Black Holes

The nearest black hole is located in the centre of our Milky Way. It bears the name Sagittarius A*, is more than 4 million times heavier than our Sun. In a black hole, an enormous amount of mass is concentrated in an extremely small volume. The gravity in it is therefore so strong that not even light can move away if it gets too close to the black hole. When matter approaches a black hole, it often begins to circle in a spiral, heats up and glows as a so-called accretion disk before disappearing into the black hole.

Everything is black beyond the event horizon

The boundary around the black hole beyond which even light can no longer escape is called the event horizon. Everything that happens beyond the event horizon remains invisible to us. Calculations also show that space and time are so strongly distorted by the extreme ratio of mass to volume that we can no longer describe them with our physical models. Cosmologists then speak of a singularity.

First the theory, then the evidence

The first scientist to imagine objects so massive that they themselves retain light was John Mitchell in the eighteenth century. Some time had to pass before the correct calculations were available. Albert Einstein published his equations of the general theory of relativity in 1915, with Karl Schwarzschild finding a solution in 1916. This solution described a black hole for the first time as a singularity surrounded by an event horizon.
​Practical evidence for black holes gradually followed. In 1972, Tom Bolton observed an object that he suspected was a black hole. It was given the name Cygnus X-1. A few years later, scientists agreed by a process of elimination that it was indeed a black hole. In 2016, Advanced LIGO detectors in the United States succeeded in detecting gravitational waves that could only originate from the merging of two black holes. And in 2019, researchers used the Event Horizon Telescope to photograph the event horizon around the black hole in the Messier 87 galaxy as part of a global project. "Our" black hole, Sagittarius A*, was discovered by the two teams led by astronomer Andrea Gehz and astrophysicist Reinhard Genzel in numerous observations from the 1990s onwards. The first image of Sagittarius A* was also captured with the Event Horizon Telescope in 2022.

Frequently created from stars

Researchers differentiate between different classes of black holes, depending on how they are formed and their mass.

• Many black holes are formed from stars. Stars that are at least three times as heavy as our Sun can collapse into a black hole in a supernova at the end of their lives. These are then called stellar black holes.
• Another possibility is primordial black holes which formed shortly after the Big Bang, when the density distribution in the universe was different from today.
• "Our" black hole, Sagittarius A*, is a so-called supermassive black hole. Their mass is millions or even billions of times that of our Sun. It is still not known how they are formed.
• If a black hole has hundreds to thousands of times the weight of the Sun, it is called an intermediate-mass black hole (IMBH) and was probably formed when two stars collided.
• Micro black holes are also possible, at least in theory. Large masses do not necessarily have to be involved for a black hole to form. The decisive factor is the ratio of mass to volume. Such micro black holes could be created when particles, such as electrons, collide at high speed. Such a black hole has never been observed before.