Image: NASA
Not every galaxy looks this pretty, but a lot of them do! There are many kinds of galaxies, and today we're going to talk about spiral galaxies. Spirals are near and dear to my heart - we're in one right now! The Milky Way, our home galaxy, is a spiral, as is the Pinwheel Galaxy pictured above. You'll never guess why we called it that 
The neat thing about spiral galaxies is that they're very flat. Almost all their stars are located in a disc, orbiting a central supermassive black hole. In our galaxy, we call that black hole Sagittarius A*. Those of you who read my last space post might be able to guess why it's called that!
In a physical disk, like a CD or vinyl record, the outer rim spins faster than the inner rim. However, when dealing with orbits (like a solar system), the general rule is that further away things are moving slower because there's less gravity to keep them held in the system.
Spiral galaxies don't really do either of these things.
It turns out that, due to some weird dark matter physics, most spiral galaxies have a (alright time for a fancy phrase) flat rotation curve; stars on the very outer rim are moving at just about the same speed as stars closer to the center. This is one way we can determine things like the density of dark matter!
Here's where we start to run into a problem - galaxies are old. REALLY old. Billions of years, usually. However, orbital periods of stars (how long they take to make one full loop of the center) are much, much shorter, on the order of hundreds of millions years.
"Why is this a problem?" I hear you ask. Well, if everything is moving at the same speed, but the further away stars have to travel further to complete an orbit, they'll "fall behind" all the inner stars. So, if we assume that the gorgeous spiral arms are made of stars that have always been in them, those spiral arms would twist themselves into oblivion and ruin any pretty pictures that JWST might want to take.
What does this mean?
Clearly it means that the assumption I just dropped into your lap is wrong. Yeah, it's ok, you can throw it away on the way out. (Please don't leave it under your chair, I have to clean up afterwards)
The spiral arms aren't made of static populations - stars move in and out of them as they orbit! But it seems awfully convenient that there would be clear differences between the bright blue arms and the dimmer orange rest of the disk, right? This seems like a much harder thing to organize.
Enter the star of our show - the density wave model. It turns out that spiral galaxies have waves of higher pressure/higher density (these are synonymous) moving through their disks, a lot like how water waves can "move" through the ocean while each water molecule is just doing its own thing.
So, that at least explains why the arms exist.
But why are they so blue?
The density wave model is leaning against the wall in the corner, looking all smug, because it has an answer for this too. See, if gas and dust gets smooshed, the higher gravity in that zone will attract more and more matter, eventually igniting a star. These big density waves provide a LOT of smooshing, so the arms have lots of new star formation!
The new stars have all sorts of colors, from red to orange to blue-ish white and everything in between. And, as we've said, they'll all eventually leave the arm as they orbit out of it. However, the brighter bluer stars live MUCH shorter lives than dimmer orange ones. The blue ones explode and die before they have a chance to leave, but the redder stars can leave and frolic amongst the rest of the galaxy. Hence, the arms look much bluer because that's where stars are being born!














