Simply put, a black hole is a super dense object that has an intense gravitational pull. A black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull.

There are two parts to a black hole, a singularity and a event horizon.

The event horizon is where the force of gravity becomes so strong that even light is pulled into the black hole. Although the event horizon is part of a black hole, it is not a tangible object. If you were to fall into a black hole, it would be impossible for you to know when you hit the event horizon.

The singularity is not really a tangible object either. According to the General Theory of Relativity the Singularity is a point of infinite space time curvature. This means that the force of gravity has become infinitely strong at the center of a black hole. Everything that falls into a black hole by passing the event horizon, including light, will eventually reach the singularity of a black hole. Before something reaches the singularity it is torn apart by intense gravitational forces. Even the atoms themselves are torn apart by the gravitational forces.

Schwarzschild Radius:

The Schwarzschild radius is a characteristic radius associated with every mass. It is the radius for a given mass where, if that mass could be compressed to fit within that radius, it will continue to collapse into a gravitational singularity.Simply put, it is the radius, according to the general theory of relativity, at which a body would become a black hole.

It can naively (although incorrectly) be derived by letting the escape velocity of a black hole equal to the speed of light. The formula for the Schwarzschild radius can be found by setting the escape velocity to the speed of light, and is

R_s = 2*G*M/c^2

The proportionality constant, 2G / c2, can be approximated as 1.48×10−27 m/kg. For the Sun, Rs = 2.5 km; for Earth Rs = 0.9 cm.

You can use the Schwarzschild radius to calculate the “density” of the black hole – i.e., the mass divided by the volume enclosed within the Schwarzschild radius. This is roughly equal to (1.8×1016 g/cm3) x (Msun / M)2, where M is defined as above. From the point of view of an outside observer, this might as well be the actual black hole density, since the distribution of matter within the Schwarzschild radius has no effect on the outside.

However, it is conjectured that at the singularity the density is infinite. This is because, all the mass of the black hole is assumed to have fallen into the singularity which has zero spatial extent. But since we do not “see” inside a black hole, we never know for sure what actually happens at the singularity.

Q.Is it true that black hole is the most dence object in universe ?

Yes.

Q. A cubic milimeter area of black hole equals to 10,000 kg, is it right ?

Much more actually. You can work out the “density” from the formula given above. It actually depends on the mass of the black hole.

Einstein’s general theory of relativity describes gravity as a curvature of spacetime caused by the presence of matter. If the curvature is fairly weak, Newton’s laws of gravity can explain most of what is observed. For example, the regular motions of the planets. Very massive or dense objects generate much stronger gravity. The most compact objects imaginable are predicted by General Relativity to have such strong gravity that nothing, not even light, can escape their grip.

Scientists today call such an object a black hole. Why black? Though the history of the term is interesting, the main reason is that no light can escape from inside a black hole: it has, in effect, disappeared from the visible universe.

Do black holes actually exist? Most physicists believe they do, basing their views on a growing body of observations. In fact, present theories of how the cosmos began rest in part on Einstein’s work and predict the existence of both singularities and the black holes that contain them. Yet Einstein himself vigorously denied their reality, believing, as did most of his contemporaries, that black holes were a mere mathematical curiosity. He died in 1955, before the term “black hole” was coined or understood and observational evidence for black holes began to mount.

Why Study Black Holes?

Here are some good reasons:

Human curiosity: they are among the most bizzare objects thought to exist in the universe.

They should be strong sources of gravitational waves.

As such, black holes should reveal much about gravity, a fundamental force in the cosmos.

Confirmation that they exist will strengthen confidence in current models of cosmic evolution, from the Big Bang to the present universe.

Black holes are all very well in theory, but if they really exist, how do they form?