Glossary term: Nuclear Fusion

Also known as Thermonuclear fusion or fusion

Description: Nuclear fusion is the process where the atomic nuclei of lighter elements join together to form the nucleus of a heavier element.

In the Universe, nuclear fusion plays two important roles. For one, it provides the energy supply for the radiation emitted by stars like our Sun. When sufficiently light nuclei fuse, the total rest mass of the resulting nucleus is slightly smaller than the combined rest masses of the initial nuclei. This "mass deficit" corresponds to the energy released by the fusion reaction, via Einstein's famous formula E=mc2, which links mass m, energy E, and the speed of light c. For example, in the core of the Sun, hydrogen nuclei fuse together to form helium and release energy in the form of radiation as well as neutrino particles.

The second role of nuclear fusion is that it is responsible for producing elements in the Universe that are more complex than hydrogen and helium. After the Big Bang, only hydrogen, helium, and trace amounts of lithium nuclei existed in the Universe. Fusion reactions in the cores of stars, in the course of supernova explosions, and from explosions caused by colliding neutron stars, are the source of (essentially) all the remaining heavier chemical elements in the Universe. The chemical elements that make up the greatest part of the human body by mass, notably oxygen and carbon, were formed by nuclear fusion in the core of stars or during supernova explosions, prompting the expression "we are star dust".

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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