We explore the thermal state of the neutron star in the Cassiopeia A supernova remnant using the recent result of Ho & Heinke that the thermal radiation of this star is well described by a carbon atmosphere model and the emission comes from the entire stellar surface. Starting from neutron star cooling theory, we formulate a robust method to extract neutrino cooling rates of thermally relaxed stars at the neutrino cooling stage from observations of thermal surface radiation. We show how to compare these rates with the rates of standard candles – stars with non-superfluid nucleon cores cooling slowly via the modified Urca process. We find that the internal temperature of standard candles is a well-defined function of the stellar compactness parameter x=rg/R, irrespective of the equation of state of neutron star matter (R and rg are circumferential and gravitational radii, respectively). We demonstrate that the data on the Cassiopeia A neutron star can be explained in terms of three parameters: fℓ, the neutrino cooling efficiency with respect to the standard candle; the compactness x; and the amount of light elements in the heat-blanketing envelope. For an ordinary (iron) heat-blanketing envelope or a low-mass (≲ 10−13 M⊙) carbon envelope, we find the efficiency fℓ∼ 1 (standard cooling) for x≲ 0.5 and fℓ∼ 0.02 (slower cooling) for a maximum compactness x≈ 0.7. A heat blanket containing the maximum mass (∼10−8 M⊙) of light elements increases fℓ by a factor of 50. We also examine the (unlikely) possibility that the star is still thermally non-relaxed.