|Spiral arms come in different "flavors":
Spiral arms are sites of strong star formation: we see dust, HII regions, blue stars, lots of gas. In fact, spiral arms are much more prominant in blue light than in red. Here's a morphing movie of the spiral galaxy M81 showing how the appearance of the spiral arms change with wavelength.
But what are they? How do they form? How long do they last?
Spiral rotation curves are flat: Vc ~ constant. Therefore the angular rotation rate goes like Vc/r, so that the outer edge takes longer to complete an orbit than the inner regions.
So no solid arm could last very long -- it would quickly
get "wound up":
Well, that's okay, it looks like a spiral pattern, what's wrong with that? It happens too fast. It happens in a few rotation periods (say, half a billion years). The age of the Universe is something like 10-15 billion years -- if spirals made their arms this way, they should all be so tightly wrapped we wouldn't see them as spiral arms anymore!
So spiral arms can't be physical objects. What else could they be?
You zoom up behind them doing 60 mph, then hit the jam. You slow down to 40 and carefully work your way through the jam. Once you pass the grannies you speed back up to 60 mph. Everybody else is doing the same thing.
So we have a situation where the traffic jam moves along at 40 mph, while the cars on the highway are typically going 60 mph. This is an example of a density wave, a density perturbation which moves along at a speed different from the speed of the objects within it.
How does this apply to galaxies?
Imagine the spiral pattern is a density wave. It rotates through the galaxy at a fixed angular speed, called the pattern speed. In the inner parts of the galaxies, stars are moving faster than the pattern speed, and overtake the density wave (like you passing the grannies). In the outer parts of the galaxies, stars move more slowly than the pattern speed, and the spiral arms overtake the stars.
What happens when a star encounters the density wave?
This can't be the whole story, though. Why is there so much more star formation in spiral arms?
One idea: as gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse ( the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
Another idea: as clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.
|Do we see this happening for real? Look at the
grand-design spiral galaxy M51:
In our own galaxy, the spiral arms may be excited by a
complex interaction between our companion galaxies the Large and Small
Magellenic Clouds, our dark matter halo, and the disk of our Galaxy. This
is not well understood, though.