In recent times, there has been greater awareness of the major but overlooked contributions that women have played in the sciences, and attempts to lift them out of the obscurity to which they had been consigned. The recent film Hidden Figures told of the women mathematicians, African-American women in particular, who worked in the US space program in the 1960s doing critical complex calculations despite the Jim Crow laws that heaped all manner of indignities on them.
These women were given the dehumanizing term ‘computers’ and just recently I came across another ‘computer’ even before them and whose work was crucial in figuring out how to measure distances to stars. She laid the groundwork for modern cosmological models of the universe and yet Henrietta Swan Leavitt was never given all the credit that she deserved but instead had it appropriated by her superiors.
In investigating the properties of the universe, the distances to the stars are crucial pieces of information. To get those, we use the relationship between the absolute brightness of the star (i.e., the rate at which energy is radiated by it), the apparent brightness of the star (i.e., the rate at which energy from the star is received by the observer), and the distance from the star to the observer. Given any two, we can find the third. The apparent brightness is a measured quantity, so given one of the other two, the third can be found.
After studying thousands of stars, Leavitt zeroed in on a class of stars known as Cepheid variable stars whose brightness pulsated. She discovered a relationship between the absolute brightness of the star and the period of pulsation so by observing their period, the absolute brightness could be inferred. This enabled astronomers to use those stars as standards for that particular galaxy to determine the distance to that galaxy, and thus calculate the absolute brightness of the other stars in that galaxy. This was a crucial step in mapping the distances to stars and galaxies.
Gael Mariani tells us Leavitt’s story and it is a fascinating one, and yet sad because of the lack of recognition she received in her own lifetime.
Henrietta Swan Leavitt was born in Massachusetts in 1868 and entered Radcliffe, then named the Society for the Collegiate Instruction of Women, at the age of 20. The rigorous education she received, spanning from classical Greek to fine arts and philosophy, to analytical geometry and differential calculus, would have been sufficient to lead to a successful academic career – if she had been a man. Sadly, at that time, the scientific profession was almost exclusively a male prerogative: thus when Leavitt took up employment at the Harvard College Observatory in 1893 it was not as an astronomer, nor even a junior astronomical researcher, but as a lowly ‘computer’ whose job of cataloguing the brightness of stars more resembled book-keeping than research and was rewarded by the princely wage of 25 cents per hour, comparable to a servant’s pay at that time. Leavitt worked in a tiny, dingy office alongside a number of other highly-educated women relegated to similar routine tasks under the supervision of their boss, Edward Pickering, and known collectively as ‘Pickering’s harem’. Cecilia Payne, another Harvard ‘computer’ who would later make the revolutionary (and disparaged, at the time) discovery that the sun was composed largely of hydrogen, wrote that ‘Pickering chose his staff to work, not to think.’
Whilst recording and cataloguing the data on her variable stars, Leavitt found that she could accurately and consistently relate the period of a given star’s brightness cycle to its absolute magnitude. The discovery of this simple and hitherto-unknown relationship made it possible, for the first time, to calculate their distance from Earth. Henrietta Leavitt had just become, in the words of George Johnson, author of the book Miss Leavitt’s Stars, ‘the woman who discovered how to measure the Universe’.
Leavitt’s discovery ignited a scientific powder trail whose blaze brought fame and glory to many of her peers: without it, Edwin Hubble might never have been able to show that the spiral nebula Andromeda was not located at the edge of our galaxy, as had been previously thought, but almost a million light years away. Nor might the Danish astronomer Ejnar Hertzsprung been able to measure the parallax of many of the nearby Cepheid variables; even though while using Leavitt’s discovery to his advantage, a slip of his pen is alleged to have caused him to underestimate the stars’ distance by a factor of 10.
As she had lived quietly, unnoticed, so her death left barely a ripple among her peers: to the extent that when, in 1925, the Swedish mathematician Gösta Mittag-Leffler wrote her a letter: ‘Honoured Miss Leavitt, your admirable discovery … has impressed me so deeply that I feel seriously inclined to nominate you to the Nobel Prize in Physics for 1926’, he had to be informed that she had in fact been dead for four years. As the Nobel Prize is not awarded posthumously, Leavitt never received her nomination. Instead, Harlow Shapley, the new director of the observatory, suggested in his reply to Mittag-Leffler that the real credit belonged to him, Shapley, for his interpretation of her findings.
One wonders how many more such overlooked people there are.