Why are some galaxies Spiral Shaped?
Apparently, this is still under research. The Wikipedia article on Spiral Galaxies has a discussion of a “density wave” explanation that’s been around for some time, along with mention of a paper published this year suggesting that local mutual gravitation alone can explain how stars group and move in spiral arms. One thing that seems clear from a physics view is that it’s hard to predict the motion of a system when you don’t know what’s driving it. There doesn’t seem to be a good explanation of the observed variation of rotation rate with distance from the center of our own Milky Way. I don’t find the notion of “dark matter” any more appealing that the idea that gravitation laws being warped to fit the data. At any rate, the article was an interesting read. Check it out.
