Hypervelocity Stars Escaping Milky Way Baffle Scientists

By Rhonda J. Miller on January 12, 2014 9:28 AM EST

Stars Of The Milky Way
The Spitzer Space Telescope's infrared cameras reveals the stars of the crowded galactic center region of the Milky Way. (Photo: NASA / MCT / Getty Images / Rhonda J. Miller)

Astronomers have discovered a new class of speeding stars called "hypervelocity stars" that are moving so fast they've escaped the gravitational pull of the Milky Way. The puzzling part of the discovery is that scientists aren't sure how the stars were ejected from the galaxy. The international team of astronomers found that the stars don't appear to be coming from the core of the galaxy, according to lead author of the study Lauren Palladino, a graduate student at Vanderbilt University in Nashville, Tenn.

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"These new hypervelocity stars are very different from the ones that have been discovered previously," said Palladino in a Vanderbilt University press 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 discovery was made by Palladino as she was mapping the Milky Way by calculating the orbits of sun-like stars in the Sloan Digital Sky Survey, a huge census of stars and galaxies that covers nearly one-quarter of the sky. Palladino was working under the direction of Vanderbilt Assistant Professor of Astronomy Kelly Holley-Bockelmann.

"It's very hard to kick a star out of the galaxy," said Holley-Bockelmann. "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 big question is 'what boosted these stars up to such extreme velocities? We are working on that now," Holley-Bockelmann said in the press release.

The findings of the study, "Hypervelocity Star Candidates in the Segue G and K Dwarf Sample," were published in the Jan. 1 issue of the Astrophysical Journal and reported at the annual meeting of the American Astronomical Society in Washington, D.C. held from Jan. 5-9.

Since 2005, astronomers have discovered 18 hypervelocity stars that were all similar in three ways, according to Time. They were massive and very bright, they moved at a minimum speed of 1.5 million miles per hour — escape velocity from the Milky Way's gravitational pull is one million miles per hour — and they were observed shooting out of the center of the galaxy.

The 20 newly discovered hypervelocity stars, which Palladino said were "coming from random directions," leaves astronomers with a puzzle. "If these candidates are truly hypervelocity stars, they were not ejected by interactions with the Milky Way's supermassive black hole," the researchers wrote. "This calls for a more serious examination of alternative hypervelocity-star ejection scenarios."

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