Nuclear fusion

Nuclear fusion is a nuclear reaction in which atomic nuclei are fused, converting matter to energy in the process. This can be done with any nuclei less heavy than iron. However, the fusion of heavier nuclei absorbs energy rather than releasing it.

In the Sun and other stars, four hydrogen nuclei fuse to make a helium nucleus. This may be done via the Proton-poroton chain or the Carbon-nitrogen cycle.

In thermonuclear weapons (the hydrogen bomb). two deuterium nuclei fuse to make a helium nucleus.

It requires very high temperatures and densities, and so is currently impractical as a source of energy for peaceful uses. However, research is under way to find ways to harness it.