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The fusion of helium-4 nuclei (alpha particles) is known as the triple-alpha process, because fusion of just two helium nuclei only produces beryllium-8, which is unstable and breaks back down to two helium nuclei with a half life of 1×10-16 to 2.6×10-16 seconds. If the core temperature of a star exceeds 100 million kelvins (100 megakelvins), as may happen in the later phase of red giants and red supergiants, then a third helium nucleus has a significant chance of fusing with the beryllium-8 nucleus before it breaks down, thus forming carbon-12. Depending upon the temperature and density, an additional helium nucleus may fuse with carbon-12 to form oxygen-16, and at very high temperatures, additional fusions of helium to oxygen and heavier nuclei may occur (see alpha process).
The fusion of helium-3 with itself or with helium-4 occurs during the fusion of hydrogen in main sequence stars (see proton-proton chain), and is not ordinarily referred to as helium fusion.
Alak K. Ray (2004) Stars as thermonuclear reactors: their fuels and ashes (arxiv.org article)
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Helium_fusion". A list of authors is available in Wikipedia.|