Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles. WHAT IS FUSION? Fusion is the process that powers the sun and the stars. Fusion is the reaction where two lighter atoms join to form a heavier one. Fusion occurs when two light atoms bond together, or fuse, to make a heavier one. With fusion, the nuclei of atoms are joined, or fused together to make a new, heavier nucleus. Fusion is the energy source of the Universe, occuring in the core of the Sun and stars.