# Why are more massive white dwarfs smaller?

I understand that a white dwarf is supported by the Pauli Exclusion Principle and that the larger the gravitational force against them, the closer the electrons must pack.

But I have two queries: One is that for more massive white dwarf stars, there would be more particles present in the star's core, which should take up more space. Is there any way in seeing why this isn't a significant effect?

Also, we haven't studied quantum mechanics and the Pauli Exclusion Principle in detail so it seems weird to me that electrons can just pack closer. This is my understanding, the Pauli Exclusion Principle means no two electrons with the same spin can share the same quantum state. For this effect to produce a force, wouldn't it mean that the electrons were already the closest they could be, as any closer they would be in the same quantum state. How does the Pauli Exclusion Principle work to counteract gravity here?

• This question appears to be primarily about white dwarfs, but you mention neutron stars in the first sentence, which is a little confusing. Apr 25, 2019 at 3:01
• I have edited my question. Apr 25, 2019 at 9:51
• Closely related: physics.stackexchange.com/questions/141865/… Apr 25, 2019 at 10:01
• And almost a duplicate of physics.stackexchange.com/questions/459970/… Apr 25, 2019 at 10:07

As for why a more massive white dwarf becomes smaller, this is just a consequence of the particular equation of state. If the pressure is proportional to density to the power of $$\alpha$$ with $$4/3 <\alpha < 2$$, then the equilibrium radius is a decreasing function of mass. Essentially, the requirement of an increased pressure gradient to support the increased weight can only be satisfied by a decrease in radius, because the central pressure does not increase enough with density. As a consequence of being supported by electron degeneracy pressure, $$\alpha$$ varies between about 5/3 in a low-mass, low density white dwarf to nearly 4/3 in a more massive, high density white dwarf.