Bok, Bartholomeus (Bart) Jan

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(b. Hoorn, Holland, 28 April 1906; d. Tucson, Arizona, 5 August 1983)

astronomy and astrophysics, studies of the spiral shape of the Milky Way and of the formation of stars.

Bok was an essential figure in twentieth-century understanding of the Milky Way. Through a two-tracked research career, he offered a visualization of this galaxy both by helping determine its shape as a spiral galaxy and by his work on the nature of how stars are formed in it. With Edith Reilly he discovered that very small, dense dark nebulae, later called Bok globules, are really the cocoons of nascent stars. As assistant director of Harvard College Observatory and director of Mount Stromlo Observatory in Australia and Steward Observatory in Arizona, he contributed to the evolution of modern astronomy through his designs of new telescopes and observatory structures in both these places.

Early Life. In a land famous for its astronomical contributions, including the first use of the telescope almost three hundred years earlier, Bartholomeus Jan Bok was born on 28 April 1906. (His long first name was legally changed to Bart when he became a U.S. citizen in 1938.) The family lived in Hoorn for only nine months before his father, Jan Bok, a sergeant-major in the Dutch army, was transferred to Haarlem, a town near Amsterdam and less than five miles from the shore of the North Sea. The young Bok attended Nassau Laan, a small primary school. The coming of peace after World War I meant another move for the Bok family just as he was starting high school. After he had just one year at City High School in Haarlem, his father was transferred to The Hague, where the WaldeckPyrmont Kade School had first-rate mathematicians and physicists, some of whom had doctorates.

It was in The Hague that Bok, at age twelve, joined the Boy Scouts and his interest in astronomy got its start on a clear evening when he found that he could not identify a single star. As he delved deeper into astronomy, he learned of the work of Harlow Shapley, a famous American astronomer who had used Cepheid variable stars to show that our solar system was not at the center of the Milky Way galaxy, but somewhere near its edge.

With this new-found passion, in 1924, Bok entered the University of Leiden as an undergraduate where he decided to do whatever he could to build upon Shapley’s work. In the fall of 1927, Bok began graduate studies at Groningen University with a study of the structure of the Milky Way galaxy. He also enthusiastically joined in the planning for the major event that would come with the summer of 1928, the triennial Congress of the International Astronomical Union (IAU) in Leiden. The two people Bok met at that congress changed the direction of his life. Shapley, of course, was one; he offered Bok a position at Harvard. The other was the young astronomer Priscilla Fairfield. The IAU general assembly lasted ten days, which was plenty of time for Bart to declare that he wanted to marry Priscilla.

A year later Bok left for the United States to marry Priscilla and accept his new position at Harvard. As Bok studied the galaxy, he suspected that its dark clouds contained precious clues about how stars are formed. He became interested in these clouds while writing his doctoral thesis on Eta Carinae, a star surrounded by nebulosity. “The grand sweep of the swirling gases” (as he described the nebula) was even more interesting to him than the star itself. Over the course of their lives Bok and his wife spent time under the southern hemisphere sky observing, photographing, and studying this beautiful area. In early years Shapley teased Bok about this passion, suggesting that his dissertation should be entitled “Miscellaneous Nonsense Vaguely Related to Eta Carinae.” It was a joke: Shapley had suggested Eta Carinae to Bok as a thesis subject in the first place. He always supported Bok’s interest in the star that first attracted attention in 1677 when Edmond Halley noticed its brightening to fourth magnitude. In 1827, it rose to the first magnitude, faded a bit, and then in 1843, it tied with Sirius for being the brightest star in the sky. Bok’s dissertation began as a study of the distribution of stars in the region of Eta Carinae, but through that distribution Bok was hoping to shed some light on the structure and rotation of the Milky Way. He would later expand his thesis to work with problems involving the stability of the galaxy’s open clusters and how they disintegrate over time as stars leave their nests.

How Stars Are Distributed in Space. As Bok’s interests deepened into the dynamics of open star clusters, he investigated how such aggregations of stars evolve, how long they last, and what process makes them fall apart. He was particularly concerned with the more loosely concentrated clusters that might disintegrate more quickly as the forces of the rotating galaxy tear them apart. Our Sun, in fact, might once have been a member of an open cluster, which has long since dissociated, its members now spread out all over the galaxy. In the brief history of these clusters might lie a clue to understanding the Milky Way and the fact that it rotates over a certain period of time. If this galaxy does rotate, Bok reasoned, its gravity would produce massive tidal disruptions that could cause the loose open clusters to fall apart. Tides occur when two bodies interact; the Moon with some help from the sun causes tides in Earth’s oceans that can exceed 50 feet. Stars close to each other can affect their paths, causing them to spread apart.

