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What is a black hole?

Definition: A black hole is a region of space-time bounded by extremely dense mass from which nothing can escape. It is so dense that even light cannot escape.

So what's the concept behind it?

• In your first try, imagine throwing an object in the air vertically up with some force. The object goes up and after reaching certain height, it comes back to the ground. This is due to the gravity acting upon it.

• Now, in your second try, if you throw the same object with a greater force in the same direction (vertically upwards), the object goes to a greater height (more than that from the 1st try) before coming back to Earth.

• In your final try, you have to throw the same object, in the same direction, but now with the velocity that it does not come back to Earth. With this velocity, it will escape the Earth's atmosphere. This is called as Escape Gravity.

• Now imagine that you have the same object which we discussed above, and instead of throwing the object directly, you first compress the object and then throw it vertically up. Compression is nothing but squeezing the same object into smaller space (volume).

• Now if an object is compressed into smaller volumes, the effect of gravitational attraction acting on the object increases. The more you compress the object, the more dense the object becomes and more the gravitational attraction.

• Now if you have to throw the object that you have compressed or squeezed, in the same direction (vertically upwards), you will need a greater velocity for it to escape the atmosphere. Meaning, the escape velocity for that object now increases.

• Finally, compress the object so much that the escape velocity becomes more than the velocity (speed) of light. This, exactly, is the condition inside a black hole. Hence, we say that even light cannot escape through a black hole.

Image credit: NASA

So how are they formed?

• Now think of a huge star that is almost a hundred times greater than the size of our Sun. Yeah that huge! These stars are made up of gasses. Due to an intense gravitational activity, it is always trying to collapse to the center.

• But at the center, in the core, there is a constant fusion activity taking place. These fusion activity opposes the the force of collapsing and thus it avoid the star from dying.

• But when the nuclear fusion stops, these center force vanishes and it can no longer avoid the collapsing force to act upon it. As a result, all the matter ties to collapse at the center, trying to form an extremely dense space-time region called as a black hole.

• As discussed earlier, the region is so dense, that even light cannot escape through it.

Important concepts

• Rotation: Black holes rotate around itself. This is because the stars from which it formed were rotating too.

• Singularity: The central part of the black hole is called as the singularity. It is the main region of the black hole and is characterized by extremely dense region.

• Event horizon: The opening of the hole is called as the event horizon. It is also called as the mouth of a black hole. The size of a black hole is determined by the size of the event horizon.

How big are black holes?

• Black holes, according to scientist can be of varied sizes. They do not have a definite size. They can be as small as the size of an atom or they can be so stellar that their size is more than millions of Suns combined.

• According to the research, tiny black holes are formed during the time of the Big Bang. They can be extremely tiny. But due to its dense nature, the mass is centered at a very small space, squeezed so tightly that they would weight millions of tons. That's strange!

• Wile some black holes are so massive that it is hard to imagine the size of stars from which they must have been formed. These humongous black holes are greater than the size of our entire solar system.

How do we know they exist?

• Well you might think that it is impossible to trace a black hole because it does not even allow light to pass through it and all our observations takes place due to reflection of light through them. Then how do we know they exist?

• Well, the area near the black hole is observed. Strange phenomenon and intense gravity gives scientists an indication of presence of black holes.

• Black holes also radiates energy in the form of 'Hawking radiation'. This is probably another evidence of presence of a black hole.

Breaking physics laws

The concept of black holes is merely a combination of near-found evidences. But the final call of proofs will still be unclear...at least for a while from now. Well, said that, at the singularity, space and time as we know them come to an end.

What exist beyond the single point of dense mass is also a fascination to physicists. Will black holes be our answer to some of the most fundamental questions? Are laws different for some part of the universe? Can we time travel through a black hole? Well, this particular phenomenon will always remain a mystery.

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