Post by Neo~Dest on Feb 5, 2006 1:51:58 GMT -5
Two stars have just been caught speeding out of the Milky Way at more than 1.6 million kilometres (1 million miles) per hour. The discoveries came during the first three nights of a new search for such stellar speedsters. And future observations could shed light on the nature of the mysterious dark matter that seems to enshroud the Milky Way.
Three previous galactic escapees have been found since 2005. Now, a team led by Warren Brown of the Harvard-Smithsonian Center for Astrophysics, who discovered the first, has turned up two more.
The new discoveries are 180,000 and 240,000 light years from Earth, respectively, and about 45° above the galactic disc. "That puts them well away from and above the disc of the galaxy," Brown told New Scientist. "They've already left the main part of the Milky Way."
The stars are each thought to have reached their incredible speeds after a run-in with the colossal black hole at the centre of the galaxy. Many massive stars are found with partners, and if a pair of these stars got too close to the black hole, theories predict one star could be captured and the other flung into space.
Brown's team designed its search to find such stars. "We were looking for young, massive stars that have relatively short lifetimes and so shouldn't be in the outskirts of the Milky Way, where the stars are all quite old," he explains.
Quick results
The team chose several hundred stars that appeared blue – and therefore young – in data from the ongoing Sloan Digital Sky Survey. In December 2005, they began studying some of the candidates using the 6.5-metre MMT telescope in Arizona, US.
After just three nights of observing – covering 5% of the sky – they found the two "hypervelocity" stars. "We imagine finishing the entire list of candidates by the end of 2006," says Warren. "Then we should know how common these stars are."
Theorists predict a star is flung outward by the galaxy's black hole once every 100,000 years, meaning there may be only 1000 such escapees in the galaxy among more than 100 billion total stars. Astronomers hope to find as many as possible to test theories of the types of stars near the galactic black hole, and the rate at which the black hole flings them outwards. "All five stars have different travel times from the galactic centre, which suggests a slow but steady rate of these things being shot out," says Brown.
Dark matter
He adds that the stars could also shed light on the distribution of matter – including theoretical dark matter – in the galaxy. "If we're right and they are shot out of the galactic centre by its supermassive black hole, they should be travelling in a straight line to where they are now," Brown says. "Any deviation from that straight line would measure how the gravity of the galaxy affects the trajectory of the star."
Ground-based telescopes are not sensitive enough to observe the stars' seemingly slight movement across the sky. But space-based telescopes, such as Hubble, should be able to detect it over a period of several years.
Brown is submitting a proposal for Hubble to track the stars, but adds that it is not clear whether the 16-year-old telescope will be functioning long enough to do the study. NASA is considering sending a space shuttle to service the telescope, but the space shuttles are currently grounded until engineers can prevent them from shedding insulating foam on take off.
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