Thanks to Brilliant, for supporting this SciShow video.

Brilliant is offering a 30  day free trial and 20% off an annual premium subscription when you sign up at Brilliant.org/Scishow If you ever find yourself in the middle of nowhere on a cloudless night, just look up. You’ll see a spectacular sight: The Milky Way.

And, no, not the candy bar. The galaxy. Our home galaxy.

And according to certain places on the internet, including a couple NASA webpages, it measures a whopping 100,000 light years across. But NASA is… I don’t want to say wrong, but it’s not right. Because if you ask the astronomers who are studying the stars  in our galaxy’s boonies, you’ll find that we actually don’t know.

It could be ten times bigger than that! And as technology advances, we’re getting closer to an answer. [intro jingle] Compared to other galaxies, it’s really hard for us to study the Milky Way. And that’s because, well, we’re inside of it.

We just can’t get a bird’s eye view of the thing to compare it to what’s nearby, which is what we do with every other galaxy. Different bits of the Milky Way get in our way of looking at other bits. And our solar system is sitting  out in the proverbial suburbs, so trying to find the edge  of one side of the galaxy is going to involve looking through  a lot more stuff than the other.

But despite the difficulty, scientists have spent the past few centuries putting together a map of our little island floating through the cosmos. Now, before they could report  the size of the Milky Way, they had to define where the end would even be. Because much like our  atmosphere fades into nothing as you fly towards outer space, the Milky Way fades away as you fly towards intergalactic space.

Neither has a defined stopping point. So scientists have to just pick something. For example, a paper published back in 2020 tried to find the edge of the Milky Way based on gravitational interactions with neighboring galaxies.

The authors came up with a total diameter of 1.9 million light years… give or take 400,000. But most often, astronomers base it off where the density of stuff falls below a specific, but still kinda arbitrary, value. If you’re looking at a galaxy and there’s at least 200 times more stuff than there is everywhere  else in your frame of view, you’re considered “in” that galaxy.

And if it’s less dense than that, you’re considered “out”. The distance between the center of the galaxy and that boundary is called the virial radius. Double the virial radius and you get approximately how wide the galaxy is.

But defining a galaxy’s  virial radius is one thing. Finding it is another. Take that paper that estimated our galaxy was between 1.5 to 2.3 million light years across.

It suggested that stars exist in those extreme galactic boonies, but that was all based on computer models. Not stars we actually know are out there.. Astronomers who use real stars to calculate the size of the Milky Way are lucky to have a handful of stars that are over 80,000 light years from the galaxy’s center.

Because the further away a star is, the dimmer it looks and the much more subtle it appears to move. You need finer and finer equipment to calculate a reliable distance. And right now, we get reliable measurements for stars that are 80 to 100,000 light years from the galactic center.

But here’s the thing. Even if the visible part of the Milky Way only stretches 200,000 light years across, we already know the galaxy  is way bigger than that. Because our galaxy is also full of invisible dark matter.

In fact, there’s more dark matter than there is regular matter. And dark matter does a terrible job of condensing down into the nice pretty spirals you associate with galaxies. Instead, it hangs around as  a gigantic spherical halo.

So when astronomers talk about a galaxy’s virial radius, they’re basically talking  about the radius of that halo. But because that dark matter halo is invisible, astronomers can’t measure its size directly. So instead, they take advantage of the fact that dark matter has a gravitational effect on all the stars they can see, and use them calculate the halo’s mass.

And this requires knowing exactly how far away a star is from the galactic center and how fast it’s going around the galaxy. Now, if you only account  for the matter we can see, and you plot out a bunch of stars’ distances and orbital speeds, you should see that as you get further out, the speeds go up for a bit and then drop off. But for the past half century or so, astronomers know this isn’t what happens.

Where the speeds should drop off, they actually hold relatively steady. The stars are moving too fast, which means there’s more mass than we can account for with our telescopes. Ergo, dark matter.

So by tracking a subset of stars in the galaxy, astronomers have estimated the mass of this halo to be as high as 2.5 trillion  times that of our Sun, but it’s probably something closer to 1 trillion. And for comparison, the total mass of all the visible stuff is estimated to be just 60 billion times the mass of our Sun. But here’s the problem.

According to multiple papers  using the latest data, that number is wrong. By, like, a lot. These papers are all using the most recent update from the Gaia space telescope.

Its job is basically to stare at the whole sky and track the motion of stars with super high precision so astronomers can assign them distances. In this update from 2022, astronomers had access to stars that were up to about 98,000 light  years from the galactic center, with data that was twice as  precise as the previous update. Basically, they were able to get more reliable orbital speeds for the stars in the observable boonies.

And when they plotted out those speeds, they noticed a drop off that  wasn’t supposed to be there. At least, it shouldn’t be  there if the dark matter halo was as massive as they thought it was. We’ll get to what that means for  the Milky Way’s size in a moment, but the total mass of our galaxy came out to be about 200 billion times the mass of the Sun.

Not 1 trillion. Which is a massive update, but  also one that appears to be wrong… at least if you compare the Milky Way to galaxies that look relatively similar. Those galaxies appear to have dark matter-to-regular matter ratios hovering around 10-to-1.

Meaning for every kilogram of regular stuff, there’s 10 kilograms of dark matter. A total mass of 200 billion Suns puts the Milky Way’s ratio at 2.3-to-1. Now, it could be that Gaia is giving us a more accurate view of the Milky Way by showing us that stars out in the boonies don’t act like we thought because there’s less dark matter.

But it could also be that the stars that Gaia happened to sample are not representative of all  the stars orbiting that far away. Or maybe the dip in speeds  that Gaia reported is real, but it’s sort of an anomaly that exists at that specific distance. If we track stars even farther out, maybe they’ll be back to  moving at the expected speeds.

Gaia is set to provide astronomers yet another update in late 2025, which may provide new information  on more distant stars, and reduce the uncertainty on the ones used in this latest round of research. Plus, there’s a whole new observatory, called the Rubin Observatory, that’s scheduled to come online in 2025. And once it does, it can provide a check on Gaia’s observations.

But until then, we’re still left pondering  the size of the Milky Way. If the dark matter halo is really only 200 billion solar masses, it means the virial radius is just shy of 400,000 light years, making the Milky Way about 800,000 light years across. That’s much bigger than the 100,000 light years you may have had rolling around in your head for the last few decades.

And much less than the 1.9 million light years that other paper was claiming based on computer simulations. The true size is probably somewhere in between those extremes, but while we wait for astronomers to find out exactly what it is, we can always head into the middle of nowhere, and look up. Either way, the Milky Way is large, but they don't call it the large way, which is uncharacteristic of astronomers.

Just think about how they named the Very Large Telescope or the Extremely Large Telescope. It's a missed opportunity, but not one that the I people  were about to miss out on. They hopped right on the bandwagon in naming their large language models.

And if you don't know what an LLM is, you could learn a thing or two from brilliant, the interactive online learning platform with thousands of lessons and problem solving from computer science to math to just science. They have an AI workshop on LLMS that uses real language models to help you learn what they're all about In this workshop, you'll not only learn how  these models build vocabulary and choose their next word,  but also how to ask the models for different kinds of  output like poetry or emails. And brilliant keeps you engaged in the topic with case studies and puzzles, and they show you how to use fun examples like Taylor Swift song  lyrics to train your models.

So if you're trying to think bigger this year you can head to Brilliant.org slash sideshow or click the link in the description down below. That link also gives you 20% off an annual Premium Brilliant subscription, and you get your first 30 days for free. Thank you to Brilliant, for supporting this SciShow video! [ OUTRO ]