In physics, nuclear fusion is the process by which two nuclei join together to form a heavier nucleus. It is accompanied by the release or absorption of energy depending on the masses of the nuclei involved. Iron and nickel nuclei have the largest binding energies of all nuclei and therefore are the most stable. The fusion of two nuclei to produce a nucleus lighter than iron or nickel generally gives off energy while the fusion of nuclei heavier than them absorbs energy.

Nuclear fusion of light elements releases the energy that causes stars to shine and hydrogen bombs to explode. Nuclear fusion of heavy elements occurs in the extreme conditions of supernova explosions. Nuclear fusion in stars and supernovae is the primary process by which new natural elements are created.

This article deals with the fusion reaction itself. See the article on fusion power for information on controlling the fusion reaction to produce useful power.

It takes considerable energy to force nuclei to fuse, even those of the least massive element, hydrogen. But the fusion of lighter nuclei, which creates a heavier nucleus and a free neutron, will generally release more energy than it took to force them together — an exothermic process that can produce self-sustaining reactions.

The energy released in most nuclear reactions is much larger than that for chemical reactions, because the binding energy that holds a nucleus together is far greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to hydrogen is 13.6 electron volts -- less than one-millionth of the 17 MeV released in the D-T (deuterium-tritium) reaction shown to the right.

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