How big is the universe? This is a central question in cosmology. I know we know how big the observable universe is. It's about 90 billion light years across. But we don't know how big the actual whole entire universe is. We do not know or we didn't have a good answer to that question. And this is a major question now. And it was a major question 100 years ago. In 100 years ago, we were hearing different observations that were telling us different things. You know, we could do some observations like star surveys. Just go out and look in every direction possible, encounter how many stars and measure how far we're there and build a map and see where we are and how big the place is. But then we could use other things like globular clusters, these clumps of old, old, red, dying stars. And these clumps are like obviously moving around. And you can look at the different clumps and you can see like what are they orbiting? It turns out they have a common center and they're all orbiting something. And these different observations were telling us different things. Some observations were saying that the universe was very large. Some observations were telling us that the universe is very small. Some observations were telling us that we are at the center of the universe. Some observations were telling us that we are like way out near the edge. And this all came to a head in something called the great debate or the Curtis-Chapely debates where we're two prominent astronomers gotten to a big debate. I mean, it was hosted. It was all very formal. But it was a big debate about the size and nature of the universe. And one of the most compelling pieces of evidence of what everyone was debating were these strange kinds of nebulae known as spiral nebulae. You know, there's all sorts of nebulae out there with all sorts of shapes and sizes and colors and places on the sky. But there are a few like the Andromeda nebula that have very distinct spiral patterns look like they have dark dust lanes have like a billion stars inside of them. We didn't know what these nebulae were. Were they part of our galaxy? Like we knew we lived in the Milky Way galaxy, but was our Milky Way galaxy it? Were we just a big clump of stars and nebulae surrounded by nothing? Were we a big clump of stars and nebula? And that was it for the entire scope of the universe? Or were were these strange spiral nebulae like something else laying far outside of our galaxy? And evidence waved back and forth. We really didn't know what it was. The answer came in the early 1920s with the work of Edwin Hubble and the work of Edwin Hubble rested on previous work done by another astronomer Henry and a Swan Levit. Now Henry and a Swan Levit studied a particular kind of star known as a Sephyad variable. As in the name suggests, a Sephyad variable is very with time. They get brighter and they get dimmer. Sometimes it takes like a couple days, sometimes it takes a couple weeks, but either way they get brighter and dimmer with time. And she found the longer a Sephyad variable takes to get brighter and dimmer, the more overall bright it is. And then the shorter it does it the overall dimmer it is. This gives you an way to measure distances because if you know how something an object, how bright an object truly is, then you can compare it to how bright it looks and you can do a little bit of trigonometry and you can calculate a distance. So you can go out and find Sephyad variables all over the place, measure how long it takes for them to get brighter and dimmer. Turn that into a true brightness, turn it compare that to the actual brightness and calculated distance. What Edwin Hubble did, so Henry and a Swan Levit established that relationship, then what Edwin Hubble did was to find Sephyad variables in the Andromeda Nebula. He watched how long it took for them to get brighter and dimmer. He calculated an absolute brightness compared that to the measured brightness, which was very, very dim, which meant these objects which were in the Andromeda Nebula were incredibly far away. We're talking millions of light years away. Edwin Hubble discovered that the Andromeda Nebula was way farther away than anything else we had ever observed in our entire history of it as a species. And that settled it. That settled it. Our Milky Way galaxy is isolated. It is a clump of stars and gas and dust and it is separate by a vast gulf of nothingness from other galaxies like Andromeda, like Triangum, like the Pinwheel, like the Whirlpool, like all these other galaxies are incredibly far away from us. Galaxies are like little island universes. And then slowly over time, we discovered more and more about the nature and the makeup of the Milky Way galaxy. In the Milky Way gets his name because in the Greek and Roman tradition, we have this splash of color, of light, like a band across the sky. And it looks like, you know, milk is spilling down if you're into that sort of thing. And so the word galaxy is literally like Greek for Milky. And so that's how we get the name. And what, uh, and for a long time, we had no clue what it was because it's just like, it's just glowing light. Like, what's the, what is that? It wasn't until Galileo looked at it with a telescope. They realized what we're seeing is the combined light of a bunch of stars, like a load of stars. They're all combining their light together where we can't see the individual stars, but there's so many of them that you get this diffuse band of light. And when you're looking at the Milky Way, that band on the sky, you're looking into our galaxy. Excuse me. So you're seeing all these stars. And then when you're looking away from that band, you're looking out of our galaxy so you don't see as many stars. So that right there tells us that our galaxy is flat like a pancake because there's more stars in one direction than there is another. More surveys of stars, more observations of globular clusters, observations of the motions of stars tells us that our galaxy is pretty large. It's about, it's about a hundred thousand light years across. It depends on how you measure the edge because it's not like there's star star star star stars stars stars and then all the stars stop and then it's just infinite black voidness. No, it's just like stars and then slightly fewer stars and then slightly fewer stars and then just a few stars out here and then like some gas that is like randomly associated with our galaxy. So it like depends on where you count the edge, but the edge of like the vast majority of the stars, we've got a diameter of about a hundred thousand light years. It's only 10 to 30 thousand light years thick. So it is much thinner than it is wider. There is a dense core. The core is not a ball, however, it is like it looks like a bar and then we have all these very, very cool spiral arms. We happen to live on a little spur associated with a much larger spiral arm. We call that arm the Orion arm. This arm where we sit is about two thirds of the way out to the edge of the vast majority of stars in the Milky Way. So we're in the suburbs, you know, we're not we're not living downtown. We wanted to get out with a yard. So we're about two thirds of the way out on a spur of a spiral arm. Our Milky Way galaxy is pretty average as spiral galaxies go. We've got John Blackhole in the center. We've got some decent magnetic fields. We have a pretty decent star formation rate. Milky Way has been around in its present form for about nine billion years and about four billion years. It's going to collide with our nearest neighbor and Dromda and that'll be, well, that's a different episode. But it took centuries of observations. Like if you if you Google search like Milky Way galaxy, you'll get a map based on our current understanding. It took centuries to build up that map. And one of the key pieces in this story is the realization that other galaxies exist, that these nebula, these spiral nebula are not just like weird looking nebula in our own galaxy. No, they are galaxies in their own right. And so once you identify that galaxy is a thing, you can come back and say, okay, what is the Milky Way galaxy look like next week. I'm going to talk about other kinds of galaxies and how galaxies form and how they get their shape. In the meantime, please go to patreon.com slash pm suitor to keep this show going. I actually really do honestly appreciate it. And please like, share, subscribe, drop me a comment. If you feel like it, I might or might not respond. 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