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reading matters 13: the glass universe

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Canto: So The glass universe, published in 2016, has a cute title, referring as it does to the ‘glass ceiling’, another clever term for that invisible barrier up there that appears to prevent women from rising in politics, business and science, but also to the glass photographic plates upon which were recorded the spectrographic signatures of a vast arrays of stars, clusters and the like, in the decades of the late nineteenth century and early twentieth century, by a somewhat less vast array of human computers – the name given to the largely underpaid female stargazers and recorders of Harvard College observatory and elsewhere.

Jacinta: Yes, Dava Sobel, author of the fascinating little book Longitude, as well as Galileo’s Daughter, which is in a stack of books here waiting to be read, has brought to life a group of dedicated women and their male supporters over a period when the higher education of women was just starting to be addressed. 

Canto: Yes, it all started with the Drapers, a wealthy and well-connected couple in the 1870s. Henry was a leading astronomer of the day, and ‘Mrs Henry’, aka Anna, a socialite and heiress. Their social evenings were mostly science-focused, with guests including the inventor Thomas Edison, the zoologist Alexander Agassiz, and Prof. Edward Pickering, of Harvard. Henry Draper was working on the chemical make-up of stars, using ‘a prism that split starlight into its spectrum of component colours’, for which he’d won great acclaim, when he died suddenly of a flu-like illness in his mid-forties. His devoted and rich widow, keen to continue his legacy, helped finance, along with Pickering, a continuation of his ground-breaking research.  

Jacinta: And so the computers of Harvard College Observatory were born. We need to explain – or try to – the science of spectrographic analysis, but I’d like to first briefly describe some of the women who did this work. They include Williamina (Mina) Fleming, a canny Scotswoman frae Dundee (our birthplace), whose first American job was as the Pickering’s maid but who soon proved her worth as a star spotter and tracker, classifier, and organiser, leading the team of computers in the early decades. In 1899 she was given the title of ‘curator of astronomical photographs’, becoming the first titled female in the university’s history. As such she presided over 12 women ‘engaged in the care of the photographs; identification, examination and measurement of them; reduction of these measurements, and preparation of results for the printer’. 

Canto: Far from just bureaucratic work – this would’ve involved a lot of learning and conjecture, noting patterns and anomalies and trying to account for them.

Jacinta: Absolutely. Antonia Maury, Annie Jump Cannon, Cecilia Payne-Gaposchkin, Henrietta Leavitt and the tragically short-lived Adelaide Ames were among the most noteworthy of these computers, and I should stop using the term, because they weren’t machines and they all made lasting contributions to the field…

Canto: And they all have their own Wikipedia pages. What more evidence do we need?

Jacinta: They contributed to academic papers, often without attribution, especially in the early years, and had their findings read out in academic institutions to which they were barred. Over time they became established teachers and lecturers, in the women’s colleges which started to become a thing in the twenties. But let’s get onto the daunting stuff of science. How were these glass plates created and what did they reveal?

Canto: So spectroscopy became a thing in the 1860s. Spectroscopes were attached to telescopes, and they separated starlight into ‘a pale strip of coloured light ranging from reddish at one end through orange, yellow, green, and blue to violet at the other’. I quote from the book. But what these changing colours meant exactly, as well as the ‘many black vertical lines interspersed at intervals along the coloured strip’, this was all something of a mystery, a code that needed to be cracked. Henry Draper had captured these spectral lines and intervals on photographic plates, which were bequeathed to Harvard by his widow. They formed the beginning of the collection. 

Jacinta: The term spectrum was first used by Isaac Newton two centuries earlier, and he correctly claimed that this coloration wasn’t due to flaws in glass and crystals but was a property of light itself. The dark lines within the stellar spectra on Draper’s plates are called Fraunhofer lines, after a Bavarian lens-maker, Joseph von Fraunhofer, who built the first spectroscope. He at first thought the dark lines between the rainbow of colours his instrument produced were somehow artificial, but continued work convinced him that they were a natural effect. He gave them alphabetical labels according to their thickness, including the letter D for a double line in the pale orange region. He mapped hundreds of them, though today we’ve detected many thousands of them in sunlight. He didn’t understand what they were, though he realised they were something significant. Later in the 19th century Robert Bunsen and Gustav Kirchov conducted experiments with various chemical elements and found that they burned in colours around those black lines, which we now know as absorption lines. 

Canto: Yes, it was Kirchov who connected the colours created by burning elements to the spectral lines that the sun’s light could be separated into, concluding that this great fireball of gases producing white light in the sky was actually a mixture of burning elements, or elements being transformed into other elements. As to the absorption lines, Sobel puts it this way:

As light radiated through the sun’s outer layers, the bright emission lines from the solar conflagration were absorbed in the cooler surrounding atmosphere, leaving dark telltale gaps in the solar spectrum.

