Alpha Process

Term used to describe the addition of a helium nucleus (4He) to existing nuclei with in a giant star to form heavier elements. At temperatures above 2 x 108 K, a 12C nucleus colliding with 4He nucleus can produce 16O. Although the 16O produced may fuse with other 16O nuclei at ~109 K to form 32S, it is much more probable that an 16O nucleus will capture a 4He nucleus (if one is available) to form 20Ne. Thus, as the star evolves, heavier elements tend to form through 4He-capture rather than by fusion of like nuclei.

The α process will continue up to the formation of 56Ni. However, 56Ni is unstable and decays rapidly into 56Co, then into stable 56Fe (the most stable of all nuclei). Thus, the alpha process leads inevitably to the buildup of iron in the stellar core. Because helium-capture reactions are so much more common, elements with nuclear masses of 4 units (4He, 12C, 16O, 20Ne, 24Mg, and 28Si) have markedly higher cosmic abundances (below).

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Some or all content above used with permission from J. H. Wittke.

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