Monday, July 20, 2015

A Brief History of Stellar Women

Tonight I'm observing at a different big telescope, Gemini North at Mauna Kea Observatory, Hawaii. But I wanted to share a great Explainer post by Dr. Amanda Bauer, a PhD Astronomer and Outreach Officer at the Australian Astronomical Observatory, that discusses spectroscopy. She was named one of Australia's "Top 5 Under 40” science researchers and communicators in March of 2015, and has written lots of blog posts about different aspects of astronomy.

This one about spectroscopy caught my interest because 1) it's my craft, too, and 2) it's one of the things in astronomy, and science in general (the basic principles come from physics and chemistry), that I explain most often/enjoy explaining most.

Here's the link. Check it out!

However, there is a bit of history missing from Dr. Bauer's post that is important. Much of what I and MANY astronomers do today for our research -- using spectra to classify and learn about stars -- is thanks to some extremely smart and dedicated women scientists. Here's a bit (er, actually, a rather long bit) of history.

In the early 1900s, the director of Harvard Observatory, Edward Pickering, had a problem similar to many scientists -- too much data and too little time or resources to analyze and understand it all. Pickering wanted to continue the work of astrophotography pioneer Henry Draper, whose great ambition it was to catalog the entire night sky with images and spectra. Unfortunately, Draper died at 45 before he was able to fulfill this ambition, but his wife Anna Mary Palmer donated money to the newly-founded Harvard College Observatory to fund the completion of this monumental survey. Imagine trying to

that could be photographed in the night sky!

So, why would Pickering hire women to help, in a time when women's work was valued even less than they are today? He could pay them less -- 25-35 cents an hour of work six days a week, seven hours a day, below what they would make in clerical work. Thus, Pickering could hire more women workers for the same price as half the number of men, and get more stars classified more quickly. The story also goes that at one point, Pickering became frustrated with his male assistant(s) and declared his maid could do a better job! I guess that was meant to be derogatory towards women, but turns out it was true --Pickering hired his maid at the time, Williamina Fleming, who went on to become the Curator of Astronomical Photographs, the first corporate appointment of a woman at Harvard ever.

Harvard Computers, 1892. Maury third from left, with magnifying glass. Flemming standing at center.

Fleming was responsible for cataloging, indexing, examining, and caring for all of the photographic plates in the Harvard catalog, as well as directing about a dozen other women "Harvard Computers". She examined over 10,000 stars (WHOA) and developed classification system for the stars from A to Q, based on how much hydrogen they showed in their spectra. In 1890 Pickering published the first Henry Draper Catalog.

Around this time, Antonia Maury, the niece of Henry Draper himself, was hired as a Computer. She had studied at the all-women's Vassar College under the first American women to work as a professional astronomer, Maria Mitchell. (Connection: I did my first undergraduate NSF REU at the Maria Mitchell Observatory!) She developed her own classification system for the tens of thousands of spectra, more complex and based on differing line widths (rather than just the presence or absence of lines). Maury's line width classification system (normal 'a', hazy, 'b', sharp, 'c', with intermediate cases) was not appreciated by Pickering at the time, and she left the project for a time. Dorrit Hoffliet, one of the other Computers, noted about Maury, "She was one of the most original thinkers of all the women Pickering employed; but instead of encouraging her attempts at interpreting observations, he was only irritated by her independence and departure from assigned and expected routine." However, Danish astronomer Ejnar Hertzsprung recognized the significance of Maury's different classification when saw that c- and ac-stars were brighter than a- or b-stars. Maury's work contributed greatly to what we know now as the Hertzsprung-Russell diagram, a plot of magnitude and luminosity against temperature that is a cornerstone of stellar evolution and classification today.

Around the same time as Maury joined "Pickering's Women" (ugh, I don't like that), a woman named Annie Jump Cannon also joined. Cannon also came from one of the top colleges for women, Wellesley College, and had studied under another female pioneer in science, Sarah Frances Whiting. Cannon took time off after Wellesley, during which she was stricken with scarlet fever and lost nearly all of her hearing. She returned to school at Radcliffe, near Harvard, and was hired by Pickering as his assistant. Cannon was able to refine the classification systems of Fleming and Maury, reducing the number of categories and rearranging them by temperature, leaving us with the famous OBAFGKM; she also included numbers 1-10 within each category as a finer distinction. Cannon's system was adopted as the standard in 1910 by the International Astronomical Union, and today with only minor changes it is known as the Harvard Spectral Classification.

