blue stragglersStars that are frequently found in open and globular clusters and that lie on the extension of the main sequence beyond the turnoff point: they are ‘blueward’ of the turnoff in color-magnitude diagrams (see illustration at globular cluster). Because such stars have main-sequence lifetimes much shorter than the age of the cluster, they should already have evolved away from the main sequence. Their existence has been a long-standing puzzle. In one model, they are stars that have been rejuvenated by mass transfer from a binary companion. Alternatively, they may represent the result of the merger of two stars after a stellar collision. Recent observations have provided some support for both scenarios. Some blue stragglers are now known to be members of close binaries. In addition, observations with the HST have revealed large numbers of blue stragglers in the cores of several globular clusters, where collisions between stars are significant. See also tidal capture.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006