SIM will make definitive measurements of fundamental structural and dynamical parameters of the Milky Way. The important niche in dynamical parameter space afforded by SIM can be exploited to resolve, with unprecedented precision, a number of classical problems of Galactic astronomy.

By selecting suitable "test particles" -- such as the SIM astrometric grid stars, and samples of the major components (bulge, disk, halo, satellite system) of the Galaxy -- SIM will perform a definitive characterization of the mass distribution within the Milky Way.

The metal-poor stars in the halo of the Milky Way galaxy were among the first objects formed in our Galaxy. These Population II stars are the oldest objects in the universe whose ages can be accurately determined. Age determinations for these stars allow us to set a firm lower limit to the age of the universe and to probe the early formation history of the Milky Way. The age of the universe determined from studies of Population II stars may be compared to the expansion age of the universe and used to constrain cosmological models. The largest uncertainty in estimates for the ages of stars in the Milky Way's halo is due to the uncertainty in the distance scale to Population II objects. SIM will obtain accurate parallaxes to a number of Population II objects (globular clusters and field stars in the halo) resulting in a significant improvement in the Population II distance scale and greatly reducing the uncertainty in the estimated ages of the oldest stars in our galaxy. These SIM observations and resultant age determination for the universe will provide an important, independent check of the preferred cosmological model.

About a dozen globular and open star clusters will be observed by SIM in order to obtain accurate determinations of their chemical abundances, ages, and the amount of foreground interstellar reddening. The needed measurements are precision parallaxes and proper motions. Results from these rich clusters illuminate the behavior of different evolutionary stages with star age and metal abundance when applied to rare stars in the tip of the red giant branch, horizontal branch, and asymptotic giant branch. In addition, the results derived from these measurements permit calibration of stellar evolutionary isochrones, applicable to other clusters and models of integrated light.

In turn, these calibrations can be used for much better estimates of the age and metal abundances of high-redshift galaxies, and thus a better understanding of galaxy formation. These measurements can also be used to improve the cosmic distance scale in several ways, and also better constrain the age of the universe.

SIM Science Investigations