planetary nebula

planetary nebula: see nebula.

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planetary nebula

Any of a class of bright nebulae that may somewhat resemble planets when viewed through a small telescope but are, in fact, expanding shells of luminous gas around dying stars. A planetary nebula is the outer envelope shed by a red giant star not massive enough to become a supernova. Instead, the star's intensely hot core becomes exposed (see white dwarf star) and ionizes the surrounding shell of gas, which is expanding at tens of miles per second.

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planetary nebula [′plan·ə‚ter·ē ′neb·yə·lə]

(astronomy)

An oval or round nebula of expanding concentric rings of gas associated with a hot central star.