Planetary Nebulae

Planetary nebulae are created when a main sequence star grows into a red giant and casts off its outer layers. This is how they get their (usually) circular shape, for the material is thrown off the star in a roughly symmetrical manner.
The remains of the central star can usually be seen in the center of the nebula.
The term "planetary" comes from the nineteenth century, when astronomers saw that they looked vaguely like the newly-discovered Uranus and Neptune in their primitive telescopes. The name has stuck ever since.

Examples:

M57 The Ring Nebula

M27 The Dumbbell Nebula

NGC7048

The Eagle Nebula