  Massive neutrinos can have helicity $s_{\parallel}\neq -1$. Neutrino helicity
changes when the neutrino interacts with an external magnetic field and it is
possible that the left-handed neutrinos born inside the Sun or a supernova
could leave their sources with a different helicity. Since Dirac and Majorana
neutrinos have different cross sections in the scattering on electrons for
different neutrino helicities, a change in the final neutrino helicity may
generate a different number of events and spectra in terrestrial detectors when
astrophysical neutrinos have travelled regions with strong magnetic fields. In
this work, we show that looking for these effects in solar neutrinos, it could
be possible to set bounds in the neutrino properties such as the neutrino
magnetic moment. Furthermore, for neutrinos coming from a supernova, we show
that even in the case of an extremely small neutrino magnetic moment, $\mu_\nu
\sim 10^{-19}\mu_B$, there will be measurable differences in both the number of
events and in the spectra of Majorana and Dirac neutrinos.
