(4) nuclear fusion Explanation: The Sun is a main-sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. Nuclear fusion is a nuclear reaction in which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to photons which are released through a cycle that even our sun uses.