Saturday, September 20, 2008

About half the stars in our celestial neighborhood may have traveled great distances through the Milky Way, according to a new study, which suggests our sun may be one of them. People have generally assumed that once a star forms inside a galactic disk, it stays in a more or less fixed orbit around the center of its galaxy, said lead study author Rok Roškar, a graduate student in astronomy at the University of Washington in Seattle.

The reality "might be a lot more complicated and interesting than that," he said.

In their study, Roškar and his colleagues performed computer simulations of the past ten billion years for a hypothetical Milky Way-like galaxy—a galaxy with roughly the same mass, size, and dynamics as ours.

The team found that under the right conditions, a spiraling galactic arm can knock a star into a bigger or smaller circular orbit.

The simulations support the theory that stars migrate, an idea that has been suggested before, Roškar said. "Stars can move toward the center of the galaxy or away from the center of the galaxy while staying on a circular orbit," he said.

And there's a 50-50 chance that our sun did just that, according to the simulation, which involved more than 100,000 hours of computer time.

About half the stars within 130 light-years of our sun have made such galactic voyages, the researchers determined.

Galactic Archaeology

Stars are made of the heavy metals and materials they draw from their environment at birth.

Conventional wisdom holds that star ingredients—measured with spectrographs or color analysis—tell us about the galactic region where the star formed.

But if the findings of Roškar and his colleagues are true, stars would not necessarily originate from the place they're observed, making the analysis of galactic regions based on the makeup of stars difficult. "It makes galactic archaeology, if you will, more complicated," he said

Roškar added that the findings "provide a really nice explanation" for why there appears to be so much variation in the chemical composition of similarly aged stars that are close enough for us to observe and analyze.

Jerry Sellwood, an astronomer at Rutgers University in New Jersey who has studied star migration, said Roškar's results help shed light on previously puzzling aspects of our galaxy.

Source : National Geo

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