Evolution of a red giant star, illustration. Red giant stars are much more massive than the Sun, and evolve through different stages. Second from left, normal nuclear fusion has come to an end. The core shrinks and heats up, causing a hydrogen-burning shell to ignite and the outer layers to swell outwards. Next, the core re-ignites and the hydrogen-burning shell dies down as the star contracts again under its own weight. Once the now carbon-rich core has burnt out again, the hydrogen fusion shell re-ignites, and a helium fusion shell forms around the core. The thermal pressures eject the outer layers of the star to form a planetary nebula (second from right), leaving the exposed core which is now a white dwarf star (right).