In reply to David Roberson's message of Mon, 23 Jul 2012 00:51:37 -0400 (EDT): Hi, [snip] > >Relative is the operational word here Robin. I misunderstood your description >of the 2 billion years half life of He2.
He2 doesn't have a half-life of billions of years. The pp reaction does, but that's a reaction between two separate protons, not He2 (i.e. the protons are separate for most of their lives, only combining to form He2 for very brief instants during their life-time). He2, only exists for a tiny instant, I would guess < 1E-20 seconds. This fact combined with the "long" half-life of beta decays in general, is why the half-life of the pp reaction is so large. (Note that a long half-life implies only a small chance of a reaction occurring). IOW the chance of the pp reaction occurring is the product of two small chances, so it's very small, which in turn makes the half-life of the combined reaction very long. [snip] >Your suggestion about the percentage of H that reacts per second begs a >question. Would it not be possible for the core temperature to adjust >downward to accommodate the rate that exists? I have always wondered why >stars did not in fact explode immediately upon first ignition unless some form >of negative feedback controls the rate of energy production. One would think >that the region that first ignited would generate a large amount of energy >that would raise the temperature in the immediate area. A higher temperature >would suggest that more fusion would occur leading to ever higher temperature >until boom. > >Dave You may be right about the negative feedback. As a region heats up it should expand reducing the density, which in turn should drop the fusion rate. However a fast reaction would overcome this, witness the H-bomb. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/project.html
