Future Sun. Artwork of a planetary nebula produced by the dying Sun in five thousand million years time, as seen from the Earth & Moon. Planetary nebulae are expanding, roughly symmetrical clouds of gas that are ejected by dying stars. The gas is illuminated and ionised by radiation from the star at its centre. Planetary nebulae are a normal evolutionary stage of stars with a mass of 1 to 6 solar masses. They occur when the star runs out of fuel to power its nuclear reactions. The star's core collapses under the intense gravitational pressure to form a dim white dwarf, whilst some or all of the surface layers are given off as a nebula.