The process of fusing the nuclei of atoms. In fusion, the repulsive electrostatic forces between the atomic nuclei are overcome and the nuclei are brought within range of strong nuclear forces, so that they fuse into a single new nucleus. In most cases, very high temperatures are required for fusion, hence we speak of thermonuclear fusion. When the nuclei of light atoms fuse, large amounts of energy are released. Fusion takes place in the core of stars and produces a variety of chemical elements. On Earth, we are trying to achieve controlled thermonuclear fusion as a clean and inexhaustible source of energy.