Animation showing the internal structure of the Sun. At the centre of the Sun is the core, where the nuclear reactions that power the Sun take place. Here hydrogen nuclei (protons, red) collide, forming deuterium (a proton and a neutron, blue) then helium-3 (two protons and a neutron) before finally forming helium-4 (two protons and two neutrons). This results in a release of energy. Zooming out, the second layer is shown: the radiative zone. Energy from the core is emitted as gamma rays, but the radiative zone is so dense that this radiation is absorbed and emitted many times as it moves outwards, cooling and lengthening in wavelength as it does so. It takes a photon an average of 170,000 years to pass through the radiative zone. The next layer is the convective zone, where the heat generated moves outwards through convection of regions of plasma, with hotter regions rising to the surface, cooling and sinking again. This forms cells on the surface of that Sun that are visible and call