Back in the Netherlands after his summers in the United States, Kapteyn discovered that his ties to Mount Wilson naturally made him the contact person of choice for northern European astronomers who dreamed of access to the world's largest telescope. Among these was Adriaan van Maanen, who later played a prominent, if not infamous, role in twentieth-century debates concerning island universes. Between 1908 and 1910, van Maanen, although enrolled as a student at the University of Utrecht, resided at Groningen so he could use the plate-measuring equipment in Kapteyn's Astronomical Laboratory. He was among the first European astronomers to join the staff at Mount Wilson, thanks to Kapteyn's recommendation to Hale.

In 1911, Kapteyn received a visit from another young hopeful, the Danish astronomer Ejnar Hertzsprung, who worked at the Potsdam Observatory in Germany. Hertzsprung, 38 years old, happily got more than he bargained for from the visit. He gained Kapteyn's support for a letter of recommendation to Hale, and he met Henrietta Kapteyn, his future wife.

Hertzsprung had studied chemistry and worked in that field in St. Petersburg and Leipzig until family circumstances dictated that he return to his mother's home in Copenhagen. There, as a private scientist, he pursued his interests in photography, spectroscopy, and astronomy.

In 1905, he came out with a remarkable paper on the classification of stars according to their spectra. He showed that stars that are as red or redder than the Sun can be divided into two unequal groups: those of approximately the same brightness as the Sun, and those, less numerous, that far outstrip the Sun in luminosity, or intrinsic brightness. The physics of the relationship between color, size, and luminosity implied that the more luminous stars must be physically larger than their counterparts of the same color, so Hertzsprung designated them as ''giant'' stars. Solar-like stars, in contrast, came to be called ''dwarf'' stars, although ''normal size'' might have been a better term, since there is nothing unusual about the size of a solar-like star.

The existence of these two classes of stars, while not yet universally accepted, had implications, if true, for the kind of star-gauging that Kapteyn was perpetually engaged in, and for the study of the evolution of the stars in time, which Hale was interested in. Hertzsprung brought his work to the attention of established astronomers, and within a few years was recognized as an expert on stellar spectroscopy and had made the transition to professional astronomer. He remained particularly interested in establishing the possible connections between the spectrum and luminosity of stars.

Hertzsprung found support for his ''giant and dwarf'' theory in research conducted at Harvard, carried out by Antonia Maury. Maury was a niece of Henry Draper, and was one of the first women Pickering hired to help analyze spectra. She had attended a women's college, Vassar, and like Hertzsprung himself had studied chemistry.

Pickering assigned Maury to the task of analyzing in detail the spectra of the brightest stars in the northern sky. Maury compared the stellar spectra to a spectrum of the Sun represented on the same scale—several inches wide, and showing thousands of the lines across the wavelength range of visible light. She classified the spectra, as usual, according to the relative strength of certain lines. Maury went further than her colleague Williamina Fleming, however, in dividing the classes more finely and in adding the designations ''a,'' ''b,'' ''c,'' or a combination thereof as a shorthand way to represent the sharpness and width of the lines.

Pickering dismissed Maury's system as too complex and time-consuming to implement. He argued that the lines might appear sharp or blurry according to the conditions under which the photographer had captured them. Hertzsprung, however, had discovered a real difference between the c-type stars and the rest: c-type spectra pertained to giant stars. In 1908, he chided Pickering for abandoning Maury's a-b-c distinction, writing in a letter, ''To neglect the c-properties in classifying stellar spectra, I think, is nearly the same thing as if the zoologist, who has detected the deciding differences between a whale and a fish, would continue classifying them together.''21 Pickering stood firm—but in 1922, the astronomical community adopted Maury's ''c'' notation, thanks to Hertzsprung's attention to her work.

When Hertzsprung appealed to Kapteyn, he had yet to convince the astronomical community of the giant and dwarf distinction, and some of his most important work was yet to come. But he showed promise, and Kapteyn approved of Hertzsprung's short-term goal, which was to use Mount Wilson resources to study the spectra of the faintest stars. He wrote to Hale, and Hale in turn offered Hertzsprung observing time on the 60-inch telescope and a stipend to support his trip. Hertzsprung arranged to accompany the Kapteyn couple on their annual voyage to the United States the following summer.

In June 1912, Hertzsprung made his way to Groningen for the start of the voyage to the United States. On the 8th of that month, he and Henrietta surprised their families by announcing their engagement; on the 9th he left for six months overseas, accompanied by her parents. Henrietta spent her summer at home studying Danish.

Hertzsprung and the Kapteyns stopped in London on their way to Liverpool, where they would embark for New York. Eddington, who had adopted the star-streaming problem, was always glad to see Kapteyn. He later wrote, ''We rejoiced to hear again the familiar gutteral exclamations and quaint expressions, as with youthful spirit and enthusiasm he unfolded his latest ideas.''22 Hertzsprung and the Kapteyns also visited Kapteyn's old friend David Gill, who had retired to London from his duties at the Cape Observatory.

Although their ultimate destination for the summer was Mount Wilson, both Kapteyn and Hertzsprung had business with the Harvard College Observatory on the way. Kapteyn was counting on Pickering's help on the Plan of Selected Areas, and in return performed some data analysis for Pickering. Hertzsprung was even more keenly interested in the work going on at Harvard than Kapteyn, for much of it related directly to his research on stellar spectra. As it turned out, Hertzsprung's research and some of the work going on at Harvard contributed to the development of new distance measurements and directly affected discussions of the scale of the galaxy, the validity of the ''Kapteyn Universe,'' and the question of the island universe hypothesis later in the century.

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