The second step of the proton-proton cycle . This step involves the weak interaction because a neutrino interaction is necessary to trigger the transmutation of one of the protons to a neutron in order to form deuterium. A proton-proton interaction can produce deuterium by a quark transformation which converts one of the protons to a neutron. The third step of the proton-proton cycle . The fifth step of the proton-proton cycle .
Alpha particles are the end product of the proton-proton fusion cycle.
They are formed by the fusion of two helium-3 nuclei .