http://www.youtube.com/watch?feature=player_embedded&v=sI23cwbbNqs
Spiral galaxies are a stunning sight. As Don Lincoln explains in one of this week’s TED-Ed lessons, these galaxies were thought to rotate in highly predictable dances, with stars close to the center moving slowly and stars further away rotating quickly because they are pulled by the mass of the stars between them and the center. The stars located very far from the galaxy center were thought to rotate extremely slowly because of the reduced pull of gravity.
