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You're probably used to real galaxies having curves, except not all of them seem to have gotten the memo.

Hosted By: Hank Green

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Imagine a galaxy. Any galaxy. You’ve got your classic spiral, you’ve got your funny-looking Sombrero, you got your Antennae Galaxies.

No matter what you pictured, though, I bet it had curves. Because that’s what galaxies look like, right? Except, not all of them actually do.

In 2012, a group of astronomers announced one galaxy that didn’t get the memo. And they called it the “emerald-cut galaxy.” It is a rectangle. Technically, the emerald-cut galaxy’s name is LEDA 074886.

It’s a dwarf galaxy around seventy million light-years away. Now, there are lots of dwarf galaxies out there. There are far, far fewer that look like this one.

And to anyone who has seen pictures of galaxies or, really, anything in space, those straight sides just look weird. See, stars and planets are more or less spherical because gravity pulls everything toward the center. If there are any bumps get too big, they collapse under their own weight.

On the other hand, a large number of galaxies tend to be disks. Early galaxies started out pretty blobby, but that didn’t last very long because of gravity. Scientists still don’t know everything about how today’s galaxies evolved, but they do know that those blobs collided a lot in the early universe.

The early universe was a traffic jam, and gravity is a terrible driver. When two galaxies collide, everything in the newly combined galaxy orbits completely randomly. But if the new galaxy has a lot of gas, over time, gravity will drag that randomness into a disk.

And gas clouds drag one another around, either directly during their own collisions or gravitationally when they get close as they orbit. Every cloud’s gravity tends to slow down and stretch the others, and this effect is strongest when the clouds orbit in opposite directions. Now, if there were exactly the same amount of gas clouds orbiting in all directions, the orbits would drag each other into nothing over the eons, and the galaxy would shrink like a deflating balloon.

But it’s incredibly unlikely that every single orbit has its match. So, there’s bound to be some path with more stuff going one way than the other. So the galaxy shrinks in all directions except the one that includes that path, just like having a ball of dough in a tortilla press.

Eventually, everything in the galaxy orbits along that one dominant path, in the one winning direction. Since orbits are curvy, the result is a flat pancake of a galaxy, which is an aspect of the conservation of angular momentum. Which is why LEDA 074886 looked so strange.

It is missing that characteristic curve. Now, even before they found the emerald, astronomers knew of a couple sorta-kinda rectangular galaxies. But none of them was as extreme as this one.

The team’s first clue about what was going on here came from looking closely at its very center, where matter does seem to be in more of a conventional disk. So they switched to the Keck Telescope in Hawaii, right next door to the Subaru Telescope they used before. By using both, they could discover why the center was disk-shaped, but the edges were more boxy-looking.

Between the two telescopes, they could measure how the colors of stars changes as you move across the galaxy. And again, the center stuck out. It’s filled with blue stars, while the rest of the galaxy is redder.

Blue stars are big stars, and big, blue stars are young stars. Older stars tend to be redder in color. So the disk-y center is young, but the boxy edges are old.

So, researchers hypothesized that LEDA may be something of a train wreck. They think that two small, disk spirals collided edge-on. The gas of the two galaxies fell to the center of the newly formed one, dragging and squeezing into baby stars that orbit in a disk at the center.

Without much gas left in the outskirts of the new galaxy, the stars from each original galaxy were thrown out in two directions. So you have stars going side-to-side and stars going up-and-down from our perspective, forming a rectangle. This hypothesis is backed up by computer simulations, and by the discovery of an even boxier galaxy in 2016.

That one even has two distinct centers, which researchers believe is further evidence that two galaxies combined into one. Today, the boxiest galaxies we’ve seen are dwarfs, but there is a small chance that won’t always be true. Because in about four billion years, two much bigger spirals will collide much closer to home: Andromeda and our Milky Way.

We cannot know exactly what the ensuing carnage will look like this far in advance. But there’s a chance, just a chance, that there could be a rectangle in our future. Or perhaps something in the shape of a pod from today’s sponsor, Cometeer!

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