"When a star like our Sun comes to age, having longly burned away all the hydrogen to helium in its core in its main sequence phase, and also (in the consequent red giant stadium), the helium to carbon and oxygen, its nuclear reactions come to an end in its core, while helium burning goes on in a shell. This process makes the star expanding, and causes its outer layers to pulsate as a long-periodic Mira-type variable, which becomes more and more unstable, and loses mass in strong stellar winds. The instability finally causes the ejection of a significant part of the star's mass in an expanding shell. The stellar core remains as an exremely hot, small central star, which emits high energetic radiation. The expanding gas shell is excited to shine by the high-energy radiation emitted from the central star; the material in the shell is moreover accelerated so that the expansion gets faster by the time. The shining gas shell is then visible as a planetary nebula. In deep exposures, the matter ejected in the Mira-variable state can be detected as an extended halo surrounding many planetary nebulae."
Source -
http://seds.lpl.arizona.edu/messier/planetar.html
So you are partly correct, as the star ages, it burns through different phases from Hydrogen through to carbon and oxygen. Hence the outer shell is red (Ha) and the inner core is typically blue/green (OIII). Though not all planetary nebula exhibit such a uniform structure as displayed in your images. Such examples are those planetary nebula known as bipolar -
http://en.wikipedia.org/wiki/Bipolar_nebula which are caused by the star ejecting material from its poles (strong magnetic fields).
I haven't crunched your optical system numbers to determine the arcsec/pixel etc, but I think the images look over sampled. They may improve with a few iterations of deconvolution to bring out more detail. Good work none the less. Thanks for sharing.