Today, the American Astronomical Association has an Annie Jump Cannon Award for "outstanding research and promise for future research by a postdoctoral woman researcher." Two of the modern day women astronomers highlighted on this blog have won it -- Alycia Weinberger and Anna Frebel. How inspiring! I hope in the future more Las Campanas Belles will be AJC Award recipients.

Annie Jump Cannon, 1922

A few years before Cannon joined the Harvard Computers, another Radcliffe College alum also started volunteering at Harvard Observatory, working seven years for no pay. Henrietta Swan Leavitt, who also lost her hearing, was a classifier who was specifically tasked with studying "variable stars", whose luminosity (brightness) varied over time. During her career Leavitt discovered over 1200 variable stars, half of *all* known variable stars at the time of her death (WHOA). While studying stars in the Magellanic Clouds, Leavitt noted that a few of the variables showed a pattern -- brighter ones appeared to have longer periods of variation. She determined that these "Cepheid variables" had bright-dim cycle periods that were inversely proportional to their magnitude. By assuming that all the Cepheids within each Magellanic Cloud were about the same distance away, such that their intrinsic brightness could be deduced from their apparent brightness (which depends on distance, and was measured from the photographic plates), Leavitt derived one of the most fundamental relations in astronomy, the period-luminosity relationship. This makes Cepheid variables the first "standard candle" in astronomy, a "rung' on the distance ladder allowing astronomers to measure distances to galaxies too remote for more local measurements of distance (stellar parallax). The later discovery by Edwin Hubble that the universe is expanding was made possible by Leavitt's groundbreaking research. Hubble himself said that Leavitt deserved a Nobel Prize, but she died before her nomination could be considered.

Henrietta Swan Leavitt, not sure what year

Love this workup of an American Girl Doll to resemble Henrietta Swan Leavitt!
A few decades after the Harvard Computers got started, Cecilia Payne-Gaposchkin continued the groundbreaking work in her pursuit of a doctorate at Radcliffe College in his new graduate program in astronomy. She was the first person (let alone woman) to do so! This was made possible by a fellowship to encourage women to study at Harvard Observatory, granted by the then-Director Harlow Shapley. In the time period since the Harvard Computers started classifying star, the science of astrophysics had been born -- the lines in stellar spectra were connected to the same lines observed when different chemical elements on Earth were heated and made to emit light. When Payne arrived at Harvard, some elements like calcium and iron had been identified in stars, since they were the most prominent lines, and it was assumed these were major components stellar composition. Payne inherited Cannon's classification system, thought to be based on stellar temperature. Payne saw the connection between the atomic physics causing spectral lines and the different (high) temperatures in stellar atmospheres. Indian physicist M.N. Saha had recently demonstrated how the ionization of atoms (causing spectral lines) was related to the temperature and pressure in stars. Payne showed that the many differences in the lines of stellar spectra were due in large part to the different atomic ionization states, and hence different temperatures, and *not* to different *amounts* of elements. This seems like a subtle distinction, but it is fundamental and crucial to our understanding of the composition of anything that is not on our planet (and many things that are!). Through her work, Payne discovered that the Sun and other stars are mostly made of hydrogen and helium, not heavier elements, which account for less than 2% of the mass of most stars.

Cecilia Payne-Gaposchkin, not sure what year

Unfortunately, unsurprisingly, Payne's work was considered "clearly impossible" by male professors at the time, and in her thesis, "Stellar Atmospheres, A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars," she included the statement, that the calculated abundances of hydrogen and helium were "almost certainly not real" to protect her career. Though Princeton Professor Henry Norris Russell did not believe Payne's conclusions at first, he later changed his mind after having derived the same result and publishing it, acknowledging Payne's previous work only briefly in his paper. Russell is still often given credit for this monumental discovery. According to also-famous astronomers Otto Struve and Velta Zeberg, Payne-Gaposchkin's thesis was "undoubtedly the most brilliant Ph.D. thesis ever written in astronomy."

So, there you have it. Spectroscopy as we use it today as astronomers is in large part thanks to the contributions of these, and many more, women who worked passionately, for little or no pay, and were not discouraged by the sexism and discrimination challenges they faced. I find their stories remarkable, and am so thankful for their intrepid character and curious, persistent, brilliant minds.

Resources used to compose this summary:

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