The Great Debate

On the evening of 26 April 1920, members of the National Academy of Sciences and their guests, and interested members of the public, made their way to the Smithsonian's Natural History Museum and through its cavernous rotunda to hear Shapley and Curtis speak on the scale of the universe. The debate began at 8:15 p.m. in the museum's auditorium. Each speaker was allotted 40 minutes.

Shapley spoke first. Historians are not aware of any newspaper or eyewitness account of the debate, but it appears that Shapley read his lecture and illustrated it with lantern slides. His notes indicate that he aimed his talk at the general public, tarrying through a rather lengthy introduction, showing photographs of open and globular clusters and defining for his listeners the term ''light-year.'' He was careful to build up an image of the Milky Way system as a collection of clusters: he spoke of a ''clustering motive'' or motif that ranged from the loose open clusters in the plane of the Milky Way to the rich, dense globular clusters farther away from the plane.44 Implicit in his argument was the idea that the ''Kapteyn universe'' constituted a local cluster within the Milky Way system.

Having prepared his audience to accept that various types of clusters pertained to a single, large system, Shapley outlined the methods of determining distances to the globular clusters. Without going into ''the dreary technicalities,'' as he called them, he discussed the use of B stars and Cepheid variables as distance indicators, emphasizing, too, the lack of absorption by dust and gas in space, except in the plane of the Galaxy. He then summarized the results: the distances of the globular clusters, and their arrangement, implied that the Milky Way system is 300 000 light-years in diameter. A consequence of this ''cluster theory of the galactic system,'' he noted, is that the Sun is very distant from the center of the Galaxy — 60 000 light-years, according to his reckoning. In conclusion, he offered the opinion that the spiral nebulae ''can hardly be comparable galactic systems,'' if the Milky Way system was as large as he maintained. Only if his distances were off by a factor of about 10, he suggested, was it reasonable to suppose that the Milky Way was a spiral system and an ''island'' in the universe.45

Curtis, a more experienced teacher and speaker, presented a completely different sort of lecture. He had been expecting a spirited but technical give-and-take on the subject, and had summarized his arguments on typewritten slides. Curtis was taken aback by Shapley's general, and hence mostly uncontrover-sial, presentation. He later wrote to Shapley that five minutes before Shapley's time was up, he thought about changing the character of his presentation to match Shapley's, but decided to go ahead with what he had planned.

In a sense, Curtis and Shapley talked past each other. Shapley followed the established theme of the debate—the scale of the universe—more closely, and rather neglected to consider the implications for the island universe hypothesis. Curtis's agenda, on the other hand, was to demonstrate the strength of the arguments for spirals as systems comparable to ours. He attacked Shapley's distance determinations and presented cogent arguments for the spiral nebulae as distant stellar systems, not associated with the Milky Way.

Curtis's main arguments, some of which he emphasized more in the published version of his lecture, were as follows.

1. The blue (B-type) globular cluster stars that Shapley used as distance indicators were not as intrinsically bright as Shapley believed. Shapley was comparing local giant stars—very bright—with distant dwarf stars that are intrinsically dimmer, so the distances he derived were too great.

2. Spirals demonstrate a stellar-type spectrum. Curtis may not have dwelled on the long history behind this assertion, but he could have made the case that the stellar spectrum of the spiral nebulae had been known since Julius Scheiner captured that of the Andromeda nebula in 1899 (see chapter 6).

3. The distribution of spiral nebulae in the sky seems to delineate a ''zone of avoidance,'' which makes sense if the spirals are island universes like the Milky Way. Many spiral nebulae seen edge-on have dark bands running along the center line, indicating a layer of obscuring matter. If the Milky Way system is a spiral galaxy and has such an obscuring layer, we would—as is in fact the case—see the spirals arrayed around the north and south galactic poles, and apparently avoiding the plane of the Galaxy.

4. About 25 novae are known in the spiral nebulae; 16 of these are in the Andromeda nebula. We can compare them with about 30 known novae in the Milky Way system. Most of the novae seen in the spiral nebulae are relatively dim and indicate a distance of about one million light-years for the Andromeda nebula. The nova known as S Andromedae is much brighter, and gives a conflicting result, but it may be anomalously bright. Perhaps there are two types of nova.

