Donald E. Osterbrock (1924–2007)
Received 2007 July 24; accepted 2007 July 25; published 2007 August 29
Donald Osterbrock, one of the leading astronomers of the second half of the 20th century, died in Santa Cruz, California, on 2007 January 11. During his long and productive career, he made many fundamentally important contributions to both theoretical and observational astrophysics, working on a wide range of problems, including the internal structure of lower main‐sequence stars, the physical conditions in ionized interstellar gas, and the nature of active galactic nuclei. In the latter years of his career, he wrote several books and papers dealing with the history of astronomy. An extensive summary of his “Fortunate Life in Astronomy” appeared in the 2000 volume of Annual Reviews of Astronomy and Astrophysics, which contains many details that won’t be repeated here. Don’s last official Lick Observatory photograph is shown in Figure 1.
Fig. 1.— Donald E. Osterbrock, circa 2000 (Lick Observatory photograph).
Don was born in Cincinnati, Ohio, in 1924. His father was a professor of electrical engineering at the University of Cincinnati, and he stimulated an interest in science and mathematics in his son, starting at a young age. By the time he was in high school, Don had a small, second‐hand reflecting telescope and decided he wanted to be an astronomer, but his college education was delayed by the outbreak of World War II. After basic training, the Army decided he could best serve the war effort as meteorologist and sent him to the University of Chicago for training. However, that training consisted mainly of taking all of the physics and mathematics courses required for a bachelor’s degree in physics, and these courses were taught by some of the leading figures in the world. He spent the remaining war years making weather observations on islands in the Pacific Ocean.
Following the war, he returned to the University of Chicago to complete his bachelor’s degree in physics. In the 1940s, the University of Chicago was one of leading centers of physics research, and Don’s teachers included such distinguished figures as Enrico Fermi and Gregor Wentzel. Don stayed on at Chicago for his graduate work in astronomy, eventually moving up to Yerkes, where the graduate astronomy program was centered. The astronomy faculty, many of whom are shown in Figure 2, was as distinguished as Chicago’s physics faculty, including such renown figures as S. Chandrasekhar, Otto Struve, Gerard Kuiper, William Morgan, Jesse Greenstein, and Bengt Strömgren. The combination of top theorists and leading observers in a single group proved to be an extremely powerful and effective way to build a world‐class research and teaching department. In the late 1960s, this Yerkes group served as a model for the development of the new Astronomy Department at the University of California, Santa Cruz, when the Lick Observatory staff moved there and began to add a number of theorists to the observational astronomer faculty.
Fig. 2.— Yerkes staff, students, and visitors (1950). Front row, from left: Lyman Spitzer, Jr., Paul W. Merrill, S. Chandrasekhar, Otto Struve, Dirk Brouwer, Gerald P. Kuiper, Otto Heckmann, and Nicholas U. Mayall. Second row: Guido Münch is between Chandrasekhar and Struve, T. D. Lee between Struve and Brouwer, and Karl‐Otto Kiepenheuer between Kuiper and Heckmann. W. Albert Hiltner is at far right end of second row, and Su‐Shu Huang is behind and to right of Lee. Second row from rear, at left: Stewart Sharpless, Irene L. Hansen (now Osterbrock), and Donald E. Osterbrock. Thornton L. Page is the tall man near the center of rear row, William W. Morgan is second to right of him, and Aden B. Meinel is at right of that row. (Caption courtesy of Donald Osterbrock’s Yerkes Observatory, 1892–1950. Photograph courtesy of Yerkes Observatory.)
Don’s Ph.D. thesis advisor was Chandrasekhar, who became a lifelong friend. His thesis was an extension of a well‐known theoretical problem to a more general, realistic situation. Previous studies had investigated how stars, generally initially formed with low velocities with respect to one another, acquire higher velocity dispersions with age as they gravitationally interact with other stars. Don also included the gravitational interactions of stars with a fluctuating‐density interstellar medium of gas and dust in his thesis investigation. Chandrasekhar was a serious, stern taskmaster and visited his students daily to check on their progress. For some students, these daily visits were a source of great anxiety. Later, when Don had his own students, he tended to follow the same practice, checking in every day he was present to monitor progress. However, his students didn’t find him anywhere near as intimidating a figure as Chandrasekhar could be to his.
