The triple alpha process is how stars fuse helium nuclei into carbon and oxygen nuclei. It requires high temperatures and helium density. The process involves the fusion of three alpha particles, which eventually creates carbon. The reaction rate depends on temperature and density. In small stars, it can lead to an uncontrolled helium flash.
The triple alpha process is the means by which stars fuse helium nuclei into carbon and oxygen nuclei when they have used up their hydrogen fuel. Initiating the triple alpha process requires sustained temperatures of over 100,000,000 K and sufficient helium density. This happens when a star starts accumulating significant amounts of helium “ash” in its core from burning hydrogen. Helium has nowhere to go and doesn’t produce its own energy, so it clumps in the core and contracts. Contraction greatly increases heat and pressure. At 100 megaKelvin, the triple alpha process, also known as helium burning, begins.
The triple alpha process gets its name because the process is the fusion of three alpha particles. An alpha particle consists of two protons and two neutrons bonded together, which is the same thing as a helium nucleus. Under colossal pressures at the stellar core, two helium nuclei can be caused to combine into a beryllium nucleus, releasing a gamma ray in the process. The beryllium nucleus is unstable, within 2.6×10-16 seconds, it collapses back into helium nuclei. But if enough beryllium nuclei are continuously created, eventually one will fuse with another energetic helium nucleus and create carbon, a nucleus with a total of six protons and six neutrons.
The triple alpha process occurs in all low to intermediate mass stars (0.6-10 solar masses) at the end of their lifetimes. After the Red Giant stage, which features traditional hydrogen burning in a compressed shell around a helium core, the core collapses and begins to burn helium, throwing the star into the asymptotic giant branch of the Hertzsprung-Russell diagram, which compare the brightness of the star to the spectral type.
The rate of the triple alpha reaction is strongly dependent on the temperature of the core: the reaction rate is the product of the temperature times the 30th power and the density squared. In small stars, the helium nucleus becomes so dense that it becomes a form of degenerate matter, where increases in temperature do not correspond to increases in volume. This can lead to an uncontrolled triple alpha reaction called a helium flash, where 60-80% of the helium in the core is incinerated within minutes. For larger stars, helium begins to fuse on an outside shell of a carbon core, preventing it from reaching the state of degenerate matter. In these larger stars, carbon burning eventually begins.
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