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Helium fusion and the fate of massive stars.

The destiny of a massive star hangs on a number: the proportion of carbon and oxygen created in the final stages of the nuclear fusion reactions at the star's high-temperature core. This ratio determines how quickly and in what order various elements form within the star and, indirectly, the timing and outcome of its explosion as a supernova.

Now, two teams of physicists have for the first time found experimentally the rate at which alpha particles (helium nuclei) fuse with carbon-12 nuclei to produce oxygen-16. Previously, astrophysicists could do little more than roughly estimate what this rate might be, based on astronomical observations of the abundance of various elements in stars.

"The number one unknown in nuclear astrophysics [has been] the process of helium burning," says Moshe Gai of Yale University,

A star approximately 25 times more massive than the sun lasts about 7.5 million years before exploding as a supernova. For the first 7 million years, hydrogen nuclei fuse to produce helium and release energy. In the last 500,000 years, helium fusion takes over.

Stars initially create carbon-12 by fusing-three alpha particles. A carbon-12 nucleus can then capture another alpha particle to make oxygen-16. The carbon-oxygen ratio at a massive star's core depends on the relative rate of these two fusion reactions. Nuclear physicists originally knew only the first rate.

Because of the extreme difficulty of duplicating on Earth the temperatures and pressures at which alpha particles fuse with carbon-12 and for a variety of technical reasons, researchers had to study the reverse reaction -- the decay of oxygen-16 into carbon-12 and an alpha particle - to deduce the appropriate reaction rate. "That's a very difficult, but possible, task," Gai says.

In the Yale experiment, Gai and his coworkers aimed a deuterium beam at a titanium nitride target laced with nitrogen-15. Collisions between deuterium and nitrogen produce the radioactive isotope nitrogen-16, which then decays into oxygen-16 by emitting an electron (or beta particle). Oxygen-16 then decays via the emission of an alpha particle into carbon-12. The researchers monitored the emission of alpha and beta particles.

Sifting through these particles and removing extraneous background effects in order to identify the relevant interactions wasn't easy "For every three alpha particles, there would be 1 billion electrons," Gai notes.

Lothat Buchmann of the TRIUMF high-energy accelerator in Vancouver, British Columbia, and his collaborators chose a somewhat different route. The researchers initially produced a beam of nitrogen-16 ions, then fired the beam into a carbon target. They used special detectors to look for coincidences between the emission of alpha particles and the recoil of carbon-12 nuclei from the breakup of oxygen-16 nuclei created in the target. The TRIUMF team eventually recorded 1 million such events.

Using these data, both groups then applied theory to derive the required reaction rate. The two experiments produced remarkably similar results. The findings - to be refined further -- also approximately fit the estimates of nuclear reaction rates that astrophysicists have used for modeling the evolution of massive stars. Such studies suggest that a star a little heavier than 25 solar masses would contain sufficient oxygen near the end of its life to reach the supernova stage more quickly than a less massive, carbon-rich star.

Gai and Buchmann described their findings at an American Physical Society meeting held last week in Washington, D.C.

"We have done completely different experiments - with very different production mechanisms and very different detection [techniques]," Gai says. "But the two experiments are in perfect agreement."
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Title Annotation:rate of helium fusion in stars derived
Author:Peterson, Ivars
Publication:Science News
Date:Apr 24, 1993
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