Why can't we find a black hole
"We have discovered a silent black hole"
Astronomers estimate that there are probably between one hundred million and one billion black holes in the Milky Way. However, they have only discovered two dozen of them. Because most black holes are inactive - and therefore invisible to telescopes. In an interview with Welt der Physik, astrophysicist Dietrich Baade from the European Southern Observatory in Garching reports how researchers have tracked down such a "silent" black hole and what the find means for science.
World of Physics: You and your colleagues have discovered the closest black hole to date. Where is it exactly?
Dietrich Baade: The black hole we tracked is located in the constellation Telescopium and is about a thousand light years away from us. For comparison: our galaxy is around a hundred thousand light years across.
What is known about this black hole so far?
It is a stellar black hole. Such objects are created when a massive star explodes as a supernova at the end of its life. The star's shell is repelled and the core of the star collapses into a black hole. The entire mass of a black hole is concentrated in its center. As a result, the gravity of a black hole is so strong that even light can no longer escape it.
That would make it difficult to observe these objects?
Yes, since black holes do not emit radiation that could be captured with telescopes or satellites, they can never be observed directly. However, if there is matter - for example gas clouds - in the vicinity of a black hole, it is captured by the enormous force of attraction. The matter moves faster and faster towards the center of the black hole, heating up to temperatures of up to several million degrees Celsius and thereby glowing brightly. We can then use our satellites to detect the X-rays released - and thus indirectly observe the black hole. So far, around two dozen black holes have been discovered in the Milky Way in this way. But we assume that there must be many more black holes in our galaxy.
How do you know there are so many black holes?
We roughly know how many stars in our galaxy have sufficient mass to collapse into a black hole at the end of their lifespan. If you compare the life expectancy of these stars with the age of the Milky Way, you can estimate how many stars have already developed into a black hole. This leads to numbers between a hundred million and a billion black holes. And there are most likely numerous specimens that are even closer to our solar system than the object that has now been detected. The short distance to earth is not the really exciting thing about our find. Rather, it is the fact that most of these many black holes are inactive: there is no matter in their vicinity that makes these "silent" black holes visible. Nevertheless, we have now succeeded in discovering such a silent black hole.
How did you do that?
To find a black hole wasn't our aim originally. We observed a star in the Telescopium constellation that is visible to the naked eye from the southern hemisphere. We first noticed that there is a second star in the system called HR 6819, which circles around a common center with the first star. Further observations then showed that the second star had to orbit another object - in a much smaller orbit and at a speed of about sixty kilometers per second. An enormous force is required to bring the enormous mass of a star to these speeds. This can only be explained with an extremely massive object in the vicinity of the star, which forces the star on a circular orbit via its gravity. However, because we could not directly discover any other object in our observation data, it was clear to us that it must be a silent black hole.
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