nucleosynthesis or nucleogenesis, in astronomy, production of all the chemical elements from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleus; nuclear energy). A star obtains its energy by fusing together light nuclei to form heavier nuclei; in this process, mass ( m) is converted into energy ( E) in accordance with Einstein's formula, E= mc2, in which c is the speed of light. The reactions are initiated by the high temperatures (about 14 million degrees Celsius) at the center of the star. In the course of producing nuclear energy, the star synthesizes all the elements of the periodic table from its initial composition of mostly hydrogen and a small amount of helium.
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