Nuclear Fusion Animation
Nuclear fusion is the joining (or fusing) of the nuclei of two atoms to form a single heavier atom. At extremely high temperatures in the range of tens of millions of degrees the nuclei of isotopes of hydrogen (and some other light elements) can readily combine to form heavier elements and in the process release considerable energy.
For fusion to occur, the electrostatic repulsion between the atoms must be overcome. Creating these conditions is one of the major problems in triggering a fusion reaction.