Ring nebula

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Ring nebula,

planetary nebulanebula
[Lat.,=mist], in astronomy, observed manifestation of a collection of highly rarefied gas and dust in interstellar space. Prior to the 1960s this term was also applied to bodies later discovered to be galaxies, e.g.
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 in the northern constellation Lyra; cataloged as M57 or NGC 6720. It is perhaps the most famous and beautiful nebula of this type. Its name describes the appearance of the expanding shell of gas. The nebula is estimated to be more than 5,000 light-years distant.

ring nebula

A ring or arc of nebulosity centered on and ionized by ultraviolet radiation from a Wolf-Rayet star. The Ring nebula is, confusingly, not a ring nebula.

Ring nebula

(M57; NGC 6720) A ring-shaped planetary nebula, about 1′ across, in the constellation Lyra. The inner regions closest to the centrally placed ionizing star are green as a result of light emitted by doubly ionized oxygen; there is an approximately symmetrical transition to the reddish outer edge where the lower-energy transitions of hydrogen and nitrogen are responsible for the color. See emission nebula.

Ring Nebula

[′riŋ ′neb·yə·lə]
A nebula in the summer constellation Lyra; it is an example of the planetary type of gaseous nebulae.