Miss Leavitt's Stars, by George Johnson

The Story of the Woman Who Discovered How to Measure the Universe

Nov 14, 2009

An unsung heroine of astronomy finally receives well-deserved credit for her tireless work and momentous results.

Even the best science writer in America, George Johnson, admits the challenge of making Henrietta Swan Leavitt the star of her own story.

“Henrietta Swan Leavitt deserves a proper biography,” Johnson writes. “She probably will never get one, so faint is the trail she left behind.”

Along with her own self-effacement, Miss Leavitt owes her comparative anonymity to the subordinate role assigned to women in astronomy (and elsewhere) in the early decades of the 20th century. Miss Leavitt belonged to the class of female assistants known as “computers” at the Harvard Observatory.

Low Wages at Harvard Observatory

She began as a volunteer hoping to learn astronomy at the observatory in 1893. She had recently graduated from Radcliffe (then called Society for Collegiate Instruction of Women) with a “certificate” that Johnson describes as equivalent of BA from nearby Harvard (men-only at the time).

At a wage of 25 cents per hour, the duties of Miss Leavitt and other “computers” consisted of cataloging and recording data on thousands of stars. They peered at glass plates that were exposed on the telescope with chemical emulsion, the forerunner of photographic film. The plates were the equivalent of negatives – stars showed up as black dots on white background. Since the plates were time exposures, the size of a dot indicated its brightness or magnitude.

Miss Leavitt (it seems only proper to refer to her that way) had bouts of poor health, and she was gradually losing her hearing. But her eyes and her mind were the critical faculties for her grindingly repetitive tasks.

Cataloging Variable Stars

Edward Pickering, director of the observatory, asked her to concentrate on variable stars, which brightened and dimmed with various regular intervals or periods. By 1902 she was well-paid (30 cents an hour) and prized by Pickering. After six years of meticulously examining thousands of dots on glass slides, cataloging more than 1700 variable stars in the Large and Small Magellanic Clouds, she published a paper in “Annals of the Observatory of Harvard College.”

Near the end of her report, she stated: “It is worthy of notice. . .the brighter variables have the longer periods.” That single observation unshackled astronomy and freed it for a new era in mapping the size and scope of the universe.

Thanks to Miss Leavitt, the variable stars (called Cepheids) could be used as yardsticks or “standard candles” to calculate distances across the Milky Way galaxy and beyond, to other more remote stars and clouds (nebulae).

Pickering was gracious in crediting Miss Leavitt’s work. In a 1912 scientific paper under his name, he declared: “The following statement regarding the periods of 25 variable stars in the Small Magellanic Cloud has been prepared by Miss Leavitt.”

Pickering’s successor as director of the Harvard Observatory, Harlow Shapley, wrote that Miss Leavitt’s discovery “is destined to be one of the most significant results in stellar astronomy.”

Hubble and the Expanding Universe

Shapley compiled a table of values for luminosity and period that became a standard reference for astronomers. Edwin Hubble used Shapley’s chart in his pioneering work of the 1920s and 1930s, establishing the scale of the universe as unimaginably large – and expanding all the time.

Henrietta Swan Leavitt didn’t live to see these far-reaching consequences. She died of stomach cancer in December, 1921, at age 53. Just a year earlier, in the census of 1920, (“perhaps defiantly,” Johnson observes) she had listed her profession as “Astronomer.”

The copyright of the article Miss Leavitt's Stars, by George Johnson in Science Books is owned by Mike Perricone. Permission to republish Miss Leavitt's Stars, by George Johnson in print or online must be granted by the author in writing.