As Bok’s interest in the distribution of stars and the structure of the galaxy continued to grow, in the summer of 1936, he gave a series of lectures at Yerkes Observatory about his research. In the audience was his friend Otto Struve of Yerkes. As an editor of the Astrophysical Monographs, Struve suggested that Bok write the series’ opening book about this subject. Bok was well prepared for the task; he had a summary of each lecture he had given and had even taken the trouble—as he would do throughout his career—to photograph the notes he had written on the blackboard. By early 1937, Bok had begun work on The Distribution of the Stars in Space, and the first draft went quickly. But on the Memorial Day weekend of 1937, as Bok was completing the final stages of Distribution from his home in Lexington, he suffered an attack of polio, which left him no permanent disability save for a somewhat disfigured right thumb. When Bok completed the 124-page Distribution in 1937, he was only thirty-one years old. It is a remarkable book, written with a prescience and understanding of the subject not shared by most of his peers.

Dark Nebulae . From the time of his doctoral dissertation, Bok always had an interest in the Milky Way’s dark clouds. In The Distribution of the Stars in Space, he described in detail the numbers, sizes, and studies of the then-known dark nebulae in the Milky Way. Around 1947, Edith Reilly, a technical assistant suffering from multiple sclerosis, began work at the Harvard College Observatory. One afternoon Reilly asked Bok if she could study dark nebulae with him. Realizing that Reilly’s multiple sclerosis would prevent her from handling Harvard’s heavy 8-by-10-inch photographic plates, Bok asked her to examine the catalogs of dark nebulae kept decades earlier by the famous American astronomer Edward Emerson Barnard, to select candidates for further study. As work progressed, Reilly noted a particular kind of small, round, and unusually dense nebula. Bok began photographing these nebulae, and in 1947, a preliminary paper discussed their work on these “small dark nebulae”—typically roundish, from three to five arc minutes wide (about one-sixth the apparent diameter of the Moon) and located in regions of the Milky Way with no bright nebulae or unusual stars nearby. Through a telescope, Bok later described, “you would come to the leading edge of one of these things and suddenly the stars would just disappear. And then you would push the telescope’s slow motion button a bit and bloop! the stars come back.” (Bok, interview by David Levy, 1983).

Bok and Reilly found about two hundred of these dark objects within the relatively close distance of some 500 parsecs (somewhat more than 1,500 light-years) outside the solar system, the best examples being in the constellations Taurus and Ophiuchus. These tiny nebulae were optically extremely thick, with possibly thirty magnitudes of extinction; if one could, for example, cover a first magnitude star with one of these clouds, it would become invisible even through the Hubble Space Telescope. Bok thought that these nebulae marked the birthplaces of new stars. As their dark gases move about slowly, he reasoned, they begin a slow collapse under their own gravity that intensifies until stellar fusion starts. In 1956, a search of two prints of the newly completed Palomar Sky Survey revealed seventeen thousand new dark objects. When astronomers began using radio telescopes to study these objects around the same time, the nature of these small dark nebulae as star precursors became much more credible.

Radio Astronomy and Discovery of Spiral Structure. In the early 1950s, Bok embraced the new idea of studying the Milky Way at radio wavelengths. On the horizon was a new 21-centimeter radio telescope whose penetrating ear would soon hear into the galaxy’s heart. This completely new approach was only part of the story of how the Milky Way’s spiral structure was actually found. The discovery was a long process involving both optical and radio telescopes. Although Bok was not the one to make the discovery, his approach to the problem certainly helped point the way. As the acknowledged master of statistical studies of the structure of the galaxy and its interstellar material, Bok’s work on galactic structure had concentrated on the analysis of counts of stars; however his results did not go far enough to indicate the concentrations of matter necessary to establish the existence of spiral arms.

