What causes blue stragglers in globular clusters?
It is thought that blue stragglers are caused by colliding stars.
The stars in globular clusters follow a distinct pattern on the Hertzsprung Russell diagram. Blue stragglers don't follow the pattern as they are too big.
The most likely theory is that they are formed when two stars collide. This can happen when binary stars collide. Blue stragglers are often found in the denser areas of clusters where stars are quite close together and more likely to collide.