Published online by Cambridge University Press: 19 July 2016
One of the most striking attributes of planetary nebulae are their complex, yet highly symmetric shapes. The process(es) which shape planetaries are only beginning to be understood. It is proposed that the morphologies of most PNs can be understood within the context of the “interacting winds” wherein a fast but light wind driven by the nucleus rams into an older, slower, and more massive wind, or red giant envelope (“RGE”) ejected earlier.
In order to explain the shapes of noncircular PNs, it seems necessary to hypothesize that the slow wind was originally ejected with an enhanced density along an equatorial plane. The morphologies of nearly all PNs can be understood through two basic parameters: the equatorial density contrast in their RGEs, and the degree of interaction between the fast and slow winds.