Following Alan’s talk last week there is a publication in Science about why some planetary nebulae have the shapes they do. A summary can be found at http://www.skyandtelescope.com/community/skyblog/newsblog/Butterflies-in-the-Sky-Planetary-Nebulae-Explained-177883611.html. ESO details at http://www.eso.org/public/news/eso1244/
This concentrates on the Nebula Fleming one but the results are applicable to others as well.
John Murrell
Astronomer Without Portfolio
Website www.JohnMurrell.org.uk