planetary nebula

Definition

Astronomy

  • Nebula formed by a red giant star emitting its outer layers into space after they start burning helium. The observable planetary nebulae are a few tens of thousands of years old at most, since once the gas which forms the nebula expands too far it ceases to be visible. Early observers of these nebulae thought that their round form made them look like planets, to which they are unrelated.
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