<P> Research into developing controlled thermonuclear fusion for civil purposes also began in earnest in the 1950s, and it continues to this day . </P> <P> The origin of the energy released in fusion of light elements is due to interplay of two opposing forces, the nuclear force which combines together protons and neutrons, and the Coulomb force, which causes protons to repel each other . The protons are positively charged and repel each other but they nonetheless stick together, demonstrating the existence of another force referred to as nuclear attraction . Light nuclei (or nuclei smaller than iron and nickel), are sufficiently small and proton - poor allowing the nuclear force to overcome the repulsive Coulomb force . This is because the nucleus is sufficiently small that all nucleons feel the short - range attractive force at least as strongly as they feel the infinite - range Coulomb repulsion . Building up these nuclei from lighter nuclei by fusion thus releases the extra energy from the net attraction of these particles . For larger nuclei, however, no energy is released, since the nuclear force is short - range and cannot continue to act across still larger atomic nuclei . Thus, energy is no longer released when such nuclei are made by fusion; instead, energy is required as input to such processes . </P> <P> Fusion reactions create the light elements that power the stars and produce virtually all elements in a process called nucleosynthesis. The Sun is a main - sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium . In its core, the Sun fuses 610 million metric tons of hydrogen each second and gives helium 606 million metric tons . The fusion of lighter elements in stars releases energy and the mass that always accompanies it . For example, in the fusion of two hydrogen nuclei to form helium, 0.7% of the mass is carried away from the system in the form of kinetic energy of an alpha particle or other forms of energy, such as electromagnetic radiation . </P> <P> It takes considerable energy to force nuclei to fuse, even those of the lightest element, hydrogen . This is because all nuclei have a positive charge due to their protons, and as like charges repel, nuclei strongly resist being pushed close together . When accelerated to high enough speeds, nuclei can overcome this electrostatic repulsion and brought close enough such that the attractive nuclear force is greater than the repulsive Coulomb force . As the strong force grows very rapidly once beyond that critical distance, the fusing nucleons "fall" into one another and result is fusion and net energy produced . The fusion of lighter nuclei, which creates a heavier nucleus and often a free neutron or proton, generally releases more energy than it takes to force the nuclei together; this is an exothermic process that can produce self - sustaining reactions . </P>

Energy released by the sun is a result of