It looked like a ring on the sky. Hundreds of years ago astronomers noticed a nebula with a most unusual shape. Now known as M57 or NGC 6720, the gas cloud became popularly known as the Ring Nebula [ http://seds.lpl.arizona.edu/messier/m/m057.html ]. It is now know to be a planetary nebula [ http://zebu.uoregon.edu/~imamura/208/feb22/feb22.html ], a gas cloud emitted at the end of a Sun-like star's existence. As one of the brightest planetary nebula [ http://antwrp.gsfc.nasa.gov/cgi-bin/cossc/apod_search?planetary+nebula ] on the sky, the Ring Nebula [ http://www.ezaccess.net/sbb/M57.html ] can be seen with a small telescope in the constellation of Lyra [ http://galileo.gmu.edu/constellation/LYR.html ]. The Ring Nebula [ http://www.seds.org/billa/twn/n6720x.html ] lies about 4000 light years away, and is roughly 500 times the diameter of our Solar System [ http://antwrp.gsfc.nasa.gov/apod/ap961214.html ]. In this recent picture [ http://www.astro.psu.edu/users/rbc/NGC6720.html ] by the Hubble Space Telescope [ http://antwrp.gsfc.nasa.gov/apod/ap970306.html ], dust [ http://antwrp.gsfc.nasa.gov/apod/ap980104.html ] filaments and globules [ http://antwrp.gsfc.nasa.gov/apod/ap961229.html ] are visible far from the central star. This helps indicate that the Ring Nebula is not spherical, but cylindrical. Perhaps the Ring Nebula [ http://antwrp.gsfc.nasa.gov/apod/ap950727.html ] would appear differently [ http://antwrp.gsfc.nasa.gov/apod/ap971223.html ] if viewed sideways [ http://antwrp.gsfc.nasa.gov/apod/ap980427.html ].