We consider how first-order phase transitions in systems having more than one conserved charge (multicomponent systems) differ from those in systems having only one. In general, the properties of the transition are quite different in the two cases. Perhaps most importantly the pressure varies continuously with the proportion of phases in equilibrium, and is not a constant in the mixed phase as in the example of the gas-liquid transition in familiar one-component systems. We identify the microphysics responsible for the difference. In the case that one of the conserved charges is the electric charge, a geometrical structure in the mixed phase is expected. As an example, possible consequences are developed for the structure of a neutron star in which the transition to quark matter in the core occurs. It is also pointed out that the general results pertain to relativistic nuclear collisions in the so-called stopping or baryonrich domain where there are three conserved charges (baryon, electric, and strangeness), and impact the expected phase transition from confined hadronic matter to quark matter as regards signals that are supposedly driven by pressure. The physics discussed here is also relevant to the subnuclear gas-liquid transition that is under study in lower-energy nuclear collisions.

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Neutron stars are studied in the framework of Lagrangian field theory of interacting nucleons, hyperons and mesons, which is solved in the mean field approximation. The theory is constrained to account for the four bulk properties of nuclear matter, the saturation binding and density, compressibility and charge symmetry energy. The cores of the heavier neutron stars are found to be dominated by hyperons and the total hyperon population for such stars is 15-20%, depending on whether pions condense or not. The rho meson, which contributes to the isospin symmetry energy, has an important influence on the baryon populations. Lepton populations are strongly suppressed and charge neutrality is achieved among the hadrons. A possible consequence for the decay time of the magnetic field of pulsars and hence for their active lifetime is mentioned.

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First order Bose condensation in asymmetric nuclear matter and in neutron stars is studied, with particular reference to kaon condensation. We demonstrate explicitly why the Maxwell construction fails to assure equilibrium in multicomponent substances. Gibbs conditions and conservation laws require that for phase equilibrium, the charge density must have opposite sign in the two phases of isospin asymmetric nuclear matter. The mixed phase will therefore form a Coulomb lattice with the rare phase occupying lattice sites in the dominant phase. Moreover, the kaon condensed phase differs from the normal phase, not by the mere presence of kaons in the first, but also by a difference in the nucleon effective masses. The mixed phase region, which occupies a large radial extent amounting to some kilometers in our model neutron stars, is thus highly heterogeneous. It should be particularly interesting in connection with the pulsar glitch phenomenon as well as transport properties.

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