When fusion occurs in the sun, it releases energy according to the mass loss due to fusing two nuclei into one. Apparently this keeps the sun from collapsing due to its gravitational force. However, I am confused on how the energy released by fusion translates into pressure/force to combat the gravitational force and pressure created by that force.
2 Answers
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At the centre of the Sun, the entirely ionised gas is, to within a couple of percent, an ideal perfect gas, with pressure $P = nk_B T$. Here, $n$ is the number density of all particles and $T$ is the temperature.
Nuclear fusion releases energy. The energy escapes from the fusing nuclei in the form of gamma/X-ray photons, the kinetic energy of the resultant particles and neutrinos (2%).
Whilst the neutrinos can easily escape from the Sun, taking that energy with them, the photons are absorbed within a cm or so of where they are created and the particles collide with other particles on similarly tiny length scales (compared with the size of the Sun). Thus their energy is deposited into the gas and that provides it with heat.
The interior of the Sun has evolved into an equilibrium, where the heat deposited by the fusion reactions equals the heat escaping from the surface.
The heat keeps the interior temperature high, and that leads to the gas pressure that supports the Sun.
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To answer the question exactly and simply :
The pressure of the stellar gas results from the incessant collisions and rebounds of particles on each other.
Everywhere, this pressure balances the action of gravity. If it were to suddenly cancel out, the Sun would completely collapse on itself in barely three minutes!
PV = nRTor something. I am not sure if that applies to the Sun but maybe. Another answer is the photons have some amount of momentum, and as they try to escape they impart some of their momentum outwards on the material inside the star. $\endgroup$