Stellar orbits in the Galactic Center (Stefan Gillessen, Max Planck Institute for Extraterrestrial Physics-Germany)

Since the discovery of the non-thermal, compact radio source Sgr A* it has been suspected that the center of the Milky Way harbors a massive black hole. A breakthrough in measuring the associated mass became possible with the advance of infrared astronomy. Within a decade the observations have progressed from velocity dispersion arguments to tracking individual stars as test particles for the gravitational potential. Technically this was possible due to high-angular resolution techniques, in particular adaptive optics, at a very large telescope. The observations revealed a star that moves apparently on a Keplerian orbit with a period of 15 years, allowing to measure the mass with unprecedented accuracy. Today, the 4 million solar masses black hole in the Galactic Center is the best case for the existence of an astrophysical black hole in general. For more than twenty stars orbits have been determined and further progress can be expected - both from the ever growing time base as well as from future techniques such as near-infrared interferometry.