The big galaxy hypothesis

Sometime before November 1916, Shapley came up with a distance measurement that surprised him even more than the globular cluster distances had. Looking in the plane of the Milky Way—or what Shapley called the galactic system—in the direction of Scutum, near Sagittarius, he examined the stars of the open or galactic cluster known as M11. He found faint blue stars there, similar to the ones in the globular clusters. Applying the same techniques he had used on the globular clusters, he was forced to the conclusion that M11, firmly a member of the Milky Way system, was some 50 000 light-years distant.28 This was as far as some of the globular clusters, and larger than the diameter of about 20 000 light-years that he had previously ascribed to the Milky Way system. Here was a clue that the Galaxy might be larger than anybody expected—though still an ''island'' in the universe. Perhaps a new, larger estimate of the size of the Galaxy was what Shapley had in mind when he predicted to Kapteyn in February 1917 that he would ''get something out of the problem yet.''

At about the same time that he was considering revising his estimate of the size of the Galaxy, Shapley's thoughts about spiral nebulae were thrown into confusion, also. In 1917, several novae had appeared in a number of spirals, and these inspired a search through old photographic plates at Mount Wilson and Lick Observatories for more ''new'' stars. Shapley joined the fray and in October 1917 announced that the novae he had examined in the Andromeda nebula—with the single exception of one that had flared in 1885, S Andromedae, whose brightness had been exceptional—indicated that the nebula was about 1 million light-years away.29 This result from the novae posed no challenge to Shapley's belief in the ''island universe'' theory.

But contradicting such a large distance for a spiral nebula was the work of Shapley's friend Adriaan van Maanen. Shapley had paid scant attention to it at first.

Van Maanen had been studying pairs of images of the spiral nebula M101 with a blink stereocomparator. This instrument allowed him to view an old photograph and a new one of the same subject in rapid alternation through the same optics. The resolution of the images was not fine enough to permit him to see stars, but distinct nebulous points that he took for star clusters appeared in the nebula. Van Maanen searched for movement of those nebulous points. If any had shifted position within the nebula between the times when the two photographs were taken, he would detect the shift by eye. To obtain the amount of shift, van Maanen selected comparison stars on the image, which he assumed were foreground stars not associated with the spiral nebula, and measured the shifted points with respect to the comparison stars using a kind of micrometer.

To his own surprise, van Maanen noted shifts indicating what he called internal proper motions or rotation of the spiral. Not that rotation, per se, was unanticipated; ever since Lord Rosse had drawn the spiral M51, which he had seen through his enormous telescope, astronomers had speculated that the whirlpool structure of the spirals indicated their rotation. Heber D Curtis voiced a widespread opinion in 1915 when he said, ''The spirals are undoubtedly in revolution since any other explanation of the spiral form seems impossible.''30 But if the spirals were vast assemblages of stars so distant that individual stars could not be seen telescopically, then their undoubtedly stately pace of rotation should not be so easily discerned in photographs taken only a few years apart.

Van Maanen published the first of his results in the spring and summer of 1916, and began searching for similar motion in other spiral nebulae. Shapley, when he finally took note of van Maanen's work, was perturbed. If the spirals rotated at the rate van Maanen indicated, they must be much closer than anticipated. In September 1917, Shapley wrote to his former teacher, Russell: ''Do you sometimes suspect the internal motions in M101? V. M. [van Maanen] does a little, Hale more, and I much.'' 31 About two months later he sent off another letter to Russell reporting on the distances of about 30 globular clusters, the nearest about 20 000 light-years away, and the farthest about 200 000 light-years. ''This is a peculiar universe,'' he wrote.32 Evidently Shapley still clung to the framework of island universes, was inclined to think the Galaxy might be exceptionally large, and wished he could dismiss van Maanen's results.

Russell replied in a letter dated 8 November 1917. ''I am at present inclined to believe in the reality of the internal proper motions, and hence to doubt the island universe theory.''33 He puzzled over what the spiral nebulae might be, if not distant agglomerations of stars.

Russell's opinion may have swayed Shapley and persuaded him to try to fit the puzzle together with van Maanen's piece. Shapley held Russell in very high esteem; one of Shapley's graduate students in the 1920s said many years later, ''Russell had been Shapley's teacher and mentor, and his word was law. If a piece of work received his imprimatur, it could be published; if not, it must be set aside and its author had a hard row to hoe.''34 In any case, Shapley reconsidered his distances, the novae in spirals, van Maanen's rotations, and the island universe theory itself.

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