Miss Leavitt's Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe (Great Discoveries)

Henrietta Swan Leavitt was a computer. No, this is not a tale of artificial intelligence, but rather of the key discovery which allowed astronomers to grasp the distance scale of the universe. In the late 19th century it became increasingly common for daughters of modestly prosperous families to attend college. Henrietta Leavitt's father was a Congregational church minister in Ohio whose income allowed him to send his daughter to Oberlin College in 1885. In 1888 she transferred to the Society for the Collegiate Instruction of Women (later Radcliffe College) in Cambridge Massachusetts where she earned a bachelor's degree in 1892. In her senior year, she took a course in astronomy which sparked a lifetime fascination with the stars. After graduation, she remained in Cambridge and the next year was volunteering at the Harvard College Observatory and was later put on salary.The director of the observatory, Edward Pickering, realised that while at the time it was considered inappropriate for women to sit up all night operating a telescope, much of the work of astronomy consisted of tedious tasks such as measuring the position and brightness of stars on photographic plates, compiling catalogues, and performing analyses based upon their data. Pickering knew that there was a pool of college educated women (especially in the Boston area) who were unlikely to find work as scientists but who were perfectly capable of doing this office work so essential to the progress of astronomy. Further, they would work for a fraction of the salary of a professional astronomer and Pickering, a shrewd administrator as well as a scientist, reasoned he could boost the output of his observatory by a substantial factor within the available budget. So it was that Leavitt was hired to work full-time at the observatory with a job title of “computer” and a salary of US$ 0.25 per hour (she later got a raise to 0.30, which is comparable to the U.S. federal minimum wage in 2019).There was no shortage of work for Leavitt and her fellow computers (nicknamed “Pickering's Harem”) to do. The major project underway at the observatory was the creation of a catalogue of the position, magnitude, and colour of all stars visible from the northern hemisphere to the limiting magnitude of the telescope available. This was done by exposing glass photographic plates in long time exposures while keeping the telescope precisely aimed at a given patch of the sky (although telescopes of era had “clock drives” which approximately tracked the apparent motion of the sky, imprecision in the mechanism required a human observer [all men!] to track a guide star through an eyepiece during the long exposure and manually keep the star centred on the crosshairs with fine adjustment controls). Since each plate covered only a small fraction of the sky, the work of surveying the entire hemisphere was long, tedious, and often frustrating, as a cloud might drift across the field of view and ruin the exposure.But if the work at the telescope was seemingly endless, analysing the plates it produced was far more arduous. Each plate would contain images of thousands of stars, the position and brightness (inferred from the size of the star's image on the plate) of which had to be measured and recorded. Further, plates taken through different colour filters had to be compared, with the difference in brightness used to estimate each star's colour and hence temperature. And if that weren't enough, plates taken of the same field at different times were compared to discover stars whose brightness varied from one time to another.There are two kinds of these variable stars. The first consist of multiple star systems where one star periodically eclipses another, with the simplest case being an “eclipsing binary”: two stars which eclipse one another. Intrinsic variable stars are individual stars whose brightness varies over time, often accompanied by a change in the star's colour. Both kinds of variable stars were important to astronomers, with intrinsic variables offering clues to astrophysics and the evolution of stars.Leavitt was called a “variable star ‘fiend’ ” by a Princeton astronomer in a letter to Pickering, commenting on the flood of discoveries she published in the Harvard Observatory's journals. For the ambitious Pickering, one hemisphere did not suffice. He arranged for an observatory to be established in Arequipa Peru, which would allow stars visible only from the southern hemisphere to be observed and catalogued. A 24 inch telescope and its accessories were shipped around Cape Horn from Boston, and before long the southern sky was being photographed, with the plates sent to Harvard for measurement and cataloguing. When the news had come to Harvard, it was the computers, not the astronomers, who scrutinised them to see what had been discovered.Now, star catalogues of the kind Pickering was preparing, however useful they were to astronomers, were essentially two-dimensional. They give the position of the star on the sky, but no information about how distant it is from the solar system. Indeed, only the distances of few dozen of the very closest stars had been measured by the end of the 19th century by stellar parallax, but for all the rest of the stars their distances were a complete mystery and consequently also the scale of the visible universe was utterly