The Galactic Center: The Stellar Cluster and the Massive Black Hole

The luminosity of the entire central parsec of our Galaxy is dominated by a cluster of young and massive stars. Already less than a dozen of the most luminous (L > 105.75 L ), moderately hot (T < 104.5 K) blue supergiants contribute half of the ionizing luminosity of that region. Due to their strong 2.06mum Helium line emission they are also often referred to as He-stars. These stars are generally classified as Ofpe/WN9, although a few might be luminous blue variables (LBV) or Wolf-Rayet stars. The exact mechanism of star formation in the GC remains still enigmatic: The extreme tidal fields due to the presence of a super-massive black hole and extremely dense stellar cluster, as well as strong magnetic fields and stellar winds in this region, need to be included in discussing if star formation can occur in the immediate vicinity of the center. In addition to infrared bright bow shock sources that interact with the ISM in the central parsec there is also a population of fainter infrared excess sources which is discussed here in more detail.As proven by the Keplerian orbits of several high velocity stars within the central arcsecond at the position of the compact radio source SgrA* the Galactic Center harbors a ~3.5 X 106M massive black hole. Simultaneous NIR/X-ray observations of SgrA* in 2003/2004 have revealed first insights into the emission mechanisms of both the powerful near-infrared flares and the `quiescent' emission from within a few 10 - 100 Schwarzschild radii of the super-massive black hole at the center of the Milky Way. SgrA* shows synchronous NIR/X-ray flare variations and indications of quasi-periodicity within the NIR flares.