Animation of the proton-proton chain reaction that powers the Sun by converting hydrogen to helium. In the Sun's core, it is hot and dense enough that protons (red) can collide and fuse, forming a diproton. One proton then undergoes beta decay, turning into a neutron (yellow) and emitting a positron (orange star) and a neutrino (pink), forming a deuterium nucleus (hydrogen-2). Another proton then fuses with this, forming helium-3 and emitting a gamma ray (yellow wave). When this helium-3 collides with another helium-3, two protons are emitted and a helium-4 nucleus is formed. This nuclear fusion releases the energy that powers the Sun. In more massive stars, a catalytic cycle called the CNO cycle dominates. See clip K003/7243 for that process.