  Many massive stars travel through the interstellar medium at supersonic
speeds. As a result they form bow shocks at the interface between the stellar
wind. We use numerical hydrodynamics to reproduce such bow shocks numerically,
creating models that can be compared to observations. In this paper we discuss
the influence of two physical phenomena, interstellar magnetic fields and the
presence of interstellar dust grains on the observable shape of the bow shocks
of massive stars.
  We find that the interstellar magnetic field, though too weak to restrict the
general shape of the bow shock, reduces the size of the instabilities that
would otherwise be observed in the bow shock of a red supergiant. The
interstellar dust grains, due to their inertia can penetrate deep into the bow
shock structure of a main sequence O-supergiant, crossing over from the ISM
into the stellar wind. Therefore, the dust distribution may not always reflect
the morphology of the gas. This is an important consideration for infrared
observations, which are dominated by dust emission.
  Our models clearly show, that the bow shocks of massive stars are useful
diagnostic tools that can used to investigate the properties of both the
stellar wind as well as the interstellar medium.
