When two stars collide and merge they form a new star that can stand out
against the background population in a starcluster as a blue straggler. In so
called collision runaways many stars can merge and may form a very massive star
that eventually forms an intermediate mass blackhole. We have performed
detailed evolution calculations of merger remnants from collisions between main
sequence stars, both for lower mass stars and higher mass stars. These stars
can be significantly brighter than ordinary stars of the same mass due to their
increased helium abundance. Simplified treatments ignoring this effect give
incorrect predictions for the collision product lifetime and evolution in the
Hertzsprung-Russell diagram.Comment: 8 pages, 5 figures to appear in the proceedings for "Unsolved
Problems in Stellar Physics", Cambridge, 2-6 July 200