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Saturday, May 31, 2014

Reading List: Miss Leavitt's Stars

Johnson, George. Miss Leavitt's Stars. New York: W. W. Norton, 2005. ISBN 978-0-393-32856-1.
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 enormity 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 realised 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 2013).

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.

In the Kindle edition (as viewed on the iPad) the quotations at the start of each chapter are mis-formatted so each character appears on its own line. The index contains references to page numbers in the print edition and is useless because the Kindle edition contains no page numbers.

Posted at May 31, 2014 21:54