It is not possible to see black holes either by the naked eye or through any telescope – and that is why they are called black holes. The reason for their invisibility is their tremendous gravitational pull that does not allow the rays of light to escape. After all an object can be seen only when we see the rays of light coming from that direction through the lenses that are our eyes.
As the light of black hole remains imprisoned within the black hole it remains invisible to us. When a star with a mass much greater than that of our Sun exhausts its fuel (hydrogen) that constitutes much of its mass it implodes and contracts. As the distance between its center and the surface shrinks, its gravitational pull increases considerably which in turn constricts it further. This process continues till its original diameter is reduced from millions kilometers to just about 3 kilometers. At this stage its gravitational force is so strong that even light can not escape.
Any object trying to escape from a star or a planet has to acquire a certain velocity to break away from its gravitational pull. This velocity is called its escape velocity. For example, a rocket launched from the Earth has to acquire a minimum velocity of 11.19 km. per second in order to break out of the Earth’s gravitational pull. Even if the velocity is marginally less the Earth’s gravity will pull it back. Velocity of the rays of light being 3,00,000 kilometers per second, the Earth is not able to imprison them. On the other hand, gravitational pull of the black hole is so strong that an object must have escape velocity greater than 3,00,000 kilometers per second. As a result escaping rays of light descend back on the black hole like a cricket ball that has been hit high in the air. This is the reason why we are unable to see the star which has collapsed and become a black hole. The possibility of existence of many black holes in the universe can not be ruled out because many giant stars have become extinct after exhausting their fuel.
Although one may not be able to see a black hole, we are able to infer its existence as it sucks gaseous matter of the stars in the vicinity as shown in the picture.
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