Curtis added a fifth argument based on data taken at Lowell Observatory in Arizona by Vesto M Slipher. In the 1910s, Slipher recorded the faint spectra of stellar nebulae with very long exposures and found, to everyone's surprise, that the spectral lines were shifted to the red end of the spectrum. A notable exception to this rule was the spectrum of the Andromeda nebula, which showed a blue shift. No one was sure what to make of these results, but they clearly implied, based on the kind of analysis that Huggins had first made on the radial motions of individual stars, that these stellar or spiral nebulae were almost all receding from the Milky Way as though repelled by some force, with velocities up to several hundred kilometers per second. While this repulsion was not easy to explain, Slipher's data, Curtis believed, were solid. These space velocities were unlike those of any galactic objects; the spirals must be extra-galactic. Curtis was willing to take Slipher's data at face value; years later, Hubble confirmed the data and investigated the phenomenon more closely, as we shall see.

Curtis did not trust van Maanen's measurements of the apparent rotations in spirals, which, if true, implied that the spirals were small and nearby. He was aware of other measurements that reported rotations in the opposite sense—van Maanen claimed the spirals were rotating such as to ''wind up'' the arms, while Slipher, at Lowell Observatory, showed they were ''unwinding.'' Unfortunately, as Curtis knew, there would be no quick way to resolve the issue, because the rotations were in any case slow and would require a long time interval between comparisons on photographic plates. In the published version of his talk Curtis said diplomatically that ''should the results of the next quarter-century show close agreement among different observers'' indicating a certain minimum speed of rotation, then the island universe theory ''must be definitely abandoned.''46

Curtis came away from the evening feeling that he had won the debate—or rather, since it had not been a debate, that he had made the best impression with his arguments. He reported to his family back in California that his friends told him he ''came out considerably in front.'' Shapley must not have felt as confident; in later years he acknowledged that Curtis had been more articulate. Russell felt the same way, and wrote to Hale that Shapley should teach a lecture course to cultivate a ''gift of the gab.''47

The debate was not reported in the general press. It did, however, find its way onto the pages of science-oriented semi-popular publications, at least in Europe. Bart Bok, a Dutch astronomer who began working with Shapley in the 1930s, claimed that ''the impact of the Shapley and Curtis presentations upon astronomical thought was terrific.''48 He read about the debate in a Dutch magazine and immediately petitioned the public library in The Hague to obtain a copy of the published versions of the talks. Walter Baade, an astronomer about seven years younger than Shapley who worked at Mount Wilson from the 1930s through the 1950s, did not comment on the debate specifically but emphasized that Shapley's ideas contributed to stirring up the astronomy community on the scale of the universe. He said, ''It was a very exciting time, for these distances seemed to be fantastically large, and the 'old boys' did not take them sitting down. But Shapley's determination of the distances of the globular clusters simply demanded these larger dimensions.''49

With the benefit of hindsight, we can arbitrate this debate-that-was-not-a-debate. But it is not easy to call a winner. Both contestants were right in some respects. Shapley was correct to draw attention to the asymmetrical distribution of globular clusters, and to suggest that the center of the globular cluster distribution coincides with the center of the Galaxy.

Shapley's distance determinations were riddled with errors and invalid assumptions, as we shall see. Most importantly, he assumed, incorrectly, that interstellar absorption could be discounted. However, his errors did not mask the fundamental truth that the Galaxy is, in fact, much larger than his contemporaries assumed, and that the globular clusters constitute a sub-system of the Milky Way. (See chapter 10 for more on globular clusters and the structure of our galaxy.)

Curtis's defense of the popular island universe hypothesis was very strong; his instinct about what evidence to consider and what evidence to view as suspect was excellent. His remark that there might be two categories of nova, for example, was prescient. S Andromedae turned out to be a supernova, 10 000 times brighter than an ordinary nova. He was, of course, correct to point out that spiral nebulae have exactly the type of spectra one would expect if they are made up of stars. And Curtis was right to suspect van Maanen's measurements of the rotation of spirals.

Years passed before the dust settled on the Great Debate; astronomers did not resolve all the issues raised by discordant data until the 1950s, and most astronomers who attended the debate or read about it did not change their views as a result. However, the debate did help sway the astronomical community on one important point that Shapley argued: the Sun is located far from the center of the Milky Way galaxy. The center lies in the direction of Sagittarius, behind vast star clouds and obscured by interstellar dust and gas.

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