What happens when a star collides with another star?

It depends on several factors. The size and kind of star and the rate of speed at which they collide. Two binary neutron stars can cause a black hole to open up. Some stars may actually merge and become hotter and larger than the two stars by themselves were. Another possibility is that a star collision could result in a supernova with a nebula as the result, which could trigger nearby intergalactic clouds to produce star births. Wormholes do not normally, if ever, cause star collisions. Wormholes are too small to influence stars. A black hole, an aggregate of wormholes, are not known to cause star collisions either. It takes great speed for stars to overcome their gravitational forces to collide. This happens, rarely to sometimes, in binary and multiple star systems as stars orbit one another.