Redshifted temperature as a function of mass density inside a neutron star with M = 1.4 M_⊙ at different ages (marked near the curves) according to different theoretical models. Line styles correspond to theoretical models in the same way as in Fig. 2. The three parts of the figure show the thermal structure of the envelopes comprising the ocean and the outer crust (left), the inner crust (middle), and the core (right) at different density scales. In the gray-cross-hatched domain, the nuclei are arranged in a crystalline Coulomb lattice. The nearly vertical dotted lines in the left part show the position of the outer boundary for the cooling code, beyond which (to the left in the figure) the heat transport problem is solved using quasi-stationary approximation: the left (black) and right (red) lines delimit a quasi-stationary envelope of mass ΔM = 10^-12M_⊙ and ΔM = 10^-6M_⊙, respectively.