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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What is the Intercloud Medium?


Stars are like headlights in a fog bank that’s impossibly thick in some places, and so thin as to be transparent in others. Sometimes, we get lucky enough for starlight to light up the fog. Other times, stars shine straight through it.

That “fog” is the interstellar medium. I’ve covered it in several posts already. We’ve gone over nebulae, the visible evidence of the stuff between the stars. I’ve talked about ways to study the interstellar medium. And I’ve introduced you to cool clouds, the clouds of mostly neutral hydrogen gas.

Now I want to introduce you to the intercloud medium. It’s different from cool HI clouds in that it’s ionized, rather than neutral.

But what exactly does that mean?

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What are Cool Clouds?


Have you ever driven through fog?

I’m going to guess, for the sake of this post, that you have. And if you haven’t…well, I think the image above should give you a pretty good idea of what it’s like. Low visibility—you can’t really see anything.

Imagine a fog bank that sort of thickens and thins out as you drive through it. In some areas, it’s so thick you can hardly hope to see the cars around you. In others, you can see for miles ahead.

That’s actually a pretty good description of the interstellar medium…or, at least, the cool clouds between the stars.

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