A few years following the discovery of pulsars, other mysterious signals disturbed the night-time studies by astronomers: this time the disturbing forces were x-rays, that were also emitted at regular intervals. What object could be responsible for the new "message"? If sooner or later everyone reached an agreement about the existence of a neutron star, debates are still raging about the presence of an even more extraordinary source, with characteristics at the limits of the understandable. Its supporters call it a "black hole" because this object is so dense and has such a strong gravitational force that nothing, not even light, can escape. Even the black hole probably was created from a star, a star with enough mass to overcome the repulsion between the atoms of which it is formed, contracting until being reduced theoretically to a single point: here matter is so strongly compressed that a small sphere with a radius of just one centimeter would weigh ten thousand billion tons and our planet wouldn't be any larger than a tangerine! Therefore, there is such an intense gravitational field around the black holes that space and time are warped and curved: in their center space disappears and clocks stop, and the universe as we know it no longer exists. But if a black hole swallows everything and lets nothing escape, why does it emit x-rays? The fact is that, unlike pulsars, the x-ray sources generally belong to a binary system, formed by two stars which rotate around a common center. When one of the two stars is a black hole, it attracts the matter from its larger and brighter companion in a continuous flow: while the gaseous substances "fall" into the black hole, they are heated until reaching several billion degrees and before disappearing, emit intense x-ray radiation. However, the x-rays emitted reach the Earth in intervals because they are "eclipsed" periodically by the larger star when it passes in front of the black hole.
A few years following the discovery of pulsars, other mysterious signals disturbed the night-time studies by astronomers: this time the disturbing forces were x-rays, that were also emitted at regular intervals. What object could be responsible for the new "message"? If sooner or later everyone reached an agreement about the existence of a neutron star, debates are still raging about the presence of an even more extraordinary source, with characteristics at the limits of the understandable. Its supporters call it a "black hole" because this object is so dense and has such a strong gravitational force that nothing, not even light, can escape. Even the black hole probably was created from a star, a star with enough mass to overcome the repulsion between the atoms of which it is formed, contracting until being reduced theoretically to a single point: here matter is so strongly compressed that a small sphere with a radius of just one centimeter would weigh ten thousand billion tons and our planet wouldn't be any larger than a tangerine! Therefore, there is such an intense gravitational field around the black holes that space and time are warped and curved: in their center space disappears and clocks stop, and the universe as we know it no longer exists. But if a black hole swallows everything and lets nothing escape, why does it emit x-rays? The fact is that, unlike pulsars, the x-ray sources generally belong to a binary system, formed by two stars which rotate around a common center. When one of the two stars is a black hole, it attracts the matter from its larger and brighter companion in a continuous flow: while the gaseous substances "fall" into the black hole, they are heated until reaching several billion degrees and before disappearing, emit intense x-ray radiation. However, the x-rays emitted reach the Earth in intervals because they are "eclipsed" periodically by the larger star when it passes in front of the black hole.
