BOWEN, IRA SPRAGUE

(b. Seneca Falls, New York, 21 December 1898; d. Hollywood, California, 6 February 1973)

astronomy, physics.

Bowen was the son of James H. Bowen, Wesleyan Methodist minister, who could trace his ancestry to Welsh settlers who had emigrated lo Massachusetts in 1643, and of Philinda Sprague Bowen. Both his parents were graduates of Geneseo State Normal School. James Bowen died in 1908; and Ike, as he was called by his friends, was brought up by his widowed mother, who became a teacher at the Wesleyan Methodist Seminary in Houghton, New York.

Bowen, who was interested in science from boyhood, graduated as valedictorian of his high school class in 1915. His first three years of college were at Houghton Seminary’s junior college, where all the courses in mathematics, physics, and astronomy were taught by the president, J. S. Luckey. For his senior year Bowen transferred to Oberlin College, where he received the A.B. in 1919.

He then entered the University of Chicago as a graduate student in physics, studying for two years under A. A. Michelson and Robert A. Millikan. Bowen was Millikan’s assistant in his spectroscopy laboratory. When Miilikan moved to the California Institute of Technology as chairman of its executive council (1921), Bowen went with him as a junior faculty member. He did the laboratory work and analysis for their series of papers on the ultraviolet spectra and energy levels of highly ionized atoms. Bowen also taught physics to many of the top undergraduates at Caltech and received his Ph.D. in 1926. In 1929 he married Mary Jane Howard, a child psychologist. They had no children.

His great discovery was the identification of the “nebulium” lines in the spectra of gaseous nebulae. These two emission lines in the green spectral region, the strongest emitted in these nebulae, were of completely unknown origin. By 1926 physicists understood the periodic table and knew there were no unknown elements such as the hypothetical “nebulium.” Henry Norris Russell speculated that these lines might he emitted only in gases of very low density if, for example, it took a long time for atoms to make the relevant transitions. In 1927 Bowen, who had been thinking about this problem for years, suddenly realized that the nebulium lines might actually be emitted as “forbidden” transitions in commonions, such as O⁺⁺, that occur too infrequently to be observed in laboratory sources. His accurate data on the energy levels allowed him to check this hypothesis at once, and he found it to be correct. Rapidly following up his discovery, he identified many of the remaining lines in the spectra of nebulae with similar forbidden transitions in O⁺, N⁺, and other common ions, Bowen quickly developed the essentials of the physical picture of gaseous nebulae as we understand them today.

In 1934 W. H. Wright, at Lick Observatory obtained the first good ultraviolet spectra of planetary nebulae and noticed that the permitted O⁺⁺ emission lines in this region had quite abnormal relative in tensities. He sent his data to Bowen, who was able to explain them immediately as resulting from fluorescence. He had noticed years before that an accidental coincidence between the strongest resonance line of He⁺ and an O⁺⁺ line from the ground term would lead to selective excitation of a single energy level of the latter ion. This process would lead through a long chain of radiative decays to the emission of just those lines that Wright had observed to be strong in the planetary nebulae, thus observationally confirming Bowen’s physical idea.

Bowen spent the summer of 1938 at Lick Observatory and observed the spectra of planet an nebulae in collaboration with Arthur B. Wyse. They identified many more forbidden fines and from then quantitative intensity measurements were able to show that the abundances of the elements in the nebulae are approximately the same as in the sun and stars.

As a physicist deeply interested in astronomy. Bowen was a member of the Advisory Committee to the Observatory Council that guided the construction of the Palomar 200-inch telescope. In 1946 he was appointed director of Mt. Wilson Observatory. He supervised the completion of the 200-inch, then became director of Palomar Observatory in 1948. Boweins superb knowledge of optics was essential in bringing the 200-inch Hale telescope and the coudé spectrograph he designed for it into successful operation. He directed the research staff of Mt. Wilson and Palomar observatories during some of the most productive years of American observational astronomy. With the coudé spectrograph and the 200-inch, Bowen measured with high accuracy the wavelengths of main nebular lines and, inverting his procedure of thirty wars before. used them to determine more precisely the energy levels of the ions that emit them.

Bowen applied his knowledge and experience of optics to optimizing the design of astronomical spectrographs. His papers on this subject, on the 200-inch telescope, on Schmidt cameras, and on telescopes of the future were all highly influential in affecting the course of astronomy.

After he retired as director in 1964, Bowen devoted himself to telescope design. His masterpiece was the 100-inch Du Pont telescope at I as Campanas Observatory, Chile, a wide-field. very-high-definition instrument. He thus made important contirbutions to astronomy as a research worker, as an instruments maker, and as a scientific leader.

BIBLIOGRAPHY

I. Original Works. Horace W. Babcock. “IraSprague Bowen,” in Biographical Memoirs, National Academy of Sciences, 53 (1982), 8–119, contains a full bibliography of Bowen’s published papers and review articles. Some of his papers are in the California Institute of Technology Archives: the remainder are in the director’s files of Mt. Wilson and Las Campanas observatories, still closed but no doubt ultimately to be deposited in the obseravatories’ archives in Pasadena.

Donald E. Osterbrock