planetary nebula

Astron.

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]

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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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▪ astronomy

 any of a class of bright nebulae (nebula) that are expanding shells of luminous gas expelled by dying stars (star). Observed telescopically, they have a relatively round, compact appearance rather than the chaotic, patchy shapes of other nebulae—hence their name, which was given because of their resemblance to planetary disks when viewed with the instruments of the late 1700s, when the first planetary nebulae were discovered.

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