Wiktionary
n. (context astronomy English) A nebula composed of ionized gases which produce visible light (as distinct from a reflection nebula, which reflects the light produced by neighboring stars).
Wikipedia
An emission nebula is a nebula formed of ionized gases that emit light of various colors. The most common source of ionization is high-energy photons emitted from a nearby hot star. Among the several different types of emission nebulae are H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.
Usage examples of "emission nebula".
Alnitak too had collected a haze, contributed by the Flame Nebula and the emission nebula IC434.
Nita silently recited her favorite shieldspell, the one that could stop anything from a thrown punch to an ICBM, and for safemission nebula, flung across the darkness like a transparent gauze burning in hydrogen red and oxygen blue.
Nothing was in the sky but unfamiliar stars and the flaming, motionless curtain of an emission nebula, flung across the darkness like a transparent gauze burning in hydrogen red and oxygen blue.