Black holes are regions of space from which no radiation can escape at all. In classical physics, to explain this effect, one can imagine that the escape velocity, which would be necessary to leave these objects, is greater than the speed of light.
In theory, black holes have been known for a long time, but they have been detected with high probability only in the last few years. The detection of objects which are invisible is by nature not easy and only indirectly possible. One uses either the enormous gravity, which black holes exert on surrounding stars and gas clouds, or the interaction with matter, which is sucked by neighboring objects into a black hole and emits highly energetic radiation thereby.
Black holes are objects in which a huge mass has been compressed to such a small volume that neither matter nor light can escape from their immediate surroundings because of the enormous gravitational pull. The outer edge of this region is called the event horizon, because everything that happens inside this "shell" remains unobservable to the outside world. Thus black holes are apparently no longer part of our universe. The existence of these objects follows from the general relativity theory which ALBERT EINSTEIN (1879 -1955) had described in 1915. In the spring of 1916, shortly before his death, the German astronomer KARL SCHWARZSCHILD (1873 -1916) recognized that EINSTEIN’s equations had solutions possessing the described properties of a black hole. The event horizon of non-rotating black holes is also called SCHWARZSCHILD radius in his honor. SCHWARZSCHILD’s calculations of black holes assumed that these objects do not rotate on their own axis and are electrically neutral.
Formation of black holes
A possible way to the formation of black holes in imploding stars was shown by the physicist JULIUS R. OPPENHEIMER (1904-1967) 1939 on. Nevertheless, black holes remained of theoretical interest until the late 1960s, because no way to observe them seemed conceivable. Also the name "black hole" was coined only at the end of the 1960s. The first astronomical objects detected in X-rays as well as the extreme radiation emission of so-called quasars (see separate keyword), however, finally led to a change of thinking. Nowadays, several black holes of different sizes are known, which can be observed by their influence on their cosmic environment.
The British physicist STEPHEN HAWKING (b. 1942) showed in the 1980s that physical effects can occur in the vicinity of some black holes that cause these objects to emit radiation to the outside – something that seems to completely contradict the original picture of the black hole. With the help of this " HAWKING radiation " such black holes could be detected directly in principle. They lose mass continuously by the radiation emission, so that small holes of this kind can even dissolve in the course of time. Because they become visible by their radiation, they are also called white holes.
Supermassive black holes
The largest and most massive black holes are probably found in the centers of star systems like the Milky Way system, i.e. in galaxies. Their masses are in the range of one million to one billion solar masses and thus already in the order of spherical star clusters or small galaxies. Their presence caused widespread activity in galaxies, especially in the early days of the cosmos, which has gradually subsided as the age of the star systems has progressed.
The details of such supermassive black holes are only accessible by model calculations, but especially with the help of the high-resolution HUBBLE space telescope their effects on the surrounding matter could be observed. For example, a black hole of three billion solar masses has been detected in the center of the radio galaxy M87. Due to its gravitational pull, the gas and stars in its vicinity move around it at high speeds, which still increase steadily toward the black hole.
The different phenomena of active galaxies are explained by different amounts of gas, which collapse into the black hole. In quasars, which are mostly found in young galaxies, there is still a lot of gas available, which accumulates in the form of a disk, the accretion disk. Collisions between the atoms in the disk dissipate the kinetic energy and use it to heat and ionize the gas, i.e., the gas becomes partially electrically charged. Since the gravitational pull of the black hole continues to act, gas moves on spirals in the direction of the black hole. Magnetic fields inside the gas are aligned essentially perpendicular to the plane of the disk, so that some of the electrically charged matter flows away at high velocity along the magnetic field lines. It forms the jets observable in radio and sometimes in visible light, jet-like streams of matter that often extend over distances of several hundred thousand light-years.
Stellar black holes
Black holes formed from stars should not be less abundant than supermassive black holes, but they are harder to detect. According to theory, they are formed in a specific type of stellar explosion, supernovae, in which a star of at least eight solar masses is completely torn apart. The explosion is triggered when nuclear fusion, which generates energy in the star’s interior (and in various shells surrounding the core), comes to a halt for lack of fuel.
If energy production stops, the star begins to collapse under its own gravitational pull. A shock wave forms in the interior of the star, and as it travels outward, it tears off the outer regions of the star. Part of the shock wave travels inward, giving the core collapse even more momentum. In the case of low-mass stars, the collapse is slowed down by the formation of a star consisting entirely of neutrons, consuming energy in the process. However, if the mass of the star is greater than eight solar masses, this loss of energy is not enough to stop the collapse. The resulting neutron star continues to collapse and its radius eventually falls below the event horizon corresponding to its mass, it becomes a black hole.
The detection of such a stellar black hole is only possible, if it orbits another star, thus forms a double star with this one.
Detecting stellar black holes is very complicated, because there are different groups of binary stars, which are nearly identical in structure. Also in them is a compact object (a neutron star or a white dwarf, the final stage of a normal star) surrounded by an accretion disk. Matter flows out of it onto the surface of the star and emits X-rays there. Unlike black holes, neutron stars have an upper mass limit of about three solar masses. The problem in detecting stellar black holes, then, is to prove with certainty that the collapsed object has more than three solar masses.
Primordial black holes
All kinds of objects can in principle become a black hole, if only they are squeezed together enough – the Earth, for example, would have to be brought to a size of about one centimeter. In today’s universe, this is rarely possible, so most black holes are confined to the two mass ranges "stellar" and "supermassive". Shortly after the formation of the universe this was different: The average mass density in the cosmos was comparable or larger than the density in (present) atomic nuclei, and from local density fluctuations tiny masses could evolve to so-called primordial black holes.
Primordial black holes could populate the universe in large numbers even today.