The fusion of hydrogen nuclei (protons) into helium is a nuclear reaction that releases energy through a process called nuclear fusion. In this process, the mass of the resulting helium nucleus is slightly less than the combined mass of the original hydrogen nuclei. The energy released is a result of the conversion of mass into energy, as described by Einstein's famous equation, E=mc².
The mass that is converted into energy comes from the difference in mass between the reactants (hydrogen nuclei) and the products (helium nucleus). During fusion, some of the mass is converted into energy according to the principle of mass-energy equivalence. This energy is released in the form of electromagnetic radiation, typically in the form of high-energy photons (gamma rays).
While it is true that the mass of a neutron is slightly larger than the mass of a proton, the overall fusion reaction of hydrogen nuclei into helium involves a combination of protons and neutrons. The total mass of the helium nucleus is less than the combined mass of the individual hydrogen nuclei due to the binding energy that holds the nucleus together.
The binding energy is a measure of the energy required to break apart the nucleus into its individual components. In the fusion process, when the hydrogen nuclei combine to form a helium nucleus, some of the mass is converted into binding energy. This release of binding energy per the mass-energy equivalence principle results in the overall release of energy in the fusion reaction.