  We demonstrate the necessity of using realistic stellar models taken from
stellar evolution codes, as opposed to polytropes, for starting models in
smoothed particle hydrodynamics calculations of collisions between main
sequence stars. Evolved stars have mean molecular weight gradients, which
affect their entropy profiles and therefore affect how they react during a
collision. The structure of stellar collision products of polytrope parent
stars is significantly different from that of collision products of realistic
parent models. These differences strongly affect the future evolution of the
collision products, particularly products of collisions between unequal mass
stars which have undergone significant chemical evolution. The use of
polytropes as parent star models is likely to result in qualitatively mistaken
results for the structure of the collision product.
