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Zero Signal
2003-Jun-26, 06:01 PM
This has been nagging at me for a while. I've known for a while that certain fusion reactions, especially the proton-proton reaction, produce antimatter in the form of positrons (IIRC, it's; p + p = H-2 + positron + neutrino + energy). So what exactly is the role on these positrons? Would they quickly encounter an electron and get annihilated? Would this contribute any significant amount of energy to the star's output? So what exactly is the role of antimatter in stellar interiors?

tracer
2003-Jun-26, 07:53 PM
Well, the energy released in the proton-proton chain equals the mass difference (expressed in energy units) between what goes in and what comes out, right? So, let's calculate that:

What goes in: 6 protons.
What comes out: 1 alpha particle, 2 protons, and 2 positrons.

(We can ignore the mass of the neutrinos emitted, or assume that they just count toward the total "energy" liberated.)

Okay, here are the rest-mass figures:
proton = 938.3 MeV
positron = 0.511 MeV
alpha particle = 3727.4 MeV

So, what goes in has a total mass of 6*938.3 = 5629.8 MeV.
What comes out has a total mass of 3727.4 + 2*938.3 + 2*0.511 = 5605.0 MeV.
The difference is a net mass-energy of: 5629.8 - 5605.0 = 24.8 MeV released in the reaction, not counting subsequent positron-electron annihilation.

If both positrons annihilate with electrons, the total energy of the gamma rays produced will be 4*0.511 = 2.044 MeV.