  We study the collective density modes which can affect neutron-star
thermodynamics in the baryonic density range between nuclear saturation
($\rho_0$) and $3\rho_0$. In this region, the expected constituents of
neutron-star matter are mainly neutrons, protons and electrons ($npe$ matter),
under the constraint of beta equilibrium. The elementary excitations of this
$npe$ medium are studied in the RPA framework. We emphasize the effect of
Coulomb interaction, in particular the electron screening of the proton plasmon
mode. For the treatment of the nuclear interaction, we compare two modern
Skyrme forces and a microscopic approach. The importance of the nucleon
effective mass is observed.
