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New thinking about the ages of old stars.

Whether it's truly a crisis in cosmology or just a matter of incomplete knowledge, astronomers are up against a conundrum: The universe, according to several observations, appears to be younger than its oldest stars.

To resolve this paradox, researchers must prove that the universe is older than recent estimates of 9 to 11 billion years or that the oldest stars in our galaxy are younger than 12 to 18 billion years.

Two new reports examine the question of stellar age but come down on opposite sides of the issue. One team suggests that researchers may have overestimated the ages of globular clusters, dense groupings of elderly stars scattered around the Milky Way. The other team, which describes a new method of determining the ages of certain mature stars, suggests that at least one star in the galaxy is truly old.

In the Jan. 1 Astrophysical Journal Letters, Allen V. Sweigart of NASA's Goddard Space Flight Center in Greenbelt, Md., reports that helium's outward movement from the core of a star causes the mature star to burn more brightly than previously calculated. Such migration of interior gases must occur, Sweigart says, because the surfaces of older stars show varying abundances of several other elements-especially aluminum-that are only produced deep inside a star.

Sweigart used computer models to analyze a set of stars known as RR Lyrae. They all have the same intrinsic luminosity, like light bulbs of a single wattage. Astronomers use the brightness of RR Lyrae stars to infer the ages of the globular clusters in which they reside.

If RR Lyrae stars are intrinsically brighter than had been thought, then their globular clusters lie farther from Earth. And if the clusters are more distant, all the stars within them must also be brighter and younger. Sweigart calculates that if an RR Lyrae star were 10 percent brighter-which would require only a small addition of helium to its surface-then the cluster might be as much as 1.5 billion years younger.

Because the extent of helium mixing is not yet known, Sweigart cautions that he cannot determine how much brighter RR Lyrae stars really are. Michael Bolte of the Lick Observatory in Santa Cruz, Calif., agrees that helium mixing affects brightness. He adds, however, that astronomers also use other methods to determine the age of globular clusters and that helium mixing does not influence those estimates. Bolte notes that he's intrigued by another report, which describes a seemingly more straightforward method of determining the ages of elderly stars.

In a study to appear in the May 1 Astrophysical Journal, John J. Cowan of the University of Oklahoma in Norman and his colleagues estimate the age of an elderly Milky Way star from the extent to which its abundance of thorium, an extremely long-lived radioactive element, has declined since the star's birth.

The team finds that the star is between 13 billion and 21 billion years old. Further studies should narrow that range, Cowan says, but he notes that the age paradox has yet to be resolved.
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Title Annotation:research indicating that universe is younger than oldest stars is reviewed
Publication:Science News
Article Type:Brief Article
Date:Dec 14, 1996
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