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 between any nuclei larger than C require such high temperatures that they are actually quite uncommon in stars and formation of most heavier elements occurs by addition of He nuclei (the α process). For example, the repulsive force between two C nuclei is three times greater than the force between the nuclei of C and He. Thus, C-He fusion occurs at a lower temperature than C-C.
Some or all content above used with permission from J. H. Wittke.