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Tuesday, November 15, 2005

Mysterious Runaway Star Stymies Experts

Catapulted Star
Mysterious Runaway Star Stymies Scientists
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Nov. 14, 2005 ? Astronomers have stumbled onto a runaway star inbound to our galaxy that might have been kicked out of our nearest galactic neighbor by a supermassive black hole.
The star, HE 0437-5439, was found on a star survey and initially led its discoverers to suspect their instruments were out of whack.
"We first thought our wavelength calibration was off," said astronomer Ralf Napiwotzki of the University of Hertfordshire's Center for Astrophysics Research in the U.K.
The light from the star was both the wrong color (wavelength) for where the star is located and showed spectral signs that it is traveling inbound to the Milky Way at an unusually high speed ? about 1.6 million miles per hour (2.6 million kilometers per hour).
The light from HE 0437-5439 indicates that it's a rather healthy young star, like those found all over the Milky Way, explained Napiwotzki.
Yet this star is located out in the Milky Way's hinterlands, the geriatric ward of the galaxy.
Napiwotzki's first thought, after confirming the star's spectral information with the European Southern Observatory's (ESO) Very Large Telescope in Chile, was that the star had been slingshot out of the core of the Milky Way after a close call with the giant black hole there.
That would account for the speed. But after calculating the star's velocity, location and age, it just didn't add up.
The bottom line: HE 0437-5439 is too far away from the center of the Milky Way to have made the trip and still be so young, said Napiwotzki, who, with several German colleagues, has authored a paper on the matter in a coming issue of Astrophysical Journal Letters.
"I agree that ejection from the Galactic center is not very plausible," said University of Texas astronomer John Kormendy, who was not involved the HE 0437-5439 discovery. "The star's lifetime is too short, given the necessary travel time."
It's more likely that the star is from the Large Magellanic Cloud (LMC) a irregular galaxy very near, but outside the Milky Way and visible in the night skies of Earth's Southern Hemisphere. But the LMC poses another problem, said Kormendy.
"Almost certainly the LMC does not have a supermassive black hole to do the same job," said Kormendy.
That means something even more unusual might have thrown the star towards the Milky Way, something like a sideways blast by a much bigger companion star that exploded as a supernova, he said.
Napiwotzki, for his part, is still holding out for a supermassive black hole at the center of the LMC.
To confirm this, however, will require a lot more information on the runaway star's trajectory, he said, and that could take many years of observing the star as it moves.

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