The black hole is result of the death of a star (Though it is not the absolute end as we will see later. There exits life beyond this for a star.) A star may gain so much gravity that it starts pulling it self into itself. Like our sun. It is producing nuclear energy. As the energy depletes, the sun’s gravity with get more powerful and start to crumple itself into itself. This will then become a white dwarf. A white dwarf is a ball of nuclei and free electrons. The mass will remain the same, but the size will be smaller, thus becoming very dense. As the energy is depleting continuously, the collapse continues. For some the collapse will stop into a steady state (like our sun) due to exclusion principle. As that law states, in any given orbital, only 2 electrons can stay. But with some stars with very large initial mass, the exclusion principle will be over-ridden by gravitation. The gravity will destroy the nuclei and then it will eventually become a neutron star. A lump of neutrons. As the collapse continues, the neutron star becomes a black hole.
There are many small black holes, often called the black spots. The concept is that we can take a small rock and keep pressing it till it becomes a black hole. But no such powerful source of energy exists on earth or in visible universe. The only time such energy existed was when universe was being created. So after big bang many such black holes were created and universe has many of them.
The black holes are everything hogs. Anything that goes inside is help down b gravity. Including light (photons). So how do we observe a black hole. Roger Penrose came up with an interesting idea. Imagine a tornado that pull everything it passes by into itself and manages to keep everything inside. Imagine this big twister. It will “eat” everything it passes upon. Now imagine a car at the edge of the tornado. The force at tornado’s edge will not be string enough to pull it in, but it will throw the car tangentially at greater force. This was the way he proposed we can observer rotating black holes. On edges we will see radiation, not absorbed by the black hole, but thrown at greater speed.
We know that black hole absorbs everything. So if we light up a torch from a space ship at it, a person on surface of the black hole will see the light. But when he lights a torch back, the light will never the black hole. So we can never observe the features of the black hole as we “see” features based on emitted light and radiations. Thus for us, all black holes are alike. This is often referred to as black hole has no hair.
Now imagine a probe going into the black hole form a space ship. The people in space ship will see i go down as they see the light it is reflecting. Soon, it will enter a region at some distance from surface of black hole, where the gravity of black hole will draw down the light and not let it escape. This edge is referred to as event horizon. Thus, the characteristics of a black holes, it’s identity is the event horizon as anything beyond it is unobservable.
