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going around the Sun. But by 1914 it was recognized that this type of variable was actually a single, pulsating star, its atmosphere regularly ballooning out and then shrinking back in, over and over again. When the origin of a star's power was at last understood, astronomers came to see that a Cepheid is a star far more massive than our Sun, anywhere from five to twenty solar masses, that has reached a particular stage in its evolution. Having used up its main supply of hydrogen, the Cepheid becomes unstable for a while (about a million years) as it adjusts to burning new sources of nuclear fuel. When the star is compact, pressures build up, causing the star's outer atmosphere to expand and thus become more luminous. But once stellar pressures are reduced, gravity takes over and causes the star to contract back and become dimmer—that is, until stellar pressures build up once again. In this way, the Cepheid comes to pulsate in a regular fashion. More important, the brighter and more massive Cepheids oscillate more slowly than the fainter and smaller ones.

In 1908 Leavitt was wary that her initial sample of sixteen Cepheids was too small to secure a firm and predictable “period-luminosity” law. She needed more, but chronic illnesses and the death of her father delayed her a few years. Moreover, though very bright, allowing them to be seen over long distances, Cepheids are also very rare. Not until 1912 was Leavitt able to add nine more Small Magellanic Cepheids to her list. With twenty-five in hand, she could at last establish a distinct mathematical relationship between a Cepheid's blinking and its perceived brightness.

Science often involves discovering patterns, spotting regularity and order where none before had been noticed. And the pattern that Leavitt made plain, with such patient care and shrewd insight, in time opened up the universe. The connection was immediately apparent when Leavitt plotted her data on a graph. “A remarkable relation between the brightness of these variables and the length of their periods will be noticed,” she wrote, with a decided animation rare in scientific discourse. On a logarithmic scale, the visible brightness of her Cepheids rises steadily as the stars' periods get longer and longer. Her variable stars huddle along a sure, straight line from the bottom left to the upper right of the graph paper. This historic finding was published as Harvard College Observatory Circular, No. 173, a three-page paper titled “Periods of 25 Variable Stars in the Small Magellanic Cloud” and now considered a “masterpiece” of scientific literature.

Henrietta Leavitt's historic 1912 graph showing how a Cepheid's

brightness increases as the variable star's period gets longer (From

Harvard College Observatory Circular, No. 173 [1912], Figure 2)

Cepheids stood ready to be the perfect standard candles, but first she needed to know the true brightness of at least one, the luminosity she would observe if she were essentially right next to the star. If she could determine the brightness of just one, her graph would let her know all the others. Once her graph was calibrated in this way, an astronomer could pick out a far-off Cepheid anywhere in the sky, measure its period, and infer its actual luminosity. The distance to the Cepheid then followed: By measuring the Cepheid's apparent brightness in the sky (a much fainter magnitude), you could figure out how far away it must be to appear that dim. Cepheids held the promise of being astronomy's handiest cosmic measuring tape. Astronomers could at last gauge the distance to celestial objects farther out than they ever conceived possible. Leavitt knew this, but she wasn't one to state things so daringly. Besides, Pickering chose his women computers “to work, not to think,” according to one Harvard astronomer. So, in a far quieter tone, Leavitt simply wrote at the close of her paper, “It is to be hoped, also, that the parallaxes [essentially, distances] of some variables of this type may be measured.”

What was needed was an indisputable distance to a bona fide