turn-off point

A globular cluster's or open cluster's turn-off point (or main-sequence turn-off point or just turn-off) is a point in the cluster's H-R diagram (HRD) main-sequence beyond which there are relatively few stars (i.e., very few stars hotter, bluer, and brighter) even though a random population of main sequence stars would extend further across the diagram. The turn-off point is a sign of the age of the cluster: assuming all the cluster's stars began their life at roughly the same time, then it is expected that the hot and bright stars, which have short lives, are all past their main sequence, and the age of the cluster should be just shorter than the longest expected main-sequence lifetime of stars still in the cluster.

Blue stragglers (BSS or BS) are the stars that are exceptions to the turn-off point: they are hotter and more luminous than the general run of stars in the cluster, but (as suggested by the word straggler) there are fewer than one would find in a younger cluster. There is still a question as to how the blue stragglers came to be there. It is generally held probable that they result from interactions between stars, such as binary stars. Stars might actually merge, or mass could be transferred from one star to the other, significantly increasing its luminosity later in its life.

The terms yellow straggler (YSS or YS) and red straggler (RSS, RS) refer to other groupings of stars not in the usual places in stellar cluster HRDs, also in the vicinity of the turn-off point. An interpretation is they are former blue stragglers on their way to the red-giant branch.