Horsehead nebula, optical image. North is at left. The Horsehead nebula (centre) is formed by the dark nebula B33 blocking the light from the bright red emission nebula IC 434 behind it. The emission nebula glows as the hydrogen it contains is ionised by the radiation from hot young stars within it. To the left of the Horsehead nebula is the pink Flame nebula (NGC 2024), which is divided by another dark patch of dust. Above the Flame nebula is the star Alnitak (Zeta Orionis). These nebulae lie around 1500 light years from Earth in the constellation Orion.