Eclipsing binaries at Princeton

At Princeton, Shapley once again had the good fortune to associate with distinguished scholars. Outstanding among these was his thesis supervisor, Henry Norris Russell.

Russell, only eight years older than Shapley, already claimed the status of an important personality on the Princeton campus.

Shapley described him as ''a high-class Long Island clergyman's son and very high hat.''11 He found Russell aloof and ''shy'' at first, although Russell's excitable, driven personality must have quickly overriden any reserve he exhibited initially around his new graduate student.

Shapley recalled that Russell became friendlier after Shapley used one of his methods to solve a problem in orbital mechanics. Thereafter they treated each other more like colleagues than student and teacher. Wrote Shapley, ''Students were much interested when Shapley, the Missourian, and Russell, swinging a cane, would stroll across the campus. If students got in the way, Russell would just brush them off with the cane. We got along well, and we both learned a great deal.''12

As a graduate student, Shapley took courses related to his thesis work, including theoretical astronomy and classes in the astronomical uses of the photographic camera, spectroscope, and photometer. As he had been at Missouri, he was attracted to other fields too. His natural curiosity always made other subjects very enticing. At Princeton he found time to audit classes in physiology and paleontology.

Russell kept abreast of developments in many branches of astronomy, but his steadfast quest, beginning with his postgraduate studies at Cambridge University, was to apply the principles of physics to the problem of stellar evolution—how stars form and evolve. Shapley, as Russell's graduate student, naturally contributed to this area of research, but his interests took him in a different direction and eventually led him to the quite distinct ''sidereal problem'' of the arrangement and extent of stars in space.

At the time of Shapley's move to Princeton, the basic idea Russell worked with was that stars condensed out of nebular gas and dust and gradually became smaller and denser as gravity pulled the stellar material together. The denser a star, the older it must be. What happened to the temperature of a star as it aged was unsettled; Russell was inclined to think that the temperature of the visible, outer layers of a star would rise as the star contracted. Double stars, too, were thought to form together from a common cloud. The leading exponent of the theory of double star formation from a nebular cloud was George Darwin, Plumian Professor of Astronomy and Experimental Philosophy at Cambridge and son of the famous naturalist.

Russell was mainly a theorist, but the resources of Princeton's Halsted Observatory, manned by his junior colleague Raymond Dugan, were at his disposal. To put his ideas about stellar evolution on a quantitative basis, Russell examined data on so-called eclipsing variables that Dugan and, later, Shapley collected. These objects are binary systems, consisting of two stars in orbit around each other. Their name comes from the fact that they have variable light output. The plane of their orbits is such that, from our vantage point, each star periodically passes in front of the other, causing an ''eclipse'' of the star on the far side.

The advantage of studying binary systems is that detailed information about the component stars can be obtained—more information than is available for isolated stars. Kepler's laws of motion allow one to deduce the combined mass of the two stars from the orbital size and period. Knowing the combined mass of the system allows one to put limits on the possible mass of each star. Furthermore, spectroscopic measurements of the stars' velocities as they orbit each other give the size of the orbit. Finally, the information on mass and orbit, combined with an analysis of the lightcurve — the variation of light as each star is eclipsed in turn—yields the dimensions of the stars themselves. Russell was interested in deriving from these quantities the density of stars, to see how the density related to age.

Shapley's arrival gave Russell's program a boost. During the two and a half years that it took him to complete his doctoral dissertation, Shapley made thousands of observations of variable stars at the Halsted Observatory and computed the orbits of 90 eclipsing binaries. The calculations involved were laborious; before Shapley published his thesis, fewer than a dozen orbits had been computed although some 50 000 eclipsing variables had been discovered. Shapley and Russell used slide rules and ''little calculating machines,'' as Shapley called them—electric multipliers and adders.13

Shapley apparently benefited from his fiancee's mathematical expertise in reducing all these data for his dissertation. In 1913, Martha was affiliated with Bryn Mawr, a women's college near Philadelphia, as a ''scholar in German.''14 She intended to pursue a PhD in Teutonic Philosophy.15 According to an astronomer who collaborated with her in the 1940s and

1950s, Shapley would take a train from Princeton to meet Martha at the Broad Street Station in Philadelphia, where he would collect finished calculations from her and give her new ones.16 Certainly it is true that she became interested in the mathematical challenges of eclipsing variables, and published a number of papers on eclipsing systems under her own name or as coauthor in the Astrophysical Journal, beginning in 1916.17

Among the first exciting results to emerge from the study of eclipsing binaries were estimates of the sizes and densities of the stars. Shapley and Russell found hundreds of stars that far surpassed the Sun in radius—giant stars like those Hertzsprung described. Because their masses were not too different from the Sun's, the densities of the stars had to be very low. Shapley called them ''enormous gas bags.''

