  Neutrinos with a magnetic dipole moment propagating in a medium with a
velocity larger than the phase velocity of light emit photons by the Cerenkov
process. The Cerenkov radiation is a helicity flip process via which a
left-handed neutrino in a supernova core may change into a sterile right-handed
one and free-stream out of the core. Assuming that the luminosity of such
sterile right-handed neutrinos is less than $10^{53}$ ergs/sec gives an upper
bound on the neutrino magnetic dipole moment $\mu_\nu < 0.2 \times 10^{-13}
\mu_B$. This is two orders of magnitude more stringent than the previously
established bounds on $\mu_\nu$ from considerations of supernova cooling rate
by right-handed neutrinos.