These absorption lines, which together with emission lines, are spectral lines in the visible spectrum which ‘can be used to identify the atoms, elements or molecules present in a star, galaxy or cloud of interstellar gas’, to quote from this Swinburne University site

Jacinta: So we’ll try to keep within the confines of the book, and the scientific developments of the period which these women, in particular, contributed to. So, rather, surprisingly to us modern wiseacres, these revelations about the sun as a super-hot fireball and a producer of elements was a bit hard for 19th century folk to take in, but scientists were excited. Henry Draper described spectral analysis as having ‘made the chemist’s arms millions of miles long’, and in 1872 he began photographing the spectra of other stars. It was long known that they had different colours and brightnesses – called ‘apparent luminosities’ – but spectral analysis provided more detailed data for categorisation, and sets of photographs revealed changes in luminosity and colour over time. Williamina Fleming, Harvard’s principal computer, took charge of Draper’s thousands of plates, which provided the most detailed spectral data of stars up to that time, and was able to analyse them into classes, via their absorption lines, in new and complex ways. It was cutting edge science.

Canto: There was also an interest in throwing more light, so to speak, on variable stars. They were so numerous and complex in their variability that Pickering needed more computers to track them. Lacking funds, he advertised for volunteers, emphasising the role of women in particular, whose effectiveness he’d seen plenty of evidence for. 

Jacinta: Not to mention their willingness to work for less, or effectively nothing. These were often siblings or partners of astronomers or other scientists, with unfulfilled scientific ambitions. Later, though, came from the newly created ‘Ladies’ Colleges, such as Radcliffe and Wellesley.

Canto: The Orion Nebula was a particularly rich source of these variable stars, and Pickering found an ideal computer, Henrietta Leavitt, a Radcliffe graduate, to explore them. Within six months, she’d confirmed previous identifications of variables in the nebula and added more than 50 others, afterwards confirmed by Fleming. Then, using a combination of negative and positive glass plates, she found hundreds more, in the Orion Nebula and the Small Magellanic Cloud. As Pickering pointed out, due to the lack of resolution in the plates, this number was likely the tip of the iceberg. In writing up a report of her findings, Leavitt described a pattern she’d found: ‘It is worthy of notice… that the brighter variables [aka cepheid variables] have the longer periods’. This brightness (or luminosity) and its relationship to periodicity (the time taken to go through a full cycle of change) is now known as the Leavitt Law, though of course it took decades for Henrietta Leavitt to receive full recognition for discovering it. 

Jacinta: Yes, it’s worth noting that these women worked painstakingly on data analysis, developing new and more rigorous classification systems, studying and theorising about anomalies, and communicating their findings to leading astronomers and researchers around the world. And it’s also worth noting that they were supported and highly appreciated at Harvard by Edward Pickering and his successor as Director of the Harvard College Observatory, Harlow Shapley – though of course there were plenty of naysayers. 

Canto: Okay so we’ve spoken of two or three of the computer stars’, and there were many more, but let’s finish with the work of Antonia Maury. 

Jacinta: Well we must also mention Annie Jump Cannon (great name), star classifier and photographer extraordinaire, suffragist and generally formidable persona, in spite of being almost completely deaf. She classified around 350,000 stars and contributed greatly to the Harvard Classification Scheme, the first international star classification system. Antonia Coetana de Paiva Pereira Maury (I’m not kidding), a graduate of Vassar College, was a niece of Henry Draper. 

Canto: Not what you know but who you know? 

Jacinta: It is partly that – and that cliché is worth a whole book to itself – but Maury was no slouch, she was a keen and observant star observer and systemiser. One important discovery she shared with Pickering was one of the first known binary star systems, in the handle of the Big Dipper. This required months of careful observation from 1887 through 1889, as they noted one spectral line separating into two then the lines merging again, then separating, with one line shifting slightly to the red end of the spectrum and the other to the blue. Once they recognised that they were dealing with binary star systems, others were soon found. And once these systems were confirmed, Maury carefully calculated their orbital periods and speeds.

Canto: There were many other important breakthroughs. Spectral colours, as we’ve pointed out, were connected to particular chemical elements, and Cecilia Payne, whose major focus was the measurement of stellar temperatures, found a superabundance in the elements hydrogen and helium, which confounded other experts and soon made her doubt her own calculations. Payne wrote up her findings in the Proceedings of the National Academy of Sciences in 1925, ‘admitting’ that the percentages of hydrogen and helium were ‘improbably high’ and ‘almost certainly not real’. 

Jacinta: Yes, it’s well worth noting that the knowledge we have of stars today, which seems almost eternal to us, is in fact very recent. The book also covers the dispute between Harlow Shapley and Edwin Hubble – with many on either side of course – as to whether other galaxies existed. That dispute was only resolved in the thirties, and now we count other galaxies in the trillions. So the period covered in Sobel’s book was a truly transformative period in our understanding of the universe, as well as transformative in terms of women’s education and women’s participation in the most heavenly of all the sciences. 

Canto: Whateva.


The glass universe, by Dava Sobel, 2016

Written by stewart henderson

October 22, 2020 at 1:22 pm

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