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Optical photo of the Ring Nebula (M57, NGC 6720), taken with the four-metre telescope at Kitt Peak Observatory in Arizona. M57 is a planetary nebula about 2000 light years away in the constellation of Lyra. The nebula is a shell of gas ejected from the central star some 20,000 years ago and is now around half a light year across. The gas glows because it is illuminated by ultraviolet radiation from the hot, blue central star. Planetary nebulae are so called because they resembled planets in 19th century telescopes; M57 was the first such nebula to be discovered.
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