Stellar black holes are as heavy as three to a few dozen suns, but only a few kilometers tall. It has been known for a long time that they are the bodies of burned-out, massive stars whose cores support each other. The researchers still puzzle over the exact processes. It is unclear, for example, whether a star can collapse directly to a black hole or whether it explodes first as a supernova. At the other end of the spectrum of cosmic maelstroms are the galactic or supermassive black holes. They weigh as much as a million to a hundred billion suns. Maybe every galaxy has such a heavy, dark heart. In the meantime, there are increasing indications that these central concentrations of mass have influenced the development of the Milky Way, and perhaps even started it up in the first place. No matter how they came into existence, the dark "hearts" influence the evolution of galaxies, such as the shape and star formation rate. So, black holes are closer to us than we think: if they were not, the cosmic conditions would have developed quite differently, and then probably there would be no humans at all. It is amazing that an object smaller than the solar system can affect a huge galaxy of billions of stars.
Stellar and galactic black holes are not all that nature has to offer in cosmic colossi. Recently, several Japanese and American research groups independently found indications that there are also medium-sized black holes - celestial bodies 100 to 10, 000 times heavier than the Sun, but smaller than our Moon. They betray themselves by subtle changes in the X-ray spectrum of galaxies, which were recorded with the German X-ray satellite Rosat and the Japanese ASCA. The collision of stellar black holes is the most likely way to form medium black holes. Although a frontal collision in space is extremely unlikely. But the majority of the stars in the universe is not alone, but circles around a common focus in twos or threes. When they have enough mass, they collapse at the end of their lives to black holes that carry on the cosmic dance. But gravity energy is lost. This energy loss sets in motion a spiral of destruction: the black holes are getting closer and finally collide. Physicists are already lying in wait to measure these cosmic shocks. "Colliding black holes are among the most promising candidates for detecting gravitational waves, " says Edward Seidel, a professor at the Max Planck Institute for Gravitational Physics in Golm near Potsdam. With his colleagues Bernd Brügmann and Werner Benger, as well as foreign researchers, he has calculated for the first time exactly what happens in such a collision.
The calculations show that the event horizons of the black holes distort peanut-shaped before merging like two drops of water. The strength of gravitational waves increases rapidly within a few millionths of a second. Their frequency is also increasing rapidly. Two black holes of 10 and 15 solar masses, falling into each other at a distance of about 50 kilometers in less than 0.2 thousandths of a second, emit about one percent of their total mass in the form of gravitational waves. "This is a gigantic amount of energy, which is about a thousand times larger than the radiation that our sun has released in the course of their five billion years of life, " says Bernd Brügmann. Finally, as in a church bell, the vibrations gradually fade away, while the event horizon of the united black holes again takes on a spherical shape.
On earth, of course, hardly anything can be felt from these cosmic supercrashes. But with new gravitational wave detectors, which will begin their measurements in the coming months, physicists want to prove the vibrations. It will not be easy - but that's not to be expected of cosmic heavyweights.=== Rüdiger Vaas