Horsehead nebula, optical image. North is at left. The shape of a horse's head (centre) is formed by the intrusion of the dark dust cloud B33 into the glowing red emission nebula IC 434. Emission nebulae (or HII regions) glow red as the hydrogen gas they contain is ionised by ultraviolet light from nearby hot stars. The bright star to the left of the horsehead is Alnitak (Zeta Orionis). Below Alnitak is the pink Flame nebula (NGC 2024). The Horsehead nebula lies around 1600 light years from Earth in the constellation Orion.