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The gaseous component of the interstellar medium is confined almost entirely to a thin disk in the plane of the Galaxy. In the vicinity of the Sun the disk is only about 1000 ly in thickness. About 90 percent of it by number is hydrogen, of which perhaps half is in molecular form and half in atomic form. Atomic hydrogen occurs in both neutral and ionized forms. Molecular hydrogen and ionized hydrogen are found in only a small fraction of interstellar space compared with the vast volume in which neutral hydrogen atoms are located. And where either hydrogen molecules or hydrogen ions are located, they are the most prevalent form of hydrogen, the other being absent. Because hydrogen is the main ingredient in the interstellar gas, astronomers generally designate a region in which hydrogen is predominantly ionized as an H II region and a region where hydrogen is predominantly neutral as an H I region. The Balmer alpha line of hydrogen is responsible for the vivid red color of many H II regions. These H II regions are produced by hot stars and are associated with interstellar clouds either by being surrounded by them or by being on the edge of a cloud complex. |