It's hard to imagine a bigger catastrophe than colliding galaxies.
Picture it: Two behemoths, each a hundred thousand light years across and with hundreds of billions of stars, approaching each other at hundreds of kilometers per second. Even before the actual collision the destruction is wreaked on a colossal scale, the gravity of each distorting and stretching the other like taffy, flinging away stars in long, sweeping arcs sometimes far larger than the galaxies themselves.
Then the actual collision. Sometimes the galaxies pass through each other, but their gravity pulls them back together, and, after hundreds of millions of years of chaos and turbulence, they merge into a single, larger galaxy.
Of course, a picture is worth 1k words:
The Antennae Galaxies, a pair of colliding spiral galaxies 45 million light years away. Credit: NASA/ESA/ESO/NAOJ, Rolf Olsen and Federico Pelliccia
[Note: I had to shrink the image to fit the blog; the highest-resolution (3800 x 3200 pixel) image is a jaw-dropper.]
That is NGC 4038 and 4039, a pair of galaxies caught mid-collision. They are also called the Antennae Galaxies due to their resemblance of insect antennae, though I've always thought they look more like a bird flying directly away....