A planetary nebula is a kind of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from old red giant stars late in their lives. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.
A mechanism for formation of most planetary nebulae is thought to be the following: at the end of the star's life, during the red giant phase, the outer layers of the star are expelled by strong stellar winds. Planetary nebulae seemingly only occur at the end of the lives of intermediate and low mass stars between 0.8 to 8.0 solar masses.
After most of the red giant's atmosphere is dissipated, the ultraviolet radiation the hot luminous core emits ionizes the ejected outer layers of the star. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.
Planetary nebulae may play a crucial role in the chemical evolution of the Milky Way by expelling elements to the interstellar medium from stars where those elements were created.