planetary nebula

American  

noun

Astronomy.

1. 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.

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planetary nebula British  

noun

1. 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

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planetary nebula Scientific  

/ plăn′ĭ-tĕr′ē /

1. 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.

2. See more at white dwarf

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Etymology

Origin of planetary nebula

First recorded in 1850–55; so named for its resemblance to the planets Uranus and Neptune