unknown. Because the intrinsic brightness of stars varies over an enormous range (some stars are a million times more luminous than the Sun, which is itself ten thousand times brighter than some dwarf stars), a star of a given magnitude (brightness as observed from Earth) may either be a nearby star of modest brightness or an brilliant supergiant star far away.One of the first intrinsic variable stars to be studied in depth was Delta Cephei, found to be variable in 1784. It is the prototype Cepheid variable, many more of which were discovered by Leavitt. Cepheids are old, massive stars, which have burnt up most of their hydrogen fuel and vary with a characteristic sawtooth-shaped light curve with periods ranging from days to months. In Leavitt's time the mechanism for this variability was unknown, but it is now understood to be due to oscillations in the star's radius as the ionisation state of helium in the star's outer layer cycles between opaque and transparent states, repeatedly trapping the star's energy and causing it to expand, then releasing it, making the star contract.When examining the plates from the telescope in Peru, Leavitt was fascinated by the Magellanic clouds, which look like little bits of the Milky Way which broke off and migrated to distant parts of the sky (we now know them to be dwarf galaxies which may be in orbit around the Milky Way). Leavitt became fascinated by the clouds, and by assiduous searches on multiple plates showing them, eventually published in 1908 a list of 1,777 variable stars she had discovered in them. While astronomers did not know the exact nature of the Magellanic clouds, they were confident of two things: they were very distant (since stars within them of spectral types which are inherently bright were much dimmer than those seen elsewhere in the sky), and all of the stars in them were about the same distance from the solar system, since it was evident the clouds must be gravitationally bound to persist over time.Leavitt's 1908 paper contained one of the greatest understatements in all of the scientific literature: “It is worthy of notice that the brightest variables have the longer periods.” She had discovered a measuring stick for the universe. In examining Cepheids among the variables in her list, she observed that there was a simple linear relationship between the period of pulsation and how bright the star appeared. But since all of the Cepheids in the clouds must be at about the same distance, that meant their absolute brightness could be determined from their periods. This made the Cepheids “standard candles” which could be used to chart the galaxy and beyond. Since they are so bright, they could be observed at great distances.To take a simple case, suppose you observe a Cepheid in a star cluster, and another in a different part of the sky. The two have about the same period of oscillation, but the one in the cluster has one quarter the brightness at Earth of the other. Since the periods are the same, you know the inherent luminosities of the two stars are alike, so according to the inverse-square law the cluster must be twice as distant as the other star. If the Cepheids have different periods, the relationship Leavitt discovered can be used to compute the relative difference in their luminosity, again allowing their distances to be compared.This method provides a relative distance scale to as far as you can identify and measure the periods of Cepheids, but it does not give their absolute distances. However, if you can measure the distance to any single Cepheid by other means, you can now compute the absolute distance to all of them. Not without controversy, this was accomplished, and for the first time astronomers beheld just how enormous the galaxy was, that the solar system was far from its centre, and that the mysterious “spiral neublæ” many had argued were clouds of gas or solar systems in formation were entire other galaxies among a myriad in a universe of breathtaking size. This was the work of others, but all of it was founded on Leavitt's discovery.Henrietta Leavitt would not live to see all of these consequences of her work. She died of cancer in 1921 at the age of 53, while the debate was still raging over whether the Milky Way was the entire universe or just one of a vast number of “island universes”. Both sides in this controversy based their arguments in large part upon her work.She was paid just ten cents more per hour than a cotton mill worker, and never given the title “astronomer”, never made an observation with a telescope, and yet working endless hours at her desk made one of the most profound discoveries of 20th century astronomy, one which is still being refined by precision measurements from the Earth and space today. While the public hardly ever heard her name, she published her work in professional journals and eminent astronomers were well aware of its significance and her part in creating it. A 66 kilometre crater on the Moon bears her name (the one named after that Armstrong fellow is just 4.6 km, albeit on the near side).This short book is only in part a biography of Leavitt. Apart from her work, she left few traces of her life. It is as much a story of how astronomy was done in her days and how she and others made the giant leap in establishing what we now call the cosmic distance ladder. This was a complicated process, with many missteps and controversies along the way, which are well described here.

