Monday, November 28, 2011

I shoudn't be blogging this but ...

Vero Beach
27.39.56 N 080 22.27 W

This post is very technical. It isn't about sailboat cruising, but it might be revealing about me. If that doesn't suit you, stop reading now.

For as long as I remember, I think about certain things while trying to go to sleep. It is analogous to counting sheep, except that my tastes lead me to think about scientific puzzles. For the past two decades, one of my favorites has been trying to visualize what happens on a molecular scale when a giant star collapses in a type II supernova. Go ahead an laugh; it's true.

Anyhow, recently I greatly surprised myself by actually figuring out something about the physics. I discovered something. Let me be clear. I'm sure that my discovery is nothing new or remarkable to the the scientists who study such things. It might even be written in their elementary textbooks. However, it was not written in anything I ever read. The discovery was new to me.

What is it? I'll try to explain as briefly as possible.

Dust clouds collapse on themselves because of gravity. The cores are compresses. Compressing increases the pressure and temperature (P & T). At some point, P*T reaches the critical point for nuclear fusion. Hydrogen nuclei fuse to make Helium and energy is released. The process is exothermic, i.e. it releases energy. The energy released opposes further collapse and a semi-stable equilibrium is reaches. In other words, a star is born.

When all the H is consumed, collapse restarts. P*T increases until Helium fusion begins in the innermost core. This happens again and again, until the star looks like an onion, with layers of heavier elements at the center. See the picture.



When fusion of Si into Fe ceases, the cycle ends (because Fe fusion does not release more energy). What happen then is that the center of the star collapses. The product is a remnant neutron star (or a black hole) in the center plus a massive supernova explosion casting off all the outer layers. The mysterious part, and the subject my my nocturnal musings for decades is that the collapse and explosion happens so fast. Indeed it happens in 0.2 seconds. That sounds far too fast to be believable. How can that be?

Here is what I abruptly realized after two drowsy decades. During collapse, P*T becomes so extremely high, that the heavy nuclei get smashed and split up. Instead of nuclear fusion, we have nuclear fission. For those nuclei, all the energy released by the many cycles of fusion must be put back!!! Instead of exothermic, the process is endothermic i.e. it absorbs energy! Instead of resisting further collapse, it accelerates it. Ah so.

That's not all. P*T is still so high that free electrons begin fusing with protons. The product is a neutron and some neutrinos. Neutrinos are reluctant to interact with matter so they fly away out of the core at the speed of light. It takes only microseconds for the neutrinos to exit the core, carrying energy away with them. In terms of the remaining  core material, that process is also endothermic. It causes the speed of core collapse to redouble!  Ah so.

There you have it. Instead of core collapse being mitigated and slowed by negative feedback, it is exacerbated and accelerated by positive feedback. That is why it happens so unbelievably fast.  Ah so. That was my "discovery".  It startled me and made me sit up in bed.,

Thank you for your interest. I hope my explanation was entertaining and understandable. 

2 comments:

  1. When I can't sleep, I read blogs. That did it! :)

    ReplyDelete
  2. That's great! Myself, I discovered that Hell is exotherimic.

    ReplyDelete

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