1. Two protons fuse to make a deuterium nucleus (1 proton and 1 neutron). This step occus twice in overall reaction.

2. The deuterium nucleus and a proton fuse to make a nucleus of helium-3 (2 protons, 1 neutron). This step also occurs twice in the overall reaction.

3. Two helium-3 neuclei fuse to form helium-4 (2 protons, 2 neutrons), releasing two excess protons in the process.

Gamma rays and subatomic particles known as neutrinos and positrons carry off the energy released in the reaction.