In astronomy, nebulae was originally all luminous, planar objects on the celestial sphere. This also included star nebulae, i. e. galaxies and star clusters, as they appear as nebulae in the telescope or with the naked eye at low resolution. Some of the proper names of such objects have remained unchanged, e. g. the Andromeda Nebula M 31 or the Triangle Nebula M 33.

Today, however, the term cosmic nebulae is almost exclusively used for interstellar clouds of dust and gas, which are classified into different categories depending on the type of light they emit: Gas nebulae that emit or reflect light and dark nebulae that absorb light.