Planetary Nebulae

Although it is called a planetary nebulae, it actually has nothing to do with planets. These nebulae were given this name because they often look like planets in small telescopes due to their round shape. A planetary nebulae is formed when a dying sun sized star begins to shed its outer layers. These nebulae are emission nebulae with a spectral emission that is similar to the emission nebulae found in star formation regions. However, planetary nebulae are denser and more compact than the emission nebulae in star formation regions.
A planetary nebula is a shell of gas produced by a star as it nears the end of its life cycle. The outer shell of gas is usually illuminated by the remains of the star at its center. Our Sun will probably become a planetary nebulae. When the Sun begins to die it will expand, and become what is called a giant star. It will grow so large that it may engulf the Earth. After millions of years as a giant star the sun will again shrink down to its normal size. As it shrinks much of its surface layers will be shed leaving behind a beautiful ring. This ring is only visible for about 50,000 years. Over time, the nebulae mixes into surrounding space eventually becoming too thin to see. A typical planetary nebula is less than one light-year across.
The Ring Nebula (M57) in Lyra is one of the best examples of a planetary nebula. The Cat's Eye Nebula is another example of a planetary nebula.

The Ring Nebula (M57)

The Cat's Eye Nebula