heliumkern

Heliumkern, literally “helium core” in German, is a term used in multiple scientific contexts to denote a mass of helium associated with a larger system. In astrophysics it refers to the helium core of a star, formed after the exhaustion of core hydrogen, while in nuclear physics it can denote the helium nucleus, the alpha particle.

In stars, the helium core consists mainly of helium and forms after hydrogen burning ends. The core contracts and heats until helium fusion begins via the triple-alpha process. In low-mass stars helium ignition occurs under degenerate conditions, producing a helium flash before stable helium burning; in higher-mass stars the ignition is nondegenerate and proceeds smoothly. The timing and size of the helium core influence later evolution, including transitions from the red giant stage to horizontal-branch or to later phases such as the asymptotic giant branch.

In nuclear physics, the helium nucleus (alpha particle) is a tightly bound cluster of two protons and two neutrons. It has a mass of about 4 atomic mass units and a charge of +2e; its binding energy is about 28.3 MeV. Helium nuclei are produced in alpha decay and feature prominently in fusion reactions in stars and laboratory experiments, where they are used as probes and fuels.