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The woman in the moon: Science of astronomy owes much to Henrietta Leavitt

Voices of the Clarion

Posted: Sunday, April 30, 2006

Over billions of years, a pulverizing rain of meteors has cratered and re-cratered the surface of the moon, forming the familiar face in the night sky.

Amid those countless pockmarks is a 66-kilometer wide, moderately eroded crater named for a little recognized champion of astronomy who worked at the Harvard College Observatory in Cambridge, Mass., in the early part of the last century. Her name was Henrietta Leavitt.

Though acknowledged in many science textbooks, references often are limited to footnotes or just a paragraph or two. She was a woman in a world where the science of astronomy was dominated by giants, all of them men, including Joseph Lockyer, who found helium in the spectrum of the sun; Enjar Hertzsprung and Henry Russell, who created the indispensable diagram that bears their names relating absolute magnitude to a star’s spectral type; Edwin Hubble, who first proposed, correctly it turned out, that the universe was expanding; and Karl Schwarzschild, who found solutions to Einstein’s General Relativity equations and investigated the idea of black holes.

In many ways, all of them owe something to Henrietta Leavitt’s work, for it was her perseverance at cataloging certain kinds of stars that ultimately led others to solve one of the most perplexing puzzles of early 20th century astronomy.

Born the daughter of a Congregational minister in Massachusetts on Independence Day 1868, Leavitt grew up to attend Oberlin College in Ohio and the Society for Collegiate Instruction of Women, which later became Radcliff. It was there that Leavitt became intrigued by astronomy as a senior in 1892. Soon after graduating, she suffered a serious illness that left her profoundly deaf. Nevertheless, after recovering she returned to the science of astronomy, offering herself as a volunteer at Harvard College Observatory in 1895.

Seven years later, observatory director Edwin Charles Pickering appointed her to the observatory’s staff, paying her 30 cents an hour to analyze a collection of photographic plates and measure and catalogue the brightness of stars. She would eventually rise to head the photographic photometry department, according to biographical data available on the Public Broadcasting System (PBS) Web site.

Though more than librarian work, it wasn’t the theoretical astronomy this brilliant scientist might have done had she been male. Nevertheless, she would leave her mark.

Among the stars she cataloged were some whose brightness appeared to wax and wane over time. She would go on to discover more than 2,400 of these variable stars, roughly half those known in her time.

More specifically, it was a certain type of variable star called a Cepheid variable that would lead to one of the most important discoveries in, and become her major contribution to, the science of astronomy.

The early 20th century was a time of heated debate among astronomers over the true scale of the universe and whether the Milky Way was all there was. It was considered the “great debate” of the age perhaps best exemplified in a 1920 clash of wits between two well-known observational astronomers, Harlow Shapley and Heber Doust Curtis, whose competing ideas each contained some truth and some inaccuracies.

Curtis insisted that so-called “spiral nebulae,” wispy objects being discovered in great numbers by ever-improving telescopes, were actually other galaxies.

Shapley disagreed, arguing the nebulae must be nearby dust clouds. He, on the other hand, put our sun nearer the edge of the Milky Way (which is true), while Curtis stuck Sol squarely in its middle.

The issue of the Milky Way’s place in the cosmos was soon settled when another astronomer, Edwin Hubble, found some of Leavitt’s Cepheid variables in one of those spiral nebulae, the Andromeda Galaxy.

Here’s how Leavitt influenced those later discoveries.

In 1908, she published her cataloging results (Annals of the Astronomical Observatory of Harvard College) and recorded notes on something odd about variable stars she’d found in photo plates of the Magellanic Clouds, later determined to be nearby galaxies.

By 1912, after further study, she was sure that the periods of variability among Cepheids could be associated quite accurately with their luminosity. That is, the time it took a Cepheid to grow bright and dim again could be correlated to its actual intrinsic brightness. This relationship became a vital yardstick for determining astronomical distances, allowing her to begin estimating distances between stars and the Earth.

Hertzsprung used Leavitt’s data to calibrate the distances to several Cepheids in the Milky Way, and thus to any visible Cepheid anywhere. Armed with this new yardstick (along with the luminosity attributes of another kind of pulsating star called an RR Lyr) Hubble and others began estimating the distances to other galaxies.

Hubble, for whom the Hubble Space Telescope is named, went on to discover that the universe was expanding. Galaxies were literally flying apart and the farther apart they were the faster was there recession speed. That, of course, led eventually to the “Big Bang” theory and all the other amazing esoterica of modern astrophysics.

Leavitt continued quietly with her work on variable stars until she died of cancer in 1921. Shapley took over as head of stellar photometry.

Today, it is estimated that the number of galaxies beyond our own island Milky Way may exceed 125 billion. A recent estimate by astronomers with the Australian National University, there may be as many as 70 sextillion stars. That’s a 7 followed by 22 zeros.

Leavitt never got to pursue the kind of astronomical research for which she was clearly qualified, performing much more than adequately the jobs she was assigned. A colleague reportedly said of her that she possessed the best mind at the observatory.

Considering how her work ultimately influenced the explosive expansion of her science over the next 90 years, one might think her worthy of more recognition. Fact is, though, you can’t even see her crater on the moon. It’s on the far side.

Hal Spence is a reporter for the Clarion.



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