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We have been studying our home galaxy for years, but even though astronomy has come a long way, there is still a lot we don't know about the Milky Way.

Host: Caitlin Hofmeister

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The Milky Way is our home, so you’d think we’d know a lot about it. And we do.

But a whole galaxy is a pretty big place, so there’s still plenty of weird stuff that we’re trying to figure out. And that’s the cool stuff! So, here are three questions scientists are still trying to puzzle out.

One is our galaxy’s shape. We know that we live in a spiral galaxy, with long, sweeping arms packed with stars. But why’s it like that?

After all, stars form from a fog of gas and dust called the interstellar medium, or ISM. And the ISM is everywhere within the galaxy, so why do stars congregate in the arms? Well, we don’t totally know -- but we’re making progress.

One clue comes from structures called filaments that astronomers in the 1980s started to find at the cores of some galactic arms. Filaments are dense regions of the ISM that are incredibly thin, up to a hundred times longer than they are wide. They can stretch for hundreds of light-years but, weirdly, seem to exist on all size scales, kind of like a fractal.

Together, they form a sort of galactic skeleton for the Milky Way, providing ingredient-rich places for new stars to form. The European Space Agency’s Herschel Space Observatory has even found filaments connecting all the closest star forming regions! But exactly how many of these structures there are and how they play a role around here is still up for debate.

That’s not the only mystery our galaxy’s stars present. As astronomers have mapped out the positions and speeds of nearby stars more accurately, they’ve found they’re less predictable than expected. Instead of mostly circling in a flat plane, a research group discovered in 2012 that many stars seem to be moving up or down about 10% as fast as they’re circling the galactic center.

When mapped out at large scale, the Milky Way’s stars even seem to be rippling with waves. Studying these waves is called galactoseismology and, like the study of seismic waves here on Earth, it may reveal clues about unseen events. The waves are probably the result of a big collision with the Milky Way... but where’s the collider?

Researchers aren’t sure, but they think there are two likely possibilities, both of which would’ve happened about 100 million years ago. One’s kind of standard: an interaction between our galaxy and one of its many satellite galaxies. The other, though, is really cool: It’s possible that the Milky Way collided with a massive dark matter structure, and its gravity stirred up all those stars.

Although we can’t see dark matter, we know it’s all around us, especially near our galaxy’s outer edges. But just because we can’t find the culprit, doesn’t mean it’s gotta be dark matter. Whatever the collision was, its trajectory probably carried the other object to a place beyond the galactic core and basically out of sight from Earth.

There’s also at least one piece of evidence suggesting a dwarf galaxy might’ve been the culprit instead. Astronomers have found a few stars traveling so fast that they couldn’t have come from our galaxy, so they might have been knocked loose during that big collision. Either way, we’ll have to work fast to figure this out because those waves of stars will probably disappear in another hundred million years.

Then again, maybe we won’t have to work that fast after all. Finally, stars aren’t the only thing traveling weirdly fast around here. For decades, astronomers have observed a few clouds of gas moving at strangely high velocities -- up to 90 kilometers per second faster than the stuff around them.

At that speed, it would take only a year and a half to travel from the Sun out to Neptune! Always on the lookout for a clever name, researchers call them high-velocity clouds. These clouds are mostly found in the outer, halo region of the Milky Way, and they’re often observed to have a low metallicity, or abundance of heavier elements.

And they’re not small, either: They can contain millions of times more material than the Sun and span tens of thousands of light-years. Scientists have four main ideas about where they might have come from: [yawn ort] The oldest hypothesis, proposed by famous astronomer Jan Oort, suggests that the clouds might be the far-flung leftovers of the process that formed the Milky Way. With only the weakest gravitational pull affecting them, that far-off material would’ve taken billions of years to get here, speeding up along the way.

Another idea is that -- because they do have such low metallicity -- high velocity clouds are the remnants of an ancient collision between the Milky Way and another galaxy. Hey, where have we heard that before? A related hypothesis is that the material is made up of gas stolen gravitationally from a satellite galaxy.

That seems especially likely for the clouds found in the southern hemisphere, which is in the direction of the Magellanic Clouds, our closest galactic neighbors. One last idea is that these clouds actually came from the Milky Way all along. Perhaps a powerful supernova explosion hurled them away from the galaxy’s disk, only for gravity to claw them back before they could fully escape.

There’s even a chance different clouds could have different origin stories. To figure that out, astronomers are studying the composition of each cloud in detail. If they were created in different ways, there might be a signature of their home still waiting to be detected.

We’ll just have to wait for the results. We don’t yet know the answers to any of these Milky Way mysteries or many others out there. But put them all together and our galaxy suddenly seems a lot less isolated and our cosmic neighborhood a lot busier.

Just watch out for any flying galaxies. One place within the milky way where we can figure out the answers to big questions relatively quickly is Inspired by these existing questions about the milky way, I’m working on the Dark Matter quiz.

Leading up to it was a lesson and quiz on the fate of the universe that I looked through to prep, but this starts off pretty easy. So this question wants to know what is the dominant source of mass in our solar system. So the Sun!

So I got it right and so did 72% of the other people in the Brilliant community. Which is a neat thing about Brilliant because you can see how you did but also see how other people did when they answered the question. I think what sets Brilliant apart from other education sites is this community aspect.

On Brilliant you can answer questions that other people have submitted or you can even submit your own questions to be answered. So check it out for yourself and let us know if you submit a question and I’ll try to solve it. And the first 777 people to use that url to sign up for an annual premium subscription will get 20% off.

Plus, you’ll be helping to support SciShow Space when you do, so thank you! [ ♪ Outro ].