We examine the thermal state of the neutron star in the Cassiopeia A supernova remnant using the recent discovery that the thermal X-ray radiation seen from this star originates from a carbon atmosphere (and the emission arises from the entire stellar surface). Utilizing neutron star cooling theory, we formulate a robust method to extract neutrino cooling rates of thermally-relaxed stars (i.e., those with isothermal interior temperatures) from observations of surface thermal radiation. We show a comparison of these rates with the rates of standard candles — stars with non-superfluid nucleon cores cooling slowly via the modified Urca process. We demonstrate that the data on the Cassiopeia A neutron star can be understood in terms of three parameters: f_l, the neutrino cooling efficiency with respect to the standard candle; the stellar compactness x=2GM/Rc²; and the amount of light elements in the heat-blanketing envelope. For an ordinary (iron) heat blanketing envelope, we obtain f_l∼ 1—0.25 (implying a standard cooling efficiency) at x <∼ 0.5 and slower cooling, f_l ∼ 1/50, at maximum compactness x≈0.7. A heat blanket containing the maximum mass of light elements (∼10⁻⁸ M_Sun) increases f_l by a factor of 50. Finally, we consider the (unlikely) scenario where the neutron star still has not thermally-relaxed.

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