For astronomical bodies, this is typically expressed in terms of how the mass of an object is related to the radius of the object (more fundamentally, the pressure and density). Neutron stars are excellent laboratories for nuclear physics because we need extreme physics to explain how we observe very massive, very compact objects. Pulsar timing provides precision measurement of pulsar masses while instruments like NICER will be able to precisely measure the radii of neutron stars, thus telling us about the equation of state and the internal composition of neutron stars.