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Horsehead nebula. True-colour optical image of the Horsehead nebula in the constellation Orion. North is to the left. The horsehead shape is formed by the intrusion of the dark nebula B33 into the bright emission nebula IC 434. The emission nebula shines red due to the ionisation of its hydrogen gas by radiation from hot young stars embedded in it. This image was produced by a CCD (charge- coupled device) camera, which stores images electronically rather than on film. (KEYSTONE/SCIENCE PHOTO LIBRARY/MOUNT STROMLO AND SIDING SPRING OBSERVATORIES)