Thermonuclear fusion comprises a wide range of processes during which the nuclei of light atoms fuse, creating a heavier nuclei and releasing energy. Processes like p-p fusion, the CNO cycle, or the triple alpha process power stars; D-T fusion with an ignition temperature of 150 million kelvin is tried on Earth. Positively charged nuclei need to overcome repulsive forces and come close enough together to fuse. This could be done by external power (e.g., strong gravity) or by giving particles enough velocity (e.g., by a high temperature). The Lawson criterion determines the conditions for harnessing an effective fusion reaction.