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Black holes are among the most extraordinary celestial objects of the universe and, surely, the most difficult to describe, given that they don't have precise limits in the space. A black hole is an object with an enormous mass, exerting such an intense gravitational attraction that matter and even light of every frequency can't get out from it, once attracted. For this reason, a black hole is totally dark and we cannot observe it directly in the visible light.

To make a gravitational comparison between the gravitational attraction of the earth and that of a black hole, the escape velocity to abandon the earth is about 11.2 Km/sec (about 25,000 Km/h), but that of a black hole is higher than the speed light (300,000 Km/sec); just for this reason, neither the light can escape from it.

Around a black hole, there's a spherical invisible surface, called "event horizon", beyond which an object can't come back anymore in any way, neither the light. Before this limit, we can still see the object during its fall, after, no more because also its own light is captured. After few minutes or hours, depending on the dimensions of the black hole, the object will be squashed into a single point with an infinite density, becoming one with the black hole.

According to the relativity theory, black holes create deep curvatures in the space-time, swallowing whatever object like in a deep water eddy and the most massive black holes are created by the union of many minor ones or neutron stars.

The density of mass in a black hole is extremely high, very difficult to calculate, so that all mass can be considered as concentrated in a point, at least in the theory. In the practice, it has been calculated that a black hole with the same mass of the Sun would have a diameter of about 6 Km, while the same mass of the Earth would have only 2 cm of diameter!

But what creates such an extreme mass concentration? The main explanation of their origin is the gravitational collapse of a super-massive "blue-giant" star (>30 times the solar mass), as soon as it has consumed all its H and He necessary for the nuclear fusion reactions that keep stable the star. In fact, without the enormous energy produced by these reactions, the matter of the blue giant, after only 4-5 millions years from its birth, collapses violently on itself within few minutes because of its immense gravitational force.

This collapse generates a violent shock-wave that makes explode the external gaseous layers of the star, at a speed

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