This isn't entirely true: near the surface of a neutron star, there's a coexistence of neutrons, protons, and electrons in a type of thermal equilibrium. (I'm

This isn't entirely true: near the surface of a neutron star, there's a coexistence of neutrons, protons, and electrons in a type of thermal equilibrium. (I'm embarrassed to say that I don't remember offhand how to estimate the ratios involved, but I do remember my PhD candidacy exam committee looking pretty happy with me for deriving it on the fly during my oral. A few days later, one of them commented to a group of us who'd just finished the exam, "Congratulations: at this moment, you know more physics than you ever have or ever will again". I guess he was right!)