Hubble's half-dozen cosmic train wrecks

One of the most fantastic statements that can be made about the Universe is: Sometimes, galaxies collide.

A galaxy is an immense thing, billions or even trillions of stars, uncountable clouds of gas and dust, (usually) a huge halo of dark matter, a central supermassive black hole, all in one place and held together by their mutual gravity. A typical galaxy is tens of thousands of light years across, and some, like our home galaxy the Milky Way, well over 100,000. Mind you, a single light year i...