Thickness of the plane of the Milky Way as determined from 21 cm observations.The plot shows the layer thickness (measured as a gaussian width), with dotted lines for range 20°< 0 <160°, the dashed for 200°< 0 <340°, and the solid line the average of both ranges. 0 is galactocen-tric azimuth, where 0 = 0 is the direction I = 0 . [Butler Burton, Sterrewacht, Leiden University]
confined to the plane. Fig. 16.13 shows the thickness of the plane as measured by various tracers. In general, we think that components that are more closely tied to the plane are younger. This means that HII regions and molecular clouds have formed more recently than HI clouds or globular clusters.
Molecular hydrogen is more closely confined to the galactic plane than atomic hydrogen. The
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