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What is a Black Hole ?

What is a Black Hole?


As we have discussed before the stability of the star is maintained by the pressure balance between the gravitational inward pull and the radiation pressure. Depending on the mass of the star, its end fate is decided. We have discussed briefly about the white dwarf and the neutron star. But what is the fate of the star when the mass is so high not to be balanced by even the neutron’s degeneracy pressure? BLACK HOLE is the answer. Such high mass stars end their fate by forming a black hole.

black_hole
A black hole accreting matter (www.space.com)

What is a Neutron Star ?

What is a Neutron Star ?

As we have seen in the last section the inward gravitational pull of the material is balanced by the outward radiation pressure to make a star stable. However, when the nuclear fuel inside the star is depleted the inward gravitational pull takes over and the star comes to a collapse state. The star forms a white dwarf when the mass is enough to be balanced by the degeneracy pressure of the electron. But what is the fate of the star when the mass is high enough not to be balanced by the electron’s degeneracy pressure?

The star collapses further, and a stage is reached where the electrons and protons combine to form neutrons and neutrinos. Again according to Pauli’s exclusion principle no two neutrons (fermions) can be in the same quantum mechanical state. The collapse enforces these neutrons to follow Pauli’s exclusion principle. Now, at this stage the neutrons become degenerate. . Now it is these degenerate neutrons that balance the inward gravitational pull. These degenerate neutrons maintain equilibrium with the gravitational inward pull.  Such stars are called as neutron stars.

The size of neutron star is around the size of a city around a radius of 10km. The density of a neutron star is around 1017 kg/m3.  The magnetic field of a neutron star is 1012 Gauss. Some neutron stars do spin around their axis and they are known as pulsars. They spin around at a speed of about 600 rotations per sec

In 1934, Walter Baade and Fritz Zwicky proposed the concept of neutron star. In 1965, Antony Hewish and Samuel Okoye discovered an unusual source of high radio brightness temperature in the Crab Nebula. Finally, in 1967, Jocelyn Bell and Antony Hewish discovered regular radio pulses from CP1919.

neutron star
The first direct observation of a neutron star in visible light. 
The neutron star is RX J185635-3754 (Wikipedia.org)


What is a White Dwarf star ?

What is a White Dwarf star ?

As we have seen in the last section, the inward gravitational pull of the material is balanced by the outward radiation pressure to make a star stable. However, when the nuclear fuel inside the star is depleted the inward gravitational pull takes over and the star comes to a collapse state.

The star collapses, and a stage is reached where the electrons balances the inward gravitational pull. According to Pauli’s exclusion principle no two fermions can be in the same quantum mechanical state. The collapse enforces these electrons (fermions) to follow Pauli’s exclusion principle. Now, at this stage the electrons become degenerate. These degenerate electrons maintain equilibrium with the gravitational inward pull.  Such stars are called as white dwarfs.

The size of white dwarf is around the size of Earth. The density of a white dwarf star is around 109 kg/m3 a million times denser than water. The temperature of a white dwarf is around 30,000K.


white_dwarf
Above Fig shows a structure of a white dwarf (Image credit: cse.ssl.berkeley.edu)


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