While at Yerkes, he met, fell in love with, and married Irene L. Hansen. Irene was a native of Williams Bay and was employed as a member of the Yerkes staff (Fig. 2). They had a son, William, now living in Santa Cruz; two daughters, Laura of Seattle and Carol of Santa Cruz; and three grandchildren.
Don achieved early widespread name recognition while he was a graduate student for work that established that our Galaxy had spiral arms. Because we are imbedded in our Galaxy, and our ability to see very far in it is severely hampered by the widespread obscuring dust between the stars, it is very difficult to obtain a global view of the structure of the Galaxy. However, it was known from studies of other galaxies that the youngest stars and H ii regions are generally found distributed along the spiral arms of the galaxies. In this study led by William Morgan and including Stewart Sharpless, the distribution of young stars in our Galaxy was mapped out and revealed the existence of segments of three spiral arms in the vicinity of the Sun. This work was announced in a meeting of the American Astronomical Society, where it was greeted with a standing ovation. It was published as an abstract the following year and attracted a great deal of attention. Don became very interested in the clouds of ionized gas that surrounded the hot, young stars during this time.
After receiving his Ph.D. degree in 1952, Don spent a single but very productive year as a postdoctoral fellow at Princeton. Intensive work was going on there on the subject of the internal structure and evolution of stars, and he became interested in the problem of the structure of lower main‐sequence stars like the Sun, the red dwarfs. At that time, theoretical models of the structures of these stars failed to give the correct radii and luminosities for the chosen masses, while the models for high‐mass stars were very successful. Don reexamined the question of energy transport inside the red dwarfs. Energy generation takes place in the central regions, where the temperature is very high, and this energy flows out toward the cooler layers and ultimately to the surface of the star. It had generally been assumed that in the outer parts of low‐mass stars, the energy was carried by radiation, in the form of photons, but Don proposed that convective energy transport could make better physical sense. When he made calculations of the structure of red dwarfs using convective energy transport in the outer layers, his models had the radii and luminosities of observed stars; the internal structure of low‐mass stars was finally understood. His first treatment of the problem was done with a calculating machine, an advanced electronic adding machine, but he redid the problem with the new electronic computers that were being developed at Princeton by John von Neumann. The general result was the same, but much more accurate. From that time on, Don was committed to using computers to solving astrophysical problems. Inspired by these results, Martin Schwarzschild and Fred Hoyle, another visitor to Princeton at this time, computed the first successful models for red giant stars by including convective energy transport for the outer parts.
In 1953, Jesse Greenstein invited Don to join the developing astronomy department at Caltech. After the 200 inch Hale Telescope had been dedicated in 1948, Greenstein had been recruited from Yerkes to build up a strong astrophysics department. Greenstein had already hired Guido Münch and added Art Code in 1956. Osterbrock, Münch, and Code were all students of Chandrasekhar and were thus experienced theorists, but they were all also very capable observational astronomers and could take excellent advantage of the powers of the 200 inch. In those early days of this new department, telescope time was considered a “right” of being a faculty member, with no need for a time assignment committee, and Don received a generous allotment of 200 inch time. His early observational research was on several topics, including studies of stellar evolution in globular clusters and nearby galaxies. This was based on stellar photometry derived from photographic plates and was challenging even for the biggest telescope in the world.
Walter Baade and Rudolf Minkowski were also in Pasadena at this time, and both had interests in emission‐line objects. These ranged from supernova remnants to what are now called active galactic nuclei. With large telescopes, it became possible obtain spectra of these objects, many for the first time. Of course they all had strong emission lines. With the brighter objects, even high‐resolution spectra were possible, allowing close doublets, such as the [O ii] λλ3726, 3729 lines, to be measured.