Just having finished playing Williamina Fleming in Silent Sky, this fleshed out three of the characters based on the "computers". I wish there was more written about them. Williamina had a crater named after her as well, which the author left out. Other than that I quite enjoyed the book.

Good read, good helper for outlook to fields open.

I am not one to normally write a negative review, but I was really disappointed in this book. My disappointment was not due to the author's skills however. The book itself is well-written and articulate as far as it goes.Rather, I was disappointed because there is not much depth at all provided about Ms. Leavitt herself. In fairness, I should have been forewarned by the apologia given by the author at the beginning of the book. The author indicated in the first pages that there was not a lot of additional or archival information available about Ms. Leavitt, aside from the achievements that she is already famous for. Thus, we do NOT learn much about Ms. Leavitt's life, motivation or personality. In a real sense, I kind of wonder why the book was written at all; as filler, a lot of the book is about Hubble and his discovery of an expanding Universe and there is much about Shapley, Pickering, and other figures. And of course there is a description of Ms. Leavitt's notable contribution to astronomy concerning her observations of Cepheid variables and the period/luminosity relation.In fairness, if you were expecting to learn a lot about Ms. Leavitt's life, motivation and personality, the book is short on this. As indicated, the author honestly admits as much at the beginning of the book. Unfortunately, as a result, he was forced to fill the book with material about others (Hubble, etc.) and we really end up with no better insight into Ms. Leavitt than would have been the case had the book not been written. The title is appropriate in a sense because "The Untold Story" of Ms. Leavitt's life still remains untold.

This short book (130 pages of text) is an essential addition to the history of astronomy. Very little data is available about Henrietta Leavitt, the woman who made one of the most important discoveries in astronomy. As the author notes, she left no diaries, no boxes of letters, no memoirs, and she did not brag about her work. Given the lack of information available, George Johnson does a great job of weaving what we do know of Leavitt's life and work into the story of astronomy in the early 20th century. Johnson is a gifted writer. Sentences and paragraphs are easy to follow; all the references to the people discussed in the book are clearly explained. His use of the "village in the canyon" analogy to explain the strengths and weaknesses of determining parallax is excellent. The relationship of Leavitt's incredibly detailed work on Cepheids to Hubble and Shapley is developed in a way that shows an often gross omission of credit on their part, yet the book is not about blame. Johnson points out the sexist hierarchical structure in astronomy at the time and the role of women as human computers. (They were actually called "computers.") Hubble and others, who should have gone beyond that social limitation, simply assumed that their human computer was not to be given credit. Johnson lets the facts about Leavitt's work speak for themselves and the reader can draw his or her own conclusion. It is true that we never get to know Leavitt in any deeply personal way but that can hardly be held against the book. Instead of speculating based on nothing, Johnson takes the information that we do have and turns this into a testament to a brilliant woman whose work became foundational for modern astronomy. The book is well worth obtaining for anyone interested in the history of science.

It is really sad that Ms Leavitt received so little recognition at the time for her foundational work. Maybe she could a cluster of galaxies named after her.

Women never got the credit they so richly deserve

A detailed account of a very specific astronomical breakthrough. The scientist involved was a woman, and was therefore never acknowledged. If you enjoy astronomy, or have an interest in women in science, you will enjoy this book.

１９世紀から２０世紀への変わり目。天文学の主導権は英国から米国へと移ろうとしている。米国では大型望遠鏡の導入とその観測結果を大がかりに「計算」する女性たちの組織という手を考えた。ハーバード大学天文台の台長ピッカリングの指揮のもとにこうした女性（人間コンピュータと呼ばれる）が集められ、写真乾板の解析にあたった。　その中の一人で、いわゆるセファイド型変光星の光度変化周期と絶対等級の相関を発見したのがリービットである。この発見はのちにハッブルらにより、宇宙の構造を知るための天体までの距離指標として利用される。２０世紀の宇宙観を左右した超大発見と言える。リービットはのちにノーベル賞候補となるが、すでに世を去っていたため受賞はしなかった。　科学実験と理論研究の関連、学者のやりたい仕事と雑務の関係、学界における女性の地位、英国や米国の当時の学界の状況などリービットとその周辺の人々を描くことで、浮き彫りにしようとした作品である。しかし残念ながらリービットは日記や手紙など個人的文書をまったく残さなかったため、彼女の人となりは推測の域を出ない。　天文学を愛し、星のデータを調べることが生き甲斐であったと思われるリービットは、この発見をしたことで満足かも知れない。世の中はこのような「無名」の人々の「仕事」で支えられる。

An interesting read. The quality of the copy was very good.This is probably a book in the library class.

Excellent Biography of Henrietta Leavitt.