Formation of planetary nebulae

[pauln]

Okay, this is officially a \'quacky idea\' that I\'m supposed to find data for and publish in a properly reviewed paper. But I\'m not an academic, have no affiliations with any kind of research organisation, and don\'t have access to the data I would need, nor even any good idea where I would begin to look for it. So I\'m posting here anyway because I had a thought that seems plausible to me and would like to know if it has any merit. I\'m asking the people who should know.

I\'m one of the (nearly) silent audience of APOD... but if APOD were to be discontinued then one source of beauty and wonder in my life would be ended with it.

Planetary nebulae (eg October 17, 2004) are the ideal illustration of both; they are beautiful but they also beg the question of processes could be at work to form them, particularly the spiral, filamentary structures. Well, here is my idea: I wonder if they could be the result of a \"flicker\" in the central fusion reaction that takes place in the star. It would happen like this...

As helium is produced by hydrogen fusion it will accumulate in the centre of the star, forming a basically inert core. This will grow as the star ages causing the region where the remaining hydrogen is sufficiently compressed for fusion to occur to become progressively thinner. Eventually this layer will become so thin that it can be locally disrupted by a combination of the pressure generated by the fusion process itself and by the turbulence within the star\'s inner atmosphere, producing a local \"flame-out\" as fusion ceases.

The gas above this \'extinguished\' area would then begin to collapse, increasing the pressure in that region, thus re-initiating fusion. This would produce a shockwave of a strength dependent on the size of the region that flamed out in the first place, as the infalling gas is halted and then repelled by the energy being generated in the area. As hydrogen continued to be consumed and the helium \"ball\" grew in size, these regions would become progressively larger. Eventually they would reach a size that would produce shockwaves strong enough to reach the surface of the star and begin to expel bubbles or sheets of gas.

The episodic nature of the shockwaves would produce the intricate, layered effect that can be seen in some nebulae. Earlier episodes would produce slower-moving gas than later ones involving larger areas of flame-out and reignition, accounting for the regions of shocked gas that are also observed in the more distant regions of some nebulae.

After a time the star would develop to a stage where the process of hydrogen fusion was extinguished across much of the helium ball\'s surface. Then there would be regions that would briefly ignite long enough to repel the infalling gas and create a low-pressure region once more, when fusion would again cease. As the pressure fell around the helium core, the momentum of the infalling gas would progressively increase at the time hydrogen recommenced, producing stronger shockwaves.

Eventually the infall would reach such a momentum and would be occurring over such a large area of the helium ball that the shockwave reaction into the helium (rather than into the gas) would be strong enough to ignite helium fusion and the star would explosively enter the next stage of it\'s life.

I thought of this idea while considering the image of a cotton ball soaked in methylated spirits as it began to run out of fuel. It would flame out in certain areas as the production of vapour was no longer sufficient to maintain a flame. Without any flame, that area of the cotton ball\'s surface could accumulate fuel vapour which would then be reignited (with a small shockwave) by adjacent areas where burning continued. In the very final stages, you would have a small burning area racing all around the surface of the cotton ball. Of course combustion is barely related to nuclear fusion, but you can see how this would lead this idea.

So, anybody, somebody... does it sound right? Can I find more (not too deeply technical)information somewhere about the current thinking on this subject?

[editor]

Not right on two counts. Formation of nebulae and what goes on in the sun. Read John Gribbin\'s book - Blinded by the Light.
For nebulae check web site http://home.cwru.edu/~sjr16/stars_birth.html

[Xerxes314]

I think it\'s a very good description of how planetary nebulae form, but according to my source, it occurs during the helium-shell-burning phase rather than the hydrogen-shell.

From Tayler\'s The Stars: their structure and evolution:

Asymptotic branch stars are believed to produce planetary nebulae, but the cause is believed to be thermal pulses, which occur before the onset of carbon burning and, indeed, prevent the carbon from being ignited. ... [There may be] instability in a thin nuclear shell source, even when the gas is non-degenerate. This arises in the case of helium shells because of the very high temperature dependence of helium burning. ... Calculations show that such instabilities, called thermal pulses, occur in asymptotic giant branch stars and can terminate with a phase of substantial mass loss, which produces a planetary nebula.

Xerxes