Revolutionary observation black holes – what are they actually??

Black hole in the galaxy Messier 87

A black hole in the galaxy Messier 87. Before it was never succeeded to photograph such a hole.

Supermassive black hole with torn-off star (illustration)

Photo: ESO, ESA/Hubble, M. Kornmesser

This is how researchers imagined a black hole before the first photo was taken

The international research project Event Horizon Telescope has succeeded for the first time in photographing a black hole. But what is a black hole and what does it do?? For this we have Prof. Dr. Jochen Liske from the Observatory of the University of Hamburg interviewed.

As a professor of observational astronomy, can you explain to us: What are black holes?

A black hole is an object whose gravity is so immense that nothing that has once come too close to it can ever escape it again: what has once fallen "into" it never comes out again. This is even true for light.

To illustrate this, let us first think of our earth. On the earth’s surface there is a certain gravitational force. If z. B. throws a tennis ball vertically upwards, it will reach a certain height before it falls to the ground again. The height depends on the initial velocity. The greater this initial velocity, the higher the ball rises. If the initial velocity would be more than 11.2 km/s (approx. 40.320 km/h), the ball would never fall back – it escapes the gravity of the earth. This special initial velocity is called the escape velocity, an illustrative measure of the strength of gravity. The strength of gravity of a body depends on its mass as well as on its size. If the earth would be smaller with the same mass, its escape velocity would be much larger. With the size of a pea the escape velocity of the earth would reach the speed of light. Since nothing can travel faster than light, nothing could escape from the surface of this pea-earth – we would have a black hole.

Black holes were already predicted by Albert Einstein. The proof of their existence succeeded however only much later on the basis of astrophysical observations. As for example the observation of stellar motions over many years or the detection of gravitational waves.

Where to find black holes?

As far as we know, black holes occur only in the vastness of the universe. We know black holes of different sizes resp. Masses: so-called stellar black holes, which have approximately the mass of some suns – a solar mass corresponds to 1.99 quadrillion tons – and are the remains of burned out giant stars. They are found everywhere in our home galaxy, the Milky Way, and probably in all other galaxies as well. However, there are only so few of these holes that we do not have to be afraid that tomorrow one will sweep us through the solar system. Beyond that we know also so-called. supermassive black holes, which have millions or even billions of solar masses. However, these are found only in the centers of galaxies – as well as in the center of our Milky Way.

Which effect have black holes?

The effect of a black hole lies in its enormous gravity, which plays a role, however, only if one comes too close to it. Seen from a great distance, the effect of a black hole is not at all different from the effect of a normal object of equal mass. Would z. B. the sun were to collapse into a black hole tomorrow, this would have no effect at all on the orbits of the planets in our solar system, since these are determined by the sun’s gravity. However, if you get too close to a black hole, it gets uncomfortable. The hole is only one point and is surrounded by the so called event horizon. If this, from z.B. of a rocket, it can no longer return. Near the horizon, there are also enormous tidal forces – comparable to those responsible for the ebb and flow of the earth – which would tear everything apart. By measuring the motion of matter in the immediate vicinity of a black hole, one can draw conclusions about the properties of the black hole and test the validity of Albert Einstein’s predictions.

Further information

The photograph of a black hole was taken by the researchers of the international research project Event Horizon Telescope. More information can be found on the homepage of the project (engl.).

A video for the presentation of the image can be found on the homepage of the European Southern Observatory.).

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