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Meaning of emission nebula

emis'sion neb"ula

Pronunciation: [key]
— Astron. Astron.
  1. a bright diffuse nebula that emits light as a result of ionization of its gas atoms by ultraviolet radiation, as the Orion Nebula, planetary nebulae, and supernova remnants, as the Crab Nebula.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.