Reinhard Genzel, 2020 Nobel Prize laureate in Physics and Director of the Max Planck Institute for Extraterrestrial Physics in Garching, started his lecture on Sunday afternoon (June 27, 2021) with a little story. Astronomy is in many ways like a journey through the universe with its wonderful peculiarities. It is like entering a forest: enchanting trees, flowers, an enormous complexity that the cosmos has produced. In rare cases it happens that one begins to see order in beauty. Blue flowers, for example, that only grow to the right of the path. Then the physicist's brain is asked to answer the question of “why”. His own way through the cosmic forest,who over a period of forty years - from initial hypotheses about the nature of the extreme object in the center of our galaxy on the basis of increasingly precise observations - led him to believe that it was a supermassive black hole, he then looked back on it again .

Sibylle Anderl

Editor in the features section.

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    The black holes, as Genzel might have continued his forest allegory, were initially something like invisible forest spirits. Albert Einstein described in his general theory of relativity in 1915 how mass and energy bend space-time. Extremely extreme and strange objects, in which a compact mass bends space-time so strongly that not even light can escape, were a consequence of this theory. For a long time it was doubtful whether these objects really existed. Distant galaxies with tremendous luminosity, the creation of which seemed hardly to be explained by anything as extreme as a black hole, were a first indication of their actual existence in the 1960s: When gigantic black holes in the center of galaxies swallow matter,enormous amounts of gravitational energy are converted into heat and radiation. But it turned out that smaller and closer black holes also left observable traces. With the strong X-ray source Cygnus X-1, discovered in 1964, a stellar black hole was indirectly detected for the first time, which snatches matter from its partner star, which emits high-energy radiation in the course of this.

    Improved observation techniques confirm earlier estimates

    Proving that there is a supermassive black hole in the center of our galaxy was much more difficult and required decades of continuous improvement in observation technology.

    The basic idea of ​​the argument is relatively simple: From the movement of stars and gas around the mysterious massive object at the location of the radio source Sagittarius A *, its mass can first be deduced.

    This had already been measured in the 1970s and 1980s, among others by Reinhard Genzel's mentor and Nobel Prize winner Charles Towns.

    The result was a mass estimate of several million solar masses.

    However, this estimate of the mass alone is not sufficient to prove that it is a black hole. After all, the mass distribution must be so compact - located in such a small space within the so-called Schwarzschild radius - that, according to Einstein, the spatiotemporal conditions for a black hole are given. The first observations had shown that the expansion of the mass was at most a million times the size required for a black hole - not sufficient for a reliable identification of the nature of the source.