Astronomy. Black holes.

First, we will learn what they are and how they are formed.

A black hole is a region of space-time that has such a strong and intense gravity that even objects moving at the speed of light (photons of light itself as well) cannot leave it. Such a boundary is called the event horizon, its diameter is considered the size of a black hole.
Initially, they were just a theory, a solution to complex mathematics.

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When a star with a mass of about 10 solar masses collapses (is strongly compressed), a supernova occurs and a neutron star is born. But if the mass of the resulting neutron star remains greater than a certain limit, which is approximately equal to 3 solar masses, then a further collapse of the star occurs, the energy density in the core at the same time goes to infinity. This is how black holes of stellar masses are formed.  The radius of such a hole will be very small, but over time it can grow significantly due to the absorption of matter, for example, a star located nearby in a binary star system. Often it can grow due to a merger with another black hole, so supermassive black holes with a mass of more than millions of solar masses can be formed. It is generally accepted that such holes are located at the center of most galaxies, in particular Sagittarius A, which is the center of our galaxy. They also play a key role in their evolution. TON 618 is currently the largest black hole with a mass of 66 billion solar masses.

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A black hole is filled with a huge mass of matter, which is compressed into a critically small volume, that is, to a point where infinitely large values of density and gravity are reached, this point is called a gravitational singularity. 
 Due to the force of gravitational influence, it distorts space-time, and the closer you are to it, the slower time passes for you.
The greater the mass of a black hole, the lower its density.

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There are four types of black holes (three real and one hypothetical)

Stellar: they are small with a mass of 5 to several tens of solar masses.
Supermassive: the largest black holes with a mass of hundreds of thousands to billions of solar masses.

Intermediate: significantly more massive than stellar, but smaller than supermassive.

Primary: hypothetical black holes that could have formed after the Big Bang with a mass less than stellar.

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