A black hole represents the final stage of the evolution of a star. When a star runs out of its nuclear fuel, it dies. The increased pressure associated with the heat produced by nuclear reactions is not enough to counteract the contraction process of the star. When a star dies it can mutate into three different entities depending on its mass, it could therefore generate into a white dwarf when its mass is about the same size as our Sun, a pulsar (neutron star) if its mass is few times the size of our Sun, and a black hole if its mass exceeds about ten or fifteen times the size of our Sun.
Black holes are extremely dense objects with a strong gravitational pull, so strong that nothing can escape its attraction, not even light. Because light cannot escape from a black hole, they are invisible and can only be identified by checking its gravitational effects on nearby objects.
When the surrounding matter falls into the black hole it reaches high temperatures and emits X-rays. It is thanks to this phenomenon, that it is possible to observe black holes in space. For example, in the Cygnus constellation there is a binary system consisting of a giant star and a black hole. The black hole constantly draws matter from the star, so X-rays are generated and therefore, it can be studied indirectly.
A black hole can also be detected thanks to a phenomenon known as gravitational lensing. Basically, the light propagates into a straight line, but when it passes near a black hole, the light’s beam is bent by its gravity. When a black hole is positioned between us and a star or a galaxy we are observing, their position appears to be out of place, due to gravitational lensing causing the illusion of observing two or more images of the same object.
It seems that the number of black holes in the Universe is enormous, indeed when studying the Universe under the X-ray, a diffuse glow is noticeable, as opposed to the blanket of darkness normally seen without the use of this technology. The glow is therefore proof of the existence of a huge number of black holes. There is already proof of the existence of an enormous black hole (Super massive black hole) at the center of our galaxy as well as in other galaxies.
When an object is swallowed by a black hole, its speed increases dramatically, so, adhering to Einstein’s Theory of Relativity, its time slows down. When the object reaches the Event Horizon, time stops. But what happens afterward? Perhaps the object travels back in time?
Those who would observe the object from the outside of the black hole would attest that the object is actually swallowed inside.
According to the Law of Relativity, in the vicinity of a black hole gravitational forces significantly alter the space-time relationship. In particular, time slows down more and more as one approaches the Event Horizon (the center of the black hole), and then stops completely above the Horizon itself.
Stars are born and die according to a well-defined path. First, we see them take the form of dust and clouds made of interstellar gas in the process of contraction, until they reach a point that will trigger their real activities: the stellar nucleosynthesis. Then, once the fuel in the star core is no longer able to counteract the gravitational forces of its external parts, the star collapses on itself and finally gives life to a black hole.