1. A nebula, such as the Ring Nebula, consisting of a hot, blue-white, central star surrounded by an envelope of expanding gas.
2. A low-mass star in a later stage of its evolution during which the star ejects its outer layers, often in an expanding spherical shell, exposing the hot stellar core.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.