ROBERT GENDLER / SCIENCE PHOTO LIBRARY ROBERT GENDLER / SCIENCE PHOTO LIBRARY
Reflection nebula (NGC 2170), optical image. This nebula (bright object at bottom right) is a large cloud of dust and gas lying around 2,400 light years from Earth in the constellation Monoceros. It does not emit light of its own, but reflects light from nearby stars. It appears blue because short-wavelength blue light is scattered more than red by the dust particles. The other bright objects are also reflection nebulae. 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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