The SIM Interferometer observatory will be a highly capable tool not only for the discovery of Exoplanets. Because of its ability to locate stars with stunning precision, SIM will also revolutionize several fields of Astrophysics. Those of you who are amateur astronomers may not realize that the properties of Main Sequence stars are not fully understood. One of the main reasons for this is the lack of data concerning stars' Masses and Distances. SIM will change all of that.
The SIM project has put out a book titled, "SIM Lite Astrometric Observatory." It goes into detail on the science and technology of the SIM space observatory. Here is a link to Chapter 8, which discusses the contributions of SIM to our basic knowledge of how stars work.
To pick one of the target areas that will benefit from SIM, let's consider O and B giant stars. For the first time, with SIM's data, we will be able to know the distances to a statistically significant number of these stars. This will allow, for the first time, accurate models of how these stars evolve.