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First 'brown dwarf' star discovered AMERICAN ASTRONOMERS MEETING


PITTSBURGH -- Dr. Gibor Basri went looking for brown dwarfs and says he's found one.

Dr. Basri is an astronomer at the University of California at Berkeley, and the brown dwarf he sought is a kind of star long thought to exist in a broad gap between the smallest known stars and the biggest planets.

"Astronomers don't like this gap, so people have been looking for a long time to find these objects," Dr. Basri said.

The problem has been that brown dwarfs, if they existed, were so small and cool, compared with regular stars, that they would be very hard to see.

Missing piece of science

Finding one, however, would not only fill in a missing piece of science.

It would also provide a big new class of "dark" matter to astrophysicists trying to find enough mass in the universe to account for the gravitational effects they see.

Other astronomers have reported finding very small, very faint stars that seemed to be reasonable candidates for brown dwarfs. But none of the discoveries was conclusive.

Yesterday, at a meeting of the American Astronomical Society, Dr. Basri claimed discovery of the first-ever brown dwarf star, called PPL15. It is in a beautiful young star cluster called the Pleiades, visible in the Northern Hemisphere's winter sky.

Small and dim

Not only is the star small and dim, he said. It also bears a chemical signature in its light that is strong evidence that it is a brown dwarf.

Using the powerful Keck Telescope on Mauna Kea in Hawaii, Dr. Basri and his team have detected the spectrographic signal of the element lithium in the star's light.

Based on current understanding of stellar evolution, Dr. Basri said, lithium created along with far greater amounts of hydrogen and helium at the birth of the universe 12 billion to 14 billion years ago is incorporated into all new stars as they form.

As the larger stars age, however, the lithium is destroyed in the nuclear fusion that powers them. The big stars in the Pleiades cluster, for example, reveal no lithium in spectra of their starlight.

In stars of the same age, but with very low mass, however, the nuclear fires would cool before the lithium was consumed. Their spectra should show significant amounts of lithium.

Evidence of lithium

Aiming the 33-foot Keck telescope's powerful high resolution spectrograph at PPL15, Dr. Basri and his team found strong evidence of lithium in its faint starlight.

"That is the basis for our claim," Dr. Basri said. "We don't rely just on the fact that it's faint and cool."

He is continuing his search for more brown dwarfs.

But from now on, he said, "everything now that is fainter and cooler than this object had better show lithium."

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