It contains about 100 billion solar masses of material, and every star you can see, even with a moderately large telescope, is in our Galaxy. The Sun definitely resides in its outskirts, at a distance of about 28,000 light years from the center. The whole galaxy would be roughly 100,000 light years across. The central part of the Galaxy is inflated into a spherical structure, or bulge , and some matter is distributed in a thinner spherical halo which extends to large distances. The old globular clusters Shapley studied are associated with the halo.
Interstellar dust is the fine haze of smokelike particles which is distributed between the stars.
Dust is visible as the dark lanes in the star forming regions illustrated in Lecture 5 and also in the dark rifts in the Milky Way in the picture at the beginning of the next section. Dust scatters and absorbs optical light. If there is enough dust in a given direction, it can totally obscure our view of distant regions. The Sun is roughly centered vertically in the plane. The stars we can easily see are therefore mostly associated with the disk.