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.