From: mperrin+news@cymric.berkeley.edu   
      
   Henry Spencer  wrote:   
   > In article <406e1635$0$436$afc38c87@news.optusnet.com.au>,   
   > 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   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)