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- Dictionaryplanetary nebula
noun
- 1. a ring-shaped nebula formed by an expanding shell of gas round an ageing star.
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noun
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A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to planets. The term originates from the planet-like round shape of these nebulae observed by astronomers through early telescopes. Wikipedia