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Planetary Nebulae
A planetary nebula is a cloud of matter that was ejected from a red giant star near the last stages of the red giant's life. It is theorized that these clouds form when stellar winds push the outer layers of a red giant away, exposing the core and dissipating the atmosphere. The exposed core emits ultraviolet radiation, which ionizes the matter and gives it a visible multicolored appearance. Planetary nebulae return matter formed during a star's evolution to the interstellar medium (ISM). Compared to stars, planetary nebulae are relatively short-lived, existing for only several tens of thousands of years.
"Nebula" means cloud, and "planetary nebula" is a misnomer. William Herschel first observed these, thinking that they were distant planets. The name has never been changed. In recent years, the Hubble Space Telescope has shown that only about 1/5 of planetary nebulae are spherical, and the rest have complex asymmetric structures.