We study the collective density modes which can affect neutron-star thermodynamics in the baryonic density range between nuclear saturation ($rho_0$) and $3rho_0$. 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.
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