New Class of Runaway Stars Escaping Milky Way

Astronomers have discovered a new class of solitary runaway stars. These hypervelocity bodies are moving incredibly fast- about a million-plus mile-per-hour relative to the motion of Milky Way. This speed enables them to break away from the grasp of the galaxy.

Smithsonian astronomers found the first outcast star in 2005. Two more castaway stars were discovered in 2006. In 2010, astronomers using NASA's Hubble Space Telescope reported detecting HE 0437-5439- a hyper-velocity star that had sped away from the galaxy at a speed of 1.6 million miles (2.5 million kilometers) an hour.

Now, a team of researchers at Vanderbilt University and colleagues have found 20 new hypervelocity stars while scanning the sky.

"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," said Lauren Palladino, lead author on the study.

Palladino and team were working on a project called Sloan Digital Sky Survey, when they chanced upon the new class of stars. The survey aims at studying stars and galaxies covering a quarter of the sky.

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

According to estimates by the researchers, the black hole at the center of our galaxy has a mass equivalent to four million suns, which exerts a lot of gravitational force and is enough to accelerate stars to hypervelocities. Their estimates suggest that to escape the Milky Way, a star needs to achieve million-plus mile-per-hour boost in relative to the motion of the galaxy.

One mechanism by which stars can get thrown out is that of a binary star system that has gotten too close to the black hole. As the pair is sucked in by the hole; one star spirals towards the core, while its partner gets a tremendous kick and gets flung out of the galaxy.

So far, researchers have found 18 hyper-velocity, blue stars, whose expulsion can be explained by this mechanism.

Palladino and her colleagues have discovered 20 sun-sized stars that could also be hyper-velocity stars.

"One caveat concerns the known errors in measuring stellar motions," she said in a news release. "To get the speed of a star, you have to measure the position really accurately over decades. If the position is measured badly a few times over that long time interval, it can seem to move a lot faster than it really does. We did several statistical tests to increase the accuracy of our estimates. So we think that, although some of our candidates may be flukes, the majority are real."

Additional research on the stars has shown that they are similar in composition to other normal stars. The similarity in composition refutes the idea that these rogue stars are from some other galaxy.

A related study had suggested that supernova explosions within a binary star could help stars get a kick that would fling it out of the galaxy.

The study is published in the Astrophysical Journal.