The black hole Sagittarius A * in the center of the Milky Way (here a photograph of its direct surroundings of the Chandra X-ray telescope) could have a smaller brother. Image: NASA / CXC / MIT / FK Baganoff et al.
Reading aloud In the heart of the Milky Way, not only a black hole hides, but two. According to astrophysicist Youjun Lu of the University of California in Santa Cruz, there is a simple test to find out: a binary star system moving at breakneck speed from the center of the galaxy can only have been accelerated by two black holes, he believes researchers. The black hole in the center of the Milky Way has a mass of about 3.6 million suns. However, some astronomers suspect that there is another black hole lurking there, which is perhaps a thousand to ten thousand solar masses heavy. The evidence for this theory is an accumulation of very young stars less than a light-year away from the monster hole. Normally no new stars can arise in this death zone. The existence of the young star cluster could be explained, however, if it originated at a greater distance and contains a medium-heavy black hole that was attracted by the huge black hole in the Milky Way Center.

However, this theory has not been proven so far. Lu has now discovered a test: The Milky Way hole does not eat all the stars that come too close. Some also hurl it outward, accelerating the stars to speeds sufficient to eventually leave the Milky Way. A total of ten of these "hyper-speed stars" have already been discovered.

According to Lu, for closely spaced binary stars less than one-third the distance between Earth and the Sun, it would make a difference whether they were cast under the spell of one or two black holes: a monster hole would devour a partner of such a double star and catapult the others into space. However, if the gravity fields of two black holes overlap, the binary system would be thrown completely into space.

Whether one of the ten known racers is a double star, can not be recognized by normal telescopes. A look at the spectrum could bring clarity. The presence of a second star would have to be noticeable by a characteristic "wobble" of the spectral lines. display

A second black hole in the heart of the Milky Way would fit in with common theories of galaxy formation. According to this, large galaxies like the Milky Way grow up by the merging of smaller galaxies and star clusters. In the process, the black holes that are located in the center of each galaxy must finally merge.

New Scientist, July 28, 2007, p. 15 Ute Kehse

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