<P> A planetary nebula, often abbreviated as PN or plural PNe, 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 . The word "nebula" is Latin for mist or cloud, and the term "planetary nebula" is a misnomer that originated in the 1780s with astronomer William Herschel because when viewed through his telescope, these objects resemble the rounded shapes of planets . Herschel's name for these objects was popularly adopted and has not been changed . 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 . </P> <P> 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 . After most of the red giant's atmosphere is dissipated, the ultraviolet radiation of the hot luminous core, called a planetary nebula nucleus (PNN), ionizes the outer layers earlier ejected from 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 . </P> <P> Planetary nebulae likely 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 . Planetary nebulae are also observed in more distant galaxies, yielding useful information about their chemical abundances . </P>

The major source of light in the expanding shell of gas in a planetary nebula is
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