The Day We Found the Universe - Marcia Bartusiak [66]
“I had not thought of making the very pretty use you make of Miss Leavitt's discovery,” Henry Norris Russell wrote Hertzsprung when this result came out. Russell had employed a similar technique around the same time, but his aim was to determine the average magnitude of a Cepheid. In the process, he concluded that they were giant stars, far bigger than our Sun. Inspired by Hertzsprung, Russell proceeded to make his own distance calculation to the Small Magellanic Cloud, arriving at 80,000 light-years. Both estimates were highly uncertain and turned out to be far less than current distance measurements (210,000 light-years), but each figure was still astoundingly huge for its day.
Shapley soon adopted Hertzsprung's approach, although he used only eleven of Hertzsprung's thirteen Cepheids in his calibration, suspecting that two of them were peculiar. Just like Hertzsprung, he counted on a simple rule of perspective: The farther away a moving object is located from you, the slower it will appear to travel. A far-off plane seems to crawl along the sky, while a plane closer in going at the same speed would zoom right past you. After estimating an average velocity for a star, Shapley checked how his eleven Cepheids were journeying across the sky. The slower the apparent velocity, the more distant the Cepheid.
It was at this point, though, that Shapley parted company with Hertzsprung. He didn't use Leavitt's period-luminosity relationship, which was based solely on stars in the Small Magellanic Cloud, but instead constructed his own relationship based as well on the Cepheids in the Milky Way, in order to obtain an “improved and extended” period-luminosity law combining both sets of variable stars. He then applied his new rule to the Cepheids found in the globular clusters. He would monitor a Cepheid to peg its period and then calculate the star's distance from his graph.
This worked as long as Shapley could find Cepheids in his globular clusters. Some of the clusters had none at all, as far as he could observe. What they did harbor were variables that were not quite the same. These variables changed quite rapidly, in a matter of hours rather than days or months. There was no guarantee that they behaved in the same way as Leavitt's Cepheids.
Shapley tried mightily to check with Leavitt on this question, writing several times to her boss, Edward Pickering, on whether she had detected fast variables in the Magellanic Clouds and found them to obey her rule. Pickering assured him that photographs were being taken. But progress on the question was occurring at a glacial pace. Pickering was keeping Leavitt busy with work he considered more important. “Routine stuff,” decried Russell to Shapley at one point. “I fear, however, that I am not the man who may justly raise my voice in criticism.”
Eager to move forward, Shapley simply decided to treat his fast variables as if they did follow Leavitt's rule. He extended the Cepheids' period-luminosity relationship to include all these variables, both slow and fast. “This proposition scarcely needs proof,” he had boldly asserted in one early paper, though it was a very controversial decision. But by doing this, Shapley was able to determine the distances to the nearest globular clusters—a formidable task, as the stars were very faint. For clusters farther out, too remote to spot any variables, he resorted to using the brightest stars as distance markers. He just