The simple and abundant molecule CO forms only through gas-phase reactions. Its strong binding energy of 11.1 eV then helps preserve the molecule against further destructive reactions. Like H2, therefore, CO is self-shielding in the ambient field of ultraviolet radiation. In the outer regions of a molecular cloud, the two species build up in a similar manner, although CO remains dissociated to a greater depth. (See Chapter 8.)
It is fortunate for astrophysics that the CO molecule does have a permanent electric dipole moment and emits strongly at radio frequencies. Since its 1970 discovery in the Orion molecular cloud, CO has served as the primary tracer of molecular gas, both in our own and in external galaxies. The most abundant isotope, 12C16O, is naturally the easiest to detect, but 13C16O, 12C18O, and occasionally 12C17O and 13C18O, have also proved useful.
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