emission nebula

Astron.

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.

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▪ astronomy

 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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