  The early evolution of dense star clusters is possibly dominated by close
interactions between stars, and physical collisions between stars may occur
quite frequently. Simulating a stellar collision event can be an intensive
numerical task, as detailed calculations of this process require hydrodynamic
simulations in three dimensions. We present a computationally inexpensive
method in which we approximate the merger process, including shock heating,
hydrodynamic mixing and mass loss, with a simple algorithm based on
conservation laws and a basic qualitative understanding of the hydrodynamics of
stellar mergers. The algorithm relies on Archimedes' principle to dictate the
distribution of the fluid in the stable equilibrium situation. We calibrate and
apply the method to mergers of massive stars, as these are expected to occur in
young and dense star clusters. We find that without the effects of microscopic
mixing, the temperature and chemical composition profiles in a collision
product can become double-valued functions of enclosed mass. Such an unphysical
situation is mended by simulating microscopic mixing as a post-collision
effect. In this way we find that head-on collisions between stars of the same
spectral type result in substantial mixing, while mergers between stars of
different spectral type, such as type B and O stars ($\sim$10 and $\sim$40\msun
respectively), are subject to relatively little hydrodynamic mixing.
