What is a model photosphere, and why build one? It would seem logical to take our stellar observations and deduce from them the physical conditions existing in the atmosphere of the star – somewhat like a parallax measurement yields the distance to a star. Alas, the formation of the stellar spectrum involves many physical variables, and a rigorous deductive interpretation cannot generally be made. Instead we hypothesize a model through which we organize and relate the information conveyed in the starlight. The model resembles a scientific theory in that it is constructed on the basis of our observations and known physical laws. We may then gather additional observations to test our model much like we would test a theory. The model is modified and improved as information is added. When our model closely reproduces all the available observations, we begin to feel that our model is worthy of some trust. Then properties associated with the model, or deduced from further application of the model, are associated with the star, properties such as effective temperature, surface gravity, radius, chemical composition, or rate of rotation.

The model photosphere consists of a table of numbers giving the source function and the pressure as a function of optical depth for an assumed chemical composition. Additional columns may be added to the table depending on the use intended for the model.