Differential gravity stretches stars into long tidal arms during close passages. The Antennae Galaxies show classic double tails 500,000 ly long.
Colliding gas clouds compress and fragment, triggering massive star formation. Star formation rates can spike 100x normal during mergers.
After multiple passes, orbital energy dissipates. Stars randomize into an elliptical galaxy. The Milky Way–Andromeda merger is expected in ~4.5 Gyr.