Galaxies collide. Spiral galaxies swing past each other. Each star only feels the pull of the galactic cores, nothing else, and yet the encounter tears out long tidal tails and bridges of stars, like the ones seen in the Antennae and the Mice. Alar and Juri Toomre showed it this way in 1972, with one of the first computer simulations in astronomy.
Galaxies collide
Galaxies are mostly empty space, so when two of them meet, their stars almost never touch. Gravity alone reshapes them.
How it works
Tidal tails. As two spiral galaxies swing past each other, each one pulls harder on the near side of the other than on the far side. Stars are stripped into long curved tails and bridges, the shapes seen in pairs such as the Antennae and the Mice.
Toomre & Toomre, 1972. Alar and Juri Toomre reproduced these shapes with one of the first computer simulations in astronomy, with a beautifully simple idea: every star feels only the pull of the two galactic centres. That is what runs here, with tens of thousands of stars, plus a gentle braking that lets the two centres finally merge.
Throw your own. Drag on the sky to launch a galaxy: the arrow is its speed. Galaxies that turn in the same sense as their orbit grow long tails; those that turn against it hardly change.