nuclear fusion (thing)

Stars on the main sequence radiate energy by the fusion of protons, which can be thought of as hydrogen nuclei. This is known as hydrogen burning, and ultimately it provides the energy by which humans live.

First, two protons are fused together to form a deuterium nucleus - a proton and a neutron. The creation of a neutron from a proton entails the release of a positron (an anti-electron) and a neutrino, a massless, chargeless particle. The nuclear equation can be represented as follows: note that the numbers in subscript represent charge and the numbers in superscript mass, and that both are preserved in all the following reactions.

₁¹H⁺ + ₁¹H⁺ ----> ₁²H⁺ + ν + ₁⁰e⁺.

Two particles of charge 1+ and mass 1 (protons) have been transformed to a particle of charge 1+ and mass 2 (a deuterium nucleus), a massless, chargeless neutrino, and a positron of charge 1+.

This deuterium nucleus then fuses with another proton to produce a helium-3 nucleus:

₁¹H⁺ + ₁²H⁺ ----> ₂³He²⁺

Finally, two of these helium-3 nuclei fuse together to form a helium-4 nucleus (the common isotope of helium) and release two protons:

₂³He²⁺ + ₂³He²⁺ ----> ₂⁴He²⁺ + ₁¹H⁺ + ₁¹H⁺

This reaction releases energy for the same reason as all nuclear reactions - helium has a higher binding energy per nucleon than hydrogen, so energy is released equal to the difference in binding energies of the reactants and products.

Later in a star's life, while it is in the red giant phase, the core temperature becomes great enough (around 100 million degrees Celsius) to ignite further nuclear fusion reactions. First, beryllium (Be) is created from helium. Then carbon (C) is created from helium and beryllium. Finally, oxygen (O) is created from carbon and helium. These reactions are exothermic (they release energy), and thus they are spontaneous in the sense that they will continue unaided as soon as enough energy has been provided. However the activation energy in this case is too great for the reactions to occur in a main sequence star, so it does not occur until the red giant phase.

Stars which become supernovas release so much energy that they initiate endothermic fusion reactions: those whose products are actually less stable than the reactants. These reactions are responsible for all the elements heavier than iron-56, which has the highest binding energy per nucleon of any possible nucleus. Without supernovas, there would be no nickel, copper, zinc, silver, gold, iodine, platinum, lead, mercury, uranium or plutonium, to name some of the most familiar.