[With the combined strength of astronomy and nerdery, we can end this destructive conflict and bring order to the galaxy. Maybe. Credit: Lucasfilm]
When Star Wars* came out 40 years ago this year — holy crap, 40 years! — I saw that opening line -- “A long time ago, in a galaxy far, far away …” - and, knowing what a horse’s ass I was at that age, I probably snorted.
“Oh sure,” I remember thinking. “It’s probably all happening here in this galaxy, and they’ll reveal it was Earth all along, or that it’s really the future, or something like that.”
It turns out I was wrong. It really was a different galaxy. Probably. At the risk of having a million padawans emailing me with details from novels, cartoons, and things muttered under the breath of some minor character in one of the movies, I will argue that the Star Wars galaxy probably wasn’t ours ... probably.
After all, today is May the Fourth, so how can I resist? This will take a moment to explain, so patience you must have.
First, what is a galaxy? It’s a huge collection of dust, gas, stars and dark matter, all held together by its own gravity (a force that surrounds them and penetrates them; you could say it binds the galaxy together). There’s no real definition for how small a galaxy can be, though some are known with only a few million stars and barely any gas and dust. The biggest are ridiculously huge. One, called UGC 1382, may be three quarters of a million light-years across! The Milky Way is considered a big galaxy, and it’s only 100,000 light-years in diameter. It’s not clear just how big a galaxy can get, but a million light-years end to end is probably about it.
Galaxies come in four main shapes: elliptical, spiral, irregular and peculiar. Elliptical galaxies are, well, elliptical, and spiral galaxies are flat disks with spiral-shaped arms of material. Irregulars are self-explanatory, and while peculiars have a shape, they’re weird, like ring galaxies, tidally distorted galaxies and so on. As viewed from Earth, most galaxies are easy to classify, though not always. Sometimes, extreme ellipticals can look like disk galaxies, and irregular galaxies can be in the middle of forming themselves into an actual shape. Still, while this isn’t cut and dried, in lots of cases, it’s not too hard to put a galaxy into one box or another.
To my recollection, in the original trilogy of movies, we always see the galaxy from the inside. The only exception may be the final scene of Empire Strikes Back. There, we find C3PO, R2D2, Luke and Leia standing in front of a window on a ship, looking out over what looks like a spiral galaxy. The actual identification of that object is a bit controversial in fandom; some do claim it’s the Star Wars galaxy. Perhaps the rebels went far outside the galaxy to hide in empty space, which does make sense. But I also heard it could be a solar system in the process of forming, a protoplanetary disk, which can bear a resemblance to a spiral galaxy (I’ll note in the movie it can be seen to spin slowly, which in real life wouldn’t be visible for either a galaxy or a protoplanetary disk). That would be a good place to hide as well, since they can emit lots of radio waves and the debris would make good camouflage.
Either way, we don’t have to guess: The Force Awakens came along. Near the end of the movie (no spoilers), a map of the Star Wars galaxy is projected holographically, and we finally get a good look at it. And we see it is, in fact, a spiral galaxy!
And, as much as I liked the movie, I felt a little disappointed. Despite the resemblance, that galaxy is not the Milky Way.
How do I know? Because — despite the presence of the Mos Eisley cantina — the Star Wars galaxy doesn’t have a bar.
OK, hang tight. Let me say that it’s actually hard to know what a galaxy looks like when you’re inside it and don’t have a hyperdrive. Imagine being stuck inside a vast, smoke-filled room and being asked to figure out the shape. The smoke obscures your view, so it can be hard to figure out! Our galaxy is filled with dust (long chains of carbon and silicate molecules) that absorbs visible light, so our vision is blocked. However, at night, you can see a broad swath of stars across the sky, like a milky river (that’s where our galaxy name comes from; in Latin, it’s Via Lactea, or literally “Milky Way”). That strongly implies it’s a flat disk.
With better technology, we could see a lot more. Dust doesn’t block infrared light and radio waves, so looking in those parts of the spectrum we were able to map out the size and shape of our galaxy. And, voila! It’s a spiral!
But there’s more. That broad, fuzzy structure across the middle of the galaxy is what astronomers call a “bar.” Most spiral galaxies have this elongated structure. They come in different proportions and specific shapes, but, in general, they look like, well, a bar. The Milky Way has one, shaped roughly like a peanut. From our position inside the galaxy, it resembles a stubby Tic Tac, but that’s due to perspective and stuff like dust blocking our view of it. Clearer maps made in the infrared reveal its bi-lobed shape, extending for about 7000 light years on either side of the galactic core.
As you can see in that illustrated map, the inner spiral arms of our galaxy start at the tips of the bar. Now, take a look at the Force Awakens map again. No bar. And the galaxy has a different shape and structure, with well-defined inner arms, and less obvious outer ones.
If there’s a bright center to the Milky Way, the Star Wars galaxy is the one that it’s farthest from. That’s not our galaxy.
So, kid-version of me (and maybe a wee bit of adult version me still holding out) was wrong.
... or was he? It turns out, bars don’t last forever. They form from the gravitational interaction of all those stars in the disk of the galaxy with the stars orbiting closer to the center. Over billions of years, this interaction can actually change the orbits of the more central stars, modifying them, so that the bar eventually goes away. It’s also thought that these same interactions can reform the bar over more billions of years, so that it’s a cyclic phenomenon, forming and dissipating many times over the life of the galaxy.
So, it’s entirely possible our Milky Way didn’t have a bar ... a long time ago.
Hmmm. If true, then all this has happened before, and will happen again.
No! Wait! Wrong franchise! But I know that fans will argue over this until long after Episode IX is a part of history. But I’ve observed from one side of this galaxy to the other, and I’ve never seen anything that makes me believe there’s one all-powerful explanation controlling everything in the Star Wars universe.
Search your feelings. You will know it to be true.
*Yes, just Star Wars. You can call it A New Hope if you want, but when I was in middle school I sat my butt in a theater seat and watched the movie when it came out, so it’ll always be just Star Wars to me.