In Fig. 1.2 we see an image of the very center of our galaxy. This picture is taken with the Northern branch of the two Gemini telescopes, which is located in Hawaii on top of the mountain Mauna Kea.
 |
In the very center of our galaxy, a black hole resides with a mass a few million times larger than the mass of our sun. Although the black hole itself is invisible, we can infer its presence by its strong gravitational field, which in turn is reflected in the speed with which stars pass near the black hole. In normal visible light it is impossible to get a glimpse of the galactic center, because of the obscuring gas clouds that are positioned between us and the center. Infrared light, however, can penetrate deeper in dusty regions. Fig. 1.2 is a false-color image, reconstructed from observations in different infrared wavelength bands.
In the central few light years near the black hole, the total mass of stars is comparable to the mass of the hole. This region is also called the galactic nucleus. Here the stellar density is at least as large as that in the center of the densest globular clusters. However, due to the strong attraction of the black hole, the stars zip around at much higher velocities. Whereas a typical star in the core of M15 has a speed of a few tens of km/sec, stars near the black hole in the center of our galaxy move with speeds exceeding a 1000 km/sec. As a consequence, the frequency of stellar collisions is strongly enhanced.
Modeling the detailed behavior of stars in this region remains a great challenge, partly because of the complicated environmental features. A globular cluster forms a theorist's dream of a laboratory, with its absence of gas and dust and starforming regions. All we find there are stars that can be modeled well as point particles unless they come close and collide, after which we can apply the point particle approximation once again. In contrast, there are giant molecular clouds containing enormous amounts of gas and dust right close up to the galactic center. In these clouds, new stars are formed, some of which will soon afterwards end their life in brilliant supernova explosions, while spewing much of their debris back into the interstellar medium. Such complications are not present in globular clusters, where supernovae no longer occur since the member stars are too old and small to become a supernova.
Most other galaxies also harbor a massive black hole in their nuclei. Some of those have a mass of hundreds of millions of solar masses, or in extreme cases even more than a billion times the mass of the sun. The holy grail of the study of dense stellar systems is to perform and analyze accurate simulations of the complex ecology of stars and gas in the environment of such enormous holes in space. Much of the research on globular clusters can be seen as providing the initial steps toward a detailed modeling of galactic nuclei.