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Diagrammatic representation of the ppI (p-p-one) chain, which is the main nuclear reaction taking place in the core of the Sun. To the left, four protons (Hydrogen nuclei) are shown coming together in pairs to form two deuterium nuclei, each consisting of a red proton and a blue neutron. An electron (shown in light blue) and a yellow flash of energy are emitted from each nucleus as it is formed. In the centre, an additional proton is added to each of the two nuclei, to form two nuclei of Helium-3 (each consisting of two protons and one neutron). A yellow flash again indicates emission of energy during this process. Finally, on the right, the two Helium-3 nuclei combine to form a single Helium-4 nucleus, consisting of two protons and two neutrons. Two protons and a yellow flash of energy are emitted as this final nucleus is formed. The overall result of this reaction is the conversion of four hydrogen nuclei into one helium nucleus. The yellow flashes indicate energy being released at each stage.

 3.2 Nuclear fusion – the source of the Sun’s energy