20th century / Science

Henrietta Swan Leavitt and the Harvard Computers

By Melanie O’Sullivan

‘What a variable star ‘fiend’ Miss Leavitt is, one can’t keep up with the roll of the new discoveries.’- Charles Young of Princeton, to Edward Pickering in a letter.

The director of the Harvard College Observatory, Edward Pickering, frustrated with the inefficiency of his male assistants, once declared his ‘maid could do a better job’. It was an epiphany which within years would materialise as a group of approximately 40 women, controversially named ‘Pickering’s Harem’, or later, the ‘Harvard Computers’, who worked under Pickering’s former maid Williamina Fleming to classify stars by their magnitude, or brightness. The ambitious project was established in 1907 with the intention to comprehensively catalogue all the stars in the night sky from photographic plates. It was a daunting task, with the production rate of photographic data far outweighing the rate at which it could be processed. The lower wage requirements of women meant more of these ‘Computers’ could be employed, increasing output. Furthermore, assigning the mundane processing task to women released their male counterparts to carry out work requiring more ‘original mental power’; in the early 20th century working as a ‘Computer’ was the closest option to a scientific career for women educated in Physics or Astronomy, giving them invaluable access to data and resources.

This article wishes to focus on the work of Henrietta Swan Leavitt, the profoundly deaf daughter of a Congregational Church minister. She joined the Observatory in 1895 as a volunteer, and within seven years was promoted to head of the photometry department where she was required to estimate the brightness (or magnitude) of stars from a series of photographic plates.

Henrietta Leavitt

At the time, no standard method existed to assess the magnitude of stars, but rather they were analysed by the naked eye and categorised on a 6 point scale. This method had many shortcomings; it was highly subjective, and due to nature of light perception by the eye, it had a logarithmic rather than linear scale. A newer techinique, using photographic methods to determine magnitude, gave differing values to visually determined data, due to the higher sensitivity of film to blue light. Pickering’s group aimed to standardise this data by cataloguing. Using 299 plates from 13 telescopes situated across the globe, Leavitt selected 46 stars from within the north celestial pole, a region within 2o of the North Star, Polaris, and derived a new method of analysis to determine absolute magnitude, the brightness of a star independent of observer distance. Leavitt’s method, classifying stars over 17 magnitudes of brightness, was accepted a year after publication in 1913 by the International Committee on Photographic Magnitudes. Over the following eight years, Leavitt continued to refine her method, and used it to comprehensively determine the magnitude of stars in 108 areas of the sky. Her method was used as standard until the 1950s when it was superseded by modern photometric measurements.

Her main contribution, however, was from her work with Cepheid variables. Cepheid variables are post-main sequence stars whose emission intensity oscillates regularly with a fixed time period. In a normal main sequence star, like our Sun, the hydrogen which fuels energy generation by nuclear fusion is plentiful so emission is relatively steady. However, as the hydrogen fuel depletes and the star enters the post-main sequence stage, the products of nuclear fusion such as helium, begin to fuse, which is a significantly less efficient process. This causes the star to expand and contract regularly, and with it the internal temperature fluctuates. On contraction, the star’s temperature elevates ionising the helium at the surface, making the star more opaque, thus less luminescent to the observer. Expansion cools the star, and the resulting transparent surface emits more light.

In her work calculating the absolute magnitudes of stars in the Milky Way’s companion galaxies, the Magellanic Clouds, she noted in 1912 under Pickering’s name ‘A remarkable relation between the brightness of these [Cepheid] variables and the length of their period’. Through the study of 25 Cepheid variables she discovered the Cepheid variable period-luminosity relationship, which after calibration, allowed the intrinsic luminosity of the star to be calculated from the oscillation period.

Cepheid variables
Cepheid variables, as studied by Leavitt

The repercussions of this discovery were enormous. On calibration, knowledge of intrinsic luminosity allowed the distance of the star from Earth to be calculated using the inverse-square law. Before her discovery, only the distance of stars within 100 light years of Earth could accurately be calculated; overnight this increased to 100 million light years, thus earning itself the name ‘the yardstick of the universe’. Her discovery later enabled E. Hubble to calculate the age of the Universe; and Pickering’s successor, H. Shapley, to calculate the size of the Milky Way.

During her time at the Harvard College Observatory, Leavitt discovered 4 novae, and approximately 2400 variable stars, half of that known until the 1930s. She also carried out work on asteroids and two-star systems called Algol type binary stars.

Leavitt was limited to the work which she was assigned. Despite her revolutionary discovery, she was not permitted additional time to pursue her findings further. Moreover, it has been suggested that Leavitt, over her entire career at the HCO, never became trained on a telescope. This, coupled with her untimely death at the age of 53 from cancer prevented her from completing her classification system.

Four years later, in 1925, Gosta Mittag-Leffler of the Swedish Academy of Sciences wrote to Leavitt requesting more information in order to nominate her for the Nobel Prize for Physics. Unfortunately, the rules of the prize meant the award could not be awarded posthumously, and as a result Leavitt, quoted in her eulogy as ‘possessing the best mind at the Observatory’, slipped into obscurity.

Johnson. G, Miss Leavitt’s stars: The Untold Story of the Woman Who Discovered How to Measure the Universe (2005)