Evolution of a red giant star, illustration. Red giant stars are stars that are much more massive than the Sun, and evolve through different stages. At upper left, the cutaway shows the star's burnt-out core as normal nuclear fusion starts to come to an end. At upper centre, as the core shrinks and heats up a hydrogen-burning shell ignites (blue) and the outer layers swell outwards. At upper right, the core re-ignites and the hydrogen-burning shell dies down as the star contracts again under its own weight. At lower left, the now carbon-rich core has burnt out again, the hydrogen fusion shell has re-ignited, and a helium fusion shell (orange) has formed around the core. The thermal pressures eject the outer layers of the star (lower centre) to form a planetary nebula, leaving the exposed core which is a white dwarf star (lower right).