PNe are low-intermediate mass stars having a bit of a mid-life crisis. During the final stages of stellar evolution, the outer layers of the star are blown off, creating a shell of gas surrounding the star. This gas is heated by the star within, thus producing a PN's characteristic emission lines.

The Ring Nebula, HST	The Cat's Eye Nebula, HST Both images stolen from SEDS

from Jacoby & Ciardullo, 1999

• PNe are readily detected through on/off band imaging.
• Because of the brightness of the emission lines, they are prime targets for spectroscopic radial velocity measurements.
• As a short phase in the normal stellar evolution of common stars, they are reasonably abundant and faithfully trace the general stellar population.

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