Reposition the arm so that one end is at the center of the rotating platform.  Mount the globe stand at the center of rotation and mount the Nerf ball at camera height.  The camera represents our position at the edge of one of the spiral arms with the Nerf ball representing the galactic center.  The view as the arm is rotated should show some parallax and a great deal of background movement in relation to the galactic center.  You can also use the dust mask plates from the 6F40.10 - Distortion due to Atmospheric Scattering - Reddening Due to Atmospheric Scattering or Interstellar Dust demo to show how the mass of stars and dust interfere with our view of the galactic center.  Some assorted size balls on Plexiglas handles can also be used to simulate large densities of stars obscuring the galactic center view.