Motions of the photospheric gases introduce Doppler shifts that shape the profiles of most spectral lines. Doppler shifts arising from the rotation of the star are equally significant, and often dominate the shaping process. (We concentrate on rotation in the next chapter.) In a very general sense, the shape of the distribution of Doppler shifts will be the shape taken on by the spectral lines. Our job is to make this quantitative, to work backward from the line profiles and deduce the nature of the velocity fields. The first task is to separate rotational broadening from the photospheric-velocities broadening. For the photosphere, we would like to know the geometry of the motion, whether we are dealing with wave motion, convective motion, prominence-like geysers, or some other kinds of flow. Measurements of the characteristic sizes and any temperature heterogeneities would help us discern what physics we are dealing with and whether or not the dynamics affects the ionization equilibrium, the excitation, and the transfer of radiation.

For the Sun we have a wealth of information with considerable spatial resolution, and there the velocity fields are dominated by granulation (the top of the convection zone) with a smaller contribution from non-radial oscillations. Since we have no spatial resolution for most other stars, we are fundamentally handicapped in having to deal with disk-integrated line profiles.