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)
