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Unformatted text preview: Writing in his doctoral thesis in 1917, Hubble noted that catalogs already included some 17,000 small, faint nebulous objects that could ultimately be resolved into groupings of stars. Perhaps 150,000 were within the reach of existing telescopes. Yet, he wrote, "Extremely little is known of the nature of nebulae, and no significant classification has yet been suggested; not even a precise definition has been formulated." The way Hubble discovered to classify nebulae is described here . After serving in World War I, Hubble joined the Mount Wilson Observatory staff. There he took photographs of nebulae with the new 100-inch reflector, the most powerful telescope in the world. Hubble discovered variable stars in an irregular nebula (cataloged as NGC 6822). By now Shapley had left Mount Wilson for the Harvard College Observatory. Hubble wrote to Shapley in Shapley had left Mount Wilson for the Harvard College Observatory....
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