I understand that nuclear fusion generates energy in the form of gamma-rays (as in the pp chain), but how does that translate into heat in the star's core? Or is the heat just generated by gravitational contraction?

After doing more research, I found that nuclear fusion puts some energy into the particles in the form of momentum, but is that enough to cause the heat?

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    $\begingroup$ Put some energy into your skin by rubbing your hands together. Does that cause heat? $\endgroup$
    – PM 2Ring
    Mar 19 at 4:17
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    $\begingroup$ Mostly, the kinetic energy of the products. I have some info on p-p chain energy here. As you can see, some reactions in the p-p chain release a gamma (in addition to the gammas released by the positron-electron annihilations). But most of the fusion energy output is just KE of the reaction products. And for that matter, a photon is "just" quantised electromagnetic KE. $\endgroup$
    – PM 2Ring
    Mar 19 at 5:58
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    $\begingroup$ A snarky comment would be that fusion doesn't always generate heat. For example Fission of Uranium produces energy, but if you Fused back together what you got from Uranium Fission, you would lose the same amount of energy as you got out of the Fission :) $\endgroup$
    – Solx
    Mar 19 at 18:34
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    $\begingroup$ This is a good question, but is more properly a physics question; 'how does fusion work' isn't precisely astronomy. $\endgroup$ Mar 20 at 22:36
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    $\begingroup$ @Solx A snarky comment would be to elaborate on user253751's comment: "Because if you discovered a process that doesn't generate heat you'd win the Nobel Prize " ;-). $\endgroup$ Mar 21 at 15:37

2 Answers 2


The products of nuclear fusion have either kinetic energy (the products with mass) or just plain energy in the case of massless photons. The kinetic energy of particles in the gas is heat. The energy of the photons is also rapidly converted into kinetic energy when they interact with other particles by absorption or inelastic scattering.

The only fusion energy that does not heat the core is the 2% (in the Sun) lost in weakly-interacting neutrinos.


In nuclear fusion, generally a nucleus plus impactor splits into two tunnelling-separated particles. Those two particles, having overcome the tunnelling barrier are now repelling each other via the electrostatic Coulomb force, and hence are accelerated.

The momentum they gain from this is quickly randomized (or "thermalized") as the particles bump into other particles, and the microscopic momentum becomes macroscopic thermal energy.

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    $\begingroup$ This is at least misleading. The energy, thus heat, does not come from coulomb forces. $\endgroup$ Mar 19 at 5:29
  • $\begingroup$ @planetmaker Hmm good point, the energy is in the end nuclear. The Coulomb force in the end is just the mediator giving the particles their initial direction. I'll update the answer. However for the sake of explaining what's going on, the Coulomb force is still a much clearer example, as no simple "Quark gluon quantum weirdness" explains why those two particles should fly apart, as no such thing as a simple nuclear tunneling potential can be visualized. $\endgroup$ Mar 19 at 5:46

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