The mystery of Black Holes!

Black Holes have been 

Let us consider an example how a black hole is formed. On the stars nuclear fusion of four hydrogen atom to form a helium atom goes on continuously. Slowly in the later time when the gases i.e. fuel goes on decreasing on a star the star approaches its death. A stage comes when the fuel over it completely depletes. This is the time when gravitational force wins and the star shrinks to a high density ball. Ratio of the radius is just like earth compared to a football. According to the formula of escape velocity

V_esc = square root of (GM/R)

Where;

 G stands for gravitational constant

M stands for mass of star

R for radius of star

Now refer the table and see the effect on escape velocity.

Class	Mass	Size
Supermassive black hole	~105–109 times MSun	~0.001-10 AU
Intermediate-mass black hole	~103 times MSun	~103 km = radius of earth
Stellar black hole	~10 times MSun	~30 km
Micro black hole	up to ~MMoon	Up to ~0.1 mm

See this, the escape velocity becomes greater than even the highest speed limit i.e. velocity of light. This means even  light cannot escape a black hole.