Brainlet question: how can galaxy shapes, like spirals, ever be considered "stable"? The function describing the required speed relations, based on "different distances to the galactic center", is linear (like $2 \times \pi \times r$), and pic related is far from showing a linear function? I can even hardly imagine a periodicity worth mentioning in galaxy shapes over time. And if we categorize galaxies based on their shape, don't we categorize them on a temporary feature?