The importance of Shapley's thesis work for our story is that the eclipsing variables got him interested in another type of variable star, the Cepheid variable (see chapter 7). In the early part of the twentieth century, astronomers assumed that Cepheid variables were a type of eclipsing binary system. But the Cepheid variables did not seem to fit the pattern of the other variable stars Russell and Shapley studied, and during Shapley's tenure as a graduate student, he and Russell discussed the possibility that the Cepheids might be single stars whose brightness varied intrinsically. Shapley later elaborated on this theory, and showed that Cepheids could be used, along with other distance indicators, to study the distances of globular clusters and the structure of our stellar system.

Shapley completed his dissertation in 1912 and turned to the question of his future. He wrote to his former Missouri teacher, Seares, at Mount Wilson. The 60-inch reflector there had been in operation since 1908, and the five-year task of grinding the glass mirror base for the 100-inch telescope was already underway. Seares arranged for George Ellery Hale, the founder and director of the observatory, to meet Shapley in New York. The meeting went well, and in due course Shapley received an offer to go to California. However, Shapley elected to defer the start of his job there for a year.

Shapley had some loose ends to tie up in the binary work, and Seares was also recommending that he travel. In 1912, Shapley attended his first meeting of the American Astronomical

Society, in Pittsburgh. In 1913, Shapley and his younger brother John traveled to Europe.

For John, the Atlantic crossing was the beginning of doctoral studies in Vienna in linguistics, archaeology, and the history of art. For Shapley, it was an opportunity to visit observatories and meet astronomers. He got as far east as Hungary, as far north as Sweden, and as far south as Algiers. He attended a meeting of the Royal Astronomical Society in London, where he met ''almost everybody of stature,'' and a meeting of the international astronomical society the Astronomische Gesellschaft in Bonn, where he met Ejnar Hertzsprung.18 As we saw in chapter 7, Hertzsprung was working along lines very similar to Russell's, and correlating stars' spectral types with luminosities. Shapley's enjoyment of the trip came to an abrupt end in Paris, where he found a telegram informing him that his father Willis had been killed by lightning. He wandered the streets of Paris in shock.

Back in the United States in 1914, Shapley made a shorter trip up and down the East Coast, visiting American observatories. At Harvard, director Edward Pickering invited Shapley to his house for dinner, and welcomed Shapley to search the Harvard archives for data that interested him. Shapley ate a second dinner in Boston with ''the famous and jolly Miss Annie J Cannon,'' the classifier of stellar photographic spectra.19 But the most important meeting he had on this visit was that with Solon Bailey.

Bailey had come to Harvard in 1887 as a college-educated but unpaid assistant. At the time of Shapley's visit he was a full professor and, informally, assistant director. He had proven his worth by establishing Harvard's observing station at Arequipa, Peru, where he had photographed the entire southern sky with a succession of telescopes and, almost single-handedly, had measured the magnitudes of about 8000 stars.

It was during one of his multi-year stays in Arequipa that Bailey had developed an interest in variable stars. He was specifically interested in variables in globular clusters, which at the time were thought to be stellar systems comparable to our own, and in the Magellanic Clouds, which, though irregularly shaped and very different from globular clusters, were also assumed to be ''island universes'' of some sort. When Bailey began his study, only a few of the 400 known variable stars were in clusters. Within five years he discovered 300 variable stars in clusters. His favorite objects were the globular clusters 47 Tucanae and Omega Centauri; he knew them so well that on at least one occasion he identified a variable star simply by noticing visually a change in a photograph. The usual method that Bailey employed, and that Henrietta Leavitt used on the plates he sent back to Harvard, was to overlay a negative taken on one date with a positive image taken on a different date. Variable stars stood out because they left a slightly larger photographic impression when they brightened, while the stars that shone steadily matched exactly in the overlays.

Shapley sought Bailey out in his office during his 1914 visit, and found a quiet but warm reception. Shapley reported that Bailey said, ''I have been wanting to ask you something. We hear that you are going to Mount Wilson. When you get there, why don't you use the big telescope to make measures of stars in globular clusters?''20 In later years, Shapley always credited Bailey for leading him to the rich field of variables in globular clusters.

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