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Black Hole Formation

Updated on June 18, 2017

The Early Stage

A star normally burns by combining the hydrogen atoms in its core. The fusion starts (after its formed from the huge gas collapsing at a single point in an interstellar cloud) when the core is hot enough to combine two hydrogen atoms( about 18Million K) to form a single helium atom. When this happens, helium is formed and it continues to form till the hydrogen in the star is exhausted. The exhaustion of the hydrogen depends on how big the star is because if its too big, it will exhaust its fuel in few million years and if its normally sized then it will exhaust in billions of years. When the hydrogen gas gets exhausted the next layer-- Helium is combined to form carbon then carbon to silicon and so on till Iron. When the core starts forming iron in its core, it is in its last stages of its life.

This shows the stages of burning of fuels in a star's core.
This shows the stages of burning of fuels in a star's core. | Source
Expansion of outer layer of star due to exhaustion of the fuel.
Expansion of outer layer of star due to exhaustion of the fuel.

Gravity wins over nuclear Force

When the core starts to form layers after layers, the gravity has been already overtaken the force of the equilibrium which was earlier maintained by the nuclear reactions taking place. It can no longer withstand the immense pressure of the inward force of gravity because of its immense mass. The star starts to explode its outer layers to support the burning processes.

The forces acting on a star.
The forces acting on a star. | Source

How does a Star Burn?

A star goes Supernova

The star has already exhausted its nuclear fuel and its already reached the stage where the Iron atoms have started to form at the centre of the core. As we know, that Iron has the highest energy per nucleon in the periodic table, the star can no longer form any more elements in its core. The energy which was earlier released by combining elements, is used to push further the formation of other elements. But, the star is incapable of doing so due to using up of its energy.

Now, the dense core starts to collapse on itself due to immense gravity on the core. The star starts to shrink till the iron core, this immense pressure build up in such a small period of time sends a shockwave outwards of the layers of the dying star. Thus, blasting of the outer layers in a tremendous explosion known as the supernova.

In a supernova the remaining elements of the periodic tables are formed, sending this immense heat and light to huge distances across the galaxy.

When a star can no longer withstand its gravitational force due to exhaustion of its fuel, it goes into an explosion --a supernova.
When a star can no longer withstand its gravitational force due to exhaustion of its fuel, it goes into an explosion --a supernova. | Source

The Conditions

Now, generally a star goes through different stages of its life depending on its mass. Not all the stars goes supernova, normally a star about 8 solar masses go into supernova to form

  • A white dwarf
  • A black hole
  • Neutron star

Now, when the star exhausts its fuel, its mass gets collected at its core. When a star goes into the white dwarf stage there is small explosion called as nova which is the blasting of the outer layers in a controlled manner such that only dense core remains. Our Sun will not go supernova as its too small to form a neutron star and hence a black hole.

When a massive star is near its death, it collapses and goes supernova( a giant explosion which stretches many light years across) and forms a dense core-- highly rotating dense core called as a neutron star. A neutron star rotates very fast because of the sudden decrease in the size, thus increasing the angular momentum.When the neutron star is formed neutrons are formed by the process of electron capturing--protons and electrons are combined to form neutrons inside the core.

This continues till a certain point-- when the neutron degeneracy limit is reached the star can no longer withstand its immense gravity and further collapses to form a highly dense core of star which is called as a black hole.

When a black hole is formed, it starts collecting the neighbouring gas and dust to grow in size. When it reaches the schwarzschild radius or the event horizon of the black hole-- not even light can escape by its immense gravitational pull.

Normally the Chandrasekhar limit is set as r=2.95km, when this radius is crossed by a neutron star it collapses further to form a black hole. Event horizon is the point of no return, when light ray emitted at this point, it never reaches us due to immense stretching of the wavelength of the light ray. The black hole stretches the space to

  • a single point-- if its a non rotating black hole
  • circular point if its a highly rotating black hole

The highest rotating black hole ever discovered rotates about 916times per second.

The Chandrashekar Limit
The Chandrashekar Limit | Source

The Video Explains the birth of a Black Hole

A Black Hole

When a black hole is formed it starts to suck in the dust and gases surrounding it and grows bigger and bigger. Light can still escape through it, but once the black hole reaches to a size of a point called as an event horizon, the gravitational pull of the black hole reaches to a singularity and the escape velocity of light ( the velocity needed to escape from the gravitational pull of the black hole-- escape velocity on Earth is 11.2km/s) tends to infinity and thus even light cannot escape such an enormous gravitational force. The event horizon is also called as the point of no return.Dust and gases are sucked in a spiral manner thus creating a disc around the black hole, called an accretion disc.

  • When a spaceship falls into a black hole, as it goes near and near the difference in the gravitational pull on the front of the ship and the rear of the ship increases. When this happens the pull in front of the ship is greater than that in the rear, this stretches the spaceship. This continues till the spaceship is torn into shreds. This process is called as spaghettification.
  • Now, for the scientific definition if needed is that,spaghettification (sometimes referred to as the noodle effect[1]) is the vertical stretching and horizontal compression of objects into long thin shapes (rather like spaghetti) in a very strong non-homogeneous gravitational field; it is caused by extreme tidal forces. In the most extreme cases, near black holes, the stretching is so powerful that no object can withstand it, no matter how strong its components. Within a small region the horizontal compression balances the vertical stretching so that small objects being spaghettified experience no net change in volume.

An Astronaut stretched due to Spaghettification.
An Astronaut stretched due to Spaghettification. | Source
The diagram showing the nature of a Black Hole.
The diagram showing the nature of a Black Hole. | Source
The gravity of the black hole bends the curvature of space to a single point-- a singularity.
The gravity of the black hole bends the curvature of space to a single point-- a singularity. | Source

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