Horsehead nebula. True-colour optical image of the Horsehead nebula in the constellation Orion. North is at left. The Horsehead itself (lower centre) is formed by the intrusion of a large dark dust cloud (B 33) into a pink emission nebula (IC 434). Emission nebulae glow due to ionisation of their hydrogen gas by nearby hot young stars. The bright star to the left of the nebula is Alnitak (Zeta Orionis). The nebula below Alnitak is the Flame nebula (NGC 2024). The bright star at upper left is Epsilon Orionis, and the star above the Horsehead is Sigma Orionis. This image was produced by digitally combining photographs taken by the UK Schmidt Telescope in blue and red light.