USC&GS and NRL
After what in retrospect were two idyllic summers, I had to find someone to pay me to do what I wanted to do. In the fall of 1948 I got a job with the title of mathematician (!) in the Astronomy and Geodesy Section of the U.S. Coast and Geodetic Survey in Washington, D.C. It sounds pretty grand for a new college graduate. It wasnít. The Survey used observations of the position of stars to establish precise locations for their primary benchmarks. It was my job to enter data from the field into a system of well-established formulas to derive these locations. In other words to fill out stacks of forms. Not quite what I had in mind. However, I did learn about the discipline of a real job and about how much of any job, even in science, is made up of dull, demanding hours. But then, my desk was in the Commerce Department at 14th and Pennsylvania Avenue in the Nationís Capital. It was a perfect city for the young and poor - parks and monuments and libraries and museums - all free and safe.
In March of 1951 came another time of good fortune. In the Sunday newspaper I saw an amazing picture. It showed a large (50 ft.) dish antenna on the roof of a building at the Naval Research Laboratory. The final wedge-shaped section of the antenna was just being lowered into place. I knew that there had been observation of the sunís emission at radio wavelengths with such an antenna. Perhaps this was my opportunity. I discovered that not only was it planned to do "radio astronomy" with it, but there was no one at NRL with as much knowledge of astronomy as I had, small though that was. So I was hired! The men who were planning to use the antenna had the vision to see that their understanding of electronics gained in the development of radar during the war could be useful in the infant new branch of astronomy. I knew nothing of electronics, but apparently I could bring something useful to the group.
The years I worked at NRL were full of variety and surprises. When I first arrived the equipment was not ready and I busied myself in translating French astronomical articles. Again not what I had in mind, but useful at least to me. By early summer, however,
Dr. John Hagen, director of the section, put into operation his equipment to record radio radiation from the sun at a wavelength of 8.5mm. This wavelength provided a new window through which to observe objects beyond the earth; it was shorter than any so far used. (The window has now been extended to wavelengths more than ten times shorter, but at the price of observing at the South Pole!) The antenna we used was a mere two feet in diameter - a far cry from the huge one on a nearby building. It was, however, ideal for our purpose. The idea was to record the radiation from the whole disc of the sun throughout the day in order to detect any sudden increase in its intensity.
Such sudden increases, characterized by a rapid rise in intensity and a slower fall, had been observed at longer wavelengths. They occurred near the time of large solar flares observed optically. We discovered bursts of radiation on five occasions between May 1 and October 1 (resulting in my first published paper! [Hepburn & Hagen, Solar Outbursts at 8.5mm Wavelength: Nature 170: 244, 1952]). Such bursts had been observed at longer radio wavelengths, including at 3 cm at NRL. Observation of bursts at very short wavelengths is of interest since the radiation presumably comes from deeper layers in the sunís atmosphere. One type of burst occurs first at the shorter wavelengths, and then at successively later times at the longer wavelengths, suggesting that material flowing out of the sun is the source of the burst energy. However, one burst occurred at precisely the same time at 8.5mm and 3cm, suggesting that for some bursts another source of energy must be sought. Our study required data from other astronomers, including Dr. Helen Dodson at the University of Michigan.
About this time something else reminded me of her. I found that I too could "sail" to work. Well, it wasnít quite sailing. NRL had a small motor launch (manned by US Navy sailors, no less) to ferry workers across the Potomac. It ran summer and winter in the best Naval tradition; I especially remember very windy winters.
Early in 1952 came another stroke of luck. At Harvard H.I.(Doc) Ewen completed his thesis with Dr. Edward Purcell. They had discovered the radiation of neutral hydrogen atoms in the interstellar gas. It was a stunning discovery. This gas lying between the stars had previously been detected only by absorption of the light of bright stars. Therefore, our knowledge of a major component of our galaxy had been derived only from observations in directions that happened to coincide with the presence of suitable stars. In addition the components of the gas detected by the absorption of starlight did not include the most abundant element in it - neutral hydrogen. The discovery that this element emits radiation detectable at a radio wavelength of 21 cm is one that continues to yield new insights, and exploring the interstellar gas became the focus of my own research. A sensitive receiver for 21cm wavelength was developed for our 50-foot dish. The intent was to make a more precise study of hydrogen clouds in interstellar space. What happened demonstrates the fun of scientific research. With the new instrument John Hagen, Ed McClain and I (as a very junior partner) decided to try out the instrument by looking for radiation at 21cm wavelength from some astronomical objects known to emit not only light but also a broad band of radio wavelengths. These objects could probably be more easily detected than the narrow-band and faint radiation of atoms in interstellar space. Some had already been observed at other radio wavelengths (including 3cm at NRL) However, in these early days of radio astronomy it seemed that every new observation yielded unexpected results - heady days indeed.
Our survey detected twenty sources, all but three of them associated with optically known objects. Two of those were known from longer wavelength studies, but one remained a mystery. It was a complex source known as NRL#9. I remember looking at its coordinates and having the feeling that they were familiar. Indeed they should have been. The source lay precisely in the direction of the center of our galaxy!
But another discovery awaited us. It was decided to be efficient about these observations by looking simultaneously at the continuum radiation from these sources and at the hydrogen radiation originating in interstellar gas in the same direction. There were two paper tape recorders displaying two lines that varied as the antenna was swept across the position of each source. As the source came into the beam the line from the broadband continuum receiver went up. To our amazement the line from the narrow band hydrogen-line receiver went down. This was the first evidence that interstellar hydrogen was absorbing the radiation passing through it. A new tool for probing the interstellar medium at radio wavelengths had been found.
During the same period studies of the sun continued at NRL. In February 1952 there was a total eclipse of the sun visible from Khartoum, Sudan, and an expedition went to observe it. Both optical and radio observations were made. It was the eleven photographs of the solar corona that occupied many months of my time. The photographs were made on 8x10 plates (again) with a camera whose focal length was 18 feet. (How this enormous camera was set up in the desert in the Sudan is a story in itself.) As a result the image of the moon (blocking out the disc of the sun) was about two inches in diameter. The photographs made possible a photometric study of the whole inner corona. I want to describe the scheme in some detail to give an idea of the magnitude of the task.
A platform slightly less than two inches in diameter was added to a device for measuring the intensity of a small beam of light transmitted through the photographic image. With the image of the sun centered precisely on the platform it was rotated (at a rate of about four minutes per rotation) while a pen recorder registered the changing intensity along a circle around the sun. The table was then moved so that the light beam traced out a circle with a radius 0.2 mm larger, out to the edge of each of the eleven plates. All very clever, but not really necessary. When I began to analyze the data I realized that all I really needed was a series of measurements along a few selected radii, because the information of use in understanding the corona lies primarily in changes in the radial direction.
The experience of wasting so much time and effort by following someone elseís direction convinced me that I wanted to direct my own work. (It was not that I was sure I could avoid mistakes but that I wanted them to be my own.) The only way to do that was to go to graduate school for the proper credentials. Another incident in connection with my paper on the solar corona serves to emphasize a less tangible value in obtaining a graduate degree - a certain self confidence (or arrogance?). A visiting astronomer (who will remain nameless) advised me to leave out of my paper my conclusion that material appeared to be flowing out through the coronal "streamers". I took the advice and came to wish I hadnít.
Although I had nothing to do with it, in 1954 while I was still at NRL Dr. John Hagen was charged with overseeing the
Vanguard project, our countryís first attempt to launch an artificial satellite. It was a thankless task and in the end an unsuccessful one. In times to come satellites would play a significant role in my career.