n.
Any of various tenuous clouds of gas and dust in interstellar space.
Nebulae constitute only a small percentage of a galaxy 's mass. Dark nebulae (e.g., the Coalsack ) are very dense, cold molecular clouds that appear as large, obscure, irregularly shaped areas in the sky. Bright nebulae (e.g., the Crab Nebula , planetary nebula ) appear as faintly luminous, glowing surfaces; they emit their own light or reflect that of stars near them. The term nebula also formerly referred to galaxies outside the Milky Way Galaxy .