First Stellar Outcast Discovered by Astronomers
Cambridge, MA
Using the MMT Observatory in Tucson, AZ, astronomers at the Harvard-Smithsonian
Center for Astrophysics (CfA) are the first to report the discovery
of a star leaving our galaxy, speeding along at over 1.5 million
miles per hour. This incredible speed likely resulted from a close
encounter with the Milky Way's central black hole, which flung the
star outward like a stone from a slingshot. So strong was the event
that the speedy star eventually will be lost altogether, traveling
alone in the blackness of intergalactic space.
"We have never before seen a star moving fast enough to completely
escape the confines of our galaxy," said co-discoverer Warren
Brown (CfA). "We're tempted to call it the outcast star because
it was forcefully tossed from its home."
The star, catalogued as SDSS J090745.0+24507, once had a companion
star. However, a close pass by the supermassive black hole at the
galaxy's center trapped the companion into orbit while the speedster
was violently flung out. Astronomer Jack Hills proposed this scenario
in 1998, and the discovery of the first expelled star seems to confirm
it.
"Only the powerful gravity of a very massive black hole could
propel a star with enough force to exit our galaxy," explained
Brown.
While the star's speed offers one clue to its origin, its path
offers another. By measuring its line-of-sight velocity, it suggests
that the star is moving almost directly away from the galactic center.
"It's like standing curbside watching a baseball fly out of
the park," said Brown.
Its composition and age provide additional proof of the star's
history. The fastest star contains many elements heavier than hydrogen
and helium, which astronomers collectively call metals. "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.
The star is traveling twice as fast as galactic escape velocity,
meaning that the Milky Way's gravity will not be able to hold onto
it. Like a space probe launched from Earth, this star was launched
from the galactic center onto a never-ending outward journey. It
faces a lonely future as it leaves our galaxy, never to return.
Brown's co-authors on the paper announcing this find are Margaret
J. Geller, Scott J. Kenyon and Michael J. Kurtz (Smithsonian Astrophysical
Observatory). This study will be published in an upcoming issue
of The Astrophysical Journal.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center
for Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.
For more information, contact:
David Aguilar, Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7462 Fax: 617-495-7468
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
cpulliam@cfa.harvard.edu
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