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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