|an expanding shell of gas surrounding a dying star, formed from matter ejected from the star's outer layers; the gas is ionized by the remaining hot stellar core, emitting light in the process|
|[C18: named from its (occasional) resemblance to a planetary disc]|
|planetary nebula |
A nebula consisting of a rapidly expanding shell of glowing gas, mostly hydrogen, ejected from a red giant upon its collapse into a white dwarf. Ultraviolet radiation from the hot, luminous white dwarf ionizes the expanding gas and causes it to glow. The nebula disappears once the cooling dwarf can no longer ionize it, and its material eventually returns to the interstellar medium. See more at white dwarf.