blue straggler

Blue straggler. A schematic Hertzsprung-Russell diagram for a typical globular cluster, showing the location on the diagram of blue stragglers. A star that appears to belong to a globular cluster or an old open cluster but has an anomalous blue colour and high luminosity in comparison with other cluster members. When the stars of such a cluster are plotted in a Hertzsprung-Russell diagram (see illustration), there is a distinct turn-off point on the main sequence. This point marks the lower mass limit of the stars that have evolved into red giants so that they lie to the right of the main sequence. In some clusters, a small number of stars appear to lie on the main sequence above this turn-off point - these are the blue stragglers. The reasons for their anomalous properties are not fully understood, but a range of explanations might account for the observation, including binary star membership.