Ring Nebula (M57): located in the constellation of Lyra, the Ring Nebula is a planetary nebula. These form when dying stars, late in their Red Giant phase of evolution, slowly eject their outer layers.
They appear to be luminous because their gases are ionised by radiation from the compact stellar remnant - the core, usually a white dwarf - lying at the centre (as above). Thus, planetary nebulae are a form of emission nebulae. (Illingworth, 1994)
