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By Root 546 0

Peering through a magnifying eyepiece at two photographic plates of the Small Magellanic Cloud, taken at different times, she noticed that several stars in the cloud had changed in brightness. On one plate a particular star was relatively luminous; on another plate that same star had turned far dimmer. It was as if the star were undergoing a slow-motion twinkle. Over the following year, she looked at additional images of the cloud and found dozens more. With each new delivery of plates from Harvard's station in Peru (and checks on old ones going back to 1893), she readily and meticulously updated her count, so much so that a Princeton astronomer described her as a “variable-star ‘fiend.’” Soon she included the Large Magellanic Cloud in her tally, and by 1907 she found a record-setting total of 1,777 new variable stars residing within the prominent, mistlike clouds (before that, only a couple of dozen variable stars had been detected in the Magellanic Clouds). She dutifully reported her findings in the 1908 Annals of the Astronomical Observatory of Harvard College, with thirteen pages taken up with listing every new variable she had discovered, its exact position in the sky, as well as its minimum and maximum brightness.

More intriguing was what she wrote at the end of this paper. Over the course of her painstaking examination of the Small Magellanic Cloud, she came to notice a special group of variable stars, sixteen in number. They were later identified as Cepheid variables, stars that are thousands of times more luminous than our Sun. Their name was derived from one of the first and brightest discovered, δ Cephei, located in the constellation Cepheus the King, a major landmark in the northern sky. These stars regularly vary their brightness in a matter of days or months. The shortest cycle Leavitt measured for these Magellanic variables was 1.2 days, the longest 127 days. Yet no matter if the Cepheid had a long or short period, each was as regular as a metronome in its variation. “As a rule, they are faint during the greater part of the time,” reported Leavitt, with the period of maximum brightness being fairly brief. The variable δ Cephei, for example, goes from dim to bright in just a day, then gradually fades back to its faintest magnitude over the next four days, until it suddenly brightens once again.

But it was the next sentence in Leavitt's report that turned into its most venerated statement. “It is worthy of notice,” she continued, “that… the brighter variables have the longer periods.” Since all her Cepheids were situated in the same celestial cloud, Leavitt could assume they were all roughly the same distance from Earth. And that meant she could trust that the Cepheids' periods were directly associated with their actual emission of light. Leavitt was in fact getting a first glimpse at astronomy's celestial Rosetta stone, a means for astronomers to solve the mystery of the spiral nebulae. The key was that link between a Cepheid's period—the steady rhythm of its oscillation—and its luminosity. She was on the brink of finding a new cosmic yardstick, one that would allow astronomers to determine the distances to far-off celestial objects that were formerly immeasurable by more traditional means.

Leavitt had chanced upon the celestial equivalent of lighthouses on Earth. A sailor, if he is familiar with the amount of light a particular lighthouse emits, could roughly estimate how far he is from land, given how bright the beacon appears to him from offshore. Similarly, a Cepheid's period labels it as having a particular brightness. The distance to the Cepheid is then obtained by figuring out how far away it must be to be viewed as the faint point of light we see from Earth. In this way, the Cepheid becomes a valuable “standard candle” (as astronomers call it) for gauging distances deep into space, when all other methods fail.

Bright and dim, bright and dim goes a Cepheid's cycle, but not endlessly. It was long believed that a Cepheid was an eclipsing binary star—one star regularly circling another like the Earth