a glowing, expanding nebula that forms around a collapsing star
Origin:
from its resemblance, when viewed through a telescope, to a planet
See planetary nebula in American Heritage Dictionary 4
noun
A nebula, such as the Ring Nebula, consisting of a hot, blue-white, central star surrounded by an envelope of expanding gas.
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.