Force Opposing Gravity in Stars This seems like a fairly basic thing, but I've google around and I'm just not sure what to ask.
I assume that, for example, in a star, there is a force of gravity compressing the stellar matter, and an equal, opposite force opposing that compression. My question is, what is that force? Is it a centrifugal kinetic force from the rotation of that matter? Is is some atomic force keeping the atoms apart? Or something else?
Thanks
 A: It is a pressure gradient. The equation of hydrostatic equilibrium is:
$$\frac{dP}{dr} = - \rho g,$$
where $g$ is the gravitational field and $\rho$ is the density at a radius $r$.
The pressure may have contributions from kinetic gas and radiation pressure. Which dominates depends on the mass of the star (radiation pressure dominates in very massive stars).
The equation is modified in the presence of significant rotation rates to account for a centrifugal component to the local force field that acts in a different direction to gravity, when considering a rotating frame of reference.
A: There are actually many different answers, depending on the star. The most basic is gas pressure from the hot gas that makes up the star: $P=nkT$
Hotter stars are supported by radiation pressure in their cores: $P=\frac{4\sigma}{3c}T^4$ for thermal equilibrium.
Both these pressures are ultimately based on the electromagnetic force.
For denser stars like white dwarfs, the gravity overwhelms the gas pressure, and the star collapses to a degenerate core, which is held up by quantum degeneracy pressure. Not really a force, but acts like one.
At even higher densities, like a neutron star, the pressure is dominated by the nuclear strong force ($not$ neutron degeneracy, as is often believed). Neutron stars can spin quite rapidly, as high as 600-700 Hz, so in those cases the angular momentum also contributes to holding the star up from collapsing. 
