<P> Research into developing controlled thermonuclear fusion for civil purposes began in earnest in the 1950s, and it continues to this day . </P> <P> The release of energy with the fusion of light elements is due to the 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 . Protons are positively charged and repel each other by the Coulomb force, but they can nonetheless stick together, demonstrating the existence of another, short - range, 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 repulsion . 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 nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of particles . For larger nuclei, however, no energy is released, since the nuclear force is short - range and cannot continue to act across longer atomic length scales . Thus, energy is not released with the fusion of such nuclei; instead, energy is required as input for such processes . </P> <P> Fusion powers stars and produces virtually all elements in a process called nucleosynthesis . The Sun is a main - sequence star, and, as such, generates its energy by nuclear fusion of hydrogen nuclei into helium . In its core, the Sun fuses 620 million metric tons of hydrogen and makes 606 million metric tons of helium each second . 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 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 . 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 . The strong force grows rapidly once the nuclei are close enough, and the fusing nucleons can essentially "fall" into each other 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>

Where does the energy come from when hydrogen fuses
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