Why does higher surface gravity broaden spectral lines? It's always made intuitive sense to me that a higher surface gravity leads to broader spectral lines from a star, but yesterday I realized that I don't have a physical explanation for why this is so.  Why is this so?
In order to be in hydrostatic equilibrium, high surface gravity atmospheres have higher pressures at the same sort of temperatures. That means the density of atoms and ions is higher.

If the particle density is higher then there is more chance of interactions between the particles. In particular, collisions between atoms perturb their energy levels and reduce the lifetimes of atoms in particular states. The natural frequency width is proportional to the reciprocal of the radiative lifetime. If this lifetime is truncated due to frequent collisional (de)excitation, then the lifetime is shortened and the frequency width increased. This is known as collisional broadening.