TIDK = Things I Didn’t Know… until I started gathering the knowledge that in another lifetime may have made me a professional astronomer. This may become a long list…
I knew that neutron stars were stellar remnants, leftovers of stars that had finished with nuclear fusion and collapsed under their own gravity to the point where protons and electrons were crushed together to create more neutrons to join those that were there already. So my answer would have been “neutrons” whether you had asked me “what’s the surface made of?” or “what’s the centre made of?”. Neutrons all the way down and through.
As with most things in astrophysics, it’s not that simple. Now I find myself tracking data from an experiment called NICER, which is mounted externally on the International Space Station. NICER stands for Neutron Star Interior Composition Explorer, which tells you that it’s about a lot more than just neutrons.
For an idea of the complexity, and the mystery, just take a look at this diagram: