Isaac certainly had his act together. Impressive what a simple inverse square law and conservation of momentum can do. Of course, if two galaxies or planets or anything collide and fuse, i.e., become one, something has to account for the change in potential energy and that something is an increase in kinetic energy of some of the member stars. I suspect a lot of stars get thrown out into empty intergalactic space. It must be strange living on a planet circling an orphaned star, where there are virtually no other stars in the night sky, just the occasional galactic nebula, and once a year, seasonally, the midnight transit of the receding, but still quite prominent, mother galaxy, entwined in a Sapphic embrace with the one she disowned you for.
It wouldn't be strange at all, if that's all you have ever known.
Ever read the story "Nightfall" (Isaac Asimov)? It takes place on a world that's in the opposite situation as the one you just proposed. Excellent story, too.