Blue stragglers

It is becoming clearer that several mechanisms, difficult to distinguish, must be resonsible for the blue straggler phenomenon. It is also highly likely that mroe than one mechanism occurs even within the same cluster to produce blue straggler stars (BSs). There is still some ambiguity about whether BSs are single or double stars, simply because of the possibility that some BSs have merged. In the youngest clusters, perhaps high rotation in single BSs provide support for internal mixing; BSs in young to intermediate-age clusters are likely to receive this mixing support from high magnetic fields; in old-disk open clusters, globulars, and perhaps dwarf galaxies, binary mass-transfer, and binary merger are likely the major causes for the production of BSs, with a contribution fromm binary-binary collisions and coalescence. There is considerable observational evidence of the existence of binaries in these systems. Progress has been certainly made in the last forty years, but BSs remain an intriguing challenge.