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What are black holes

According to special theory of relativity, the maximum velocity that can exist is the velocity of light
c=3X108 m/s
Now we also Know that escape velocity which is the velocity needed for an object to become essentially free of the gravitational effect of another object is given
$v=\sqrt{\frac{2\mathrm{GM}}{R}}$
or
${v}^{2}=\frac{2\mathrm{GM}}{R}$
We may speculate on the mass and radius of a steller body that has an escape velocity of c.
Then
${c}^{2}=\frac{2\mathrm{GM}}{R}$
or
$R=\frac{2\mathrm{GM}}{{c}^{2}}$
This quantity R is called the Schwarzschild Radius and is usually designated by RS
Substituting the values of G and c,we have
RS=1.485 X10-2 M
The above equation gives us the relation between M and RS. It states that A body of Mass M in kg and radius RS in m or smaller produces such a strong gravtitional at its surface that no particle on its surface can escape.This even applied to electromagnetic radiation ( photons) including light.
So Even light cannot escape from such planet or body. That is the reason such body bodies are termed as Black holes
The most common way for a black hole to form is probably in a supernova, an exploding star. When a star with about 25 times the mass of the Sun ends its life, it explodes. The outer part of the star screams outward at high speed, but the inner part of the star, its core, collapses down. If there is enough mass, the gravity of the collapsing core will compress it so much that it can become a black hole. When it’s all over, the black hole will have a few times the mass of the Sun. This is called a “stellar-mass black hole”, what many astronomers think of as a “regular” black hole.