Spiral Arms

[shadron]

I have long known the theory about spiral arms in the galaxy; that they are pressure waves moving around the galaxy, causing waves of star birth, and therefore brightness along the arms.

Then I saw this video (sorry to have to subject you to the numbing ignorance at the beginning):



Interesting. He doesn't mention that inside the pressure waves star birth would be preferentially encouraged, but I can see that.

My question is, he suggests that this starts when a planar galaxy is disturbed by another gravitationally. That's OK, but what happens when that happens again and again? There are also satellite galaxies, no doubt perturbing the main galaxy even more.

This would also say that elliptical galaxies, once they settled down enough to evolve into a flat configuration (which will happen as long as the galaxy has a predominant circular momentum), will again eventually become spiral. That pleases my sense of symmetry.

 

[Jack by the hedge]

Thanks. I've often wondered how that worked and that's a very pleasingly accessible explanation.

 

[shadron]

With some further cogitation I think I've answered my own questions. More perturbations will further stretch (or perhaps diminuate) the shapes of the ellipses, but not very much disrupt them, as the stars still have to come to a gravitational energy minimum by not overlapping their orbits. The perturbations will be visible in long term changes in the spiral patterns (cf, a ring galaxy).