What If Andromeda Collides With The Milky Way? | Unveiled

What If Andromeda Galaxy Collides with the Milky Way?

Slowly but surely, the Andromeda Galaxy and the Milky Way are careening towards each other, fated to clash in an epic meeting. It’s hard to imagine a more potentially destructive event in the universe than two sprawling galaxies crashing into each other - but that’s what we’re going to do!

This is Unveiled, and today we’re answering the extraordinary question; What if Andromeda Galaxy collides with the Milky Way?

You might assume that galactic collisions were a rare occurrence, but they’re actually quite common in our universe - with many scientists arguing that they’re a necessary part of a galaxy’s evolution. Our closest major interstellar neighbor, the Andromeda Galaxy, is thought to have collided with at least one other galaxy in its past, and our own Milky Way has survived a few cosmic collisions with more minor dwarf galaxies. In fact, we’re technically still in the process of incorporating stars from an ancient collision with the Sagittarius Dwarf Galaxy - an event that wasn’t destructive for us since smaller galaxies more quickly merge into larger ones. When two similarly large galaxies meet, though, there’s much more power and energy involved, and the potential for more damage.

The Milky Way, with around 300 billion stars, and Andromeda, with 1 trillion stars, are the most massive galaxies in our local group. And the two crashing together actually isn’t purely hypothetical, because they really are speeding toward each other at around 250,000 miles per hour, and in about 4 to 4.5 billion years they are primed for impact. With each carrying so many millions of celestial bodies, it feels as though we should see planets, stars and moons smashing into one another all over the place - but that’s not how it would be.

In reality, there’s only a slightly above 0% chance that any stars will hit each other when our galaxies collide, simply because the distance between each star is enormous. Imagine trying to shoot a speeding bullet out of the sky and you have some idea of the extremely low likelihood of stellar collisions. This seeming contradiction is actually present in all of our universe, down to even the cells in our bodies… Astonishingly, an atom consists of 99.99% empty space around a nucleus, which means that your body is, in some ways, 99% emptiness. The same is true of space; it’s mostly, well, space.

So, if stars and planets aren’t colliding when our galaxies meet, what actually does happen? Do they just harmlessly pass through each other? Well, they might, if it weren’t for gravity. Floating in the space between stars is cosmic dust and gases that are the fuel for star birth. During galactic collision, these nebulas are pulled, pushed and mashed together as part of a gravitational onslaught. The gases get heated by the extreme pressure until they densify through sheer force, and new stars are created. These new stars might then disrupt the old, draining them of energy and upending any form of equilibrium that may have existed before.

In effect, the galaxies become tangled up in each other, swerving under the gravitational influence that each exerts. For Andromeda and the Milky Way, this means billions of years of instability. As the galaxies pass through, their own gravity relentlessly pulls them back together again - fighting against one another, continuously birthing and killing stars, until eventually merging together in a single, massive, elliptical galaxy - with both former galaxies having totally lost their shape.

Crucially, at the center of most galaxies there’s a supermassive black hole. Both Andromeda and the Milky Way have one at their center, with the Milky Way’s being close to 5 million times the size of our sun. And here’s where the most incredible destruction takes place.

When these behemoths move close to each other, a couple of different scenarios are likely. First; they could glide past each other and form a binary black hole system, where both exist independently, though relatively close to each other. But, the second option sees them draw closer and closer until they merge as part of a massive release of power, enough to distort space-time itself with gravitational waves rippling through space. Any stars or planets that are unlucky enough to be nearby when this particular part of the galactic collision takes place could find themselves forcefully ejected from the galaxy entirely, spat out into open space.

After their monumental meeting, if the black holes have merged, they could then form a Quasar, a distant celestial object that’s thought to reside in the middle of some of the most massive galaxies we know about. Quasars are extremely luminous and are the brightest known bodies in the universe. They can be 100,000 times brighter than the entire Milky Way, blasting out almost incomprehensible jets of dazzling energy.

Luckily for us (or perhaps unluckily), we won’t be around to see this. Assuming human beings do manage to live for billions more years, they’d still have been killed off by our own sun long before any of it happens. The sun is set to turn into a massive and deadly red giant in five billion years’ time, but it will have already grown large enough to scorch Earth and turn our planet into an uninhabitable world by the time Andromeda and the Milky Way meet in four billion years.

Which is slightly disappointing, because if life on Earth could survive, it’d witness quite the spectacle! Over centuries, the night sky would be alight in a brilliant display of colors, as gases are heated and mixed. We’d discover millions (possibly billions) more stars lighting up out of seemingly nowhere, and we’d have all new constellations on the map. There’s a slim chance that our own sun would get stuck in the crossfire by passing too close to another, larger star… And, in that case, we’d watch as its energy is inevitably drained by the new, massive body around which we’d now begin to orbit. We’d gaze in wonder as the apparently fundamental facts of our existence change forever.

When the cosmic debris finally settles, the Milky Way and Andromeda would no longer be distinguishable from one another. They’d now be one large elliptical galaxy, with a brand-new structure and far more densely populated than ever before. The destructive potential is almost unimaginable, but both would adapt and, in some way, survive. And that’s what would happen if the Andromeda Galaxy collided with the Milky Way.