The energy loss of a degenerate neutron gas by the "Urca" process is calculated in this paper; it is 7.36×104(T109)8 erg g-1 for a density of 6×1014 g cm-3. It follows from this energy loss that the Urca process alone should have cooled the core of the neutron star created in the type I supernova of 1054 a.d. to a temperature around 5×108 °K. The emission power of the star should then be about one order of magnitude smaller than that of the x-ray source discovered recently in the Crab nebula; the source cannot be interpreted therefore simply as the thermal radiation of the star. This conclusion is consistent with the result of a recent experiment performed by the method of lunar occultation, indicating an angular size of the source comparable to that of the Crab nebula. A more refined experiment performed by the same method should, on the other hand, make it possible to decide whether a neutron star exists in the Crab nebula.