When cars crash, you’d hardly describe it as ballet, but when galaxies do, the results are a lot prettier. All galaxies are on the move, whether that’s within their own local groups or carried away from each other by the expansion of space in our ever-expanding universe. Galaxies by the way don’t actually fly away from each other the way they’re sometimes shown. Instead, new space opens in the gulf between them, increasing their distances from one another to make it look as if they were hurrying away. To be clear, we’re not talking about galaxies that hang together in a cluster — they have their own random motions — but galaxies and clusters of galaxies so far apart that they’re affected by cosmic expansion.
Sometimes, as seen in this spectacular Hubble image of Arp 256, galaxies relatively near one another can collide in a crash of cosmic proportions. 350 million light-years away in the constellation of Cetus the Sea Monster, a pair of barred spiral galaxies have just begun a magnificent merger. We see them frozen in time because it takes billions of years for gigantic entities like galaxies to approach, orbit about each other and then finally merge to form a single larger galaxy. From the human perspective, an entire 80-plus year lifetime is no more than a second on the countdown clock to a galactic merger.
Already though, the intertwining gravitational pulls of each galaxies have churned up a froth of gas, dust and stars between them and distorted their shapes. The upper galaxy has long ribbons of gas, dust and stars called tidal tails. The word “tidal” comes from tides: the same way the moon tugs on Earth’s oceans and land to create tides, each galaxies raises tides in the other, warping themselves into bizarre shapes.
Often when galaxies collide, clouds of molecular gas slam together, collapse and create dazzling new clusters of hot, blue stars. Look closely at the photo and you’ll see this blue confetti everywhere. If you lived on a planet orbiting a star in a galactic merger and lived a very long time, I’m sure you’d find all this excitement appropriate to moving into your new home, a bigger, re-made galaxy.
Although large, massive objects like clouds collide, stars rarely if ever do in mergers because there’s simply too much space between them.
Arp 256 was first cataloged by astronomer Halton Arp in 1966 as one of 338 galaxies in his Atlas of Peculiar Galaxies. Many amateurs have a copy of the book as it makes a wonderful guide to tracking down these oddities with a telescope under a dark sky. The catalog’s goal was to compile examples of the weird and wonderful structures found among nearby galaxies and as a guide to the different stages of galactic evolution.
Long ago, when the universe was much smaller, interactions and mergers were more common. As astronomers continue to leapfrog across both distance and time with the largest telescopes, they’ve discovered mammoth galaxies in rich clusters that were born from the merger of not one or two galaxies but many. The galaxies in the Arp 256 system will continue their gravitational dance over the next millions of years. Like flirtatious dancers in a ballet, where the act ends in an intimate embrace, two galaxies shall become one in a final cosmic act.