Alpha Process

Term used to describe the addition of a heliumHelium (He) Second lightest and second most abundant element (after Hydrogen) in the universe. The most abundant isotope is He (99.9998%), He is very rare. Helium comprises ~8% of the atoms (25% of the mass) of all directly observed matter in the universe. Helium is produced by hydrogen burning inside nucleusCore of an atom, where nearly the entire mass and all positive charge is concentrated. It consists of protons and neutrons. (⁴He) to existing nuclei with in a giant starSelf-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. 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 fusionProcess in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. 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 coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel.. 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).