Nuclear fusion is the joining together of two light nuclei at high temperatures to form a heavier nucleus with the release of a large amount of energy. A fusion reaction occurs when two light nuclei approach each other so closely that their Coulomb (charge) repulsion is overcome, allowing the nuclei to fuse. The total mass of the fusion products is lower than that of the two original nuclei; the difference is converted to kinetic energy which is distributed between the products.
Methods being investigated in an attempt to harness this potentially huge source of energy on Earth include magnetic confinement fusion (the use of strong magnetic fields to confine plasma so as to allow fusion reactions to occur within it) and inertial confinement fusion (the use of high-powered lasers or other beam devices to implode a pellet of material to such high densities that fusion occurs).
Main sequence stars produce light and heat through the fusion of hydrogen into helium in their cores. In red giants, successively heavier nuclei fuse, at higher temperatures, to form elements such as carbon, oxygen, and silicon (see stars, evolution) which are the basis of Earth-like planets and life.