A type of nebula caused by an expanding cloud of gas ejected by a dying star. For cool pictures, check out HST's planetary nebula gallery. Hourglass Planetary Nebula
An expanding gas cloud that was created as the result of gas being expelled from a sunlike star in the later stages of its life. Ex. M57 - The Ring Nebula.
A bright cloud of dust and gas surrounding an old star, namely a red giant. Towards the end of its life the star ejects the dust and gas violently, losing most of its mass and becoming a white dwarf. The nebula disappears after approximately 100 000 years. They are called 'planetary' because originally astronomers thought they looked like planetary discs.