Nuclear Fusion in the Sun

• It looks like Hydrogen fusion will work as the power source of the Sun. We have lots of the right fuel, the conditions in the inner 10% of the Sun are right for the P-P cycle to run, and it is a very efficient source of energy.

• Exactly how does the energy get from proton mass into visible light photons flying from the surface of the Sun into space?

• -rays are released during the P-P fusion chain and almost immediately (after traveling on average 1cm) absorbed and re-emitted. The energy contained in that original -ray bounces its way outward and after ~100,000 years and absorptions and re-emissions it leaves the surface of the Sun ``downgraded'' into visible light photons (because of the temperature gradient in the Sun).

• An interesting Aside: The Sun must be losing mass as it is converted into energy.

• How Much?

  

  This comes to

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• One more fact and we can understand the Main Sequence in the H-R Diagram (!)
  1. A more massive star has a higher or lower central temperature? Higher of course - hydrostatic equilibrium demands this.

  2. Higher central temperature means P-P rate is higher and more energy is released - this means a larger luminosity.

• Pay attention now:

• So, this explains why there is a Luminosity-Temp relation and why as you move to higher Temp and Luminosity along the main sequence, you are moving along a mass sequence.