One of the principal methods for obtaining the electronic spectra of unstable radicals is to study the spectra of comets, which are almost entirely due to radicals. Many radical spectra have been found in cornets, including that due to CN. These radicals are produced in comets by the absorption of far ultraviolet solar radiation by their parent compounds. Subsequently, their fluorescence is excited by sunlight of longer wavelength. The spectra of comet Hale-Bopp (C/1995 01) have been the subject of many recent studies. One such study is that of the fluorescence spectrum of CN in the cornet at large heliocentric distances by R.M. Wagner and D.G. Schleicher (Science 275, 1918 (1997)), in which the authors determine the spatial distribution and rate of production of CN in the coma. The (0-0) vibrational band is centred on 387.6 nm and the weaker (1- 1) band with relative intensity 0.1 is centred on 386.4 nm. The band heads for (0-0) and (O-I) are known to be 388.3 and 421.6 nm, respectively. From these data, calculate the energy of the excited 51 state relative to the ground So state, the vibrational wave numbers and the difference in the vibrational wave numbers of the two states, and the relative populations of the v = 0 and v = 1 vibrational levels of the SI state. Also estimate the effective temperature of the molecule in the excited 51 state. Only eight rotational levels of the SI state are thought to be populated. Is that observation consistent with the effective temperature of the 51 state?