The interior of a neutron star might harbor exotic states of dense matter, as new particles or new phases can be present at the extreme condition prevailing in the core of compact stars. This new state might be chracterised by the presence of strange particles, hyperons or kaons, or by a completely new phase, commonly dubbed strange quark matter. In any case, the properties of dense matter are encoded in the so-called equation of state which can be read off by studying the global properties of neutron stars, their mass-radius relation, the signal in neutrinos emerging from core-collapse supernovae and the gravitational wave signal from merging neutron stars.
(Figure credit: Illustration: CXC/M. Weiss)