The cloud of expanding gas surrounding a star that has blown off its outer layers, possibly in a nova stage: A planetary nebula is a shell of gas thrown off by a star at the end of its life.

Planetary nebulae have nothing to do with planets. They were named by William Herschel, who thought their rounded shape resembled the disk of a planet.

After a star, such as the Sun, has expanded to become a red giant, its outer layers are ejected into space to form a planetary nebula, leaving the core as a white dwarf at the center.