The black holes are generated in the result of a collapse of giant stars with the mass more than three Sun's masses. Then the star is compressed so that the light cannot master its gravitation. The compression radius is called gravitational radius.
The observations show that very many stars are double, and part of them entire the wider star accumulations. But sometimes we may observe only the emitting of one components of the double star. But it must be taken into consideration that the second component is either a dim star of a small mass a white dwarf. But in some pair the invisible components mass is too great for this object. In such care it may be supposed that it is a neutron star or a black hole. But even then there is a great part of vagueness if one of the neighboring stars would be a compact massive “dead star”. Its gravitational field may be too strong to strip the substance from the normal star. In this case the gas being to separate from the external layers of a visible star and to fall on the invisible satellite. But this gas will be observed. And moreover the gas near the neutron star or the black hole will get hot and will become a source of high energetic electromagnetic radiation in X-rays and gamma range. Such radiation doesn’t come through the terrestrial atmosphere but it may be observed from the space ship’s telescopes.
In the majority of double systems, being the sources of X-rays radiation, the invisible component mass does not exceed the two solar masses, so it is a neutron star. But some objects of this type are too massive for a neutron star. It may be supposed that in this case the gravitational field is generated by the black hole.
It is very difficult to differ the black hole and the neutron star (without radiation of the last), that is why the black holes are spoken about like a hypothesis. But the discovery of the great dark bodies in Universe (with the several Sun's masses) is the serious proof to their being.