One theory suggests that we are observing a density wave sweep through material, which causes compression and heat, increasing the likelihood of new stars to be born, these stars burn bright and white. As this wave passes through, it leaves behind those stars in its wake, which over time burn less brightly than before.
One analogy that comes to mind an electronic circuit, although electricity travels at the speed of light, the actually charge carriers are moving relatively slowly, know as the drift velocity.
And (I'll note that) the drive belts are usually hidden from view.
For example, the black hole at the center of the Milky way is estimated at 4.1×10^6 M (solar mass) and the total Milky Way mass is estimated at 0.8–1.5×10^12 M (figures from Wikipedia).
Some interesting articles found while digging for these numbers:
https://ned.ipac.caltech.edu/level5/Merritt/Merritt1.html