MORE ON BLACK HOLES

 Stars come in various sizes. Our Sun is an average star in the Universe. Stars can vary with masses ranging from 0.08 solar masses to 100 solar masses.

It is found that stars with 1 solar mass is 10 times more common than stars with 2 solar masses, stars with 2 solar masses are 10 times more common than stars with 4 solar masses, and so on. The most common type is the Dwarf star with about 0.25 of the solar mass.

Like humans, stars too die, but a star's death does not mean the end of the star. It means the star has used up all the matter, the thermonuclear reactions have stopped, and the star loses its luminescence.

The more, the size of the star, the faster it uses up its matter and dies. For example, our Sun with a surface temperature of 5500 degrees K has a life span of 10 billion (1000 crore) years while the Red Dwarf star with about half the Sun's mass with a surface temperature of only 4000 degrees K can live for trillions of years.

There is a class of stars known as Super Giants. They are the largest known stars in the Universe but they are rare. The star Betelgeuse in the Constellation of Orion is a Super-giant. It has a diameter of 100 crore KM compared to just 14 lac KM for our Sun. However, the density of this star is negligible enough to be zero compared to 1.4 for our sun and its mass is just over 20 times that of the Sun. 

When these super giant stars which have masses over 20 solar masses die they become Super Novae and then form Black Holes. 

In both Novae and Super Novae a massive explosion would blow apart the matter in them. 

A Nova expends only a part of its matter in the explosion and has recurrent explosions and becomes a White Dwarf. 

A Super Nova explosion would blast massive amounts of matter with a mass of several times the sun into space and at that time it outshines an entire galaxy consisting of 10,000 crore stars. 

However, not all Supernovae explosions give rise to black holes. Stars with masses up to 20 times the sun turn into Neutron stars. Only those stars that is more massive than that turn into Black holes.

Any stellar object that collapses to the point where its radius is less than a certain limit must ultimately become a black hole. This critical radius is called the Schwarzschild radius. Only a star that has a mass of over 3 times that of the Sun can theoretically turn into a black hole. However, such conditions exist only in the most massive stars which are over 20 times the mass of the Sun.  

The Event horizon of a Black hole is its special outer boundary loosely considered to be its surface. It is the point where the gravitational attraction of the Blackhole becomes so great that even light cannot escape from it. That is why we can never receive a signal from the Event Horizon into the Black hole. The laws of Physics known to us break down inside the Event Horizon of the Black hole.