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2. Nuclear Fusion

Nuclear fusion is a very important process to understand for certain areas of physics, for it gives us an idea of what’s happening in very dense, energetic places in the universe. It explains what is happening in the cores of stars (and therefore why many of the elements in the universe are even in existence), very early conditions of the universe when it was still very hot and dense, as well as the reactions involved in the production of nuclear power. Nuclear fusion is the process in which two atomic nuclei fuse to form a single, heavier nucleus. 

The main points about nuclear fusion are:
  •  We see as a result of Einstein’s Theory of Special Relativity the following famous equation


which states that energy is equal to mass times the square of the speed of light. This means that even the nuclei of atoms at rest (so without kinetic energy associated with velocity) contain enormous amounts of untapped energy.

  • The elements in the Periodic Table up to Iron (Fe) release energy in the process of nuclear fusion, as a result of the nuclear strong force being stronger than the Coulomb force for smaller nuclei. Elements heavier than Iron actually absorb energy in order to fuse, which makes the fusion of these elements much less likely to happen. In fact, these heavier elements tend to undergo nuclear fission, the process which is mostly used today for nuclear energy.
  • In the cores of stars, many nuclear fusion reactions are taking place. When a star first forms, the dense region in the center of the condensing gas begins to fuse Hydrogen atoms together to make Helium. This continues for the fully formed star for a long time until the star begins to make Carbon, and various other heavy elements at the end of its life. All elements heavier than Iron in the universe are created by the supernovae of massive stars.
  • The sun’s nuclear reactions create high-energy photons that are emitted from the sun's surface only after losing much energy in the zones between the sun's core and the photosphere (see Convection and Solar Radiation).
  • Particles besides photons of radiation, like neutrinos, are generated in nuclear reactions as well. Detecting these particles on Earth verifies our model of nuclear fusion in the core of the sun.
Above is an illustration of a proton-proton chain, the main method by which our sun and smaller stars perform nuclear fusion.
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