When the star stops burning because heavier elements like Iron are formed in its core. Then the gas pressure stops and as you know the gas pressure helps keep a star in equilibrium because it provides pressure against the force of gravity. So Iron does not give off energy. So what stops the star from collapsing? So when the core starts becoming dense due to gravity it stops becoming dense due to Pauli's Exclusion principle. Fermions (a class of particle including Protons, Neutrons and electrons) of the same type obey the Pauli exclusion principle . In layman's terms, it says that such particles cannot occupy the same, small volume of space. Thus, if you try to force them into a small volume of space, they "push back". This "pushing" creates a pressure called "degeneracy pressure", and is what keeps white dwarfs from collapsing into black holes. So does that mean that the same fermions like electrons cant occupy the same sub shell? Does having the same quantum state mean that the fermions cant be at the same sub-energy level. Define in laymans terms quantum state? How is it different from sub shell? Also I read that the star does not collapse into a black hole due to quantum mechanical effects. So is the degeneracy pressure the so called quantum mechanical effect?