  An effect of a magnetic field of an arbitrary strength on the beta-decay and
reactions related with it by the crossing symmetry (the beta-processes) in
supernovae and accretion disks around black holes is analyzed. Rates of the
beta-processes and the energy and momentum transfered through them to an
optically transparent matter are calculated. It is shown that the macroscopic
momentum transferred to the medium increases linearly with the magnetic field
strength and can substantially affect the dynamics of supernovae and accretion
disks especially when a matter inside is degenerate. It is also demonstrated
that the rates of the beta-processes and the energy deposition in these
reactions for the magnetic field strength $B \lesssim 10^{15}$ G, which is
assumed to be typical in supernovae and accretion disks, are lower than in the
absence of the field. This suppression is more pronounced for reactions with
neutrinos.
