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Slowly Igniting A Star.

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Posted by Alexander on February 4, 2001 01:44:27 UTC

John, there are a few good sites out there which explain (unfortunately without detailing) how a star is formed, for example: http://observe.ivv.nasa.gov/nasa/exhibits/stellarbirth/opening1.html

If you calculate gravitational energy change of a cloud of (space filling) hydrogen which (cloud) shrunk from 10^15 m to 10^9 (about Sun's size), you get U = -3GM^2/5R = -2.3x10^41 Joules. This is for about 10^57 Sun's atoms 2.3x10^-16 Joules per atom, or 1.2x10^7 K. This is, of course way not enough to immediately burn all hydrogen into helium AT ONCE (and to blow off the star in a supernova firework) - for this you would need 10^9 K or so, which can be generated by atomic "spark" charge inside a hydrogen bomb. But due to Maxwellian distribution of energies of moving atoms there is small fraction of atoms [about exp(-E/kT) from total amount, where E=2x10^-14 J is the threshhold energy to start p-p fusion as you have to bring repelling protons close so nuclear short-range force can fuse them],which HAS enough speed/energy to fuse, and indeed fuses. As you can see, this fraction is tremendousely small: about exp(-80), or about 10^-34, or about 10^23 atoms out of all 10^57 star atoms in the star. So, each of THESE atoms (actually protons at such high temperatures)is fusing in He and the rate is about 10^17 times per second per average atom (collision rate inside star), so they generate about 10^27 Joules per second, which we see mostly in the form of thermal radiation from surface (light) and some neutrinoes from the core, where reactions dominate due to high temperature and density.

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