The Milky Way

How Far Away are Galaxies?

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Well, I’ll give you a spoiler: they’re ridiculously far away.

Let’s consider for a moment what a light-year actually means. It sounds like a unit of time, but it’s actually the distance that light travels in one Earth year.

Think of it this way: if your name is Bob, and you can travel a certain distance in one year, that distance could be called a Bob-year.

I know it’s strange to think of light traveling at a certain speed. When you flip a light switch, the room immediately brightens. When you shine a flashlight, its beam immediately falls across the nearest surface.

But that just goes to show how insanely fast light travels. If it takes 2 million years for light to get from one object to another…imagine how far apart those objects are?

Well, that’s the case for our home galaxy, the Milky Way, and our nearest galactic neighbor, the Andromeda Galaxy.

But…wait a second. How do we know that?

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The Milky Way Demystified

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Alright, people…time to finish off our exploration of the Milky Way Galaxy, our home in the cosmos!

For the past nine weeks, we’ve covered everything from how our galaxy was “discovered” to how it may have formed. But there’s so much more to explore–and, starting next week, we’ll begin covering the vast universe of galaxies beyond our own!

But before we do that…I want to wrap up our discussion of our own galaxy with an overview to tie the last nine posts together.

(By the way, has anyone noticed I actually managed to chug out a post a week for the entire Milky Way “module”? I’m a bit impressed with myself for that!)

Anyway…on to the Milky Way!

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How Did the Milky Way Form?

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Over the course of my last eight posts, we’ve covered just about everything there is to cover about our home galaxy–or, well, at least the basics.

We’ve explored how astronomers first discovered what that incredible, milky stream of dust across the night sky actually is. We’ve followed astronomers like the Herschels and Harlow Shapley as they tried to measure the size of our galaxy.

We’ve covered its structure–a thin disk of spiral arms, surrounded by an enormous, diffuse halo–and how truly massive this great wheel is.

We then explored those spiral arms, where all the youngest stars are–and where stars form in the first place. And we explored the chaotic nucleus at the very center of the galaxy.

Most recently, we delved into the composition of the Milky Way–that is, how much heavy elements its stars contain. We discovered that stellar compositions hint at how old certain parts of the galaxy are.

But there’s one question we haven’t answered yet, and it’s quite possibly the most important one of all.

How did the Milky Way actually become what it is today?

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The Composition of the Milky Way

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This image has an empty alt attribute; its file name is andromeda-galaxy-milky-way.jpg

What is our home galaxy made up of?

In the broadest sense, it’s made up of stars, clouds of dust and gas, and the mysterious dark matter.

We could also get a little more detailed. We could say that it is a great wheel of stars, made up of a thin disk component, a central bulge, and a broader spherical halo that surrounds the disk.

We could even build on that, and say that the thin disk is where all the youngest stars are found. We could say that within the thin disk are spiral arms, where the star formation actually happens. We could say that the oldest stars are found in the central bulge and the halo, where there is very little dust and gas to make new stars.

But…what about its chemical composition? If we could explore our galaxy and bring home test tubes of “star stuff,” what would we find? And what can that tell us about our galaxy’s history?

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Exploring the Milky Way’s Nucleus

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Here is an edge-on illustration of our Milky Way Galaxy. (Keep in mind that the disk actually stretches quite a bit farther out from the budge than is apparent in this illustration. Proportionally, its full diameter makes its thickness less than that of a pizza crust.)

What if I asked you to imagine what that central bulge would look like to us–lifeforms living inside the galaxy? What would you imagine?

Perhaps you’d imagine looking inwards toward a glowing ball of light. Perhaps you’d imagine a region of our sky unusually thick with stars and interstellar clouds. Or perhaps you’d imagine something entirely different.

But…would you imagine this?

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What Are Spiral Arms?

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Probably the most spectacular feature of our Milky Way galaxy is its spiral arms.

We can’t get a probe far enough out yet to take a galactic selfie, but astronomers are reasonably sure that we live in a spiral galaxy. Observations of other spiral galaxies offer clues to what kind of objects can help us trace out the shapes of spiral arms, called spiral tracers. Using those spiral tracers, we’ve been able to map out patterns within our own galaxy that appear to be spiral arms.

Over the years, astronomers have tested the spiral arm hypothesis against the evidence again and again, and there is now a great deal of confidence that the Milky Way is a spiral galaxy.

More than that–star formation, which we know is limited to the disk of the galaxy (rather than its central bulge or halo), appears to be specifically found in the spiral arms.

But why? And for that matter…what even are spiral arms?

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Exploring the Milky Way’s Spiral Arms

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The Milky Way–our home galaxy–is a spiral galaxy, a classification I often describe as pinwheel-shaped.

The main difference between a spiral galaxy’s shape and a pinwheel’s shape is that spiral galaxies, like the Milky Way, only have two main arms. For the Milky Way, those are the Scutum-Centaurus arm and the Perseus arm. If you study the image above, you’ll notice that all the other arms are a bit wispier, and most branch off from the main arms.

There’s just one problem, though…

How do we even know that this image is an accurate depiction of our galaxy? How do we know that the Milky Way has spiral arms?

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How Massive is the Milky Way?

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Over centuries of philosophy and research, through the times of the classical astronomers to Galileo’s observations of the Milky Way, humanity’s understanding of our universe has evolved from a simple model of the sun and planets to a vast wheel of stars we now know as our galaxy.

And since the “discovery” of our Milky Way–or, more accurately, the discovery of what that hazy band of stars in the sky is–we’ve come to realize just how massive our home in the cosmos really is.

That scientific journey started with the Herschels’ mapping of what was then called the “star system.” Later astronomers began to realize just how far out from the sun the stars of our galaxy really reached. Determining distances across our galaxy was the first step to discovering its size.

Later, we began to understand its structure–mapping the extraordinarily thin disk, the chaotic central bulge, and the visible part of the halo, a sphere of stars that extends beyond the plane of the galaxy.

And since then, we’ve begun to master the next critical part of understanding our galaxy: its mass.

So, how massive is our galaxy?

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Structure of the Milky Way

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In the vast expanse of the cosmos, the Milky Way Galaxy is our home.

You’ve no doubt seen images of the Milky Way and similar galaxies elsewhere online. It’s a large, spiral galaxy, one of the most spectacular galactic shapes. That spiral shape is fairly iconic–and for years, that’s as far as I thought galaxy classification went.

Turns out, galaxies are way more diverse than just the main three classifications I knew about (spirals, ellipticals, and irregulars). The Milky Way is fully classified as an SBbc: a barred spiral galaxy with a medium-sized nucleus.

Spirals are also described as “grand design” (two distinct spiral arms) or “flocculent” (a sort of fluffy appearance); the Milky Way is somewhere in the middle.

But even those classifications and descriptions don’t fully describe our galaxy.

So what exactly is the structure of the galaxy we call home?

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How Big is the Milky Way?

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How big is our galaxy, anyway?

And more than that–how do we know?

Consider that we can’t really take a photo like this of our galaxy. We’re inside it, and space travel has not advanced to the point where we can leave it just yet. There’s no way we can get a camera out to take a picture from this perspective.

Most things in the universe–like stars, planets, and even other galaxies–can be measured using their angular diameters. That is, we use trigonometry to find their actual sizes based on how large they appear to us in the sky.

But that doesn’t work for an object that we’re inside of.

In order measure the size of our own galaxy, early astronomers had to get a bit creative–with variable stars.

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