Astronomers recently discovered a fascinating new class of smaller "hypervelocity stars" moving quickly enough to escape the gravitational hold of the Milky Way galaxy.
How were they discovered? Vanderbilt University graduate student and lead author on the study Lauren Palladino was mapping the Milky Way by calculating the orbits of Sun-like stars in the Sloan Digital Sky Survey when she found the new class of hypervelocity stars.
The first hypervelocity stars were discovered by Smithsonian astronomers in 2005, according to the Harvard-Smithsonian Center for Astronomers. The rare hypervelocity star is a "massive star whose 2 million mph velocity can be explained only by ejection from the Galaxy's massive black hole."
"These new hypervelocity stars are very different from the ones that have been discovered previously," Palladino said 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."
Kelly Holley-Bockelmann, who supervised the work of Palladino, said that none of the observed hypervelocity stars come from the galactic center, which suggests that "there is an unexpected new class of hypervelocity star, one with a different ejection mechanism."
Palladino and a team of international researchers discovered an additional 20 sun-sized stars, each of which could possibly be characterized as hypervelocity stars. She says that "one caveat concerns the known errors in measuring stellar motions," but that they believe the majority of the star candidates are accurate.
The next big question is what got the stars moving at such a high velocity. That's what researchers are working on next.
"It's very hard to kick a star out of the galaxy," Holley-Bockelmann 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."
In a 2008 study titled, "An Alternative Origin for Hypervelocity Stars," researchers used numerical simulations to indicate that "disrupting dwarf galaxies may contribute halo stars with velocities up to and sometimes exceeding the nominal escape speed of the system."
In 2010, astronomers used the Hubble Space Telescope to make the first direct observation linking the hypervelocity stars to a galactic center origin. At that time, astronomer Warren Brown said in a news release, "These exiled stars are rare in the Milky Way's population of 100 billion stars. For every 100 million stars in the galaxy lurks one hypervelocity star."