Nuclear fusion is the process of combining 4 hydrogen nuclei to create 1 helium-4 nuclei and 2 new hydrogen nuclei. It is described in the process above. The positrons produced annialate an electron and do not provide energy. In total, the reaction produces 26.732 MeV of energy, 2.2% of which is lost to gamma rays. This process can be initiated at lower temperatures than the CNO system of fusion, which is why it dominates in stars under 1.3 solar masses. The downside of this process is that the initial hydrogen-hydrogen fusion reaction is so rare that it can take a proton in the sun's core 9 million years to fuse. That is the limiting factor of this reaction. Later on in a star's lifetime, it may start fusing different elements, through the p-p II branch, fusing helium, the p-p III branch, fusing born on beryllium, and the IV branch, though that has never been observered.
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