Nebulae are collections of gas in space. Planetary nebulae (PN) are collections of gas that are "roundish", like the shape of a planet (i.e. Mars, Jupiter, Earth, etc.). Of course, planetary nebulae really have nothing to do with planets. They are, in fact, complex and interesting gas envelops around dead or dying stars. If diffuse nebulae are the birthplaces of stars, planetary nebulae are the death places of stars. Their colors are determined by the gases and temperatures of the nebulae. Their shapes are determined by the presence of materials around the PN, the magnetic fields these stars generate (which can cause high speed polar "spouts") and other complex physical factors. PNs are relatively bright and often can be imaged from a light polluted site. For this reason, they are my favorite deep space objects to photograph.
