Sun internal structure, computer artwork. The central region of the Sun is the core (yellow/white), where hydrogen nuclei are fused together to form helium, with the release of enormous amounts of energy. It is this nuclear fission that powers the Sun. Surrounding the core is the radiative zone (orange/yellow tangles), where energy in the form of photons is emitted by nuclei and then reabsorbed almost immediately by nearby nuclei. The density of this layer decreases as it cools further from the core, until it is cool and rarefied enough for convection to take effect, in the convective zone (dark orange). Here hot plasma rises to the visible surface (photosphere, bright yellow) and cools (dark dots) before sinking back to the inner layers. The Sun is around 1.39 million kilometres in diameter.