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 nebulae and supernova remnants.