## Black hole. How is it created and can it be seen?

A black hole is an astronomical object, a region of space-time with extraordinary gravity. Astronomers know little about it yet, research is still ongoing and raises many doubts. What is a black hole? What is it characterized by? Can you see it?

## 1. What is a black hole?

A black hole is a region of space-time, a creation of gravity, to which particles both small and large in mass, including light, are subjected. According to the theory of relativity, for it to arise, it is necessary to accumulate a sufficiently large mass in a sufficiently small volume.

The black hole is surrounded by a mathematically defined surface called an event horizon that marks the boundary of no return. It is called black because it completely absorbs the light that hits the horizon, reflecting nothing like a black body in thermodynamics.

The quantum field theory predicts that this hole emits radiation like a black body with a temperature other than zero. This temperature is inversely proportional to the mass of the black hole, making it difficult to see in a black hole with a stellar mass or greater.

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## 2. How is a black hole formed?

According to Einstein's theory, time flows slower in a strong gravitational field than in a weak one. The processes taking place in it are slower (time dilation) from the observer's point of view, and strong gravitational fields change the geometrical properties of the space, which may mean, for example, that the sum of the angles in the triangle will not be 180 °.

Space and time create a curving, four-dimensional space-time. The force of gravity on the surface of the star becomes infinite, and as the size of the body approaches the gravitational radius, it tends to infinity.

When this happens, it cannot be balanced by the finite pressure, and the body has to collapse inward, creating a black hole. Time starts to run slower near it, and the flow of time disappears completely on the outer surface of the black hole.

Stellar-mass black holes are formed as a result of the above-mentioned gravitational collapse of very massive stars towards the end of their lives. There are also black holes millions of times the mass of the sun.

According to researchers, black holes of this type are found at the centers of most galaxies. There is evidence of a black hole with a mass of about 4 million solar masses at the center of the Milky Way.

We can also distinguish black holes with an intermediate mass between supermassive and stellar, while the heaviest black holes are called ultramass.

Flesh that falls into a black hole cannot get out of it, as all escape routes lead back inside. Not only matter is trapped in it, but also light that moves along geodetic lines.

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