How do astronomers explain the existence of blue stragglers?
Blue stragglers are thought to be the result of collisions.
Blue stragglers are large stars in open or globular clusters which don't fit the pattern of the other stars. A blue straggler has more than double the expected mass for stars in the cluster.
The leading theory is that a blue straggler is caused by a collision between two stars or a star and some other equally massive object. This could be a close binary star system where the stars collide. Also, blue stragglers tend to occur in densely packed regions where stars are close together and can easily collide.