what is fusion? Fusion is the process that powers the sun and the stars. It is the reaction in which two atoms of hydrogen combine together, or fuse, to form an atom of helium. In the process some of the mass of the hydrogen is converted into energy. The easiest fusion reaction to make happen is combining deuterium (or âheavy hydrogen) with tritium (or âheavy-heavy hydrogenâ) to make helium and a neutron. Deuterium is plentifully available in ordinary water. Tritium can be produced by combining the fusion neutron with the abundant light metal lithium.