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.

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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!

My Very Excellent Liz Just Brought Us Sixteen Pizzas

I hadn’t really talked about it yet, because I’ve been too busy… I don’t know… blogging about Marmite and beds and computers or something… but I had the most fabulous time at a New Year’s party hosted by Liz and Simon at their house in Macclesfield. There was drinking, and board games, and truly awful Troma films, and then at midnight we all counted down from 7, or 12, or something, and spontaneously broke out into a chorus of Auld Lang Syne. See: there’s a video and everything –


(can’t see the video? click here to watch on YouTube)

It seems that my mnemonic (as used in the title of this post) is broken, unless we reinstate Pluto as a planet and rename the fourth and eighth planets in the solar system to Lars and Septune, respectively. Which I think are better names, anyway.

It was a fantastic opportunity to catch up with folks I don’t see enough of, to talk about what had gone right (and wrong) about the year gone by, and what we were looking forward to in the year to come. Liz suggested that perhaps this should become a regular thing, a little like “fake Christmas” has begun to, and that seems like a good idea (and I’m pretty sure I heard Bryn volunteer to host it next year…).

By the way: do you remember how last year Paul, Ruth, JTA and I invented Argh! It Burns Night? We’re doing it again this year, and because so many of you expressed an interest in joining us, we’d like you to come too. It’ll be on the evening of Saturday 4th February (yes, we know this is a little late for a Burns Night, but the second part of Ruth & JTA’s honeymoon is going to get in the way otherwise): drop me an email if you want to come along for a night of haggis, whisky, and fanfiction.