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What is a black hole?
Black holes are some of the most fascinating and yet to be fully explored objects in outer space that have kept scientists and researchers on their edge since it was discovered.
Simply put, it’s a region in outer space where nothing, not even light, can escape its gravitational pull. It’s a region in spacetime where the gravity is very strong and doesn’t allow the smallest of particles, electromagnetic radiations, space objects such as comets, planets. Because nothing can get out of these regions, they are not visible to people which is why they are referred to as black holes. Space telescopes with special tools are used to identify these black holes.
The theory of general relativity, which was published by Albert Einstein in 1915 and which describes gravitation in modern physics, predicts that a sufficiently compact mass can deform spacetime which then turns into a black hole. The boundary of the black hole after which nothing can escape its gravitational pull is called the event horizon. According to the general theory of relativity, there are no detectable features of black holes despite the fact that they can have an enormous impact on objects around them. Black holes of stellar mass have temperatures on the order of billionths of a kelvin which makes it impossible to observe.
History
In the 18th century, John Michell and Pierre-Simon Laplace were the first to consider objects whose gravitational pull were strong enough to not allow even light to escape. Later in 1915, Einstein developed the general theory of relativity and showed that gravity does influence light’s motion. Karl Schwarzschild, in 1916, found a solution using relativity theory to characterize a black hole. Later in 1958, David Finkelstein interpreted it further. The discovery of neutron stars in 1967 sparked further interest in the gravitationally collapsed compact objects.
On 11 February 2016, LIGO Scientific Collaboration (LSC) and the Virgo collaboration detected gravitational waves for the first time. It also represented the first observation of a black hole merger. By December 2018, 11 gravitational wave events were observed which involved 10 merging black holes and in April 2019, the first direct image of a black hole and its surrounding was published. The observation was made using Event Horizon Telescope in 2017 of the supermassive black hole in Messier 87’s galactic centre.
How big are black holes?
Scientists believe the smallest black hole is the size of an atom but its mass can be that of large mountains. ‘Stellar’ black holes are 20 times the size of the sun and there are plenty of stellar black holes found in our Milky Way Galaxy. The largest black holes are called ‘supermassive’ and they have masses of more than 1 million suns together.
How is black hole formed?
It is believed that the smallest black holes were formed at the start of the universe. ‘Stellar’ black holes are formed when the centre of a big star collapses. The biggest black holes, supermassive black holes, are formed at the beginning of their galaxy.
How are black holes detected?
Scientists detect them through the strong gravity, which affects the surrounding objects such as stars, planets. Scientists study the behaviour of stars and other objects to find out if they are near a black hole. Also, high-energy light is emitted when a black hole and a star are close together. These lights can be detected using powerful telescopes.