What is a Planetary Nebula?

This unfortunate name has nothing to do with planets.  Early astronomers with primitive equipment noticed deep space objects that with often small, bright, and round which looked like planets. This is how they got their name.  In reality they are shells of gas blown off by intermediate sized-stars (like our Sun) late in life.  Planetary nebula are usually circular in shape and  brighter ones often feature red and green colors.

Good examples of planetary nebula include the Ring nebula (M57), the Dumbbell nebula (M27) and the Owl nebula (M97).