  In young dense clusters repeated collisions between massive stars may lead to
the formation of a very massive star (above 100 Msun). In the past the study of
the long-term evolution of merger remnants has mostly focussed on collisions
between low-mass stars (up to about 2 Msun) in the context of blue-straggler
formation. The evolution of collision products of more massive stars has not
been as thoroughly investigated. In this paper we study the long-term evolution
of a number of stellar mergers formed by the head-on collision of a primary
star with a mass of 5-40 Msun with a lower mass star at three points in its
evolution in order to better understand their evolution.
  We use smooth particle hydrodynamics (SPH) calculations to model the
collision between the stars. The outcome of this calculation is reduced to one
dimension and imported into a stellar evolution code. We follow the subsequent
evolution of the collision product through the main sequence at least until the
onset of helium burning.
  We find that little hydrogen is mixed into the core of the collision
products, in agreement with previous studies of collisions between low-mass
stars. For collisions involving evolved stars we find that during the merger
the surface nitrogen abundance can be strongly enhanced. The evolution of most
of the collision products proceeds analogously to that of normal stars with the
same mass, but with a larger radius and luminosity. However, the evolution of
collision products that form with a hydrogen depleted core is markedly
different from that of normal stars with the same mass. They undergo a
long-lived period of hydrogen shell burning close to the main-sequence band in
the Hertzsprung-Russell diagram and spend the initial part of core helium
burning as compact blue supergiants.
