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Most galaxies that we know of are spirals, including the Milky Way, but how do they form and keep their shape over billions of years?

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The universe's galaxies come in all shapes and sizes. There are shapeless blobs and smooth ellipses, but about three-quarters of galaxies have that classic spiral structure, including the Milky Way. Except, even though astronomers are surrounded by examples of galaxies like these, they still don’t fully understand how these spirals form, or how they keep their shape over billions of years.

But they’re working on getting to the bottom of it, because the better we understand the physics of spirals, the better we can understand the origin and the future of these galaxies and their stars. One of which, we live around! The thing that makes this tricky is that a spiral galaxy doesn’t work like a classic pinwheel.

It looks like one, with its sweeping arms full of stars, all rotating around the center. But if it worked like a pinwheel, the stars would be fixed within those arms, and swept around as the galaxy rotated. And in reality, that’s not what happens.

Observations have shown that spiral galaxies do rotate, but the inner parts spin faster than the outer edges. So if stars were really fixed in place in the arms, the difference in rotation speed across the disk would slowly wind the arms tighter and tighter. And so eventually, the arms would disappear.

But it doesn’t seem like that happens:. Spiral galaxies can last for billions of years without losing their arms. This creates what’s called the winding problem.

And it tells us that there must be something else going on. So, if the spiral arms aren’t fixed packs of stars, what are they? Well, in the 1960s, a pair of astronomers had a new idea:.

Maybe the arms are the areas where the stars just happen to be more densely packed. This is called the density wave theory, and it suggests that as stars orbit around the center of the galaxy, they pass in and out of these higher-density regions. It’s kinda like how cars pass in and out of a traffic jam:.

The part of the highway that’s jammed up stays that way even as individual cars move in and out. So, according to this theory, as the spiral arms move around the center of the galaxy, the stars themselves move at a different speed in and out of the arms. Now, the theory did not suggest how these waves might get started in galaxies, but we know that density waves are like ripples in a pond, they come from some disturbance that sends waves moving outward.

Except, since a galaxy is rotating instead of sitting still in a pond, its ripples create spirals instead of circles. And hypothetically, those spiraling ripples would interact to form spiral arms. At first, this was all based on simulations.

Since everything happens on cosmic timescales, it’s hard to actually see this in action. But one way astronomers have been able to find evidence for density wave theory is by testing some of its predictions:. For one, if galaxies’ spiral arms really did come from density waves, astronomers expected compressed gas in the dense regions to ignite into lots of new stars.

And since the 1960s, astronomers have noticed that these galaxies’ bright, young stars are mostly concentrated in the spiral arms. More recently, in a 2016 study, researchers tested their prediction that stars travel at a different speed than the arms, and they did this by analyzing the position of stars of different ages in dozens of spiral galaxies. Since stars change color as they age, scientists could study specific age groups by isolating different colors of light.

As expected, they found the youngest stars mostly in the arms, while older stars were typically located farther and farther away. It was pretty strong evidence that stars aren’t fixed in the spirals arms; they’re just passing through. To be fair, the density wave theory isn’t the only explanation that’s been suggested for spiral galaxies.

But so far, these lines of evidence make it the leading idea. That said, there’s still some big unanswered questions. Like, scientists still don’t know what exactly could cause these density waves in the first place.

Some have suggested that since many spiral galaxies have a bar at the center, maybe those bars set off ripples in the galactic disk as they rotate. Except, scientists think that the bars themselves are formed by density waves. Which leaves the question of where those waves came from.

Plus, not every spiral galaxy has a bar in the middle, so this can’t be the only explanation. Another hypothesis is that density waves could be caused by smaller, companion galaxies tugging at the disk from the outside. But sometimes, spiral galaxies don't appear to have companion galaxies, either.

And like, they could just be too dim for us to see, but more likely, we need a better explanation. So, maybe the answer is simpler than we think:. Some simulations have suggested that spirals get going spontaneously, from density fluctuations within the disk.

You know, just like how traffic jams sometimes happen for no reason at all. But for now, scientists will need to keep studying spiral galaxies to make sense of their mysterious swirls. And the better we understand these distant spirals, the better we can understand the galaxy we live in and how it came to be.

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