4.3 Gas Distribution Interstellar gas and dust are conspicuous in disk galaxies of type Sa and later. Most of this interstellar material is found in a disk with a scale height significantly smaller than that of the stellar disk. The radial distribution of gas in disk galaxies does not always follow the exponential distribution seen in the starlight. Molecular clouds tend to be more concentrated towards the centers of galaxies, often in the form of rings of dense gas. 4.5 Spiral Structure Disk galaxies exhibit a wide variety of spiral structure, a point explicitly recognized in the de Vaucouleurs' classification system.
Spiral arms are often sites of star formation.
However, near-IR observations indicate that in many cases the older stars also follow a spiral pattern. In barred S0 galaxies the bar is often the only structure visible in the disk. The surface brightness within the bar is often fairly constant. Many barred galaxies also contain luminous rings . Inner rings often appear to be elongated in the direction of the bar, while outer rings seem to be more nearly circular.