alpha process

The alpha process (or alpha ladder or helium capture) is a sequence of nucleosynthesis steps, by which elements are synthesized within extreme heat (e.g., inside stars), the sequence thought of as the nucleus climbing a ladder. A nucleus undergoing this repeatedly transforms into heavier elements from carbon up to iron and beyond. Each step is the combination of an alpha particle (basically, a helium nucleus) with another nucleus (e.g., of carbon or oxygen, etc.) during a collision between them. The initial elements from which the sequence begins have even atomic numbers, and each step adds two to the number, resulting in even-numbered elements (the alpha elements) that comprise the most abundant metals. At some of its later steps, the nucleus is unstable and a beta decay can occur before another alpha particle is consumed. The beta decay decreases its atomic number by one; for these elements, typically two such decays occur, which reaches a stable isotope. The alpha process has a high temperature/density requirement, only met in some early stars, essentially what is termed silicon burning, which is essentially the alpha process starting with silicon. The term burning is typically used for stars, in which each step occurs within a separate shell-shaped region formed where sufficient density and temperature have built up for one of the steps, and the term alpha process is generally used for supernovae, in which a density and temperature sufficient for all the alpha process steps occurs very rapidly.

Note that the triple alpha process, which is the combination of three alpha particles to form carbon, is generally thought of as a different process.