Black hole is a compact object which is formed by gravitational collapsing of a heavy mass star. When all the nuclear fuel of a star, which is responsible for nuclear fusion has end, then the star will gravitational collapse because of the domination of the gravitational force and became a black hole. It is not necessary to every star become a black hole. Only massive star which has mass greater then 5 solar masses, will become a black hole. Its gravitational field is so high that even light particle cannot escape from there. A black hole have only three observable parameters, mass, angular momentum and electric charge.
An event horizon is a surface of black hole from where nothing can escape. It is like one way travelling surface. If a particle crosses the event horizon, than it can never communicate with the external universe. Only general relativity is able to explain the particle motion near the event horizon. According to this, every observer has it’s own proper time. If a observer come to near the the horizon, then for a distant observer it red shift become larger and larger and become infinite at event horizon. So, according to a distant observer this person can never reach the horizon, but for this person, he has his own proper time and he crosses the event horizon with in finite time. Outside the event horizon, particle motion is time-like geodesic and on event horizon it is null-like and inside the horizon motion is space like.
It is also believe that at the center of black hole there is singularity. Singularity is a point where the curvature of space-time becomes infinite, or we can say that every observable property become infinite at center .Black hole may be of different type. They are distinguish according to their charge and angular momentum. Simple black hole is Schwarzschild radius black hole. It has no charge and no spin. It is called static black hole. Other black hole is Resinner-Nordstrцm black hole. It has charge but no spin. First rotating black hole as discover is KERR black hole. It has net angular momentum but no charge and there is also a black hole which have electric charge as well as angular momentum, this black hole is known as KERR-NEWMAN black hole. For rotating Kerr black hole it forms a ring singularity around the center. It has a static limit surface which lie beyond the event horizon. Theoretically it is possible to extract energy from the region between the static limit surface and event horizon. This region is called ergo sphere. Generally Kerr solution is stationary, axial symmetric and asymptotically flat. Event horizon and static limit surface are coincide at poles and has maximum distance and equator.
Mr. Fazley Ahmad
Research scientist at BMUCO