What are blue stragglers?
Blue stragglers are main sequence stars in open and globular clusters which don't fit the expected pattern.
In open and globular clusters, all stars are approximately the same age. As a result, all of the stars in a cluster should lie in a distinct curve on the Hertzsprung-Russell diagram. The shape of the curve is defined by the age of the cluster. Star positions are determined by their initial mass.
Blue stragglers are stars which don't fit the pattern. They lie off the expected curve and have 2-3 times the mass of other stars.
It is not known exactly how Blue Stragglers are formed. A leading theory is that they are formed when two stars collide and merge into a bigger star. This theory is supported by evidence that Blue Stragglers tend to occur in densely populated areas of clusters.