In 1951, William W. Morgan, Stewart Sharpless, and Donald Osterbrock, all of Yerkes Observatory, detected evidence of two spiral arms, which they called the Orion and Perseus arms, plus part of a third called the Sagittarius arm. The same year, using a small pyramid-shaped horn antenna mounted on a roof of Harvard’s Civics Building, Harvard physicists Harold I. Ewen and Edward M. Purcell detected radiation from neutral hydrogen atoms at the 21-centimeter wavelength, as a radio signal from the Milky Way. Before the advent of radio telescopes, the galaxy’s shape lay hidden behind a dark veil of interstellar dust that optical telescopes cannot penetrate. But radio telescopes “see” a different wavelength of sky, and through it, the Milky Way’s spiral shape could be mapped. The spiral arms are traceable by observing where hydrogen is especially concentrated. Not only could the Orion and Perseus arms be confirmed, but the arms could also be extended much further out, beyond the dark matter that blocks the view of the optical telescopes. It was possible to complete a 21-centimeter map of the galaxy. Intrigued by these developments, Bok arranged for the construction of a radio telescope at Harvard. The telescope was dedicated on 28 April 1956, on Bok’s fiftieth birthday. In a few short years Bok had so honed his skills with radio telescopes that he had become one of the country’s top radio astronomers. He later helped select the site for the National Radio Astronomy Observatory in a valley near Green Bank, West Virginia.

International Efforts and the McCarthy Era. During the Second World War, Bok understood that communications among astronomers were at considerable risk. Accordingly he launched what he described as “a quiet international newsletter” designed to keep scientists throughout Europe in touch with scientific progress of the day. After the war that operation metamorphosed into the United Nations Educational and Cultural Organization (UNECO). It would go without saying that Bok strongly supported the idea that “there should be an ‘S’ in UNECO.” Other scientists agreed: the Science Commission of the Conference of Allied Ministers of Education (CAME) proposed that the term “Scientific” be added to UNECO. On 2 September 1945, the same day that the New York Times reported the surrender of Japan, Bok coauthored a letter to the New York Times formally proposing the idea. It is to some extent because of Bok’s efforts that the organization became known as the United Nations Educational, Scientific, and Cultural Organization (UNESCO).

This effort had a negative side, for an FBI agent claimed to him that his “premature interest” in the founding of the United Nations made him suspected of being a communist. By 1954, Senator Joseph R. McCarthy’s investigation was nearing its zenith. In addition to the House Committee on Un-American Activities, several states formed their own committees. The Massachusetts variant was the “Educational Sub-Committee of the Commonwealth Commission to Study Communism, Subversive, and Related Activities.” Bok was subpoenaed to appear before this committee. On Friday afternoon, 19 February 1954, Bok answered its summons in a forty-minute interview. Their discussion centered around his wartime interest in the American Association of Scientific Workers. After the questions, Bok commented on the subcommittee’s central agenda and methods. He testified to his belief that there was little risk of communist infiltration of voluntary liberal organizations. However, he added this caveat that “excessive protective legislation” would discourage young scientists from participation in social issues. “Already there is a premium on the wearing of blinders,” he added, “by anyone of the younger generation of scientists” (Bok, interview with David Levy, 1982; Bok, personal letter to Arthur Sutherland and McGeorge Bundy, 23 February 1954).

Australian Years. Although Bok’s own testimony before a state committee was not harmful to him, his friendship and steadfast support of Harlow Shapley was. Senator McCarthy accused Shapley of being a communist, and he was forced to testify before the House Committee on Un-American Activities as well as, in absentia, a Senate foreign relations subcommittee. This political atmosphere was ultimately poisonous to Bok, and in 1956 he decided to resign from Harvard. He accepted a position as director of the Australian National Observatory’s Mount Stromlo Observatory.

As ANU’s first full professor of astronomy, Bok intended to establish a graduate school as good as the one he had left at Harvard. Of all his achievements in Australia, Bok considered his establishment of the graduate school at the Australian National University his most significant. His predecessor had started a few regular courses but nothing on the scale Bok foresaw. The top caliber students Bok attracted strengthened the work on Mount Stromlo, and Bok’s supportive and outgoing personality fed this constructive loop.

A few months after his arrival in Australia, the Soviet Union launched its artificial satellite on 4 October 1957. So exciting—and ominous—was Sputnik that Bok was invited to address both houses of the Australian parliament about it. Concerned about this evidence of Soviet military strength, the senators and representatives listened closely as Bok explained the consequences of the new technology. Although satellites were far from Bok’s specialty, he did expound on the possibilities of communications satellites. After Bok finished his talk, Sir Alister McMullin, the house speaker, inquired if it would be possible for some of the audience to go out and see the satellite. The speaker appointed a group of six to go outdoors and watch for it at the time calculated for its next appearance. Bok was a bit uneasy about the prediction but he need not have worried, for the satellite appeared right on schedule. By the end of that evening Bok had befriended the lawmakers, and his reputation as a salesman for astronomy in Australia was ensured. That evening also led to the start of a friendship with Australia’s prime minister Robert Menzies.

