the energy from nuclear fusion is produced by the conversion of mass to energy
In nuclear physics, nuclear fusion is a reaction in which two or more atomic nuclei come close enough to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifested as the release of large amounts of energy. This difference in mass arises due to the difference in atomic ``binding energy'' between the atomic nuclei before and after the reaction. Fusion is the process that powers active or ``main sequence'' stars, or other high magnitude stars.
Nuclear fusion