Undergrad What causes pressure in White Dwarfs and Neutron Stars?

[Ethan Singer]

Before today, I'd been led to believe that collections of only one (or few) types of matter could never exist: And to my surprise it is indeed possible to have degenerate matter, resulting from the death of less massive stars! So I'm curious: If these lesser massive stars collapse due to their inability to maintain fusion, how is it that these massive collections of degenerate matter can even exist? How is it possible that you could have (near) pure degenerate matter on such an immense scale?

This is an interesting dilemma, because the next obvious question is what causes pressure in these stars? Supposedly the pressure originates from the Pauli-Exclusion principle alone? But considering the size of it's constituent particles, how can these stars have such a large volume?

 

[davenn]

This is an interesting dilemma, because the next obvious question is what causes pressure in these stars? Supposedly the pressure originates from the Pauli-Exclusion principle alone?

have a read of this and see if it helps ...

https://en.wikipedia.org/wiki/Degenerate_matter

failing that maybe @Ken G can help

 

[nikkkom]

Before today, I'd been led to believe that collections of only one (or few) types of matter could never exist

What does this mean?

the next obvious question is what causes pressure in these stars?

As in any other type of star: gravity.

 

[Chronos]

That stars exist at all tells you some important facts about them. Gravity causes them to coalesce and shine and some repulsive force is necessary to prevent them from collapsing. At normal temperature electrostatic repulsion is sufficient to keep particles apart from one another. For a white dwarf electrostatic repulsion is insufficient to resist the gravity drawing particles together. At this point electron degeneracy pressure emerges to maintain the personal space between particles. Should gravity succeed in overwhelming electron degeneracy pressure, as is the case for neutron stars, neutron degeneracy pressure emerges to guard the privacy rights of particles. We do not yet know if there are any additional forces to preserve the sovereignty of particles. Ostensibly, a singularity [aka black hole] results when gravitational force exceeds neutron degeneracy pressure. It is certainly possible forces beyond our current understanding conspire to preclude compression of matter to infinite density. Nature tends to abhor infinites.

 

[Ken G]

The internal structure of stars would not be much different if there were no electrostatic forces, because they are screened out on larger scales, so I don't think electrostatic repulsion has much role in the transition from ideal-gas to degenerate stars. What prevents stars from contracting is gas pressure, and gas pressure depends on the kinetic energy of the particles, not their charge.

The problem with the phrase "degeneracy pressure" is it makes it sound like degeneracy is affecting the pressure somehow, but that's false. What "degeneracy" affects is the way kinetic energy is partitioned among the particles at any given pressure. Importantly, there is a partition that can be reached which renders the gas unable to lose any more heat, and that partition is called "degeneracy," for essentially unknown reasons (it has nothing to do with the concept of degenerate states, in fact it's much more like the opposite of that). The thing to remember is that "degeneracy pressure" actually means "the gas pressure achieved when the gas, due to quantum mechanical effects, can no longer lose heat." The reason this prevents further contraction is that contraction is always caused by heat loss, so when heat cannot be lost, the star cannot contract. So the answer to "what causes pressure" in these stars is: kinetic energy. The answer to "what prevents contraction" in degenerate stars is: an inability to lose heat. Degeneracy does not cause pressure, it causes an inability to lose heat.