Emission Nebula
Lagoon Nebula
Image from NASA/Hubble
A nebula that emits light. Emission nebulae are formed when energetic ultraviolet light from a very hot star excites a cloud of hydrogen gas; the UV radiation ionizes the hydrogen, stripping the electrons from the hydrogen atoms. The free electrons combine with protons, reddish light being produced when electrons and protons combine to reform hydrogen atoms.
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Development Notes
Text by M. Alan Kazlev
Initially published on 24 October 2001.