Starship Asterisk*

The Spirograph Nebula is always a good show. Perhaps "Harmonograph" would be a better name. Spirographs are driven by toothed gears - mostly on plastic wheels in the modern kids' toy - and so produce perfect periodic curves. Harmonographs are driven by pendulums and so are damped, with the curves usually asymptotic towards a central resting place. I guess the explosion of a nebula can be thought of as damping in reverse (well, if you choose the damping parameters carefully).

All you will ever want to know about harmonographs, including photos of a couple of handsome devices:

http://astronomy.swin.edu.au/~pbourke/c ... monograph/

PS - Chaotic winds would look chaotic, wouldn't they? Am I right in guessing that super electromagnetic fields have been proposed as part of IC 418's harmonographic behaviour? What is the K-mechanism mentioned in one of the linked adsabs abstracts?

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 how they could possibly be formed. 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 the my idea.

Any atrophysicists out there? What do you think? Is it anything new, or is this just what the physicists are thinking already?