Fig. 4.3. The spectrum of £ Oph in the vicinity of the strongest interstellar lines of CN, CH, and CH+, from our first 25 Lick spectra, ca. 1968 (Bortolot, Clauser and Thaddeus 1969). The R(1) and P(1) lines of CN are a direct result of the CMBR, and provide a measure of its intensity at A = 2.6 mm near the peak of the 3-K Planck curve. Directly above each line the transition is shown on a diagram of the relevant energy levels. "a" indicates the strength of the missing lines if the intensity of radiation in interstellar space at A = 1.32, 0.359, and 0.059 mm were that obtained by Shivanandan, Houck and Harwit (1968) from a rocket flight which suggested a large amount of short wavelength background radiation. ©1969 American Physical Society.

never again experienced in research - a shock of recognition perhaps comparable to that felt by Rutherford when he saw the back-scattered alphas and realized that our entire picture of the structure of matter was wrong. The now largely obsolete techniques and paraphernalia of optical photographic spectroscopy - the baking and cutting of the big glass plates to fit the curved plate holders, the development of the plates in total darkness in the darkroom with its characteristic pungent smell - provided pleasures which I am afraid few astronomers today will enjoy.2

2 My early work on measuring the CMBR with interstellar molecules is summarized in Thaddeus and Clauser (1966), Bortolot, Clauser and Thaddeus (1969) and Thaddeus (1972). Two other

Although the excitation of interstellar CN is remarkably simple, determined almost entirely by the photons of the microwave background, the general question of molecular excitation in the interstellar gas is still very much with us, with fascinating ramifications. In general, a molecule in space is subject to simultaneous excitation by three thermal or quasi-thermal reservoirs: the microwave background at 2.725 K, collisions with the ambient gas at a kinetic temperature typically between 20 K and 200 K, and background starlight, highly dilute radiation at 5000-10,000 K. There are astronomical regimes where each of these reservoirs predominates, and others where the competition between them can produce striking departures from thermal equilibrium: population inversion and maser amplification in OH and H2O, and refrigeration below 3K in formaldehyde, so that absorption against the CMBR is observed in the absence of any localized background source, an observation which would have been quite unintelligible before the discovery of the CMBR. A questionable but highly interesting example may be provided by the interstellar diffuse bands, the several hundred unidentified interstellar features which date from as long ago as the 1930s. It is widely thought that the width of these bands is the result of rapid radiationless transitions, but other interpretations are possible. The bands seem to be formed preferentially in low-density regions of the interstellar gas, and they are probably produced by strong electronic transitions with large f-values. Under these conditions, rotational excitation by starlight is a possible cause of the width of the diffuse bands, a mechanism which we considered early on to explain the excitation of interstellar CN, but rejected because of the small f-value of the CN optical transitions.

No one in the 1960s realized how profoundly the discovery of the microwave background would enhance our knowledge of the universe - realized that we were on the eve of great events which would alter cosmology beyond recognition in little more than a generation. (Possibly Dicke, our most far-seeing teacher, sensed this.) Comparable in its impact to Watson and Crick's model of the structure of DNA, the discovery of the background started a revolution, which has yet to run its course. To me as a practitioner of small science, it is a pleasure to realize that much of this revolution (obviously not all) has been done on the cheap, by small teams, in the classical way in which experimental science has been pursued since Jansky, Rutherford, Faraday, and before.

papers based on our Lick high-resolution optical spectra are Probable Detection of Interstellar

13 CH+ (Bortolot and Thaddeus 1969) and Weak Interstellar Lines in the Visible Spectrum of

Z Ophiuchi (Shulman, Bortolot and Thaddeus 1974).

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