A group of nuclear reactions involving fusion of light nuclei that converts hydrogen into helium. It is believed to be the principal source of energy in main sequence stars of a little more than a solar mass and of less massive stars. Completion of a chain results in the consumption of four protons (hydrogen-1 nuclei, designated ¹H), and the production of a helium (⁴He) nucleus plus two positrons (e⁺) and two neutrinos (ν). The two positrons are annihilated along with two electrons (e⁻), and the total energy release is 26.73 MeV. Approximately 0.58 MeV is released as neutrino energy and is not available as thermal energy in a star. The chain can be thought of as the conversion of four hydrogen atoms into a helium atom plus energy in the form of photons or neutrinos, or the kinetic energy of particles. The energy E = 26.73 MeV arises from the mass difference between four hydrogen atoms and the helium atom, and is calculated from the Einstein mass-energy equation E = Δmc², where Δm is the mass difference and c² is the square of the velocity of light. Because hydrogen is the fuel consumed in the process, it is referred to as hydrogen burning by means of the proton-proton chain.