Wikipedia:Today's featured article/December 31, 2004

A planetary nebula is an astronomical object consisting of a roughly spherical glowing shell of gas formed by certain types of stars at the end of their lives. They are in fact unrelated to planets: the name originates from a supposed similarity in appearance to giant planets. They are a short-lived phenomenon, lasting a few thousand years of a typical stellar lifetime of several billion years. About 1,500 are known to exist in our galaxy. Planetary nebulae are important objects in astronomy because they play a crucial role in the chemical evolution of the galaxy, returning material to the interstellar medium which has been enriched in heavy elements by nucleosynthesis. In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. The mechanisms which produce such a wide variety of shapes and features are not yet well understood. (more...)

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