Like Baby Boomers, stars are born in great batches. They live together--and grow old--in densely packed neighborhoods in space known as globular clusters. But among the aging population there always appear to be a few new youngsters that burn a hot blue. How these "blue stragglers" come to be has puzzled astronomers for more than 45 years.
Astronomers using the orbiting Hubble Space Telescope reported on October 29 that they may have solved the mystery: blue stragglers are rejuvenated stars formed when two mature stars either collide or pass close to each other. "This is an extremely exciting result," says Rex A. Saffer of Villanova University, "because it may help distinguish between competing theories of blue-straggler star formation and evolution."
Saffer and his colleagues pointed Hubble at the tumultuous heart of the nearby globular cluster 47 Tucanae (47 Tuc), located 15,000 light-years away in the southern constellation Tucana. Peering into the globular cluster's bright core, Hubble's Wide Field/Planetary Camera 2 revealed a number of blue stars (circled in yellow).
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Using data from Hubble's Faint Object Spectrograph (removed during a servicing mission in February), the astronomers analyzed the spectrum of one blue straggler, measuring its temperature, diameter and rotation rate. The team then combined these measurements with the blue straggler's apparent brightness to obtain the star's mass. The derived temperature and mass are consistent with the characteristics of a normal star with a mass about 1.7 times that of the sun. Yet the star is spinning at least two to three times faster than stars of its kind.
The group concluded from the data that the straggler was created by the merger of two smaller stars. The evidence also rules out the theory that the stars resulted from a cataclysmic head-on collision. Instead the astronomers believe that as the two stars approached each other, they became trapped by their gravitational fields and began orbiting each other, spinning ever faster as they grew closer. Gradually, the more massive member of the pair cannibalized its partner, sucking huge globs of material away from its smaller companion. The result: a new lease on life.