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 Originally Posted by Poopadoop
This part makes sense. To follow up then, in essence the centre of the sun is 'exploding', the bits are being expelled, and these radiate as heat, light, energy. The reason it doesn't all go up at once is because it's only hot enough at the centre.
Yes. That's as much as I've told you and you got it down.
Another reason is that the fusion is a "rare" occurrence. Most times a + charge proton gets close to another + charge proton, their Coulomb repulsion will cause them to scatter, not fuse. They have to "tunnel through" a potential barrier which is greater than their total energy.
This can only be understood with QM. It defies intuition. The principle is that if I roll a ball halfway up a hill with the exact same force a bazillion times, one of those times, it's get over the hill. That's certainly not a reasonable expectation in the macroscopic world you and I understand, but in Quantum Mechanics, the uncertainty principle allows for mandates this kind of nonsense.
Another limiting factor is that the protons can actually be moving too fast to fuse. This, again, defies intuition. Basically, if the protons are moving past each other too quickly, then their wave functions wont interact. There's a sweet spot of speed where they're slow enough to "see" each other, and barely fast enough to tunnel through each other's mountain of Coulomb repulsion.
This sets the energy of the core, basically. A star will collapse until the energy produced in this process is enough to prevent total collapse, which happens to be where the fusion rate is "rare" from a statistical stand point. Even in large stars, the fact that they're mostly Hydrogen and Hydrogen gives of loads of energy in its fusion, it will reach an equilibrium with Hydrogen shell burning and a high pressure non-fusing core of Hydrogen until that fills up with Helium.
I say "rare" in quotes because it's still going on like mad, due to the extreme size of a star and number of protons zipping about.
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