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Reid: For a long time, there was a strange mystery when it came to the question of how galaxies formed: The best models predicted that a whole galaxy would form at the same time, but... galaxies have stars with all kinds of different ages. So how do we explain that?

Turns out that galaxies eat each other. We used to think that galaxies formed when huge clouds of gas in space collapsed inwards under the weight of their own gravity. Denser pockets of that cloud became stars, and most of the stars then got pulled into the center of the new galaxy by gravity. This is known as the primordial collapse model of galaxy formation, and it’s not wrong.

Clusters of stars do form that way, and some of those clusters are big enough that you could call them galaxies, but if primordial collapse were all there was to it, then all of the stars in each galaxy should be about the same age, since they would have formed at about the same time. Except... they’re not. For example, the oldest stars in the Milky Way are more than 13 billion years old. Meanwhile, our Sun is only about four and a half billion years old. And in other places in the galaxy, new baby stars are forming right now.

The best explanation for how all these differently-aged stars can show up in the same galaxy is that they originally came from other galaxies -- which then merged into one. When two galaxies are close enough, they're affected by each other’s gravity, and if those two galaxies are of roughly the same size, they’ll eventually merge. The new galaxy formed during that merger will be much bigger, with twice the mass of either of its parent galaxies. And it'll have stars that could potentially be of vastly different ages, depending on the ages of its parents.

That new galaxy might then be attracted towards another galaxy, and eventually, they would end up merging, too. Eventually, you could end up with truly massive galaxies. All this merging is appropriately called the hierarchical merger model of galaxy formation, and it’s how we now think galaxies grow. Smaller things merge to make bigger things, which then merge to make even bigger things.

But that's just galaxies combining -- neither of them really get destroyed. So it's not what scientists mean when they talk about galactic cannibalism. Instead, they're referring to what happens when one galaxy is a lot bigger than the other. Because in those situations, the bigger galaxy rips the smaller galaxy apart, sucking in its stars and dust and its dark matter halo, and leaving just a trail of star-crumbs behind. That's galactic cannibalism -- and it's actually really common.

Even the Milky Way is in the process of cannibalizing at least four different smaller galaxies right now. First, there’s the Sagittarius dwarf galaxy, which we’re in the final stages of consuming. Once upon a time, this was one of the Milky Way’s brightest satellite galaxies. But over the past billion years, the Milky Way has dispersed and absorbed half of its stars and nearly all of its clouds of gas. The prognosis for the rest of this dwarf galaxy isn’t good, either: it’s still being pulled apart by the Milky Way’s gravity.

There’s also the Canis Major dwarf galaxy, which was only discovered in 1994 despite being our closest galactic neighbor, at only 48 thousand light years away. That's because it's pretty much gone. It’s been pulled to pieces by the Milky Way’s gravity so totally that, when it was first discovered, scientists weren’t even sure if it was a galaxy. Some of them thought it was just a weird bump on one of the Milky Way’s arms.

And then there’s the Large and Small Magellanic Clouds, which we’ve only just started to snack on. Like the others, these are dwarf galaxies that orbit the Milky Way. And according to a study published in 2014 by the Oxford University Press, they’re already beginning to deform as the Milky Way pulls them inwards. The astronomers who wrote the paper, who are based out of the University of Western Australia, gave these beautiful space-clouds about four billion years before they’re just another snack for the Milky Way.

But the Milky Way will eventually get its comeuppance, when it’s devoured by the larger Andromeda Galaxy in about five billion years. The Andromeda galaxy, in turn, will most likely merge into a massive conglomeration of all of the galaxies in the Virgo cluster... and they might end up being eaten by the Shapley Supercluster, a collection of galaxies about 650 million light years away.

But that doesn't necessarily mean that every star and galaxy in the universe will eventually end up in a giant, terrifying cannibal star-blob. That's because the universe is expanding, and the force of gravity decreases the farther away two objects get. So as the space between galaxies increases, mergers and collisions get less and less common. Across great enough distances, the power of attraction of even the most massive objects in the universe won’t be strong enough to overcome the speed of the universe’s expansion. But until we reach that point, it’s a dog eat dog world out there. Even when the dogs are galaxies.

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