That is an example of a protoplanetary nebulae and quite unexpectedly they display some of the most extreme structures considering they are very young. Concentric rings have been found in a number of PN haloes such as the Saturn Nebula, NGC 3918 and NGC 40. This suggests they are formed in an early evolutionary stage and they could be formed by the vibration of the progenitor star as it ejects material or maybe the result of starquakes?geckzilla wrote:Some nebulas exhibit both spherical and bipolar structures. The Egg Nebula, for example. It has it all. A ring of dust, polar ejections, concentric rings (which I suspect are sphere shells) and spotlights!Chris Peterson wrote:I wonder if it's certain this is actually a spherical structure? After all, the Ring Nebula was assumed for years to be a bubble, before it was realized that it was actually a bipolar structure. From certain orientations it isn't easy to distinguish the two.starsurfer wrote:Although rare, spherical planetary nebulae do exist although it is strange that the majority exhibit bipolar morphology, which is likely due to a binary central star.
The oldest planetary nebulae exhibit the least complex and simplest structure and examples with slight complexity is usually the result of interaction with the ISM. Some examples include HFG 1 and HDW 3. Many new low surface brightness planetary nebulae interacting with the interstellar medium have been discovered in the past 15 years. One of the more well known ones is PFP 1.