Horsehead Nebula. Composite image of the surface of an alien planet (bottom) with an infrared Hubble Space Telescope (HST) image of the Horsehead Nebula (Barnard 33) in the background. This nebula lies around 1600 light years from Earth, in the constellation Orion. The horsehead shape, several light years across, is formed by an intrusion of dust into the emission nebula IC 434. The red glow from this nebula is due to the ionisation of hydrogen by radiation from nearby stars.