ROBERT GENDLER / SCIENCE PHOTO LIBRARY ROBERT GENDLER / SCIENCE PHOTO LIBRARY
Reflection nebula (NGC 1999), optical image. This nebula (bright object left of centre) is a large cloud of dust and gas lying around 1500 light years from Earth in the constellation Orion. It does not emit light of its own, but reflects light from the nearby V380 star. It appears blue because short-wavelength blue light is scattered more than red by the dust particles. Just visible as a black dot within the nebula is the silhouette of a dark nebula, or thick dust cloud, in the foreground, blocking the light from behind. The reddish regions are due to areas of emission nebulosity, where the radiation of hot young stars ionises hydrogen gas and causes it to glow.
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