I think he was more saying something along the lines of: consider all the stars that lie on a certain circle centered at the center of the galaxy - if you take the average velocity of all of those stars, it won't depend on the size of the circle.Ã‚Â Basically, that velocity is independent of radial distance from the center of the galaxy, although there's no doubt that there's variation here and there -- in other words, the variation that does exist isn't dependent on distance from the center.
We're talking about something that can't happen over billions of years, because it is a catch 22. Bear with me and let me lay some background.
If you think of a moving merry-go-round, the farther you get from the center the faster you go. Therefore the outside perimeter of the MGR must move faster to keep up with the slower moving center circles.
1. Now, we lie on a spiral arm of the Milky Way. In order for our galaxy to stay in order we would have to move much faster than stars inside. If not we would break off the galaxy.
2. Now here's the catch 22. Just like the MGR has alot of centrifugal force on the outside perimeter, if we are indeed moving much faster than the inside stars, our band should have turned perpendicular and/or broken off by the same principle. That is if we had 16 billion years.
3. If your model says all the stars have independent speeds--then the galaxy breaks apart because of gravitational discontinuity (unless the speeds are proportionally different according to distance from the center, but then No. 2 breaks apart the galaxy)
4. If your model says that all stars move at relatively the same speed--then the galaxy breaks apart because of gravitational discontinuity, and No. 1.