The Milky Way Demystified

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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The Composition of the Milky Way

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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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What Are Spiral Arms?

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

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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