Density of the circumstellar medium in  g cm^-3 after 20 000 (left), 50 000 (centre) and 100 000 (right) yr, for a simulation without an interstellar magnetic field (right side of each panel) and a 3 μG interstellar magnetic field (left side of each panel). The field lines for the latter model are also shown. In both simulations RT instabilities form initially at the contact discontinuity ahead of the star. For the non-magnetic model these instabilities grow as they travel downstream, distorting the shape of the bow shock. As they grow, they take on characteristic of KH instabilities. In the presence of the interstellar magnetic field the instabilities shrink as they move past the star, and there is no sign of KH instabilities.