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Orion nebula, optical image. North is at left. The Orion nebula can be seen with the naked eye as a fuzzy patch in the constellation Orion. It comprises several nebulae, but the brightest is M42 (centre and upper right). M43 is the small round nebula immediately to the lower left of M42. These nebulae glow as the hydrogen they contain is ionised by radiation from hot young stars that recently formed inside them. In the case of M42, the stars that light it up are a group of four known as the Trapezium (just right of centre). The nebula NGC 1977 is at left. This nebula is not ionised, instead reflecting the light of nearby stars. The nebulae lie around 1500 light years from Earth. (KEYSTONE/SCIENCE PHOTO LIBRARY/ROBERT GENDLER/SCIENCE PHOTO LIBRARY)