Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens.
The Sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reactions in a star is complex, but overall hydrogen nuclei join to form helium nuclei. Here is one nuclear fusion reaction that takes place:
hydrogen-1 nuclei fuse with hydrogen-2 nuclei to make helium-3 nuclei
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A nuclear fusion reaction showing the nuclei involved
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