Eagle nebula (M16), optical image. This is an emission nebula, a huge cloud of gas that glows as the gases are ionised by radiation from hot young stars that formed within it. Colours have been used to differentiate the light-emitting ions here: ionised sulphur is red, ionised hydrogen is green and ionised oxygen is blue. The nebula contains numerous small dark dense globules of gas seen silhouetted against the bright nebula (for instance at lower right). These are thought to be regions in an advanced stage of star formation. The Eagle nebula lies around 7000 light years from Earth in the constellation Serpens.