  Muon and tau neutrinos ($\nu_x$) interact with protoneutron star matter only
via neutral currents and exchange energy with the stellar gas predominantly by
neutrino-electron scattering and neutrino-pair processes. In contrast, electron
neutrinos and antineutrinos ($\nu_e$ and $\bar\nu_e$) are frequently absorbed
and produced in charged-current mediated reactions with nucleons. Therefore the
emergent $\nu_e$ and $\bar\nu_e$ originate from layers with lower temperatures
further out in the star and are emitted with much lower characteristic spectral
temperatures. In addition, a major contribution to the $\nu_e$ and $\bar\nu_e$
opacities is due to absorptions, while the opacity of $\nu_x$ is strongly
dominated by scattering reactions with nucleons and nuclei in which the $\nu_x$
energy is (essentially) conserved. Therefore the $\nu_x$ distribution is nearly
isotropic when $\nu_x$ decouple energetically and their outward diffusion is
slowed down. In a generalized form to include this effect, the Stefan-Boltzmann
Radiation Law can account for both the facts that $\nu_e$ ($\bar\nu_e$) and
$\nu_x$ emerge from the star with similar luminosities but with very different
characteristic spectral temperatures. Simple analytical expressions to estimate
the effect are given. If, as recently argued, even at densities significantly
below nuclear matter density neutral-current scatterings were associated with
considerable energy transfer between neutrino and target particle, one might
expect spectral temperatures of $\nu_x$ much closer to those of $\nu_e$ and
$\bar\nu_e$. This is of relevance for the detection of neutrino signals from
supernovae.
