Surprising New Class of Hypervelocity Stars Can Escape Milky Way's Gravity

Some stars move slowly, drifting within the galaxy that they inhabit. Others, though, are another story. Astronomers have discovered a surprising new class of "hypervelocity stars" that move fast enough to escape the gravitational grip of the Milky Way galaxy. The findings reveal a bit more about these unusual, solitary stars.

"These new hypervelocity stars are very different form the ones that have been discovered previously," said Lauren Palladino, one of the researchers, in a news release. "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 first discovered these stars during the Sloan Digital Sky Survey. This massive census covers the stars and galaxies in a region covering nearly one quarter of the sky. They calculated the orbits of sun-like stars in this survey and found some interesting results.

"It's very hard to kick a star out of a galaxy," said Kelly Holley-Bockelmann, one of the researchers, in a news release. "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."

In order to escape a galaxy, a star needs a million-plus mph kick relative to the motion of the galaxy. The Milky Way's central black hole is actually large enough to produce the gravitational force to accelerate stars to hyper velocities. Yet these new stars, 20 in all, seem to use some other mechanism. They seem to have the same composition as normal disk stars, so the astronomers don't believe that their birthplace was in the galaxy's central bulge, the halo that surrounds it or in some other exotic place outside the galaxy.

"The big question is: what boosted these stars up to such extreme velocities?" said Holley-Bockelmann in a news release. "We are working on that now."

The findings are published in The Astrophysical Journal.