What is a planetary nebula?

A planetary nebula is created when a star blows off its outer layers after it has run out of fuel to burn. These outer layers of gas expand into space, forming a nebula which is often the shape of a ring or bubble. About 200 years ago, William Herschel called these spherical clouds planetary nebulae because they were round like the planets. At the center of a planetary nebula, the glowing, left-over central part of the star from which it came can usually still be seen.