Dictionary![]() ![]() emis'sion neb"ulaAstron. 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.
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