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emission nebula

noun, Astronomy
a bright diffuse nebula that emits light as a result of ionization of its gas atoms by ultraviolet radiation, as the Orion Nebula, planetary nebulae, and supernova remnants, as the Crab Nebula. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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British Dictionary definitions for emission nebula

emission nebula

a type of nebula that emits visible radiation See nebula
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
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emission nebula in Science
emission nebula
A nebula that absorbs ionizing ultraviolet radiation from nearby hot stars or star remnants and reemits it as visible light. HII regions, planetary nebulae, and nebulae created by the explosion of supernovae are different kinds of emission nebulae. See more at nebula.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Article for emission nebula

in astronomy, a bright, diffuse light sometimes associated with supernova remnants or with stars whose temperatures exceed 20,000 K. The excitation process necessary to provide observed optical and radio energies in such gaseous regions was long an astronomical puzzle. Spectral and polarization studies of the 1950s suggested that their luminosity largely results from synchrotron radiation-energy from electrons accelerated in a magnetic field, as seen in certain particle accelerators.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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