Black hole is the place in the space where gravity is too strong to prevent escaping lights from it. Reason of so strong gravity is squeezing the matter in tiny place. Even light do not escape so no one can see it. Special telescope with special tool we can see black hole. In 1916, Albert Einstein was first to predict the black hole in general theory of relativity. The term “Black hole” was announced first in 1967 by American astronomer John Wheeler.
In 1916, Albert Einstein had given idea in general relativity theory shows gravity influences on light’s motion. American astronomer John Wheeler had given “Black hole” term in 1967. After that for long time black hole remain as only theoretical object. First physical black hole was discovered in 1971. In 2019 EHT (Event Horizon Telescope) caught first image of black hole in the middle of galaxy M87. A German astronomer Karl Schwarzschild developed modern idea of black hole.
Most common process of formation of black hole is stellar death. When the stars reach at the end of their life cycle, they cool and become neutron stars. But the stars which have mass about 10 to 20 times that of the Sun reach at the end of life become super dense –Neutron stars called Supernova. When the star is alive, nuclear fusion create constant outward push which is balanced by inward pull of internal gravity force. But in end life of supernova there is no fusion force to oppose gravity of mass and core begins to collapse into itself and formation of Black hole is done.
Once the black hole has formed, it was continuous growing by absorbing additional matter from its surrounding. The process of absorbing the other stars and merging with other black holes cause the formation of super massive black hole. Every galaxy has a supermassive black hole at its centre. The milkyway galaxy has also black hole at its centre known as “Sagittarius A” which is about four million times massive than the Sun.
Black holes are in variety of masses and sizes. Whole mass of black hole is concentrated in infinitely small and dense point is called “Singularity”. It is surrounded by area which is called “Event Horizon”. Rotating black hole is surrounded by “Ergosphere”.
The Singularity is the centre point of the black hole where all the mass of black hole is concentrated which density and gravity is infinite. It is the point where all laws of physics are broken. If any object fall into the black hole and reach to singularity at the centre, due to differential gravitation attraction it losing dimensionally and become invisible forever in singularity. In non-rotating black holes, singularity look as a single point while in rotating black hole, it is ring shaped which lies at plane of rotation.
As the distance from the singularity increases, escape velocity is decreases. Escape velocity is the speed of the object at which it must to move to get away. Escape velocity for the Earth is 11km/s.
The black hole is bounded by edge through which nothing can escape even light also due to escape velocity is equal or more then the speed of light. It is the boundary in which object only enter into it but not escape. It is like one way membrane. Any object approaches to the event horizon becomes to slow down due to gravitational time dilation effect. Falling object fades away until it will disappear. This process is done very rapidly within less than second.
The radius of event horizon is called “Schwarzschild Radius”. This radius is depend upon the mass of black hole. The black hole of Milky way has the mass about 2500000 suns and the event horizon is about millions of kilometre.
The space time region surrounded the black hole in which impossible to stand is called “Ergosphere”. This is created due to frame-dragging where any object near the rotating mass tends to start rotating in the direction of rotation.
Ergosphere is the volume whose inner boundary is event horizon of black hole and outer boundary is ergosurface which overlap on poles of event horizon and wider at equator.
Gas, dusts and other stellar bodies rotating around the event horizon creates flattened disk which is called “accretion disk”. Event horizon of black has not ever seen but accretion disk is visible due to tremendous speed of spinning particles, releasing heat and powerful x-rays and gamma rays.
Types of Black Hole :
Main three types of black holes are exist in the Universe.
Stellar Black Holes
During the life cycle of the stars when all fuel burns, it may collapse or fall into itself. When the stars are smaller (having the mass up to three times the mass of Sun) they convert into neutron stars or into white dwarfs. But when larger stars collapse they collapse and continue to compress and creates Stellar Black Hole. Black holes which formed by collapsing the single stars are small but very dense. Gravitational force of stellar black hole pulls dusts and gases from surrounding galaxies and grows in size.
Supermassive Black holes
Scientists are not sure how such large black holes formed. Once they are formed, they consume dust and gas surrounding them and become giant. Supermassive black holes are generated by merging together of hundreds or thousands of tiny black holes. Collapsing of large gas clouds, collapsing of stellar clusters or large group of dark matters are also responsible for forming of supermassive black holes. Supermassive black holes are million times as massive as the Sun. Such black holes are lie at the centre of every galaxy.
Intermediate Black holes
Latest research shows that along with small and big black holes, mid size black holes also present in the universe which are known as Intermediate black holes. Intermediate black holes are present in the centre of small or dwarf galaxies. They are form when stars collide in the clusters in chain reaction.
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Authored by BK Suthar.
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