During Bok’s first years at Stromlo, The Milky Way came out in its third edition, its first appearance since 1945 and a major rewriting effort from the first two versions. Completed in his last year at Harvard, the new text reflected a better understood Milky Way than the ones of 1941 and 1945. Its leading changes reflected the new work on spiral structure and the role played by radio astronomy. For all but the fifth edition, Priscilla Bok acted as coauthor and editor.

Bok saw that radio telescopes were less useful than optical telescopes for determining distances of objects. Partly because of this weakness, he felt strongly that a healthy approach to galactic studies needed to coordinate both optical and radio astronomy. He strongly urged his students not to consider themselves as either radio astronomers or optical astronomers, but just astronomers.

This approach was far reaching and vitally important in a time of great change for astronomy.

By 1958, radio studies had painted a complex picture of a galaxy with a nucleus that seemed to be relatively smaller than those in some other spiral galaxies. However, the Milky Way appeared to be surrounded by a huge swath of at least three spiral arms, one in the direction of Orion, another toward Perseus, and a third toward Sagittarius.

Bok was troubled by a discrepancy between the spiral structure as revealed by the radio observations and the optical data. Morgan’s canonical map showed the spiral arms as elliptical, but radio studies at the 21-centimeter wavelength revealed them to be more circular, like those in other galaxies. Because of obscuring matter between the stars, it was still hard to tell whether a feature was a major arm, or simply a small spur of a larger and hidden arm. The problem is similar to trying to map the shape of a forest from a thicket of trees near the edge; the cartographer knows the nearby trees but has only a vague view of the rest of the woods. With radio telescopes, astronomers can search for the dark clouds of hydrogen between the stars, and for clouds which populate spiral arms in other galaxies, and they can trace the presence of spiral arms in The Milky Way. With optical telescopes, certain types of stars populate the spiral arms and help astronomers trace the extent of a spiral arm. But the arms are so large and extensive that it is difficult to determine what constitutes a complete arm and what is just a spur off another arm. Bok suspected that the arms were tightly wound up “like the spring of a watch.” And in one of these arms, about 27,000 light-years from the galaxy’s center, is our Sun.

Final Years. In 1966, Bok returned to the United States to direct the Steward Observatory at the University of Arizona, a position he held until his retirement in 1973. In December 1976, Bok published Objections to Astrology, a book that remained in print into the early 2000s. “Every ten years,” he reminisced, “I decide to do something about astrology.” Two years earlier, a fourth edition of his classic The Milky Way appeared, to be followed in 1981 by a fifth and final edition, this one dedicated to Priscilla’s memory.

A few days before her death in 1975, Priscilla and Bart attended the opening of the Flandrau Planetarium in Tucson. Priscilla stopped at the picture of their beloved Eta Carinae. “Bart,” she said, “when I die, this is where I want to be. I will be watching you from here.” Priscilla died just four days later, and Bart died in 1983. Whenever people look at the Eta Carinae nebula, they can appreciate the Boks, who did so much to increase appreciation of the Milky Way.



A Study of the Eta Carinae Region. Groningen: Hoitsema Brothers, 1932.

The Distribution of the Stars in Space. Astrophysical Monographs. Chicago: University of Chicago Press, 1937.

With I. Amdur. Letter to the editor. New York Times 2 Sept 1945.

With Priscilla Bok. The Milky Way. Philadelphia, Blakiston Co., 1941. 2nd ed. Philadelphia: Blakiston Co., 1945. 3rd ed. Cambridge: Harvard University Press, 1957. 4th ed. Cambridge: Harvard University Press, 1974. 5th ed. Cambridge: Harvard University Press, 1981.

With Edith F. Reilly. “Small Dark Nebulae.” Astrophysical Journal 105 (1947): 255–257.

“Radio Studies of Interstellar Hydrogen.” Sky and Telescope 13 (1954): 408–412.

Objections to Astrology. Amherst: Prometheus Books, 1975.


Gascoigne, S. C. B. “Bart Bok at Mount Stromlo.” Mercury 13, no. 2 (1984): 45–47.

Levy, David H. The Man Who Sold the Milky Way. Tucson: University of Arizona Press, 1993

“Spiral Arms of the Galaxy” Sky and Telescope 11 (1952): 138–139. This “American Astronomer’s Report” is apparently the original announcement of William Morgan’s work.

Struve, Otto. “Galactic Exploration by Radio.” Sky and Telescope 11 (1952): 214–217.

White, Raymond E. “Bart J. Bok (1906-83: A Personal memoir from a ‘Grandson’)” Sky and Telescope (October 1983): 303–306.

David Levy