Hubble and Shapley - Two Early Giants of Observational Cosmology

1921. According to Christianson (1995, p.149) " Hubble was privately anxious for Shapley to go. He was not accustomed to standing in anyone else's shadow, especially one whose conduct he considered obnoxious and whose ambitions matched his own." For his outstanding work Shapley was awarded the RAS Gold medal in 1934, Hubble received this same medal in 1940. Later Hubble was awarded the Bruce Gold medal of the ASP in 1938. Shapley received that award in 1939. Both men will primarily go down in history for a single great discovery. Shapley (1918) for using the distribution of Galactic globular clusters to show that the center of our Milky Way system is located far from the Sun in the direction of Sagittarius. The importance of this transition from a heliocentric to a Galactocentric paradigm was comparable to the change from a geocentric to a heliocentric Universe proposed by Copernicus in 1543. Shapley (1969, p. 60) himself regarded this discovery that the Sun (and hence mankind) was peripheral, rather than central, as one of the most important thoughts that he had ever had. Hubble's name will forever be associated with the discovery in 1923 of Cepheid variables in the great Andromeda nebula, which resolved the longstanding debate about the nature of spiral galaxies. Here there is also a link to Shapley (1914) who had previously hatched the hypothesis that Cepheids were pulsating variables, rather than binaries, as had previously been thought.

The careers of these two scientists diverged with Hubble’s main contributions involving the classification and distance determination of individual galaxies using the world’s largest telescopes. On the other hand Shapley is perhaps best remembered for his work on the distribution and clustering of galaxies, which he studied with wide-field telescopes of relatively modest aperture. Hubble’s longest (but perhaps least influential paper) concerned the large-scale distribution of galaxies over the sky. From deep studies of over one thousand small fields thinly distributed over the sky north of declination -30 o Hubble (1934) found that (1) absorption by dust causes a deficit of galaxies at low Galactic latitudes and (2) the distribution of galaxies on very large scales is homogeneous, i.e. there is no indication of a super system of nebulae. Perhaps his deepest insight (Hubble 1936, p. 82) into the nature of galaxy clustering is contained in the statement that “The groups (such as the Local Group) are aggregations drawn from the general field, and are not additional colonies superposed on the field.” Hubble (1936), p. 77 also adds that “Pending definite information, it is supposed that the frequency diminishes as the population increases, over the whole range of groups and loose clusters to the great clusters themselves. “Because Hubble’s survey sampled only widely distributed very small fields, it was not suitable for the study of the large-scale distribution of distant clusters of galaxies. Nor could Hubble’s sparsely sampled data provide much information on the structure and distribution of chains and clusters of galaxies across the heavens. On the other hand Shapley’s wide-field and all-sky surveys were particularly well-suited to the study of galaxy clusters and their distribution over the sky. Perhaps Shapley’s his greatest contribution to astronomy was the Shapley-Ames (Shapley, & Ames 1932a,b) catalog of the brightest galaxies in the sky, which was mainly based on observations with small wide-field telescopes. On the other hand Hubble is probably best remembered for his classification system for galaxies and for determining the distances to the nearest galaxies using the Mt Wilson 100-in. telescope. Plots of the projected distribution of galaxies in the Shapley-Ames catalog over the sky provided three profound new insights: (1) There is no concentration of galaxies towards our own Milky Way system, (2) galaxies are distributed in a very clumpy fashion with major flattened clusterings -such as the Virgo super cluster, and (3) Shapley & Ames (1932a) discovered what they referred to as “high latitude vacancies” -structures that are nowadays referred to as voids. Regarding the discovery of such voids Shapley & Ames (1932b) write: “The vacancies [in the distribution of galaxies] are important. They are not due to the insufficiency of the survey, for over the whole sky the search has been thorough and the plates adequate. Nor are the barren regions, such as that at λ = 20 o , b = + 50 o , the result of heavy obscuration, as in low latitudes, since nebulae fainter than the thirteenth magnitude appear in average abundance in these high latitude regions. " Shapley (1930) was also able to show that clusters of galaxies are not distributed at random, but are concentrated in super clusters. The greatest of these super clusters in nearby regions of the Universe is the so called Shapley (1930) concentration [see Proust et al. (2006)]. An intrinsic limitation on Shapley’s studies

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