Tidal tails look violent, yet the real shock is how little actually hits. Two disk galaxies can interpenetrate, rip out star streams longer than their own diameters, and still not register a single star-on-star impact.
The key is scale. Stars behave less like packed gravel and more like a sparse swarm, so dilute that direct contact is statistically negligible. In a typical galactic disk, mean stellar separation dwarfs stellar radii by factors of millions, turning a head-on merger into an exercise in gravitational interaction rather than demolition. Newtonian gravity and conservation of angular momentum run the show: as each galaxy’s mass distribution tugs on the other, their combined potential distorts, pulling out elongated tidal structures that carry stars and gas far into intergalactic space.
What looks chaotic is, in fact, highly organized. N-body simulations track these encounters by integrating the gravitational forces between millions of particles, and they consistently show the same pattern: orbits reshuffled, gas clouds shocked, black holes driven inward, yet individual stars mostly glide past in ballistic trajectories. Collisions do occur, but mainly among gas clouds, where hydrodynamic pressure and radiative cooling matter, igniting bursts of star formation while older stars pass by like cars on different levels of a highway interchange, close in projection, distant in reality.
