The distribution of optically observed H II regions and OB stars with galactic longitude indicates that it is primarily determined by extinction by interstellar dust. Thus optical observations can, at the best, reveal the local structure in the vicinity of the sun. Radio observations, on the other hand, are not affected by dust. Thus the distribution of galactic radio sources, which peaks in the northern part at about lII = 17°.5, must be related to the large-scale structure of our Galaxy. Two radio recombination line surveys of the northern and southern sky yield kinematic distances. If only the ‘giant H II regions’ are retained, the following distribution is obtained: (1) Only 5 giant H II regions are found within the 4 kpc arm. (2) The bulk of the giant H II regions is concentrated in a ring between 4 and 6 kpc from the galactic center. (3) There are other concentrations of giant H II regions indicating the existence of the Sagittarius and Perseus arm. (4) The three features revealed by optical observations of H II regions in the vicinity of the sun cannot be matched with the large-scale distribution outlined by giant H II regions. This is particularly true for the so-called Orion arm. (5) At distances beyond 13 kpc from the galactic center virtually no giant H II regions are found. (6) The surface density of giant H II regions attains its maximum between 4 and 8 kpc; the surface density of neutral hydrogen (H I) attains its maximum between 11 and 15 kpc, but the actual space density of H I in the region 4 to 8 kpc may still be rather high.
The Physical Properties of Large Scale Systems from Optical Observations