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Why is the red always on the outside?
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.
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