The formation of blue stragglers

This illustration demonstrates the two ways that blue stragglers in star clusters form. 

The illustration on the left shows the collision model where two low-mass stars in an overcrowded environment experience a head-on collision, combining their fuel and mass to form a single hot star. 

The illustration on the right depicts the "vampire" model consisting of a pair of stars that undergo a transformation, with the lower-mass star draining its larger-mass companion of the hydrogen that fuels its rebirth.