Astrometry - Challenging our Understanding of Stellar Structure and
  Evolution
  Stellar mass plays a central role in our understanding of star formation and
aging. Stellar astronomy is largely based on two maps, both dependent on mass,
either indirectly or directly: the Hertzprung-Russell Diagram (HRD) and the
Mass-Luminosity Relation (MLR). The extremes of both maps, while not terra
incognita, are characterized by large uncertainties. A precise HRD requires
precise distance obtained by direct measurement of parallax. A precise MLR
requires precise measurement of binary orbital parameters, with the ultimate
goal the critical test of theoretical stellar models. Such tests require mass
accuracies of ~1%. Substantial improvement in both maps requires astrometry
with microsecond of arc measurement precision. Why? First, the 'tops' of both
stellar maps contain relatively rare objects, for which large populations are
not found until the observing horizon reaches hundreds or thousands of parsecs.
Second, the 'bottoms' and 'sides' of both maps contain stars, either
intrinsically faint, or whose rarity guarantees great distance, hence apparent
faintness. With an extensive collection of high accuracy masses that can only
be provided by astrometry with microsecond of arc measurement precision,
astronomers will be able to stress test theoretical models of stars at any mass
and at every stage in their aging processes.
