Neutron stars are strange things. They can form when gravity kills a star, crushing its remains into a dense ball the size of a small city. They are so dense that only quantum forces and the Pauli exclusion principle keeps it from collapsing into a black hole singularity. The interior of a neutron star is so dense that matter behaves in ways we still don’t fully understand.
They are called neutron stars because their gravity destroys the structure of atoms. Electrons are squeezed into protons to create neutrons. Much of the star’s interior becomes a sea of neutrons as dense as the nuclei of atoms. But we know these stars aren’t purely made of neutrons. They have atmospheres only a few centimeters thick. Young neutron stars have a sky of mostly carbon and as dense as diamonds. Like Earth, neutron stars have a rigid crust that floats on a fluid interior. This crust is made of iron nuclei. It actively changes, and can undergo starquakes, much like earthquakes on our world.
But it is the deep interior where things get strange. Although the interior of a neutron star is extremely hot, the density is so high that the neutron sea becomes superfluid. Its behavior is similar to that of liquid helium when cooled to only a couple degrees above absolute zero. The fluid interior can generate tremendous magnetic fields, turning these stars into magnetars and pulsars.
We can’t observe the interior of a neutron star directly, so our understanding of them depends upon our understanding of its equation of state. For neutron stars, this is given by the Tolman–Oppenheimer–Volkoff (TOV) equation. While this equation can work well for regular stars, it poses a challenge for neutron stars because neutrons aren’t fundamental particles.
Neutrons are made of three quarks, two down quarks and one up quark. Up and down quarks are only two of the six types of known quarks. In our everyday lives, and even in the hearts of stars, the quarks of a neutron stick tightly together. For all practical purposes, a neutron can be treated like a simple particle. But in the core of a neutron star, things get complicated. Tightly packed neutrons might melt into a fluid of quarks, and when up and down quarks collide at high energies they might produce other quarks such as strange or charm. Or they might not.
To answer this question, a recent study compared the physics of quarks with observed neutron star properties. The study started with a detailed theoretical calculation of the properties quark matter would have. One of these properties involves the speed of sound in quark matter. Since pressure waves from things such as starquakes travel at the speed of sound, it plays a crucial role in the structure of neutron stars.
In turns out that in pure quark matter speed of sound is independent of the temperature and pressure of the material. This is not true of neutron matter. Given some reasonable assumptions about neutron star interiors, pressure waves in the deep interior of a neutron star could free quarks from their neutrons, creating a quark core. The size of this core depends upon the total mass of the neutron star.
The authors note that there is some small possibility that neutron stars don’t have quark cores, but there is other evidence to support the idea. Recent gravitational-wave observations of a merging neutron star confirm that its size agrees with the quark model. Astronomers have also recently found several neutrons stars with a mass greater than two solar masses. These large-mass neutron stars are much more likely to have quark cores.
While further studies are needed to confirm this result, it seems clear that the interior of neutron stars have much more structure than was earlier thought.
Reference: Annala, Eemeli, et al. “Evidence for quark-matter cores in massive neutron stars.” Nature Physics (2020): 1-4.