  In the currently-accepted model for star formation out of the interstellar
gas in galaxies, the basic construction material is assumed to be large clouds
of atomic hydrogen (HI). These clouds are thought to form higher-density
complexes of gas and dust, and turn molecular (H2). Stars then form out of this
molecular gas.
  In this paper arguments are advanced for a contrary view, in which the basic
construction material is cold molecular gas out of which the stars form
directly. HI appears in the region when the leftover H2 is illuminated with UV
photons from nearby young stars. The physics of photodissociation regions
provides a natural and quantitative explanation for the appearance of HI
envelopes around the clouds, and for CO(1-0) emission from the higher-density
parts of their surfaces. In this picture, much of the HI in a galaxy is a
product of the star formation process, not a precursor to it.
