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When massive supergiants, with a mass greater than 5 times of that of the Sun, collapse after a supernova explosion, the core may continue collapsing, eventually forming a body so dense that it forms a black hole. This extremely dense invisible celestial region, has such a powerful gravitational force nothing can escape, even light.
The reason a black hole is capable of trapping light is because of its high escape velocity. Escape velocity can be compared to throwing a ball into the air. The higher you throw it, the faster it must leave from your hand. Eventually, when you throw it hard enough, you get to the point where the ball will not return, because of the gravitational attraction will not be able to pull it back down. The bigger the escape velocity, the harder you have to throw it for the ball escape. In a black hole, the escape velocity is so high, that light, traveling at 186 miles per second is not fast enough to escape. Since, nothing is faster than light, nothing else can escape.
Figure 2.1 (above) A diagram of a black hole.
It is hard to believe that such a thing exists, but there is good evidence that supports this phenomenon. One of them involves neutron stars. If a neutron star goes over the Chandrasekhar limit of 1.4 solar masses, it starts to collapse until it finally becomes a black hole. Since there are many neutron stars, at least some must form black holes.
Black holes were only a theory until we could measure x-ray emissions in space.
Figure 2.2 (above) A black hole.