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Planetary Nebula

A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of star  late in their life.[The term for this class of objects is a partial misnomer that originated (1784 or 1785) with astronomer William Herschel, because when viewed through his telescope, these objects appeared to be clouds (nebulae) that were similar in appearance to Uranus' the planet that had been discovered telescopically by Herschel. Herschel's name for these objects was adopted by astronomers and has not been changed, even though planetary nebulae are now known to be completely unrelated to the planets of the solar system. Planetary nebulae often contain stars, but do not contain visible planets. 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.

The 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 via pulsations and strong Solar winds. Without these opaque layers, the hot, luminous core emits ultraviolet radiation that ionizes the ejected outer layers of the star. This energized shell radiates as a planetary nebula.

Planetary nebulae play a crucial role in the chemical evolution of the galaxy,
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X-ray/optical composite image of the Cat's Eye Nebula

returning material to the Interstellar medium that has been enriched in heavy elements and other products of nucleosynthesis (such as carbon, nitrogen, oxygen and calcium). In more distant galaxies, planetary nebulae may be the only objects that can be resolved to yield useful information about chemical abundances.

In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. About a fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms which produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields. may all play a role.

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