By Marina Koren
Gravity can do some pretty astonishing things out there in the universe. When it’s not ensuring the downward trajectory of your spilled coffee directly onto your shirt here on Earth, the invisible force is playing arts and crafts with cosmic matter: crushing gas and dust into radiant new stars, smoothing clumpy rock into spherical planets, and, my personal favorite, smushing entire galaxies together. Gravity nudges galaxies toward one another—sometimes two, sometimes more—until they meet, their contents whooshing and mixing, and the slow-moving chaos molds them all into one big galactic ball.
Astronomers have observed such events, known as mergers, in nearly every stage of the process. Early on, when the galaxies are clustered together, as if they’re convened for a very important space conference. In the thick of it, when gravity has begun to stretch them out of their original shapes. And at the end, when what remains is a messy sphere. By then, the only sign that a merger once occurred is a faint shimmer of stellar material around the orb.
The latest image in the catalog, shown at the top of this article and taken by the Gemini North telescope in Hawaii, captures the beginning of a merger in sparkling detail. The two galaxies involved—NGC 4567 at top, and NGC 4568 at bottom—will swing around each other, jostling existing stars and sparking new ones, until everything coalesces in about 500 million years. For now, they almost look like a little paper heart.
Galaxy mergers are some of the most imagination-sparking events in the universe. Sure, supernovas are cool, and so are collisions of black holes. But galaxy mergers have all of that and more. (Zoom into this shot from Gemini North and you’ll spot the afterglow of an exploding star.) Galactic collisions also provide great material for daydreams about extraterrestrial life far beyond Earth. Consider that NGC 4568 and NGC 4567 are full of stars, and most stars, as we’ve learned from observations of our own galaxy, have planets. That means—if we could fantasize for just a bit—the NGC galaxies could be, to someone else, home. What might it be like to exist in the midst of a galactic merger?
The nighttime view “would be quite spectacular,” Vicente Rodriguez-Gomez, an astronomer at the National Autonomous University of Mexico, told me. “The sky would be filled with newly formed stars, and we would be able to see warped streams of stars, gas, and dust stretching across the sky.” The view would be especially stunning if you lived along the outer edges of the galaxy, where the night sky would be less crowded with stars than at the busy galactic center. Here, at the outskirts, the other galaxy merging with your own would gleam in the darkness, bigger and brighter than any star. The big ol’ galaxy hanging in the darkened sky would simply be a fact of your existence, just like there’s a cratered moon in ours.
Even more exciting, you could take in that view mostly unperturbed, because, despite the galactic jumble, the possibility of your sun smashing into another would be extremely unlikely, Moiya McTier, an astrophysicist and the author of The Milky Way: An Autobiography of Our Galaxy, told me. “Have you ever watched a really good marching band, a performance where two groups walk through each other?” she asked me. That’s how stars move in galaxy mergers, passing one another as seamlessly as uniformed musicians gliding across the grass. There will be more stars around, but space is still, well, spacious, and “most of these stars are not in danger of colliding with something else,” McTier said. (If your planet was too far out in the boondocks, however, you might be in trouble: The hustle can untether stars from the very edges of a galaxy and fling them out into the depths of intergalactic space.)
Even as the stars file neatly past one another, the space between them can get a bit chaotic. “Galaxies have huge clouds of gas and dust in them, and then when galaxies interact, those huge clouds of gas and dust will collide,” Jeyhan Kartaltepe, an astrophysicist at the Rochester Institute of Technology who studies galaxy formation, told me. The process would create hazy pockets of gas and dust in the night sky that eventually collapse under their own weight and ignite into brand-new stars. Astronomers on Earth can detect these starburst regions in their snapshots of galaxy mergers, such as this one, captured by the Hubble Space Telescope. These are the Antennae Galaxies; they began to collide about 600 million years ago, and their spiral structures have already been smudged away, yielding to clusters of new stars, seen here as light-blue shimmers.
As the stellar stuff of colliding galaxies migrates in, so will the gigantic black holes that scientists believe sit at the centers of most large galaxies. The dense, invisible objects will plow through like invisible meteors, dragging stars along with them (and perhaps swallowing some) as they go. “When two galaxies start to merge, their central supermassive black holes sink to the center of this newly formed galaxy and eventually merge” into a single, bigger black hole, Chiara Mingarelli, an astrophysicist at the University of Connecticut and the Flatiron Institute’s Center for Computational Astrophysics who studies the fates of supermassive black holes in galaxy mergers, told me. In the case of three colliding galaxies, “the likeliest scenario is that two of them will find each other and form a binary system,” she said, and when the third arrives and starts interacting with the others, “the least massive one gets kicked out, and could actually be completely kicked out of the galaxy.” “You could, in the end, have a rogue supermassive black hole that’s just wandering around the universe,” Mingarelli said.
All of this action—the flash of new stars, roving black holes—would unfold over millions, even billions, of years. A hypothetical inhabitant of the NGC galaxies wouldn’t notice any changes in her already brilliant night sky in her lifetime, but she could very well be aware that she’s living within a merger. Hypothetical astronomers in her world could sort through archival observations from earlier generations and collect data for future scientists. They would be able to understand their galaxy’s past, just as astronomers here on Earth have figured out that the Milky Way has previously experienced tiny mergers and absorbed smaller galaxies. And those astronomers could outline the future of their galaxy, too, just as astronomers on Earth have discovered that our own is headed toward a dramatic merger that will reshape everything.
Our Milky Way is on a collision course with another spiral galaxy called Andromeda. Today Andromeda is visible as a speck of light in the night sky, but about 5 billion years from now, it will be tangled up with us. Our galaxy’s spiral arms will disappear, and so will our supermassive black hole. Andromeda’s central black hole has the mass of 100 million suns, and it will quickly swallow up our own, which has a comparatively tiny mass of 4 million suns. “It’s going to be like, bloop, done,” Mingarelli said.
Although mergers won’t result in perceptible changes in one person’s lifetime, they could still offer intriguing opportunities for any astronomers living within them. Being in a lonely, unentangled galaxy can be a disadvantage. For example, Earth’s position in the Milky Way doesn’t present the best views for studying our galactic home; there’s just too much gas and dust in the way, McTier said. “We have to study other spiral galaxies to learn about the behavior and evolution of spiral galaxies like the one we live in,” she said. But “if there was another spiral galaxy that’s much closer to you, and it was angled in such a way that you could see most of it, then you’d be able to study that a lot easier than you can study your own galaxy,” she said.
In other ways, being an astronomer in the middle of a galaxy merger would be frustrating; the night sky would simply be too crowded to observe very distant targets. “In such an environment, it would be difficult to find lines of sight that are uncontaminated by the luminous merging system that would be our home,” Rodriguez-Gomez said. Perhaps someone else has observed the Milky Way from his perch in the thick of a radiant, cosmic collision and exhaled a deep sigh, wondering, in a quiet moment of daydreaming, what it might be like here.