How do blue stragglers form?
There are several theories as to how blue stragglers form.
A blue straggler is a large star in an open or globular cluster which doesn't fir the size range of the stars in the cluster. They are typically 2 to 3 times the size of other stars.
One theory as to how they got there is that they were captured by the cluster. This doesn't seem likely.
The prevailing theory is that a blue straggler is forms when two stars collide and merge. These could be binary stars which collide. As blue stragglers often occur in the densely packed areas of clusters it is likely that they are caused by stars colliding.