The physics of compact stars
Compact stars, as white dwarfs and neutron stars, are the final endpoint in the evolution of ordinary stars. They are extreme astrophysical objects with the largest density encountered in the present universe. Rotation-powered neutron stars are regularly observed as pulsars and have gigantic magnetic fields. Gravity is so strong, that effects from general relativity are important. In fact, the present best test on general relativity in strong fields is provided by the analysis of binary systems of neutron stars which are emitting gravitational waves.
In the lectures we discuss the rich physics of white dwarfs and neutron stars which encompasses all four known basic forces. Electromagnetic interactions lead to the lighthouse effect of pulsars. Weak interactions control the cooling of neutron stars. The mass and radius of neutron stars are determined by an interplay of strong interactions (quantum chromodynamics QCD) and gravity. In addition, methods from quantum statistics and thermodynamics, plasma physics and even solid state physics have to be adopted for describing the properties of compact stars. White dwarfs and the crust of neutron stars have a lattice structure. The equation of state for white dwarf material and neutron star matter is computed with quantum statistical methods and used as input for the structure equations of compact stars, the Tolman-Oppenheimer-Volkoff equations of general relativity.
We give an outlook to new physics being probed by compact stars. The inner core of neutron stars contains QCD matter at extreme baryon densities. It is expected that there are new phases appearing, probably with a first order phase transition. The dense interior of compact stars might be filled with exotic matter be it in the form of hyperons, Bose condensates or a plasma of quarks and gluons. Compact stars might exist which are just made of quarks alone, so called strange stars. QCD matter at high baryon density relevant for compact stars will be explored with the FAIR accelerator facility at GSI Darmstadt in the near future. We outline the possible signatures for the presence of a new form of matter in compact stars by the emission of neutrinos, gravitational waves or gamma-rays and its relevance for future experiments on earth and in space.