Horsehead nebula. Optical image of the Horsehead nebula in the constellation Orion. North is at left. The Horsehead itself is formed by the intrusion of a large dark dust cloud (B33) into a glowing emission nebula (IC 434). Emission nebulae glow due to ionisation of their hydrogen gas by ultraviolet radiation from nearby hot young stars. These nebulae are around 1500 light years from Earth. This image was made by the CFH12K CCD camera on the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, USA.