BLACK HOLES

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The term "Black Hole" was coined by American astronomer John Wheeler in 1967 but their existence was theorised by Albert Einstein back in 1916 itself. Contrary to most people's beliefs, black holes are not some "Cosmic Vacuum Cleaners". That's not how they work. A black hole can be thought of as a place in space where gravity is so strong that even light, the fastest thing in the universe according to Albert Einstein, can't escape its pull. The strong gravity is due to the fact that all of its matter has been crushed to a very little space called a "Singularity". In order to understand that, we need to look at the formation of black holes. Mainly there are 3 types of black holes:
  • Stellar 

  • Supermassive

  • Intermediate

Stellar Black holes: 
The most common type of black holes found in the universe so far. As their names indicate, these are formed by stellar death or the death of a massive star. When a massive star burns through the last of it's fuel, it will collapse into itself due to its strong gravity and a huge explosion will take place called the Supernova. If the leftover matter (mostly its core) is less than 3 times the mass of our sun(Solar mass), it will become either a neutron star or a white dwarf. If more than 3 solar masses, the core will collapse further until all of the matter is crushed into a single point in space time called a "Singularity". These may be smaller in size but are crazy dense. For instance one of these things can pack the mass of 3 or more suns into the size of a city. They are very common throughout the universe and according to one study , in our galaxy alone there are hundreds of millions of stellar black holes.

Supermassive Black holes:
Although found in smaller numbers while comparing to the stellar ones, these are incredibly huge and massive, with the mass of millions or sometimes billions of sun's, they usually lurk at the centre of all galaxies. Our own galaxy "The Milky Way"  has one at its centre. It is named as Sagittarius A*, a giant with about 4 million times the mass of our sun. Even today Scientists cannot give a satisfactory explanation for their formation. They may be formed due to the merger of smaller black holes, or may be formed due to the collapse of dust clouds. Some also argue that the collapse of a "Stellar Cluster" (A small group of stars) may be responsible. Another hypothesis is the collapse of large clusters of "Dark Matter".But as of now, we are not entirely sure about the reason of their existence. 

Intermediate Black holes:
These are thought to be formed due to the collapse of small stellar clusters and as per the beliefs of scientists, they will eventually grow in size to become a Supermassive Black hole at the centre of their galaxy. They are very difficult to be observed and are believed to be found at the centre of "Dwarf Galaxies" (very small galaxies).


Black Hole Structure:
Normally we tend to see a black hole with 3 layers, the outer and inner "Event Horizons" and the "Singularity". The Event Horizon is the boundary of the black hole after which it's impossible for anything, even light to escape the gravitational pull of a black hole. The gravitational pull of a black hole is constant throughout the Event Horizon. It has some bizarre properties like if you happen to stand at the Event Horizon, you can literally see the back of your own head as the light from the back of your head revolves around the black hole unable to escape from its pull. The inner region of the black hole is called the "Singularity" where all the mass of the black hole is contained. The entire matter that makes up a black hole is crushed to a single point in space-time. The gravitational force of a black hole is so strong that it can even slow down time. And some theories suggest that it can literally stop "Time" at the event horizon for an external observer. 

Black Hole Death: 
Initially, Scientists thought of black holes as "Eternal" but interestingly, even black holes are not beyond the laws of nature. Like everything in the universe they too are destined to die owing to a phenomenon called "Hawking Radiation" through which a black hole loses its mass slowly over time and dies in a powerful explosion. But this process is incredibly slow. For example: a solar mass black hole will take up to 10^67 years to die. Considering the present age of the universe to be 10^9 years, that's a pretty good amount of life time. This process is faster for smaller black holes and slower for bigger black holes.

Hope that you all enjoyed this article on Black Holes. Have a nice day.

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