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The smashup that rejuvenates

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5:06pm, March 7, 2003
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For elderly stars, the fountain of youth may be only a collision away.

Evidence for the rejuvenating power of collisions comes from studying bright, young-looking stars residing in globular clusters, the oldest stellar groupings in the Milky Way. For nearly 50 years, astronomers examining globular clusters have encountered a paradox. Even though massive stars go for the gusto, burning brightly and dying out in just a few million years, several keep on shining in the dense cores of these clusters, which may be as old as 16 billion years.

According to theory, no star heavier than 80 percent of the sun's mass should reside in an ancient globular cluster. Yet, astronomers have found resident bright stars, known as blue stragglers, that are nearly twice as massive as the sun and one-tenth the age of the clusters.

Theorists have suggested that the merger of old, lower-mass stars led to the recent formation of the stragglers. There are two ways such a merger can happen, notes Rex A. Saffer of Villanova (Pa.) University. In one model, two stars that are tightly orbiting each other succumb to the pull of gravity and coalesce. In the second scenario, two or more completely independent stars smash into one another and merge.

Recent calculations have favored the collision hypothesis, but it remained unproven until now.

Saffer and his colleagues used the Hubble Space Telescope to peer into the Milky Way globular cluster NGC 6397, which contains about 100,000 stars and lies some 8,500 light-years from Earth. They confirmed that five blue stragglers reside in the cluster's core. Four of them weigh about twice as much as the most massive stars that theorists calculate could survive in the cluster.

The mass of the four stragglers could be accounted for by either collisions or coalescence. A fifth straggler, however, turns out to be so massive that it could only have formed by the collision of three or more stars, Saffer's team calculates. He and his colleagues reported the findings last month in Rochester, N.Y., at a meeting of the American Astronomical Society.

"Our result is especially exciting because stellar collisions have long been predicted on theoretical grounds" but evidence has been scant, says Saffer. "To our knowledge, this is the first observation of the product of these stellar smashups."

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