Hello my fellow curious critters! I have another special treat for you this week while my shoulder continues to heal up. My amazingly talented friend Sarah is blessing us all with a wonderful piece about our home galaxy the Milky Way. Sarah is currently working on her master's degree in Physics and also happens to be the only reason I passed my physics class (and I am eternally grateful for her help and for holding my hand while I attempted very basic physics problems. Math is hard, ok?). I am very proud of her and excited for her to teach us all something today. I will hopefully be back in business by next week, but I think I want to keep up the guest posts. Do you guys like them? I know I have loved learning things outside of my personal areas of interest and knowledge. Without further ado, here is Sarah on why our galaxy is a giant lake floating through space. Enjoy!
Our galaxy is basically one giant lake floating through space. Fight me.
Alyssa is gone, and I am your new overlord. Awkwardly, I don’t know anything about plants, animals, or biology in general. My area of non-expertise is space. Well, that’s a little general. I’ve spent the last two-ish years studying a singular part of the Milky Way galaxy, which just so happens to be the galaxy we live in.
Like anything else in nature, galaxies have structure. The Milky Way is a spiral galaxy and has three main features: the bulge, the disk, and the halo. The galactic bulge is right in the middle and is practically a giant bubble filled to the brim with stars. In the very, very center, there is an enormous black hole that keeps all the stars together with gravity. The disk is where most of the other stars live, and *surprise* it looks like a disk (or plate, or frisbee, or any other flat round thing). The halo surrounds the disk and is mostly empty space, with a few pockets of stars. If we could jump outside of the galaxy and look at it edge-on, it would look something like this diagram:
The Milky Way is called a spiral galaxy because all of the stars rotate around the central black hole. Some stars move slower than others and they clump together in giant ‘arms’. If you took an octopus and spun the poor thing, it would look a little like a top view of a spiral galaxy. That being said, I do not suggest spinning any octopodes.
So I made this somewhat absurd claim that our galaxy is like a giant lake, and I’d like to start building my argument. Those spinning stars and spiral arms? They look like a whirlpool. I don’t know if lakes actually have whirlpools or if that’s an ocean sort of thing, but let's ignore that for the time being.
Water whirlpool = spiral galaxy. Just look at them! I know the pictures I picked out are a little silly because the galaxy and whirlpool are spinning in opposite directions, but we can just pretend that I’m better at googling images than I actually am and that everything lines up perfectly.
Another piece of evidence: there are about 100,000 million stars in the Milky Way. That’s a lot. You know what else there’s a lot of? Water molecules in a lake. BOOM *drops mic*. I rest my case.
If that wasn’t compelling enough evidence for you, we can also look at the disk wiggles. Now, I know that sounds…. weird, but that’s actually what they are called. In the disk, starting closer to the center and going out towards the edge, there are a bunch of stars in the north, then a bunch of stars in the south, more stars in the north again, and finally another conglomerate in the south. Astrophysicists call them wiggles, but I think ripples fit better. 'Cause that’s what they look like. If you drop a rock into water, you get waves, and these wiggles (ripples) in the Milky Way disk look almost exactly the same. In fact, we theorize that some other galaxy was the giant rock that created our waves.
The wiggles each have a fancy name: the Near North Structure, the South Middle Structure, the Monoceros Ring, and the Tri-Andromeda Ring. Waves on a lake or in the ocean don’t last long enough for us to name them, but wiggles in the galaxy work a little differently. First off, the Milky Way is a bit bigger than your average lake. If you laid out a trillion copies of the Pacific ocean, the Milky Way would still be wider. I think we can also agree that your average star is larger than a water molecule. Since everything in the galaxy is so large and far apart, it takes a long time for features to form or dissipate.
Here’s another weird thing: the South Middle Structure dips nearly twice as much as the Near North Structure rises. Ripples don’t do that. They tend to dip nearly as much as their neighbor rises, with a decrease in height as you move further away from the impact site. Based on the shape of the wiggles, it would seem like that impact site is close to the galactic center, but the large South Middle Structure is further out than the small Near North Structure. Though it is possible that all of these characteristics could be explained by one single in-falling galaxy, the Sagittarius Dwarf galaxy in particular, there could be other factors at play.
For example: the Large Megallanic Cloud (LMC). The name is a bit misleading. LMC is actually a galaxy 1/10th the size of the Milky Way, and it’s just chilling as it follows us around. LMC isn’t the closest galaxy to us, but between the distance and size, it has a pretty large effect on the Milky Way itself. Even though our galaxy has so much gravity that it attracts other galaxies like LMC, stars within the Milky Way still want to move closer to the LMC. Think moths to a flame, or those little green aliens in Toy Story reaching for The Claw. It’s possible that the LMC or another orbiting galaxy could be ‘pulling’ the wiggles out of the disk and warping their shape.
Now, I know your average lake doesn’t have other lakes orbiting or falling into it, but the Milky Galaxy and lakes definitely share some features. They’ve got lots of stuff in them. Whirlpools and spiral galaxies are practically synonyms. And, hello, the Milky Way has ripples. What’s more water-like than ripples?
If you want to learn more about the disk wiggles, there are a bunch of cool resources, from websites to videos to actual academic articles:
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