http://en.wikipedia.org/wiki/Planetary_nebula
http://www.daviddarling.info/encyclopedia/P/planneb.html
http://www.seds.org/messier/planetar.html
http://www.nightskyinfo.com/planetary_nebulae/
http://www.news-about-space.org/story/2556.html
http://chandra.harvard.edu/photo/2001/1220/index.html
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