Planetary Nebulae

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