What is Black Hole?

Black hole is a region in the space with such strong gravity that even light can not escape from that. Theoretically if someone  want to escape from black hole , he needs to travel with speed of more than light. Well when a massive star ( more than 10 times of solar mass) exhausted his core fuel, star collapsed under his gravity and outer layer blown away to form planetary nebulae. Mass of constituents matter compressed into zero volume with infinite density to form singularity. A black hole consist of singularity in centre which is hidden by a surface or boundary called event horizon. Well as per  theory  given by Karl Schwarzschild, any object with mass M can form a black hole if its whole mass compressed within a radius given by formulae R= 2GM/c^2 where G is gravitational constant and c is speed of light. As per this formulae our earth can form a black whole if its whole mass get compressed into radius of a few milimetre.

 Black hole is a region with super gravity that even light can not escape from it. So how can we detect it? Well black hole can not see through any telescope. It can only be observed through analysing the behaviour of object roaming around it. If a massive star revolves around any object which can not be detected, we assume that it must be a black hole. Black hole has been devided in three form based on its size.

# Primordial Black Holes: It is assumed that such black holea have formed just after the event Big bang. They are in size of atom but with mass of nearly a massive moutain. There existence proposed first in 1966 and later in 1971 they have been studied in depth by Stephen Hawking. As per Hawking theory, these black holes having mass less than 10^11 kg,  very unlikely to have  survived today due to hawking radiation, which causes complete eveporation of them. 

# Stellar Mass Black Holes: These black holes formed due to the gravitational collapsed of a star. They may have mass from ten to thousands of solar mass. Any massive star tends to form black hole or neutron star in end of its life. When mass of star is enough and it get crushed to achieve zero volume, it form Black hole.