- an expanding shell of thin ionized gas that is ejected from and surrounds a hot, dying star of about the same mass as the sun; the gas absorbs ultraviolet radiation from the central star and reemits it as visible light by the process of fluorescence.
- 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