Although Don was heavily involved in observational research at Caltech, it turned out that the direction of his primary research over the next decade and beyond was strongly influenced by a paper by British theorist Michael Seaton (Fig. 3). In 1954, Seaton published calculations of the collisional cross sections for electrons interacting with O+ ions. The radiative transitions from the levels excited by these collisions are highly forbidden, but in the low‐density ionized gas around hot stars, this radiation can be very strong. Since the strengths of the different transitions are dependent on the density of the gas, measurements of relative line strengths can give direct information about the density of the emitting region. Don embarked on an extended study of [O ii] emission, concentrating first on the Orion Nebula, but later extending it to a variety of different kinds of nebulae. He later collaborated with Seaton on improving the O+ calculations, taking advantage of the much better electronic computers that were becoming available for the quantum mechanical calculations. In addition, there are many other collisionally excited forbidden lines of other ions seen in gaseous nebulae, and Don made the theoretical calculations needed to make use of several of these for deriving physical conditions in gaseous nebulae. In a short time, he established himself as one of the world’s leading authorities on gaseous nebulae.
Fig. 3.— Mike Seaton and Don Osterbrock at the 1994 Lexington Workshop on Mode l Nebulae, in front of a statue of Man O’War (Robert H. Rubin photograph).
Shortly after arriving at Caltech, Osterbrock began supervising his first graduate student, George Abell, who did a thesis on the distribution of clusters of galaxies on the sky. Abell was responsible for taking most of the photographic plates that made up the National Geographic Sky Survey, all taken with the 48 inch Schmidt at Palomar. Abell identified all the galaxy clusters he could on these plates, and examined the question of whether clusters themselves were randomly distributed on the sky. His thesis involved extensive statistical tests, and he concluded that many clusters were associated with other clusters to form very large structures of matter in the universe. Don was always a bit concerned about the highly abstract treatment of the data that gave the result, but he felt Abell was an extremely careful scientist. Abell’s result was not generally accepted, in part because many astronomers were uncomfortable with his highly mathematical, statistical treatment of the data, although today, of course, the existence of large‐scale structures is a well‐known feature of the universe. Abell was just the first of a long line of Don’s graduate students, many of them becoming highly respected astronomers; four of his students became directors of major observatories. Don was very much a “hands on” thesis advisor, typically meeting with his students every day, and he developed lifelong close friendships with nearly all of them.
At the invitation of Albert Whitford, in 1958 Osterbrock and Code left Caltech to help build up a first‐class department of astronomy at the University of Wisconsin in Madison. There was no denying that Caltech and Palomar offered the finest observing opportunities in the world, but Don and his wife, Irene, missed the Midwest, and especially Wisconsin, and they were very happy to leave the smog and congestion of the Los Angeles area. Don was able to continue his study of gaseous nebulae, using modern equipment on smaller telescopes at a dark site near Madison. Several of his Wisconsin students wrote excellent theses on topics relating to nebulae. However, in the early 1960s, interest intensified in the study of external galaxies with active nuclei; that is, nuclei radiating considerable amounts of energy derived from nonstellar sources. The discovery of a new, very luminous class of these objects, called quasars, provided a tremendous stimulus for research in active galactic nuclei (AGNs) in general. The spectra of these nuclei are often dominated by the same emission lines seen in gaseous nebulae around stars in our galaxies, but clearly the physical conditions are very different in galactic nuclei. Don was naturally drawn to developing a research program on these objects, as his understanding of spectra of emission‐line regions could be put to excellent use. Research on AGNs dominated his research for the rest of his career.
Don wrote a major textbook, Astrophysics of Gaseous Nebulae, that in a second edition added and Active Galactic Nuclei to the title. They were usually called “AGN” and “AGN2.” They became the leading textbooks for the ionized interstellar medium for several generations of astronomers. AGN was designed to cover material that was not present in Spitzer’s books, so that material in Osterbrock and Spitzer's texts would be a complete introduction to the material between the stars. A third edition of AGN (actually the second edition of AGN2) was recently published, with Gary Ferland as a coauthor.
