ProtonProtonKettenreaktionen

Proton-Proton-Kettenreaktionen, often abbreviated as the pp-chain, represent a dominant set of nuclear fusion reactions that occur in stars like our Sun. These reactions are responsible for converting hydrogen into helium and releasing vast amounts of energy in the process. The primary goal of the pp-chain is to fuse four protons (hydrogen nuclei) into one helium nucleus.

The pp-chain consists of several steps. The initial and rate-limiting step involves two protons colliding to form a deuterium nucleus, a positron, and an electron neutrino. The positron quickly annihilates with an electron, producing gamma rays. In the next step, a deuterium nucleus fuses with another proton to form a helium-3 nucleus, releasing another gamma ray. Finally, two helium-3 nuclei can fuse to produce a helium-4 nucleus, releasing two protons. These released protons can then participate in further pp-chain reactions.

There are actually three main branches of the pp-chain, denoted ppI, ppII, and ppIII, with ppI being the most common in stars like the Sun. The specific branch that dominates depends on the temperature and density of the stellar core. The energy released by the pp-chain is what powers stars, counteracting the inward pull of gravity and maintaining hydrostatic equilibrium. This process is fundamental to understanding stellar evolution and the origin of elements heavier than helium.