The Day We Found the Universe - Marcia Bartusiak [107]
Hubble more carefully tracked the ups and down of his variable's luminosity from the archival photographic plates. He also continued his survey of the heavens, making sure to check back on Andromeda again and again, as this was the time of year when Andromeda was in full view. He found more novae and another variable. He kept track of his finds, numbering each nova and variable and marking their positions in the spiral with a tiny red dot or circle on photos of Andromeda.
Three nights in February 1924 proved especially crucial. Over the fifth, sixth, and seventh of that month he directly observed his first variable in Andromeda brightening by more than a magnitude, doubling its luminosity, a tremendous break. From the data he had on hand he could now sketch a reliable light curve. The variable star went through its complete cycle—from bright to dim and back to bright again—in a matter of 31.415 days. From the length of this period and the shape of the curve (sharp rise and slow decline), Hubble now comprehended that he had captured that elusive and rare celestial beast—a Cepheid variable, a star seven thousand times brighter than our Sun. But it appeared so dim—the barest smudge on his photographic plate—that Hubble knew it had to reside at a great distance. It was on average more than one hundred thousand times dimmer than the faintest stars visible to the unaided eye.
The photographic plate of Andromeda (M31) on which Edwin Hubble
identified a Cepheid variable star, mistaken at first for a nova, in a spiral
nebula—the first step in Hubble's opening up the universe (Courtesy
of the Observatories of the Carnegie Institution of Washington)
At some point during these deliberations, Hubble went back to his logbook, page 157, and quickly scrawled an added note on the side of the page to amend the report of his October 5 observing run. Customarily reserved, Hubble at this moment is unmistakably restive. He didn't write his message in black ink, which he regularly did for his records, but instead in pencil. And his handwriting, usually so fluid and precise, was more hurried and askew. He was obviously elated: “On this plate (H 335 H), three stars were found, 2 of which were novae, and 1 proved to be a variable, later identified as a Cepheid—the 1st to be recognized in M31.” To highlight the addition, he drew a big arrow, pointing directly downward at his historic news. In its broad stroke, the arrow makes his excitement visible upon the page. For once Hubble dropped his guard and figuratively clicked his heels at this moment of discovery.
Hubble couldn't help but notify his nemesis. On February 19 he wrote Harlow Shapley about his efforts over the previous months. Hubble didn't open with polite niceties or inquiries of health. He got straight to the point. “Dear Shapley:—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.” His glee in communicating this news jumped off the page as he then provided Shapley with all the technical details on color index corrections and magnitude estimations. Shapley was, after all, the world's reigning Cepheid expert—not only in using them as standard candles but figuring out early on, soon after he arrived at Mount Wilson, that they were pulsating stars, their atmospheres repeatedly ballooning in and out.
Pages 156 and 157 of Hubble's 100-inch telescope logbook
(Reproduced by permission of the Huntington Library,
San Marino, California)
Accompanying this legendary letter was a graph that Hubble had fastidiously drawn in pencil