RUSSELL CROMAN / SCIENCE PHOTO LIBRARY RUSSELL CROMAN / SCIENCE PHOTO LIBRARY
Horsehead nebula, optical image. The famous Horsehead is formed by the silhouette of the dark nebula B33 against the bright pink emission nebula IC 434 beyond it. Emission nebulae glow pink as radiation from hot young stars ionises the hydrogen gas that they contain. Dark nebulae are clouds of dust and non-ionised gas, which do not emit light. At lower left is a third type of nebula, the reflection nebula NGC 2023. Reflection nebulae do not emit their own light either, but instead reflect the light of bright stars within or near them. They typically appear blue as blue light is scattered more effectively than red. The Horsehead nebula lies around 1500 light years from Earth in the constellation Orion.
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