There is a gaseous interstellar cloud in the constellation Ophiuchus that is illuminated from behind by the star ζ Ophiuci. Analysis of the electronic-vibrational-rotational absorption lines obtained by H.S. Uhler and RA Patterson (Astrophys], J. 42, 434 (1915)) shows the presence of CN molecules in the interstellar medium. A strong absorption line in the ultraviolet region at A= 387.5 nm was observed corresponding to the transition] = 0-1.
Unexpectedly, a second strong absorption line with 25 per cent of the intensity of the first was found at a slightly longer wavelength (∆λ = 0.061 nm) corresponding to the transition J = 1-1 (here allowed). Calculate the temperature of the CN molecules. Gerhard Hertzberg, who was later to receive the Nobel Prize for his contributions to spectroscopy, calculated the temperature as 2.3 K. Although puzzled by this result, he did not realize its full significance. If he had, his prize might have been for the discovery of the cosmic microwave background radiation.