UC Berkeley: OH
On March 3, 1965 I began an exciting time at the Radio Astronomy Laboratory of the University of California at Berkeley. The Lab and the campus were what I had been looking for (although in 1965 the Berkeley campus was a bit more than I had bargained for). Harold Weaver and David Williams had used the Berkeley 85-foot radio telescope to confirm the detection of OH in the interstellar gas, and they set out (just as I arrived) to look for the molecule elsewhere in the galaxy. This enterprise turned out to be the most exciting project of my career.
The relatively complex structure of the OH molecule (compared to the simple hydrogen atom) led to great complications in its study in the interstellar gas. The spectrum of OH shows radiation at four distinct frequencies: 1667, 1665, 1612, and 1720 MHz (all at approximately 18 cm wavelength). The 1667 "line" observed in the laboratory is about twice as intense as that at 1665, and about nine times that at 1612 and at 1720. Naturally searches for OH were made at 1667 MHz. However, no OH was found in absorption in the direction of three bright sources (one of which was the Orion Nebula). And to the dismay of those who hoped OH could become a tool like neutral hydrogen for studying the interstellar gas no emission from the molecule was found in searches around the galaxy.
Nan Dieter Conklin in the Hat Creek control room
(Image courtesy of N.D. Conklin)
With a new 100-channel receiver at Hat Creek (the Berkeley observatory) Harold Weaver, David Williams, and I searched for OH in absorption in fainter continuum sources. In due course we reached the source, designated W49. In its direction only obscuring dust clouds are observed optically, but behind them lies a large nebula similar to the Orion Nebula. Our first observations of W49 at 1667 MHz provided the greatest surprise since the discovery of OH. The line was not in absorption but emission, and its intensity was 100 times the upper limit set in searches elsewhere in the galaxy! Since the 1667 was so strong, we looked also at 1665, expecting the emission to be about half as strong. It was in fact much stronger! It was obvious that we should stop assuming anything about OH in the interstellar gas.
So, we set out to search for OH in a variety of sources at the supposedly weaker frequencies. In setting up a list of likely sources I went for advice to Rudolph Minkowski, who was a senior advisor in the Lab. He had been in charge of the Palomar Survey of the Northern Sky and had a set of the photographs in his office. This survey was intended to cover only regions north of a certain declination, but he had slipped in one just below the limit. On the photograph was a large, complex nebula designated NGC6334. "Try that one", Rudolph said. We were to be very glad we took his advice. NGC6334 showed very strong emission at 1665, but another surprise lay in store. One day in October 1965 I was looking at a new observation of NGC6334 and happened to have on my desk at the same time an observation made in July. They were different!! Such a thing had never been seen before. Things had been puzzling in the interstellar medium, but conditions did not change in a matter of months. In fact, both this source and the one in Orion changed in days. All these peculiarities led to the conclusion that something unsuspected exists - a MASER in the interstellar gas (the microwave equivalent of a LASER).
Such surprising results required confirmation. They were all confirmed although the variability of emission with time met with considerable skepticism. One of our Lincoln Lab colleagues called to say "Iím sorry, Nan, but it doesnít vary." To which I replied with a snap, "Thatís because you havenít looked long enough!" I had gained some confidence with the years.
My next project was of quite a different sort. I was asked by the editor of Reviews of Modern Physics to write an article on recent work on the interstellar medium. Flattering, but I knew that my knowledge of theoretical studies was far from adequate. Fortunately Miller Goss was at that time a Berkeley graduate student, and he joined me in writing the paper. I was particularly glad to write the long paper because I enjoyed trying to make my writing as lucid and graceful as possible within the restrictions of brief scientific communications, and here I would be able to stretch my literary wings. The last paragraph of the paper remains true 33 years later:
Our intention was to describe the progress that has been made in building up a detailed physical description of the interstellar medium. The outcome has been a picture full of uncertainties, but one that gives the impression that the discovery of a few vital pieces of the puzzle will clarify it all. The search for these pieces is likely to be a fascinating pursuit.