Star ousted from galaxy by black hole

A close encounter with the supermassive black hole at the center of the Milky Way has set a star on a one way trip into intergalactic space. The star, detected at the MMT Observatory in Tucson, Arizona, is zipping outward at a speed of 1.5 million miles per hour. Rocketing along at twice the galactic escape velocity, the Milky Way's gravitational attraction doesn't have the holding power to keep the star from disappearing into the emptiness between galaxies.

"We have never before seen a star moving fast enough to completely escape the confines of our galaxy," Warren Brown of the Harvard-Smithsonian Center for Astrophysics (CfA) said. "We're tempted to call it the outcast star because it was forcefully tossed from its home. Only the powerful gravity of a very massive black hole could propel a star with enough force to exit our galaxy."

Using measurements of the star's line-of-sight velocity, the scientists have concluded the star, cataloged as SDSS J090745.0+24507, is moving almost directly away from galactic center. Composition and age of the star also provide evidence of the star's origin and ultimate fate. "Because this is a metal-rich star, we believe that it recently came from a star-forming region like that in the galactic center," said Brown. Less than 80 million years were needed for the star to reach its current location, which is consistent with its estimated age.

Margaret J. Geller, Michael J. Kurtz and Scott J. Kenyon, along with Brown, will publish their find in an upcoming issue of The Astrophysical Journal.

The Harvard-Smithsonian Center for Astrophysics is based in Cambridge, Massachusetts, where scientists study the origin, evolution and ultimate fate of the universe.