Astronomical Superstars

Marion Cromb explores the cosmic contributions of the women of Observatory Hill.

“Oh, Be A Fine Girl, Kiss Me” is the popular mnemonic used to remember the standard stellar classification system, used by astronomers and astrophysicists all over the world. You might have assumed it was a guy that came up with this aide-memoire in the 1900s, but sources indicate it was the woman who pioneered the OBAFGKM system itself: Annie Jump Cannon.

Most of Annie’s co-workers at the Harvard College Observatory were also women, they were much cheaper to employ and did a better job than the male assistants employed before them. Williamina Fleming, who discovered the Horsehead Nebula, actually started out working for the director, Edward Pickering, as a maid before he offered her a job in the observatory. These women were known as the Harvard Computers (in the days before computers were machines).

By looking at astronomical photographic plates under a magnifying glass, the women of Observatory Hill classified hundreds of thousands of stars – Annie Jump Cannon classified an estimated 400,000 alone! These plates could have hundreds of stars on them, their pinpoints of light smeared out into small rainbow spectra by a prism. This stellar fingerprint is different for different stars, and enables them to be sorted into different categories. These women lived and breathed spectra, so rather than closely analysing each one, they could simply recognise to which category it belonged. With so much experience, it is no surprise that Annie Cannon established a system still used (with additions) today.

Stars once classified were carefully compiled by Cannon into the Henry Draper Catalogue (named after the first photographer of stellar spectra, although it was Cannon’s work). For this catalogue, and her contributions to astronomy, Annie Jump Cannon was the first woman to receive an honorary degree from Oxford University. Cannon also produced an extension categorising even fainter stars – working on it from her 60s till her death at age 77.

Annie classified stars based on spectral differences, but did not seek to explain the differences. It was another woman working at the Harvard Observatory, Cecilia Payne, who solved this mystery with her highly-esteemed PhD thesis. Payne built on the work of Indian physicist Meghnad Saha who had linked atomic energy levels and temperature, to show that the classification system was ordered by the temperature of the stars; O contained the hottest stars, M the coolest. Payne even used astronomical observations to correctly predict the energy level structure of several elements. Payne was also the first to propose that the stars were largely made of hydrogen, a controversial view at the time.

Annie Jump Cannon was the first woman to receive an honorary degree from Oxford University

Another astronomical pioneer working at the observatory at the time was Henrietta Swan Leavitt, a good friend of Annie Jump Cannon (interestingly, both Henrietta and Annie were deaf). She discovered thousands of variable stars and this work resulted in her formulation of the luminosity-period law which relates the absolute brightness to the brightness fluctuations of Cepheid variable stars. With this law, Cepheid variables could be used as an astronomical yardstick to determine the distances between astronomical objects for the first time. This work was instrumental to Hubble when he was formulating his theory of an expanding universe.

Cannon and her colleagues were not anomalies, at the time one in three American astronomers were women. However the contributions of these women were dismissed at the time and many women did not spend long in the field, feeling social pressure to give up their careers once married. It is likely many of these other women made contributions that were forgotten or attributed to men and remain truly unsung heroes

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