The Antennae Galaxies started falling into each other some hundreds of millions of years ago. These days, the party is still very much underway, and here you can see what happens when two galaxies slowly, violently become one.
The two galaxies actually get their names from a feature of the collision - the tidal forces of the merger have created long, antenna-like features that can be seen in wide-angle views. Located some 62 million light-years away (or 45 million light-years, depending on who you ask), the galaxies are in a state of constant, furious star production. The initial collision triggered the birth of massive new stars, which have already long since exploded as supernovas.
That's left behind tons of building blocks for new stars, which have accumulated in that deep red cloud of gas in the center of the image. The brightest points in the image are the accumulated matter slowly falling into black holes and neutron stars that are the remnants of the supernovas. These are some seriously massive black holes - some might be 100 times more massive than the Sun.
Unfortunately, we won't get to see the final, merged Antennae Galaxy. Astronomers estimate it will take another 400 million years before they settled into a single elliptical galaxy. Click the image below for a closer look at the full picture, created from images by the Chandra X-ray Observatory, the Hubble Space Telescope, and the Spitzer Space Telescope.