spiral density wave theory

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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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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