| A good question, and mostly beyond my pay grade. Almost certainly based on models rather than direct observation, though informed by the latter. I'm struggling to find a strong source, but this page, by M. Coleman Miller at the University of Maryland, describes properties suggesting modelling over observation. Specifically it describes "the guts of a neutron star": Even further down, you mainly have free neutrons, with a 5%-10% sprinkling of protons and electrons. With an subsequent 'graph: Yes, you may say, that's all very well for keeping nuclear theorists employed, but how can we possibly tell if it works out in reality? Well, believe it or not, these things may actually have an effect on the cooling history of the star and their spin behavior! What follows is more description of theory with a few points based in what is directly observable, largely spin (via radio astronomy) and some temperature observations largely in X-ray observations, and the occasional gamma-ray burst. <https://www.astro.umd.edu/~miller/nstar.html> Miller's bio at UMD emphasizes his theoretical work: Cole Miller's research in the last few years has focused on theory and modeling of high-energy radiation from neutron stars and black holes. <https://www.astro.umd.edu/people/miller.html> |