What is the process by which light nuclei combine to form a heavier nucleus?

The process by which light nuclei combine to form a heavier nucleus is nuclear fusion.

Fusion joins atomic nuclei through the strong nuclear interaction, producing a new nucleus and often releasing energy. The combined nucleus can have slightly less mass than the original nuclei; that missing mass becomes energy according to Einstein’s equation E = mc². Fusion is therefore different from chemical burning, which rearranges electrons rather than nuclei.

The Sun and other stars shine because hydrogen nuclei fuse into helium in their cores. In the Sun, the proton–proton chain dominates and ultimately converts four protons into one helium-4 nucleus, with energy and other particles emitted along the way. Stars more massive than the Sun can also fuse progressively heavier elements.

Fusion requires extremely high temperatures and pressures because positively charged nuclei repel one another electrically. Nuclear fission works in the opposite general direction: a heavy nucleus splits into smaller nuclei. Fusion can release energy for light elements up to roughly the iron-and-nickel region, beyond which fusion generally does not provide net energy.