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Thursday, November 02, 2006

Physicists Solve Stellar Mystery With Fast Computers

Using 3D models run on some of the fastest computers in the world, physicists have created a mathematical code that cracks a mystery surrounding stellar evolution.

For years, physicists have theorized that low-mass stars (about one to two times the size of the sun) produce great amounts of helium 3 (³He). When they exhaust the hydrogen in their cores to become red giants, most of their makeup is ejected, substantially enriching the universe in this light isotope of helium.

This enrichment conflicts with the Big Bang predictions. Scientists theorized that stars destroy this ³He by assuming that nearly all stars were rapidly rotating, but even this failed to bring the evolution results into agreement with the Big Bang.

Now, by modeling a red giant with a fully 3D hydrodynamic code, researchers identified the mechanism of how and where low-mass stars destroy the ³He that they produce during evolution.

They found that ³He burning in a region just outside of the helium core, previously thought to be stable, creates conditions that drive this newly discovered mixing mechanism.

Bubbles of material, slightly enriched in hydrogen and substantially depleted in ³He, float to the surface of the star and are replaced by ³He-rich material for additional burning. In this way the stars destroy their excess ³He, without assuming any additional conditions (like rapid rotation).

"This confirms how elements evolved in the universe and makes it consistent with the Big Bang," says David Dearborn, a Lawrence Livermore National Laboratory physicist. "The previous one-dimensional model did not recognize the instability created by burning ³He."

The same process applies to low-mass metal poor suns, which may have been more important than metal-rich stars like the sun throughout the earlier part of galactic history in determining the ³He abundance of the interstellar medium.

The research appears in the Oct. 26 edition of Science Express. A video is available.


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