The first step of the Hydrogen fusion process: a nucleus of Deuterium (2H) is formed from two protons with the emission of an antielectron and a neutrino.
University of Oregon

The Hydrogen Fusion Process

The basic Hydrogen fusion cycle involves four Hydrogen nuclei (protons) and two electrons and yields a Helium nucleus, two neutrinos and six photons. This process occurs in three steps: the first one is the Fusion of Hydrogen into Deuterium. Here two protons collide, one proton turns into a neutron emitting an antielectron and a neutrino.
The remaining proton is bound to the neutron forming a heavy Hydrogen (Deuterium) nucleus while the antielectron just produced will annihilate with an electron generating two high-energy photons.

The second step is the the formation of Helium-3: a proton is captured by a nucleus of Deuterium emitting a photon and forming then a 3He nucleus.

The third step is recombination of two Helium-3 into one nucleus of Helium with the emission of two protons.
Note that steps 1 and 2 each happen twice for each time step 3 occurs. In this process the total net energy released is to 26 MeV.

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