Description: Nuclear fusion is the umbrella term for all reactions whereby lighter atomic nuclei collide and merge to form one or more heavier atomic nuclei. In astronomy, hydrogen fusion is the nuclear fusion reaction that transforms hydrogen nuclei (each consisting of a single proton) into helium-4 nuclei (each consisting of two protons and two neutrons bound together). The helium-4 nucleus has a mass less than the sum of the masses of the protons and neutrons it is made of. By Einstein's famous formula E=mc², that mass difference corresponds to an energy difference. As the protons and neutrons fuse to form helium-4, the amount of energy corresponding to that difference is released. In this way, hydrogen fusion serves as the energy source for so-called main sequence stars like our Sun. At least for some time, such stars are in an equilibrium state: the amount of energy released by hydrogen fusion in their cores corresponds to the energy those brightly-shining stars emit in the form of light and other kinds of electromagnetic radiation as well as particles.

Hydrogen fusion proceeds via several intermediate steps. For stars with the mass of our Sun or less, it proceeds via the so-called proton–proton chain (pp chain). In the simplest version of that chain of reactions, two hydrogen nuclei (protons) fuse to yield deuterium nuclei (one proton, one neutron each), which then fuse with one additional hydrogen nucleus to yield helium-3 (two protons, one neutron). Two such helium-3 nuclei fuse to yield helium-4 plus two remaining hydrogen nuclei. In stars with more than about 1.3 times the mass of our Sun, an alternative process called the carbon–nitrogen–oxygen (CNO) cycle becomes the dominant way for fusing hydrogen into helium. Scientists on Earth have built machines to create fusion reactions with the hope that in future it can become a viable way of generating energy. Hydrogen fusion occurs not only in stars, but also took place during the early Big Bang phase of our Universe.

Related Terms:

• Hydrogen
• Nuclear Fusion

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

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