Thermonuclear fusion comprises a wide range of processes during which the nuclei of light atoms fuse, creating a heavier nucleus 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 about 150 million kelvin is pursued on Earth. Positively charged nuclei need to overcome repulsive forces and come close enough together to fuse. This can be achieved through extreme pressure and confinement, as in stars, or by giving particles enough velocity through heating. The Lawson criterion defines the conditions required for an effective fusion reaction.