Animation of the proton-proton IV nuclear reaction, one of the reactions 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 (pink) and a neutrino (orange star). This forms a deuterium nucleus (hydrogen-2). In the pp IV reaction, this collides with another proton, forming a helium-3 nucleus and emitting a gamma ray (yellow wave), before another proton collides and one decays, releasing a very high energy neutrino, a positron, and forming a helium-4 nucleus. The pp IV reaction is predicted but has not been observed, and is estimated to make up only 0.3 parts per million of the solar reactions. See clip K003 3971 for the dominant pp I branch of the fusion reaction, K003 4575 for the pp II branch, and K003 4581 for the pp III branch.