But, the arms don't rotate either! There are some great animations here: https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density

But, the arms don't rotate either! There are some great animations here: https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory The article was about stars on the edge of a galaxy. > “But regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round.”