What happens to a star beyond the Chandrasekhar Limit ?

 Chandrasekhar limit and White dwarfs

 What is the Chandrasekhar Limit 

Chandrasekhar limit is the limit beyond which, any star would die in a violent explosion called the supernova and collapse under its own gravity. The value of the Chandrasekhar limit is 1.4 times the mass of our sun. Any star that has a mass below this factor would remain as a white dwarf but if it exceeds this mass, it will turn into more massive objects like a neutron star or a black hole. Well, of course, our sun is expected to retire from its 'service' as a white dwarf after about 5 billion years.

The limit was first discovered by the Indian physicist, Subrahmanyan Chandrasekhar in 1930, who later won the Nobel prize for physics in 1983.

    

Why do stars collapse after a certain point?

We all know that stars expand and glow because of the nuclear fusion reactions that take place in it. With the starting element being hydrogen, it fuses to form helium, helium fuses to form iron and then carbon and so on. Apart from this outward expansion caused by these fusion reactions, stars do possess an inward gravitational pull on themselves that occurs out of the effect on their own mass. That's kind of how black holes and neutron stars are formed.

Electron Degeneracy Pressure 

But what about when the star runs all of its hydrogen fuel and there's no fusion reaction and there's no outward force to withstand this gravitational collapse? Well, there's this thing called electron degeneracy pressure. And what does it do? It withstands the inward pull to collapse into itself. Now, where does this pressure come from is another question? 

PS: from here onwards, quantum physics will be trying to be "the queen bee", so just try to keep up with its hustle ; )

Electron degeneracy pressure occurs when the star uses up all the hydrogen and the inward gravitational pull overtakes the expansion of the star. So now that the star is contracting under its own gravity, the electrons will have to fill in all the energy levels, because they get cramped up in a small volume. The electrons take higher and higher energy levels and get energized. This results in electrons moving at high speeds and energies to creating pressure. 

This outward pressure is called electron degeneracy pressure as it's caused by degenerate electrons. 

This pressure is what supports a white dwarf against its own gravity. But it doesn't support stars that are more massive than the Chandrasekhar limit. Stars with masses 10 or 20 times more than the sun won't be affected by electron degeneracy pressure. And that is why they end up as neutron stars or black holes. 

White dwarfs shine from the remaining heat left off the star. The now-dead star's core would be turning into a white dwarf after it blows off its outer remnants. After a billion years as a white dwarf, it'll turn into a black dwarf which emits no energy (which is still only theoretical). Now in some cases, white dwarfs may attract other matter towards it and gain mass (due to its gravity). This increase in its mass may result in the formation of neutron stars or black holes.