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In young star clusters, the density can be high enough and the
velocity dispersion low enough for stars to collide and merge with a
significant probability. This has been suggested as a possible way to
build up the high-mass portion of the stellar IMF and as a mechanism
leading to the formation of one or two very massive stars (M > 150
Msun) through a collisional runaway.
I will review the standard theory of stellar collisions, covering both
the stellar dynamics of dense clusters and the hydrodynamics of
encounters between stars. The conditions for collisions to take place
at a significant rate are relatively well understood for idealised
spherical cluster models without initial mass segregation, devoid of
gas and composed of main-sequence stars.
I will discuss the many uncertainties. They include the role of more
realistic cluster structures, the effect of interstellar gas, non-MS
stars and the structure and evolution of merged stars.
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