalent to four million Suns, large enough to produce a gravitational force strong enough to accelerate stars to hyper velocities.

The typical scenario involves a binary pair of stars that get caught in the black hole's grip. As one of the stars spirals in toward the black hole, its companion is flung outward at a tremendous velocity. So far, 18 giant blue hypervelocity stars have been found that could have been produced by such a mechanism.

Now Palladino and her colleagues have discovered an additional 20 Sun-size stars that they characterize as possible hypervelocity stars.

The discovery is published in the Astrophysical Journal.