Why are different kinds of stars found ‘between’ the spiral arms of our galaxy than in them?

This self-post was originally posted to /r/askscience. See more things from Dan's Reddit account.

I was elsewhere in Reddit when somebody made the claim that the space between the spiral arms of our galaxy aren’t empty, but are simply filled with different kinds of stars (different luminoscities, etc.). I was skeptical, but other sources claim that this is true, too. However, I can’t find any explanation as to why this might be the case.

Is our galaxy unusual in this regard? Is it that stars form throughout our galaxy, but matter ejected from the bar into the arms affects the type of stars seen there? Is this a question of what forms where, or is it a clustering question? I honestly haven’t a clue, and I couldn’t find where this question had been asked before so I’m still “in the dark”… unlike the ‘dark’ places in our galaxy!

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