Horsehead nebula, optical image. The horsehead 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 top. The gas of this nebula glows as it is ionised by the radiation from hot stars. The bright reflection nebula NGC 2023 can also be seen to the lower left of the horsehead; reflection nebulae reflect the light of nearby stars. The Horsehead nebula lies around 1500 light years from Earth in the constellation Orion.