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The Runaway Star
10 February 2005 (All day)
Astronomers have discovered a star traveling so fast that even the gravity of the entire Milky Way can't stop it. Theoretical calculations suggest that thousands of these runaway stars may be racing through our galaxy, providing further evidence that a supermassive black hole lurks in the core of the Milky Way.
The main idea, first posited in 1988, is that two stars orbiting each other can be disrupted when they pass close to a massive black hole in the core of a galaxy. One star ends up orbiting the black hole, while the other is ejected like a stone from a sling. In our own Milky Way, this is expected to happen once every 100,000 years or so. A team of astronomers from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, led by Warren Brown, has now identified one such stellar outcast.
The faint star is some 180,000 light years away in the constellation Hydra the Water Snake. Using the 6.5-meter telescope of the MMT Observatory at Mount Hopkins, Arizona, the team clocked the star at over 700 kilometers per second, almost 25 times the orbital velocity of Earth, and more than twice the so-called escape velocity of the Milky Way galaxy. Based on its position, velocity, and direction of motion, the team concludes that the star must have been ejected by the central black hole some 80 million years ago. Nothing else could crank up the velocity so high, they conclude in an upcoming issue of Astrophysical Journal Letters.
"It's a pretty interesting discovery," says Andrea Ghez of the University of California at Los Angeles. She believes the findings could also explain why so many young and massive stars are found in tight orbits around the Milky Way's black hole, where astronomers normally wouldn't expect them to be.