Black holes : the invisible shall become visible

They devour everything, even light. Nevertheless, there should soon be the first shot, a kind of silhouette, of a black hole.

Nobody has seen a "black hole" directly until now. Does not work at all. The attracting gravity around black holes is so great that even electromagnetic waves like light or X-rays are held back. That’s why black holes are absolutely black – even blacker than the universe itself. Or more precisely: they are enveloped by their so-called "event horizon". only what is outside of it we can observe. Exactly this space around the event horizon of a black hole is now to be shown for the first time by a telescope: The "event horizon telescope" will, if everything has worked out, soon deliver the first real black hole image.

Mathematically correct fantasies?

Already in 1916 the German astronomer Karl Schwarzschild predicted the existence of black holes. He realized that gravity near a sphere has very strange properties when this sphere is smaller than a limiting radius depending on its mass. With the help of the General Theory of Relativity developed shortly before by Albert Einstein, he was even able to calculate this limiting radius. Today it is called the Schwarzschild radius. To the mass of our sun of two quintillion kilograms (a number with 30 zeros) belongs a Schwarzschild radius of about three kilometers. In reality, the sun has a radius of 700,000 kilometers. If the sun were to shrink to a small sphere with a radius of less than three kilometers while its mass remained constant, it would turn into a black hole. It would disappear behind the event horizon, which would envelop the sun spherically with Schwarzschild radius, and what would happen behind it would remain hidden from us forever. From outside, however, light and matter would fall irretrievably through the event horizon into the black hole.

Einstein considered Schwarzschild’s calculations to be mathematically correct, but pipe dreams. Until his death in 1954, he thought that "Schwarzschild singularities" do not exist in physical reality.

Dozens of black holes in the Milky Way

He should be mistaken. There are actually at least two different varieties of black holes. The first variety, stellar black holes, are the last stop in the life of a large, massive star. If it explodes at its end as a supernova, and if a central region of at least three times the mass of the sun survives the explosion, then this former stellar center continues to collapse under its own weight. No force of the cosmos can stop this collapse, so that the collapsing remnant star finally exceeds its Schwarzschild radius and disappears behind its event horizon.

In the Milky Way, astronomers have discovered about a dozen such stellar black holes. The first one was discovered in 1970 in the constellation of Swan: Cygnus X-1, about 6,000 light-years away from us. With its strong gravity Cygnus X-1 pulls matter from a companion star to itself. But before this matter finally collapses into the black hole, it first swirls around the black hole outside the event horizon – similar to water around a drainpipe. Friction heats the swirling matter to such high temperatures that it emits X-rays. And exactly this X-ray radiation from the vicinity of Cygnus X-1 can be picked up by X-ray telescopes on board of satellites.

Possibly there is a black hole in the center of every galaxy

But there are also black holes of a completely different caliber: these "supermassive black holes" contain the mass of millions or even billions of suns. They sit in the center of many, perhaps even all galaxies. How they could have been formed – for example by the fusion of many stars, or by an agglomeration of gigantic gas masses – is still unclear. But it is certain that there is also a supermassive black hole in the center of the Milky Way, our home galaxy, recognizable by the strong gravity in its vicinity. It is shown by the surprisingly high velocities of stars observed near the center of the Milky Way. The fastest of them races around the center of the galaxy at speeds of up to 5,000 kilometers per second in just 15 years. From this, one can calculate the strength of the gravity that forces these stars, despite their high velocities, onto their tight curved paths around the Milky Way center. The result: The supermassive black hole in the center of our galaxy has a mass of about four million suns. The radius of its event horizon is correspondingly large: twelve million kilometers.

Our solar system flies at a sufficiently large distance of about 26,000 light-years around the central galactic black hole. Seen from Earth, its event horizon is therefore as small as a ping-pong ball on the Moon. Even if the event horizon could be seen in the center of the Milky Way – as a thin circle of light emitted by hot matter falling over the event horizon into the black hole – it could not be seen from Earth with any optical telescope.