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#11
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Nuclear detonation inside the sun
In article ,
Keith Harwood wrote: ...The other reactions in the proton-proton chain are not exactly speed demons either... What happened to the old carbon cycle? Last I heard it only mattered in stars smaller than the sun, but I never found out why? Contrariwise, if I'm remembering correctly -- it is significant only in big stars, because normal-sized stars don't get hot enough for it to proceed at a significant rate. -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
#12
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Nuclear detonation inside the sun
Henry Spencer wrote:
In article , Keith Harwood wrote: ...The other reactions in the proton-proton chain are not exactly speed demons either... What happened to the old carbon cycle? Last I heard it only mattered in stars smaller than the sun, but I never found out why? Contrariwise, if I'm remembering correctly -- it is significant only in big stars, because normal-sized stars don't get hot enough for it to proceed at a significant rate. Bigger than the sun, but not much bigger. The CNO cycle becomes dominant for core temperatures above 30 million degrees or so, and due to its extremely high temperature dependence (about T^20, as opposed to T^6 for the p-p chain), it very rapidly comes to entirely dominate stellar energy production once you pass that temperature threshold. For the sun, the CNO cycle contributes no more than a percent or two of the total energy output, but by 1.5 solar masses it gives half the luminosity and by 1.7 solar masses essentially the entire luminosity. Physically, the CNO cycle is a much harder reaction to get going than the p-p chain, due to the tremendous energy needed to overcome the Coulomb barrier between a nitrogen nucleus and a proton. It's not just 7 times harder (as one might naiively expect) but dozens or hundreds of times harder, because nuclear fusion proceeds by quantum tunnelling, which depends exponentially on the potential barrier. Conditions which would allow the CNO cycle to proceed for fusion power are thus almost certainly orders of magnitude harder to achieve than proton-proton reactions. - Marshall |
#13
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Nuclear detonation inside the sun
Henry Spencer wrote:
Contrariwise, if I'm remembering correctly -- it is significant only in big stars, because normal-sized stars don't get hot enough for it to proceed at a significant rate. However, because the reaction on C has the lowest barrier, there's almost no carbon in the center of the Sun (it's all been turned into nitrogen.) Paul |
#14
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Nuclear detonation inside the sun
Marshall Perrin wrote:
Conditions which would allow the CNO cycle to proceed for fusion power are thus almost certainly orders of magnitude harder to achieve than proton-proton reactions. That is not at all clear. Manmade CNO would probably be *pycno*nuclear, not thermonuclear. Compress hydrogen + heavy elements to very high density, but keep it cold so electron screening is more effective. Under those conditions, fusion on C/N/O would be faster than p+p. There have been projections that H + D fusion can be made to go in cold ( 1000 K) hydrogen at 20 g/cm^3. It may be feasible to reach these conditions used precompressed solid hydrogen that is further compressed by dozens of weak shocks. Paul |
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Nuclear detonation inside the sun
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