Take one part Spitzer Space Telescope, one part Hubble Space Telescope, and two galaxies. Shake well, bake covered for a few million years, and get this:
Coooool. This is NGC 6240, what used to be two galaxies but are in the process of becoming one. We see colliding galaxies all over the sky, but what makes this one special is the timing and its location. Collisions take hundreds of millions of years, starting from the first tentative approach to the complete merging of the two. But the different steps of the process take different amounts of time. The initial approach takes a long time, for example, so we see lots of those. The actual physical merging also takes many millions of years, so we commonly see that as well. But while the outer parts of the galaxy are interacting, so are the cores. The time between the outer parts settling ...
