Lingering questions from the Great Debate

When Shapley and Curtis debated the scale of the universe in 1921, both knew that the continued accumulation of observations with ever-improving telescopes and instruments would eventually make sense of the conflicting evidence they argued over. In 1921, when Shapley was still settling in at Harvard, he got news of an apparent clarification of this kind. Van Maanen wrote to say that measurements on the spiral nebula M81 showed rotation similar to that of M101. Shapley responded gleefully: ''Congratulations on the nebulous results! Between us we have put a crimp in the island universes, it seems—you by bringing the spirals in and I by pushing the galaxy out. We are indeed clever, we are. It is certainly nice of those nebulae to have measurable motions.''57

The surprise must have been all the greater, therefore, when Shapley found some pieces of his carefully constructed puzzle rearranged as early as 1924. In that year, Hubble announced in a letter to Shapley that he had found Cepheid variables in the Andromeda nebula, and they implied a large distance for it and other spirals. The news made it clear that van Maanen was wrong about the internal motions of the spirals; the spirals could not possibly rotate as fast as van Maanen said they did, given their large true size. Shapley could and did still believe in a big Galaxy—a continent among islands—and the news had no impact on his contention that the Sun lay far from the galactic center. He had been dealt a blow by a powerful rival, however, and had to admit the status of spirals as independent stellar systems.

Cecilia Payne witnessed the fateful letter and its impact. She recalled, ''I was in his office when Hubble's letter came, and he held it out to me: 'Here is the letter that destroyed my universe,' he said.''58 She also heard him say, ruefully, that he had believed van Maanen's result, in part because van Maanen was his friend.

More unraveling of the Great Debate problems was in store. In 1930, Lick Observatory astronomer Robert Trumpler showed convincingly that interstellar dust and gas dimmed and reddened starlight, and did so in all directions. His argument was quite elegant. He had estimated the distances to open or galactic clusters using two methods, and compared the results. First he assumed that clusters had similar linear diameters, therefore those that appeared smaller were more distant. Secondly, he used spectroscopic parallax: a star of a given spectral type should have a certain intrinsic luminosity, and a comparison of its apparent and intrinsic luminosity yielded an estimate of the distance. The two methods gave discordant results and pointed to the culprit: interstellar absorption dimmed the light from the stars, but did not affect measures of cluster diameters.

Shapley was, surely, disappointed over the blunders he had made in assessing the merits of the island universe hypothesis. He had been cautioned about the potential effects of interstellar absorption but had satisfied himself, through detailed observations, that it was not a problem. But even if he felt some lingering frustration over the turn of events, he did not allow his past errors to get in the way of further research. He accepted his mistakes and forged ahead with studies of external galaxies and a new favorite subject, the Magellanic Clouds.

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