The Believing Brain - Michael Shermer [171]
The one-hundred-inch Hooker telescope atop Mount Wilson in the San Gabriel Mountains in Southern California, where Edwin Hubble demonstrated once and for all that the mysterious nebulae were not small nearby gaseous objects within the Milky Way galaxy, but were instead “island universes”—galaxies—similar in structure to our own but very far away. PHOTOGRAPH BY AUTHOR.
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The year 1923 was Hubble’s annus mirabilis, starting with several months of classifying and cataloging familiar nebulae, followed by the discovery of fifteen variable stars in NGC 6822, eleven of which were Cepheid variables. Hubble employed the new standard candles to compute the nebula’s distance at 700,000 light-years, well beyond even Shapley’s 300,000-light-years-across “big galaxy.” On October 4, Hubble photographed a number of nebulae, including Andromeda. During the next day’s detailed laboratory analysis of the plates he thought he spotted a nova, maybe three. His attention heightened, he rephotographed Andromeda the next night and confirmed: “nova suspected.” Hubble then went to the archives to compare the plate with those shot previously, and there on the new plate he scratched in “N” for nova—new star—for three specs of light. Triple-checking his plate, Hubble realized that one of the dots was not new; it was, in fact, a variable star—a Cepheid variable, no less! Hubble wrote in the logbook for the one-hundred-inch telescope, “On this plate (H335H), three stars were found, 2 of which were novae, and 1 proved to be a variable, later identified as a Cepheid—the first to be recognized in M31.”19 On the plate Hubble crossed out the “N” and scratched in “VAR!” The date on the plate reads “6-Oct 1923.” (See figure 21.) This is the day the universe changed.
Over the next several months Hubble returned to Andromeda and tracked the light curve for his Cepheid, which varied over 31.415 days, from which he computed that the star was seven thousand times brighter than our sun. Yet it was barely noticeable on a photographic plate after hours of light-gathering exposure, which could mean only one thing: Andromeda was very, very far away. Hubble wrote to Shapley (who was now at Harvard): “You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula (M31). I have followed the nebula this season as closely as the weather permitted and in the last five months have netted nine novae and two variables.”20 Using the same technique that Shapley had used for measuring globular clusters and the size of the Milky Way, Hubble calculated that Andromeda was at least a million light-years away. If true, this would mean that Andromeda was an island universe.
Shapley was slow to see the new data in the same way Hubble did, telling him that he found Hubble’s letter to be “the most entertaining piece of literature I have seen for a long time,” and cautioned him that Cepheids with variable periods longer than twenty days may not be reliable indicators of distance. Hubble responded with more data, imaging nine variable stars in NGC 6822, then another dozen variables in Andromeda, three of which were the coveted Cepheids, plus another fifteen variables in M33, M81, and M101. In another letter to Shapley, Hubble opted for diplomacy in gently nudging his colleague and erstwhile competitor to shift paradigms: “the straws are all pointing in one direction and it will do no harm to begin considering the various possibilities involved” in accepting the island universe theory. In the end Shapley came around, showing Hubble’s letter to a Harvard astronomy graduate student and pronouncing: “Here is the letter that has destroyed my universe.”21 Shortly thereafter, Shapley championed the island universe theory, abandoning his previously held belief in light of this new and unmistakable data.
Figure 21. The Photograph That Changed the Universe
Edwin Hubble’s photograph of Andromeda in which he identified Cepheid variable stars—used for measuring distance—that allowed