Why do galxies spiral?

[Adrian]

If I had to guess right now I would say that spiral arms in galaxies are caused by central supermassive black holes. I would expect that elliptical galaxies do not have central black holes and that the formation of these sends gravitational ripples out through the galaxy, thus forming the spirals.

Density wave theory seems to be a good representation of how spirals work in terms of the rotation of stars around the core - the solar system has travelled through 4 different spiral arms in the past half billion years.

[interestingly, each passage through a spiral coincides with the 4 periods of cooler global temperatures in that time (including the current period). The current theory on this is that there is higher cosmic ray flux in spiral arms and cosmic rays seed cloud formation in earth's atmosphere, thus: Higher CRF = more cloud cover = higher albedo of Earth = lower global temperatures. This is also why solar minimums tend to have cooler temperatures as lower solar wind = higher CRF for Earth]

[toml42]

Elliptical galaxies do have supermassive black holes. The current best thinking is that most elliptical galaxies are formed by collisions or interactions between spirals (which are far more common).

A disc is a very natural shape in the universe due to conservation of angular momentum in rotating collapsing gas clouds. Once you have a rotating disc, density waves arise naturally.