The Collide
It began, as these things do, with something already in motion.
The Milky Way and Andromeda are twenty-five million light-years apart, but they're moving toward each other at about eleven kilometers per second. That sounds slow until you remember what a kilometer is and how big a light-year is. Eleven kilometers per second is nothing — it's a brisk jog. A fast car. But when you're twenty-five million light-years apart, a brisk jog means you'll meet in about four and a half billion years.
There's a word for this. The Milky Way and Andromeda are mutual infall partners. They've been falling toward each other since the universe was young, and they're the two most massive members of the Local Group. Everything else — the Triangulum Galaxy, the Large Magellanic Cloud, the dozens of dwarf galaxies orbiting both of us — we're all just passengers. We're going along for the ride.
Four and a half billion years is a long time. The Sun will still be on the main sequence. It might even still be habitable, though the increasing luminosity of stars over time means Earth might already be too hot for liquid water by then. Mars, by contrast, might have thawed enough to support it. But that's a different story. The point is that the collision isn't coming tomorrow. It's coming at the end of a very long afternoon.
When they meet, it won't be like the movies. There won't be galaxies slamming together like cars in a crash. Galaxies are mostly empty space. The distance between stars is so vast that even when two galaxies "collide," individual stars almost never hit each other. It's like two swarms of gnats passing through each other. The gnats don't touch. The swarm just changes shape.
But the gas will collide. The vast reservoirs of hydrogen gas that fill both galaxies — the fuel for star formation — those clouds will smash into each other and compress. And when gas compresses, stars are born. The collision will trigger a burst of star formation, perhaps a thousand times the normal rate. The sky in four billion years will be full of newborn stars, brilliant blue things that live fast and die young. It will be one of the most beautiful things in the history of the universe, and almost no one will be around to see it.
Andromeda's structure will be destroyed. Its spiral arms, the elegant pattern that has been turning for ten billion years, will be pulled apart by tidal forces. The Milky Way's disk will be similarly disrupted. The supermassive black holes at the centers of both galaxies — four million suns and one hundred billion suns — will eventually sink toward the center of the combined system through dynamical friction, and then they will orbit each other, and then, over hundreds of millions of years, they will merge. The merger of those two black holes will release more energy in gravitational waves than all the stars in the observable universe have ever emitted in light. The detector we'd need to see it would have to be larger than the solar system.
The result will be a new galaxy. Nobody's sure what to call it yet. Some people say "Milkomeda." Some say "Milkdromeda." In the papers they just call it a elliptical galaxy, because that's what the simulations show — two spirals merging makes something rounder, redder, more elliptical. The new galaxy will have few spiral arms. It will have old stars. It will have gas — the simulations suggest a significant fraction of the gas will be expelled, swept out by the merger's tidal forces and by supernova explosions. The star formation that initially booms will then starve for fuel, and the galaxy will settle into its new, quiet life.
The Solar System's fate is uncertain. The probability that any specific star — including our Sun — gets ejected from the new galaxy is maybe one in a hundred. But within our own galaxy, the gravitational disturbances will send comets from the Oort Cloud cascading into the inner solar system. The late heavy bombardment, all over again. If Earth is still habitable, that bombardment might be the last thing to happen to it before the Sun itself expands and engulfs the inner planets.
I think about the stars in that new galaxy and what they'll see. If there's someone standing on a planet in the remnant of Andromeda's disk, looking out at the night sky, they won't see a spiral galaxy. They'll see a diffuse, elliptical blob. They won't know that what they're looking at was once beautiful and orderly. They'll know the universe as a single galaxy — the merged one — surrounded by nothing but a few dwarf spheroidals. The cosmic web will still be there, larger scales still in motion, but on the Local Group scale, it will be just one galaxy. A lone elliptical in a sea of void.
Sometimes I think about the fact that this is how most galaxies end up. Merging. Growing. Becoming something else. The universe is not a place of permanence. It's a place of collision and transformation. And we, who are made of star stuff that was forged in galaxies that no longer exist, are just another product of that ongoing process.