Henrietta Swan Leavitt was one of those scientists who made major contributions to our understanding of the Universe but who is largely unknown outside her field, in this case astronomy. Her contribution was so important, so fundamental that astronomy would not have got anywhere in the 1920s and 1930s without it. Specifically, Edwin Hubble could not have discovered that the smudges in his eyepiece were in fact distant galaxies like our own, and he certainly could not have discovered the expanding universe. For Henrietta Leavitt, despite working in a male-dominated profession where it was thought that females could not be astronomers nor make any valuable contributions to science in general, discovered the significance of Cepheid variables.
Born the daughter of a congregational minister in Massachusetts in 1868, Leavitt attained a batchelor's degree in 1892 and a year later was asked by Edward Charles Pickering to join his team at Harvard College Observatory. In the late nineteenth, women were never considered for anything important in astronomy, and were kept well away from anything where they might make discoveries (specifically, women were not allowed to operate telescopes at observatories). Instead, Pickering's team of women - known as "Pickering's Harem" - were given the tedious task of studying, classifying and counting stars in photographic plates. This blatant discrimination, alas, survived the change of century and the great social upheavals of the 20th century, for even in the 1960s there were still some colleges in America where women were forbidden to take degrees.
Absorbed in this work, Leavitt became fascinated with variable stars - those whose luminosity brightens and dims over a period of time due to internal oscillations in the star itself. In the early 20th century, variable stars were regarded with interest, but more as a curiosity than objects which could tell us anything fundamental about the Universe. Leavitt discovered more than 2,400 variable stars by poring over the plates given to her by Pickering. But one discovery about variable stars was about to change the way we view the whole Universe.
Leavitt noted that a class of highly-luminous variable star known as "Cepheid variables", because the first had been identified, in 1784, in the constellation of Cepheus, exhibited a direct relationship between the period of its luminosity fluctuations and its intrinsic brightness (the brightness of a star you would measure if you were at a distance of 10 parsecs, or 32.6 light years, away from it - otherwise known as absolute magnitude). Some Cepheid variables of a particular intrinsic brightness exhibited longer luminosity cycles. This relationship, now known as the period-luminosity relationship, was so clear, so well-defined, and so lacking in exceptions, that Leavitt realised it was telling us something fundamental. Namely, all stars of this type, all stars exhibiting the same period, had the same intrinsic brightness, and therefore by measuring their apparent brightness in the sky you could calculate how far away they were.
In 1908 she published the results of her studies in the Annals of the Astronomical Observatory of Harvard College, noting that some brighter variables exhibited longer luminosity cycles, but it was another four years before she had confirmed that Cepheid variables did indeed have the same intrinsic brightness and so could be used as so-called "standard candles" - stars you could use to measure distances across the Universe.
That relationship, a way of accurately measuring distance, was exactly what astronomers were looking for. In the early 20th century, the distant galaxies astronomers were seeing were believed to be nebulae within our own galaxy. Nobody believed, indeed, that there were other galaxies. The Milky Way was everything, the whole Universe. And the major problem was that there was no way of measuring distances to the stars other than by using parallax, and even that was only useful for measuring distance to the closest stars. Without a way of measuring astronomical distances, nothing which was theorised could be put to the test. And now, here was the tool they needed...given to them by an unknown woman.
It was quite impossible, of course, for Henrietta Leavitt to take her discovery further. Four years after her tragically early death from cancer at the age of 53 in 1921, Harlow Shapley, who had become director of the Harvard College Observatory the year Leavitt died, told the Swedish mathematician Gösta Mittag-Leffler that the true credit for her work should go to him, Shapley, for the interpretation of her findings1. In such a male-dominated science, Leavitt was never going to make her mark nor receive the credit she deserved.
It fell upon Edwin Hubble to make the landmark discovery which would change the Universe. In the 1920s, after much searching, he discovered the first Cepheid variable in the Andromeda galaxy, and was able to use it to calculate how far away the galaxy is. Although he went wrong somewhere and initially pronounced it 10 times further away than it actually is, that was not important. What mattered was that we now lived in a Universe measured on vast scales, that those distant galaxies were gigantic star cities millions of light years away. The Milky Way was just one galaxy among millions in a universe whose true scale was incomprehensible.
Hubble, of course, went on to discover that the Universe is expanding. But if it had not been for the work of Henrietta Swan Leavitt, none of the momentous discoveries which Hubble made could ever have happened. Had she not discovered the period-luminosity relationship, it's certainly true that somebody would have, eventually. But discovering that relationship when she did, at the start of the 20th century, gave birth to modern astronomy. She'll never have her name in lights like Hubble or Shapley, but her work was at least as significant in the history of astronomy as theirs. Or that of any other astronomer.
1 There is a similarity between Leavitt's experience at the hands of her male peers and that of Jocelyn Bell (now Dame Jocelyn Bell-Burnell), who discovered the first pulsar. In 1974 the Nobel Prize for Physics was awarded to her supervisor, Antony Hewish, for his pioneering work in radio astronomy (specifically radio aperture synthesis) and its role in discovering pulsars, with Hewish claiming that it was his interpretation of Bell's work which had resulted in the discovery. Bell was seemingly passed over for the Nobel: the controversy rages to this day.
