the rotation of a galaxy isnt the same thing as the rotation of the stars contained within in. The rotation of the galaxy is measured by the rotation of arms/other features, which are due to a traveling density wave (think about how traffic speeds up/slows down)
If stars orbit faster than arms, doesn’t this mean that stars have to occasionally change which arm they’re in? How does that work - some sort of gradual re-clumping of stars?
Yes, our solar system travels from arm to arm! while the overall shape of the galaxy remains.
By the way, our transit between and into these galaxy arms is theorized to coincide with past mass-extinction events, across tens of millions of years.
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.”
As another commenter points in another thread, this article is explcitly about the red-shift-calculated velocity of stars at the outer edge of galaxies, and not about the spiral arms' apparent motion.
