What particle interaction is most important for maintaining the equilibrium of a main sequence star? For main sequence stars like the sun, balance against gravitational collapse is achieved with the pressure of the underlying plasma. 
My question is how much of this pressure is accounted for by radiation pressure versus electron-electron, electron-nucleon, and nucleon-nucleon interactions. 
What are the cross sections of photon-electron interactions versus electron-electron interactions at the temperature and pressures typical insides stars like the sun?
 A: The equation of state of the gas in a star can be written as it follows:
$P = P_{ions} + P_e + P_{rad}$
Let's review each terms of this equation:


*

*For ions, we have $P_{ions} = \overline{n}_{ions} k T$ where $\overline{n}_{ions}$ is the number density, $k$ the Boltzmann constant and $T$ the temperature. This term is important in most main sequence stars.

*For electrons, we have $P_e = \overline{n}_e k T$ when the gas is ideal. This term is dominant in the case of a degenerate electron gas, which can be found in white dwarfs.

*Finally for photons, the radiation pressure is given by $P_{rad} = \dfrac{4 \sigma T^4}{3c}$, where $\sigma$ is the constant of Stefan-Boltzmann. As you can infer from the strong dependance on temperature, this term will dominate in very massive stars, as they are very hot.


Thus, the answer is: it depends on the mass of your main sequence star. Low-mass stars will tend to be dominated by electron-electron, nucleon-electron and nucleon-nucleon interactions. But that will be different for massive, hot main sequence stars. As a matter of fact, as the mass of a star grows extensively, the increase in temperature won't allow the inward gravitionnal force to compensate for the outward radiation pressure, killing the stability of the star. This is what constrains the so-called Eddington limit.
Also, it depends on where you place yourself inside the star. stellar interiors are very stratified, and the temperature at the core is much different than the temperature in the outer shell, thus the relative importance of radiation pressure may vary as you approach center.
In the Sun for example, radiation pressure is quite small compared to the gas pressure.
