Henrietta Swan Leavitt


by David F. Coppedge

Behold the distant stars, how high they are.  (Job 22:12)

Let’s take a look at a remarkable woman astronomer, whose scientific work has been, and continues to be, of the utmost significance for our understanding of the universe.  She not only overcame the gender barrier to achieve greatness in science, but physical disability as well.

All of the other scientists in our hall of fame have been men.  Today, there are many women scientists, but until the twentieth century, whether from prejudice, tradition, or the needs of homemaking, women had rarely entered the almost exclusively male domains of science and technology.  Henrietta Swan Leavitt is a glorious exception, and with her, a whole group of lady astronomers who, under Dr. Edward C. Pickering of Harvard, made it their mission to survey the stars.  Pickering hired local women, who were willing to work for less money, to do the tedious work of measuring stars from thousands of photographic plates.  Annie Jump Cannon, one of “Pickering’s harem” as it was later crudely dubbed, would become famous for her star classification scheme OBAFGKM (memorialized by the guys for its mnemonic “O Be A Fine Girl, Kiss Me”).  Henrietta Swan Leavitt, however, would always be most famous of the group: she helped us measure the universe.

Deaf, and reserved in manner but charmingly sweet, Henrietta Leavitt had a brilliant mind and a capacity for detail that helped her discover an astronomical law destined to put her in the history books.  This law become the essential tool Edwin Hubble would later use to determine the distance to the “nebulae” (clouds) – unknown fuzzy objects found in all directions of the sky.  Leavitt was the daughter of a congregational minister.  She graduated from what was later named Radcliffe College, and in 1892, joined the Harvard College Observatory as a volunteer research assistant.  Soon after the twentieth century began, she rose to the head of a department measuring stellar magnitudes (brightnesses).  For the next several years, she performed very tedious work, searching thousands of plates taken from an observatory in Peru, for a special class of pulsating stars called Cepheid variables.  For reasons unknown at the time, Cepheids were known to vary regularly in brightness.  Some pulsated rapidly, in a few hours or days, and some took months, but they could be depended on like clockwork.  By 1908 she had compiled a list of well over a thousand Cepheids in a nebular patch of the southern sky called the Small Magellanic Cloud.  Her careful observations uncovered an important relationship: the longer the period, the brighter the star.

Great advances in science are often made by individuals who not only observe something interesting, but get that flash of insight that allows them to interpret the meaning of the observation.  To understand the significance of her find, we need to recall the concept of the universe in Leavitt’s day.  The Herschels and other notable astronomers had catalogued thousands of stars, but were frustrated by a common fact, that you cannot tell the distance of a star by its brightness alone.  It might be a very bright star very far away, or a very dim star close up.  If you had a “standard candle” or light source of known brightness, you could use it as a distance measuring tool.  Think of a row of uniform street lights vanishing in the distance along a city street.  If every lamp is the same, you can use the apparent brightness of a lamp, i.e., how bright it looks to your eye or film, and compare it to the lamp’s absolute brightness, or how bright it would look from a known, standard distance, to measure how far away it is.

The relationship Leavitt observed is a little more complicated, but similar.  If you know that fast-blinking lamps are intrinsically dimmer than slow-blinking lamps, they will have a relationship that allows you to infer how far away one is by measuring its apparent brightness or magnitude, and comparing that to its blinking rate, or period, which is linked to its absolute magnitude.  A simple mathematical equation then gives you the distance.  Before Leavitt, no standard candle was known.  Other than the few nearby stars that could be measured using triangulation, no stars hinted at a reliable method that could tell for sure how far they were, and by extrapolation, how far the universe extended.  Astronomers took part in the “Great Debate” – was everything inside the Milky Way, or were some objects beyond it?  Just how big was the Milky Way?

Leavitt used a fair assumption that the stars in the Small Magellanic Cloud were all, within reason, the same distance from us.  This meant she could compare each star’s absolute magnitude, or luminosity, as if she were to see a variety of lamps of different brightnesses from the same distance d (where d was still to be determined).  She found 16 Cepheids on the plates that appeared often enough to measure their periods.  Plotting the periods of these stars on a graph against their apparent luminosities, she saw that they all fell on a line: she had found a Period-Luminosity Relation.  If astronomers could determine the distance to one Cepheid, they could calibrate the relationship and use it as a measuring stick.  They would have their long-sought standard candle.

Henrietta Leavitt published her results, enthusiastic about the possibilities of the relationship for measuring objects in space.  But Pickering assigned her to other duties that he felt were more appropriate as women’s work than making fundamental discoveries.  Other astronomers like Hertzprung and Shapley became intrigued by Leavitt’s paper, however, and found ways to calibrate her standard candle.  It took several iterations and error corrections, but by the 1920s, using Cepheids with the Period-Luminosity Relation had become an increasingly accepted method of measurement, and astronomers were finally getting a grasp on the distances to the stars.

Leavitt did not live to see the epochal day in October, 1923, when Edwin Hubble, working at the new 100″ telescope on Mt. Wilson near Los Angeles, the largest in the world at the time, excitedly wrote “VAR!” on a plate taken of the Andromeda Nebula.  He had found a Cepheid variable star within it.  This Cepheid was to bring powerful new evidence into the long-standing debate about the nature of these spiral nebulae: were they clouds of dust or gas within the Milky Way, or star systems far beyond it?  Hubble noted that this Cepheid was much dimmer than most.   Applying Henrietta Leavitt’s Period-Luminosity Relation, he calculated that the Andromeda Nebula must be extremely distant; it was in fact another galaxy like our own, far beyond the Milky Way – an “island universe” in the vastness of empty space.  As the implications of this discovery began to sink in, and objections to it withered and disappeared, the age of galactic astronomy was born.  By 1935, the cosmos had multiplied in size a hundred billionfold – an unprecedented revolution in our understanding of the heavens, unlikely to ever be surpassed.  Armed with Leavitt’s standard candle, Hubble and other astronomers revealed to our telescopes a universe of unfathomably immense proportions.

The glory of this discovery was due largely to this wonderful lady scientist, Henrietta Swan Leavitt, who was nominated for a Nobel Prize posthumously in 1925.  What kind of person was she?  Solon I. Bailey eulogized her in these words:

Miss Leavitt inherited, in a somewhat chastened form, the stern virtues of her puritan ancestors.  She took life seriously.  Her sense of duty, justice and loyalty was strong.  For light amusements she appeared to care little.  She was a devoted member of her intimate family circle, unselfishly considerate in her friendships, steadfastly loyal to her principles, and deeply conscientious and sincere in her attachment to her religion and church.  She had the happy faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning.

This moving description makes it clear that Miss Leavitt exemplified the fruit of the Holy Spirit (Galatians 5:22): love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control.  Christians, creationists, women, and the disabled – all can justly look to Dr. Henrietta Swan Leavitt as a role model of an overcomer, an achiever, and an exemplary Christian.

There is one glory of the sun, and another glory of the moon, and another glory of the stars; for star differs from star in glory.  I Corinthians 15:41

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