Axions are a potential dark matter candidate, which may condense and form self gravitating compact objects, called axion stars. In recent work, we studied head-on collisions of relativistic axion stars with black holes and neutron stars. In the black hole case we find that relatively large scalar clouds are produced by mergers of low compactness axion stars and black holes with intermediate spins, which provides a dynamical mechanism for the formation of long lived scalar hair. For neutron star collisions we find that in the critical limit of black hole formation, the collision gives rise to a large ejection of baryonic mass from the system, purely due to gravitational effects. This could give rise to electromagnetic observables in addition to their gravitational wave signatures.