planetary nebula

noun Astronomy.
an expanding shell of thin ionized gas that is ejected from and surrounds a hot, dying star of about the same mass as the sun; the gas absorbs ultraviolet radiation from the central star and reemits it as visible light by the process of fluorescence.

1850–55; so named for its resemblance to the planets Uranus and Neptune Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
planetary nebula
an expanding shell of gas surrounding a dying star, formed from matter ejected from the star's outer layers; the gas is ionized by the remaining hot stellar core, emitting light in the process
[C18: named from its (occasional) resemblance to a planetary disc]

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
planetary nebula   (plān'ĭ-těr'ē)  Pronunciation Key 
A nebula consisting of a rapidly expanding shell of glowing gas, mostly hydrogen, ejected from a red giant upon its collapse into a white dwarf. Ultraviolet radiation from the hot, luminous white dwarf ionizes the expanding gas and causes it to glow. The nebula disappears once the cooling dwarf can no longer ionize it, and its material eventually returns to the interstellar medium. See more at white dwarf.
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