Process in which two lighter atomic nuclei combine to form a heavier atomic nucleus. Very high temperatures are normally required in order for atomic nuclei to collide with sufficient energy to overcome the Coulomb barrier (their mutual electrostatic repulsions). Fusion that occurs under high-temperature conditions is called thermonuclear fusion. Fusion reactions involving light elements release large amounts of energy. The mass of the resulting nucleus is less than the combined masses of the two original nuclei. The difference in mass, Δm (mass defect), is released as energy:

Fusion of elements up to 56Fe results in the release of energy. Thermonuclear fusion powers stars such as our own sun. Main sequence stars are dominated by hydrogen burning fusion reactions. In red giants, He is converted into C by the triple-alpha process. In highly evolved high-mass stars, fusion reactions synthesize a succession of elements up to Fe by helium capture.

For the elements heavier than 56Fe, scientists hypothesize there are other formation mechanisms including for example the merger of neutron stars that may be responsible for the formation of the lighter heavy elements.

Some or all content above used with permission from J. H. Wittke.