star of bluish colour (and thus hot) that is found in old star clusters and that appears to be lagging behind most of the other stars in the cluster in its evolution toward a cooler, reddish state. Blue stragglers tend to be strongly concentrated toward the centre of the cluster. Various theories have been proposed to account for the anomalous behaviour—for example, a blue straggler is the coalescence of two stars whose combined mass has altered the evolutionary state of the product—but no one theory accounts for all cases.