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Optical observations provide a wealth of information on atomic abundances, the excitation (level of ionization), the kinetic temperature, and the density of the gas in H II regions. However, obscuration by dust limits optical observations of galactic H II regions to those which are nearby (within a few kpc) and which are unobscured by the molecular clouds out of which they are formed (and so generally evolved). Radio observations are unhampered by extinction but provide much more limiteddoi:10.1017/s0074180900075239 fatcat:owszi3cjvreqldhfz4n5lckr7q