There are currently 3 astronomical definitions for the tag 'nebula'.
nebula
bipolarnebula, brightnebula, compactnebula, darknebula, densecloud, diffusenebula, dustnebula, emissionnebula, filamentarynebula, gaseousnebula, planetarynebula, reflectionnebula, ringnebula
opiktheory, forbiddenline, forbiddentransition, nebularfilament, surfacephotometry