Astronomers have discovered dozens of solitary stars that are moving fast enough to escape the gravitational grasp of the Milky Way galaxy. These cryptic hypervelocity stars appear to be traveling at speeds in excess of a million miles-per-hour, but scientists aren’t sure what force could have given them such a kick. An article describing these mysterious stellar speedsters appears in theAstrophysical Journal.
“These new hypervelocity stars are very different from the ones that have been discovered previously,” said Vanderbilt University’s Lauren Palladino, lead author on the study. “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 made the discovery 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,” said co-researcher Kelly Holley-Bockelmann, also from Vanderbilt. “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.”
The new rogues appear to have the same composition as normal disk stars, so the astronomers do not think 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? We are working on that now,” said Holley-Bockelmann.