Intro
“Neutron star” is a useful name and a slightly misleading one. The outer layers are dominated by neutrons. The deepest core may be something else entirely.
What We Think
As you descend, density climbs past the point where neat little neutrons are the obvious lowest-energy configuration. Several possibilities are on the table: superfluid neutrons, the appearance of heavier particles called hyperons, or matter so compressed that quarks stop being confined inside individual particles and roam more freely. Which of these actually happens depends on the same equation of state that governs the star’s structure.
What We Do Not Know
We cannot yet say which exotic phase, if any, sits at the center. Each option changes the predicted relationship between a star’s mass and radius, which is precisely why mass-radius measurements are so valuable: they slowly rule candidates in or out.
Why It Matters
If quark matter exists in stable bulk anywhere, a neutron star core is the most plausible address. Confirming it would be a discovery about the fundamental theory of the strong force, written in starlight.