Why is the red always on the outside?

[sculptor]

Why do planetary nebulas (Helix is a great example, but 3132 certainly shows it too) almost always show the hydrogen alpha on the outside, and the oxygen III on the inside?

Is it that the hydrogen alpha shell is ejected first, in the red giant phase, and then the oxygen III slams into it during the death phase creating a hydrogen shock on the outside? Sounds plausible.

Is it that the hydrogen atoms, being lighter, get slammed about more by the ultraviolet, and go further?

Is it that the star, pre-collapse, has hydrogen in the outer atmosphere but the oxygen is in the core, and that pattern is maintained? Sounds unlikely.

Image: 80 x 1 minute exposures, guided, Celestron 11" SCT, Canon EOS 20Da at ISO 800.

[jase]

"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.:thumbsup:

[tornado33]

Great shot of the Eightburst Planetary too, nice big image scale there.
Scott

[dcalleja]

I also like the big version - PN's are a favourite of mine. Nice to see some science discussion as well. Sometimes I forget what I'm imaging and get lost in the 'art' of getting the most photons down the tube & processed.