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Alpha capture is a nuclear reaction possible when an alpha particle (essentially a helium nucleus) collides with an atomic nucleus. It is their fusion, forming a larger nucleus with an atomic number and mass number each being the sum of those of each. If the resulting isotope is unstable, beta decay may follow soon after. Nucleosynthesis consisting of one or more alpha captures (possibly with some beta decays along the way) is termed the alpha process. Like other fusion it requires considerable temperature and pressure, such as within stellar cores.