Nebula

noun A visible, thinly spread cloud of interstellar gas and dust. Some nebulae are the remnants of a supernova explosion, others are gravity-induced condensations of the gases in the interstellar medium which in certain cases may become a site for the formation of new stars. The term was formerly used of any hazy, seemingly cloudlike object, including what are now recognized as other galaxies beyond the Milky Way; it is restricted now to actual clouds of gas and dust within our own galaxy. &diamf3; Nebulae are generally classified as bright or dark. Among the bright nebulae are cold clouds that reflect light from nearby stars ( reflection nebulae ) and hot, ionized clouds that glow with their own light ( emission nebulae ). Dark nebulae—cold clouds that absorb the passing light from background stars—are called absorption nebulae .