What occurs first, nuclear fusion or fission inside of the core of a star, or both? Stars are powered by nuclear fusion deep in their cores. If fusion was first, the star would begin to radiate light immediately. Nuclear fission is where the centers of atoms (nuclei) are split and broken apart. When they split, nuclear energy is released. If fission was first, the star would begin to radiate over a period of time. Would the solar mass then determine how a star begins to give light?