TripleAlpha

Triple-alpha refers to the set of nuclear fusion reactions that convert three helium-4 nuclei (alpha particles) into carbon-12. It is a principal process of stellar nucleosynthesis during the helium-burning phase of stars.

Two alpha particles fuse to form beryllium-8, which is highly unstable with a lifetime of about 10^-16 seconds. While Be-8 exists briefly, a third alpha particle can be captured to form carbon-12 in an excited state known as the Hoyle state, located about 7.65 MeV above the ground state. This excited carbon-12 then decays to its ground state by gamma emission, releasing energy. The combined reaction is therefore 4He + 4He → 8Be, followed by 8Be + 4He → 12C* → 12C + gamma. In stars this process operates at core temperatures around 100 million kelvin, and its rate is highly sensitive to temperature and density.

Without the triple-alpha process, the universe would have far less carbon, a key element for organic chemistry and life as we know it. The process also influences the evolution of stars, as carbon production alters the subsequent burning stages and the composition of stellar remnants. The energy release from the reaction contributes to stellar luminosity during helium burning.

Historically, the existence of the excited Hoyle state was predicted by Fred Hoyle to explain carbon abundances, and its subsequent experimental confirmation was a landmark in nuclear astrophysics. Modern measurements and theoretical models aim to constrain the reaction rate and its dependence on stellar conditions, though uncertainties remain important for precise stellar evolution calculations.