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Hubble's Campaign to Establish
the Velocity-Distance Relation

After his famous 1929 paper, into the 1930s Edwin Hubble continued his campaign to prove the reality of the velocity-distance relation he had announced. In the 1929 paper, he remarked that "Mr. Humason at Mount Wilson has initiated a program of determining velocities of the most distant nebulae that can be observed with confidence.… New data to be expected in the near future may modify the significance of the present investigation, or, if confirmatory, will lead to a solution having many times the weight."

By 1931 Hubble and Humason had velocities for 40 more nebulae, 26 of them in 8 groups of galaxies. The greatest distance was now over a hundred million light years. This was 50 times farther than the most distant spiral nebulae in the 1929 report. The linear velocity-distance relation held.

Hubble's 1931 velocity-distance relation. Circles represent mean values for clusters or groups of nebulae. Dots near the lower-left corner represent individual nebulae. These and the lowest two circles were used in the 1929 formulation of the velocity-distance relation.

In 1934 Hubble and Humason reported results for isolated nebulae not in clusters. Still the velocity-distance relation held.

Hubble's 1934 velocity-distance relation. The solid line represents the velocity-distance relation determined from isolated nebulae. It parallels the velocity-distance relation earlier determined from nebulae in clusters, represented by the dashed line. The velocity-distance relation for isolated nebulae is a magnitude brighter than the relation for nebulae in clusters. This was attributed to selection effects — the brightest nebulae were the easiest to find in isolation, whereas less bright nebulae were found easily in clusters.

By 1936 Hubble and Humason had data for a hundred more nebulae. The velocity-distance relation was "still sensibly linear to a distance of 70 million parsecs [more than 200 million light years]."

Hubble's 1936 velocity-distance relation. Numbers in parentheses following the names of the clusters indicate the number of nebulae observed in each cluster. Because the fainter nebulae in the most distant clusters were below the observing limit of the 100-inch telescope, Humason plotted, as an indication of distance, the magnitude of the fifth brightest nebula in each of the clusters.
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