Why is pressure broadening greater in low mass stars than in high mass stars?
I have a question about broadened absorption lines. In “Discovering the Universe” by Comins and Kaufmann, it is mentioned that cool main sequence stars have broader absorption lines than hot main sequence stars. I believe that this effect is due to pressure broadening. However, I don’t know why pressure broadening (i.e. surface gravity) would be higher for the less massive stars. I’ve been agonizing over this for a few weeks and any help would be appreciated. The surface gravity is higher in low mass stars because of the way radius scales with star mass (recall that cool main sequence stars are lower in mass than hot main sequence stars). On the main sequence, the radius of a star scales with the mass; so, a star that is twice as massive as another will be twice the size of it. Now, surface gravity is essentially the force of gravity exerted on the outermost layers of a star by the mass interior to those layers. Recall that the force of gravity is proportional to the inverse square of
