We must not confuse nuclear fission (a heavy nucleus splits to form two lighter nuclei, called the products of fission) and nuclear fusion (two light nuclei combine to form one heavy nucleus). In one case as in the other, there is a release of energy, but this is much greater in the case of fusion. Nuclear fusion (or in any case that involving hydrogen) does not, like fission, produce radioactive wastes.

Nuclear fusion reactions begin at temperatures greater than 15 million degrees. For this reason, we say that nuclear fusion (or thermonuclear fusion) is the energy source of the stars because, for now, only the cores of stars can attain such a level of thermal agitation.