emission from the core. The surface temperature responds to the core cooling and depends on properties of superdense core. All cooling isolated neutron stars whose thermal surface radiation has been detected seem to be at this neutrino cooling stage (or its very end). Therefore, we focus our attention on this stage. At the next photon cooling stage the star is cold; its neutrino emission dies out ($L_\nu \ll L_\gamma$) and the cooling is governed by photon surface emission.

Soon after the cooling starts (in minutes to a year, depending on the internal structure), the core temperature drops to $T \sim 10^9$ K. For this $T$, the internal thermal energy of the star is $\sim 10^{48}$ erg. The cooling theory shows how this heat emerges from the star.

Neutrino emission mechanisms
============================

  ----------------------------------------------------------------------------------------------------------------------
  Model                                       Process                            $Q_{\rm f}$ \[erg cm$^{-3}$ s$^{-1}$\]
  ----------------- ----------------------------------------------------------- ----------------------------------------
  Nucleon matter                ${\rm n \to p+ e +\bar{\nu} \quad                                  $ 
                                          p +e \to n +\nu }$                             10^{26}-3\times 10^{27}$

  Pion condensate    ${\rm \widetilde{N} \to \widetilde{N}+ e +\bar{\nu} \quad                     $ 
                            \widetilde{N} +e \to \widetilde{N}+ {\nu} } $                    10^{23}-10^{26}$

  Kaon condensate    ${\rm \widetilde{B} \to \widetilde{B}+e +\bar{\nu} \quad                      $ 
                             \widetilde{B} + e \to \widetilde{B} +\nu } $                    10^{23}-10^{24}$

  Quark matter         ${\rm d 