Planetary nebula

A planetary nebula is an envelope of expanding gas around a white dwarf. It forms at the end of the life of a giant star when the core contracts and ejects the star's outer layers, creating a white dwarf and a planetary nebula. The intense radiation from the white dwarf makes the nebula shine. Planetary nebulas disperse after 50,000 years. They acquired their name because astronomers used to think they were planets.