A cloud of interstellar gas and dust. The term was formerly also used for objects now known to be galaxies (e.g. the ╘great nebula' in Andromeda, now usually called the Andromeda Galaxy).
An emission nebula glows in presence of ultraviolet radiation; a reflection nebula shines by reflecting starlight. An absorption nebula is dark and is usually evident only in silhouette against the background of a luminous nebula or starfield.
Other objects consisting of luminous gas are also known as nebulae, in particular planetary nebula and supernova remnants.
