The outer regions are where the spiral arms are found, dominated in appearance by open clusters of young, bright, blue stars, by red emission nebula, and by dark dust.
The spiral arms reside in a disk dominated in mass by relatively dim stars and loose gas composed mostly of hydrogen.
What is not depicted is a huge spherical halo of invisible dark matter that dominates the mass of the Milky Way as well as the motions of stars away from the center.
