Planetary nebula

A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. The name originates from a similarity in appearance to giant planets when viewed through a small optical telescope, and is unrelated to planets of the solar system. 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. About 1,500 are known to exist in the Milky Way Galaxy.
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Noun
1. a nebula that was once thought to be a star with its planets but is now thought to be a very hot star surrounded by an expanding envelope of ionized gases that emit a fluorescent glow because of intense radiation from the star
(hypernym) nebula

[Picture] The outer, gaseous layers of a red giant star which have been gently blown off into space, whilst the stellar core is in the process of becoming a white dwarf . The name derived from William Herschel's belief that they resemble planets under low magnification through telescopes. Planetary nebulae have been discovered which are neither ring nor shell like but bipolar and irregular. They are all expanding and have estimated lifetimes of less than 40,000 years. After this length of time they become too tenuous to be seen. The reason they shine is because the gas in them is excited by electromagnetic radiation from the central, collapsing star.