White Dwarfs and Electron Degeneracy Pressure

White dwarfs are formed when any star having mass less than 20 times that of our sun dies. This limit on the star’s mass is called Chandrashekhar's limit. such stars do not end their lives in an explosion called a supernova, rather they die very peacefully.

Now, you may ask how and why lighter stars die peacefully leaving a white dwarf at the end?

when the star is in its stable period then the gravitational pull and pressure from fusion reactions balance each other and the star remains in equilibrium. however, when the star burns almost all of its fuel, Then very few fusion reactions are going on and the core gets accumulated by heavier elements e.g. Carbon, Oxygen & Iron. In this situation after a certain radial distance from centre thermal pressure from fusion reactions is greater than the gravitational attraction, so they seep out into space.

For the materials below the radius ‘r’, the pressure from fusion is not enough to escape the gravitational pull and they are pulled to the surface of the high-density core & the core starts to collapse, but at one point atoms gets so tight that the gravitational pull and the electronic repulsions balance each other. Here the collapse stops and the remanence is known as a White Dwarf.


Now, in order to understand electron degeneracy pressure, we need to know the Pauli exclusion principle. It states that no two electrons with the same spin can occupy the same energy state in the same volume. Once the lowest energy level is filled, the other electrons are forced into higher and higher energy states resulting in them travelling at progressively faster speeds. These fast moving electrons create a pressure (electron degeneracy pressure) which is capable of supporting a star. 

In particular, electron degeneracy pressure is what supports white dwarfs against gravitational collapse, and the Chandrasekhar limit (the maximum mass a white dwarf can attain) arises naturally due to the physics of electron degeneracy. As the mass of a white dwarf approaches the Chandrasekhar limit, gravity attempts to squeeze the star into a smaller volume, forcing electrons to occupy higher energy states and attain faster velocities. At the Chandrasekhar limit, the pressure exerted by the electrons travelling at close to the speed of light becomes insufficient to support the star, and the white dwarf collapses into a much denser state.