Don left Madison in 1973 to become the director of the Lick Observatory, after being heavily recruited for the job. Founded in 1888, the Observatory faculty and staff resided completely on Mount Hamilton until 1966, when the majority of them moved to the new campus of the University of California in Santa Cruz. The addition of theorists to the department was begun a few years after the move. With the Observatory headquarters in Santa Cruz, Don took up a faculty position as part of becoming director. One major attraction for making this move was the availability of the 3 m telescope at the Observatory. It was then the second largest telescope in the world. Furthermore, the 3 m had recently been equipped with a new, digital spectrograph that was more powerful than any available elsewhere. These were ideal facilities for the study of AGNs. He made excellent use of the equipment, and together with a string of excellent students and postdoctoral fellows, produced a series of major papers on the subject. He considerably expanded the classification scheme for different types of AGNs and also steadily improved geometrical models for the arrangement of the material in the AGN, based on studies of the physical conditions in the ionized gas there. He concentrated his efforts on the AGNs known as Seyfert galaxies and radio galaxies, as others using Lick facilities were working on quasars.
Don carefully reserved about half his time for research. He had a “research office” located in the midst of the faculty offices, and he tried to spend half of each day there working on research and meeting his students. He also took very seriously his directorial responsibilities. Lick was going though a phase of rapid advances in instrumentation, which Don fully supported. The project that came to dominate all others grew out of a desire by UC astronomers to have a new “dark sky” observatory site. The steady growth in light pollution from San Jose and nearby cities was making work on faint objects extremely difficult. After years of consideration of constructing a 4 m class telescope in a California or Arizona dark site, a new project emerged with the goal of building a much larger telescope, the biggest in the world. Two competing proposals were developed for building a 10 m telescope, one with the primary mirror made out of a single thin meniscus of glass, and the other with the primary being made up of a mosaic of segments. Each approach had strong supporters. The thin meniscus would likely be more straightforward to grind and polish than the off‐axis segments of a segmented mirror, while the segmented mirror would be much lighter and easier to support and transport. As director, Don very skillfully led a process that finally arrived at a choice: a segmented‐mirror telescope. This was not a simple process, as scientists throughout UC were involved, and each design approach had its strong proponents; emotions ran strong. The project continued and ultimately resulted in the construction of the two Keck 10 m telescopes on Mauna Kea, the largest optical/infrared telescopes currently available. History has shown that there isn’t a clear superiority of one approach over the other, as very successful large telescopes have been built using both mirror designs.
Don did not feel well‐suited to leading the Lick Observatory during a time when a major new observatory was being built, and he stepped down as director in 1981, early in the project. He returned to full‐time teaching and very active research on AGNs until his retirement in 1991.
He found time after his retirement to pursue his interest in the history of astronomy. He felt that history was too important to be left to historians, who too often pursue a deconstructionist approach. He wrote 12 books about major astronomical figures of the last century, and about the histories of the Lick and Yerkes Observatories. He very successfully made the transition from a research astronomer with a strong international reputation to a highly respected historian of astronomy. In 2002, he received the Leroy E. Doggett Prize for Historical Astronomy, Historical Astronomy Division, of the American Astronomical Society in recognition of this work. Don remained active to the very end and was engaged in historical research when he died unexpectedly.
Don received much recognition for his research. In 1976 he was made an Associate of the Royal Astronomical Society, and in 1997 he received its Gold Medal, one of the very few Americans to do so. He was a member of both the National Academy of Sciences and the American Philosophical Society. Don was awarded both the Bruce Gold Medal of the Astronomical Society of the Pacific and the Russell Lectureship of the American Astronomical Society in 1991, and served as the president of the AAS from 1988 to 1990. He received honorary D.Sc. degrees from five universities.
He produced 21 Ph.D. students who have turned out a substantial fraction of today’s researchers in AGNs and emission lines. His AGN books have educated several generations of astronomers and made the study of emission‐line regions accessible to nonspecialists. His passing had a profound impact on the unusually large number of people who were his close friends, colleagues, and former students.