Whats the reason for the Milky Ways spiral?
Actually that was in the slides that I was thwarted from showing by the animation that my poor little laptop couldn’t handle. It is somewhat complicated, but theoretical models can describe spiral galaxy shapes reasonably well. The basic idea is this: the arms are “spiral density waves”, or regions of high density in which stars are more likely to form. The stars are rotating around, moving through high and low density regions. Imagine you have some high density region of gas (perhaps it was originally compressed by a collision between galaxies, or a gravitational disturbance from a nearby galaxy). Stars and gas clouds moving through this region will slow down due to the high density, and spend more time there… thus the high density is maintained. Past the high density region, the gas and stars can move faster, and the gas will be more rarified. So you get a pattern of alternate dense and rarified regions, corresponding to the arms. Stars are more likely to form in dense regions. S
