system
Given the following post, respond as if you're commenting on Reddit:
human
Why are the motions of spiral galaxies so regular and predictable, as opposed to chaotic billion-body problems?

I know that systems of bodies linked by gravity are chaotic in the majority of cases for n>=3. Why is it that galaxies (in particular spiral galaxies) appear to have regular patterns in their stellar movement? Shouldn't it all be a jumbled mess of billions of bodies interacting chaotically?
gpt
For the same reason that the air acts fairly predictably, even though a litre of air contains *many* more molecules than a galaxy has stars. A galaxy is sort of a (pressureless) "gas" of stars. Although the individual paths of stars might be hard to predict (although easier than predicting the paths of molecules, because stars don't often have close encounters), the general picture is not too difficult. You have a big ball of millions of stars, with such and such a distribution of velocities, then you can predict if it'll collapse or expand or whatever.

*However* things can be a bit chaotic. We can predict for instance that a galaxy will form a bar or a spiral, but we can't predict exactly when it'll appear and what angle it'll be at etc. This is because the random motion of every single star actually matters here, and that's impossible to measure accurately.

Edit: Ooh also, for disc galaxies, they form from gas which can't have extremely large scale chaotic motions, because gas bumps into gas and changes its momentum until that stops happening, so the gas has to have a nice circular orbit to not hit anything, and so when it collapses into stars the stars end up with a fairly circular orbit too.