Physics Colloquium

Neutron stars are laboratories for dense matter physics. New
observations of neutron stars from sources such as radio pulsars, X-ray binaries, quasi-periodic oscillators, X-ray bursters and thermally-emitting isolated neutron stars are setting bounds to neutron star masses, radii, rotation rates, radiation radii, redshifts, moments of inertia, temperatures and ages. Mass (M) measurements constrain the equation of state at the highest densities and set firm bounds to the highest possible density of cold matter.
Radii (R) constrain the equation of state in the vicinity of the nuclear saturation density and yield information about the density dependence of the nuclear symmetry energy. Laboratory measurements of the neutron skin thickness of Pb and other experiments can extend this
knowledge to lower densities. The most reliable radiation radius estimates currently are achieved through observations of thermal emission from neutron stars, and if supplemented by redshift information from the same source, could yield precision radii. A moment of inertia measurement from a binary pulsar could ultimately yield precise radius estimates since their component masses are known. The largest pulsar rotation rates set upper bounds to the ratio R**3/M, and quasi-periodic oscillations, if associated with the innermost stable orbit, set upper limits to both M and R. Observations of cooling neutron stars up to a million years old shed light on the internal compositions, including their superfluid properties, by constraining the neutrino emission rates.