Horsehead nebula, optical image. The Horsehead nebula's (right) distinctive shape is caused by dark nebula Barnard 33 (B 33). This cloud of dust and gas obscures the light from the emission nebula IC 434 that lies behind, seen here as the red region at upper right. The gas of this nebula glows as it is ionised by the radiation from hot stars. The Flame nebula (NGC 2024, lower centre) is also an emission nebula bissected by a dark dust cloud. The bright star at centre is Alnitak (Zeta Orionis), one of the three stars in Orion's belt, which lies 817 light years from Earth. The nebulae lie around 1500 light years away in the constellation Orion.