What is black hole? Is it really hole in space? Details here

After NASA exhibited its interest in its study recently, black holes have attracted the attention of astronomers once again. A black hole is neither black in colour nor is it a hole. It is far from being either. Simply put, a black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. The concept of black holes originated from Nobel laureate Albert Einstein's theory of general relativity, which predicted that a sufficiently compact mass would form a black hole.

Black hole: Neither black, nor hole

It is called a black hole because it acts like an ideal black body, as it reflects no light. According to the theory of general relativity, a black hole has no locally detectable features, but it impacts the fate and circumstances of an object crossing it. 

A black hole is formed when massive stars collapse at the end of their life cycle. After it is formed, it grows by absorbing mass from its surroundings. It can become supermassive black holes of millions of solar masses by absorbing other stars and merging with other black holes. It can also be made by the collapse of gas clouds. It is clear that supermassive black holes exist at the centre of most galaxies.

Who discovered black holes?

English astronomical pioneer and clergyman John Michell was the first astronomer to give the idea that a body can be so big that even light could not escape. French scientist Pierre-Simon Laplace also gave this idea independently. Michell said in a letter written in 1784 that a star with the same density but 500 times the radius of the sun would not let any emitted light escape because its surface escape velocity would exceed the speed of light. He also said that such huge non-radiating bodies can be detected only through their gravitational effects on nearby visible bodies.

Albert Einstein: Theory of Relativity

Albert Einstein developed his theory of general relativity in 1915 after he showed that gravity does influence light's motion. Karl Schwarzschild found a solution to the Einstein field equations that describes the gravitational field of a point mass and a spherical mass. It is also now established that the simplest static black holes have mass but neither electric charge nor angular momentum. These black holes are called Schwarzschild black holes.