Star-counts, again

Although counting stars may seem boring, the Hipparcos satellite mission^3.3 was really a tremendous revolution, due to the incredible accuracy of its measurements. This satellite was launched in 1989 and specifically designed to measure very accurately the positions of stars, several times during the satellite's lifetime. For distant stars (some of which were in fact very bright, extra-galactic sources called quasars), this made it possible to construct an absolute reference system, that can be used define the positions of extra-solar objects. But for more nearby stars, Hipparcos could not only determine their parallax and hence their distance, but also, by comparing the positions at two different times, the proper motion of the stars with respect to the `fixed' reference system.

The parallax is the ultimate step in creating a distance scale. We first need to determine the parallax to standard candles, like e.g. Cepheids, before we can use them to look further. So getting these parallax measurements accurate is of major importance.

The proper motion, i.e. the dance of nearby stars around us, tells us about the distribution of mass. For example, the oscillation of stars perpendicular to the disk of the MW tells us how much mass there is locally in the disk.

GAIA^3.4 is a planned mission to be launched before 2012. It will measure positions of stars up to 15-th magnitude with a resolution of 10 $^{-5}{\hbox{\rm arcsec}}$. Gaia's expected scientific harvest is `of almost inconceivable extent and implication' (according to their web page). In total, about $10^9$ MW stars will be measured (as compared to $10^4$ measured by Hipparcos). Gaia will be so accurate that it can even measure proper motions of some of the nearest globular clusters and galaxies.