NGC 3918 illustration

Studying planetary nebulae is important to understanding the process of star death. A star begins to die when it has exhausted its thermonuclear fuel - hydrogen and helium. The star then becomes bright and cool (red giant phase) and swells to several tens of times its normal size. It begins puffing thin shells of gas off into space. These shells become the star's cocoon., which are known as plenatery nebula.

This illustration of the planetary nebula NGC 3918 shows how the radiation of the host star creates the oval form of the surrounding shell.