Hi Rashmi,

Let us take the example of our own sun. A day will come when the nuclear fuel of the star(sun) will come to an end. At the present situation the energy created due to the nuclear reactions on the sun is enough to counteract the gravitational force of the sun. Once the nuclear fuel gets over (this may happen after 4-5 billion years), gravity will take over and then the sun will begin to shrink. This will increase the density of the sun exponentially and this will also increase the escape velocity of the sun (currently the escape velocity of the sun is around 100 miles/second). So when the escape velocity becomes 186000 miles/sec i.e. 300000 km/s, then even light will not be able to escape the surface of the star (event horizon). This is the time when the star becomes a black hole (the intermediate stage is a white dwarf or a neutron star). The conversion of a star into a black hole is generally in the form of a supernova.

   A black hole is not entirely black (it was discovered by Stephen Hawkings).

It constantly emits out microwave radiations. The matter gulped in by the black hole is constantly emmited in the form of energy radiations.

            BUT how do u think it is possible for matter to escape from the event horizon? For that to happen, the particle should possess velocity greater than the speed of light (which is impossible according to the TOR). This is explained by the "Uncertainty Principle". At some measurable point where the position of the object can be found out, the velocity of that particle cannot be measured. So at this instant the (relative) velocity may exceed the velocity of light and escape the event horizon and be emmited in the form of microwave radiation.

            Scientists believe that black holes lead to baby universes and they open out on the other side of the universe from WHITE HOLES(White hole does not allow matter to enter but only allows matter to exit; reverse of a black hole). 

          The topic on Black holes is endless. If u wish to know more about them read books written by "Stephen Hawkings" or "Michio Kaku".

 

Hope that clears ur doubt.

Cheers and a Happy New Year.