  We show how the properties of the QCD axion change in systems at finite
baryonic density, such as neutron stars. At nuclear saturation densities, where
corrections can be reliably computed, we find a mild reduction of the axion
mass and up to an order of magnitude enhancement in the model-independent axion
coupling to neutrons. At moderately higher densities, if realized, meson (kaon)
condensation can trigger axion condensation. We also study the axion potential
at asymptotically large densities, where the color-superconducting phase of QCD
potentially leads to axion condensation, and the mass of the axion is
generically several orders of magnitude smaller than in vacuum due to the
suppressed instantons. Several phenomenological consequences of the axion being
sourced by neutron stars are discussed, such as its contribution to their total
mass, the presence of an axionic brane, or axion-photon conversion in the
magnetosphere.
