Reinhard Genzel receives Karl Schwarzschild Medal 2011

Black Holes are doubtless the most exotic objects in the Universe. Matter in a Black Hole is highly concentrated; it is compressed to such a high degree that even the speed of light is not fast enough to escape gravity. As neither radiation nor matter can move faster than the speed of light this leads to two consequences: light cannot escape a Black Hole, therefore it is invisible (hence the name). And matter falling into a Black Hole is gone forever. Nevertheless we know of the existence of Black Holes in space as their gravitational effects on their environments can be observed.

For some decades these kinds of observations have been obtained, but to find evidence for Black Holes was far from easy. For a long time astronomers suspected that the centres of so called "active galaxies" harbour Black Holes with several billion solar masses. Normal galaxies such as our Milky Way contain many hundred billion stars, planets, gas and dust. Active galaxies, such as quasars, in addition show extremely luminous nuclei and other strange phenomena, which can be well explained by a Black Hole in the centre swallowing matter. The observed effects, however, are only "indirect" evidence, since detailed measurements of these galaxies are very difficult because of the large distances involved.

In comparison to these galaxies, the centre of our own Milky Way is very close, only about 25000 light-years away. Could such a "gravity monster" lurk there as well? To solve this riddle, Reinhard Genzel and his team observed the motions of stars in the vicinity of the mysterious radio source Sgr A* in the centre of our galaxy with high accuracy. Their idea: From the movement of celestial objects around a central body its mass can be determined. And indeed, the stars in the centre of the Milky Way are orbiting an invisible "something", and they are orbiting fast. More than twenty years of observation led to only one conclusion: our galaxy harbours a Black Hole with 4.3 million solar masses! (See Fig. 2 and the image caption for details.) We now think that these central Black Holes exist in almost all galaxies. Unlike active galaxies, however, most of them do not attract attention as they are "starving", i.e. they do not feed on matter from their surroundings.

The AG honours Reinhard Genzel not only for this discovery, but rather for all his activities in observational astronomy. These include his important contributions to the development of new observing techniques and instruments in Infrared and Submillimetre astronomy, which operate at today's large observatories, such as those of the European Southern Observatory. Only these techniques made it possible to carry out the highly precise measurements in the centre of our galaxy.