Mergers and acquisitions
So what happens inside a bigger merger? While the two groups of stars and dust may have a lot of mass, there’s also a lot of space between objects in a galaxy. This means that while the effects of a merger can be extreme for the structure of the galaxies themselves, they don’t necessarily greatly affect the stars within.
“Most of the time in these mergers, stars move right between each other and really only interact gravitationally,” Moffett says. “Your orbits might get disrupted a little bit because of extra gravitational pull from other things that are around them, but they don’t typically collide.”
But things do get weird — really weird. Just look at the Antennae Galaxies (NGC 4038 and NGC 4039), which are in the process of becoming one. Trails of stars are thrown across thousands of light-years as they move inward, and the galactic cores are moving closer together. Eventually the two objects will settle into a more regular shape, but that’s not today.
In typical mergers between two large galaxies, they first slingshot past each other. This disrupts the dust of the galaxies. After the near miss, they move in close and then move apart once again, but that close pass is enough to start to destabilize their structures. The galaxies then begin the official merger, often appearing as a cloud of stars and dust that settles over time, their former central regions blending and creating a new, more powerful center of gravity.