When I was a little kid, I was transfixed by a drawing in one of my books. It showed a young girl in a 19th-century dress and bonnet, gazing up at a monstrous skeleton in a cliff face. The skeleton was an ichthyosaur--an ancient marine reptile resembling a shark, but with an alligator's mouth and huge, bone-encircled eyes. The little girl was Mary Anning, who grew up to be one of the great fossil hunters in history. The way I remember the story, it told how Mary had discovered the skeleton while she was out for a walk. I loved the idea of a kid making such a stupendous find, and I dreamed that one day I would too.
Since rediscovering her as an adult, I've realized that either the book or my memory had it wrong. Mary Anning was actually part of a family of fossil collectors who lived in Lyme Regis, a town in southern England on what is still known as the Jurassic Coast. She and her brother Joseph were the only survivors of ten children born to Richard and Mary Anning in the late 1700's. Their father was a cabinetmaker who made extra money by scouring the cliffs for fossils and selling them to wealthier vacationers. His two children often accompanied him, and became sharp-eyed fossil spotters themselves. Richard died in 1810, leaving the family impoverished. Mary and her brother Joseph kept earning money the way they knew--by fossil-hunting. It was actually Joseph who discovered the ichyosaur's skull, but it was Mary who painstakingly dug the rest of the skeleton out of the rocks. She was 12 at the time.
|Description of a plesiosaur, Mary Anning|
Mary Anning lived her whole life in Lyme Regis, selling her fossil finds. Legend has it she was the inspiration for the tongue twister, "She sells sea shells by the sea shore." The wealthy gentleman scientists of the day who bought her major finds rarely credited her in the papers they wrote. Still, by the time of her death she had earned their respect--they recognized her as an expert in the field, even if many of them thought of a scientifically-talented, lower-class woman as a kind of anomaly in the natural order of things. Perhaps the old ideas about earth history weren't the only outdated notions she helped change.
Now lets shift across the Atlantic. By the late 19th century one could find rooms full of computers at Harvard Observatory. Of course, I'm not talking about electronic computers. At the time, the word "computer" referred to someone hired to do the tedious work of processing scientific data. Most of the computers were women, because they could be hired to work for far less than men. In fact, they earned less than Harvard's secretaries, even though several of them went on to make ground-breaking discoveries in astronomy.
One of these "computers" was Annie Jump Cannon. A talented mathematician and photographer with a degree in physics from Wellesley, Cannon went to work at Harvard Observatory, where she was given the task of cataloging thousands of stars. Scientists had found that light from stars, when passed through a prism, would divide into a rainbow-like spectrum--but with dark lines corresponding to a particular set of wavelengths. These so-called absorption spectra were initially labeled using a complex scheme based on the letters of the alphabet. Cannon convinced the Harvard astronomers to drop all the letters but seven--O,B,A,F,G,K,M--and arrange these "spectral classes" in descending order by the star's temperature--from hot blue O stars, to milder yellow G stars like the sun, to "cool" red M stars. Since then, generations of astronomers have memorized this sequence with the mnemonic, "Oh Be A Fine Girl Kiss Me." I wonder how she would have felt about that?
Another woman employed at the Harvard Observatory was Henrietta Swann Leavitt. She was assigned to look at variable stars--whose brightness fluctuated over time. Eventually she noted a pattern in a kind of variable star called a Cepheid variable: the brighter (more luminous) they are, the longer the period between bright and dim phases. To understand the significance of this, we have to understand that stars can be bright in two ways. All stars have an intrinsic brightness, which is the total amount of light they produce, and an apparent brightness, which is how bright they seem from Earth (which depends on how far away they are.) If you take two stars of equal intrinsic brightness, the more distant one will have a lower apparent brightness. It will seem dimmer, for the same reason a distant streetlight seems dimmer than a nearby one. Intrinsic and apparent brightness are related in a predictable way--a star twice as far away will seem 1/4 as bright, one 3 times as far will seem 1/9 as bright, and so on.
Leavitt figured out the relationship between a variable star's intrinsic brightness and its period. That meant that if astronomers measured the period of a Cepheid variable of unknown intrinsic brightness and distance, then they would be able to estimate its intrinsic brightness. Then they could compare its intrinsic brightness to its apparent brightness and figure out how far away the star is. Before that, astronomers had only been able to judge the distance to relatively close stars, using a method called parallax. Now they had a new and longer yardstick for measuring the universe.
For all anyone knew in Leavitt's day, the Milky Way galaxy was the entire universe. Edwin Hubble showed this wasn't true, and he did so using the relationship Leavitt had discovered. A couple of years after she died, he found a Cepheid variable in stellar cloud known as the Andromeda Nebula. When he figured out how far away it was, he realized it was far beyond any other star ever seen. The Andromeda Nebula wasn't a cloud within our own galaxy--it was a galaxy in its own right. Today it's known as the Andromeda Galaxy, the closest large galaxy to our own. The relationship Henrietta Leavitt had discovered was the key to the realization that our own galaxy is just one little island in a universe of other galaxies.
Mary Anning, Annie Jump Cannon, and Henrietta Leavitt were never considered equals by many of their peers, though Cannon lived long enough to be recognized with a professional title as an astronomer. But each of them helped expand our view of the universe immensely. Anning helped establish the great age and mutability of life on earth, paving the way to the realization that it had evolved over time. Cannon helped classify the stars, and that classification helped astronomers see how the stars themselves change over time. Leavitt's discovery helped them realize they were seeing past the stars of our galaxy into the unfathomable reaches beyond. These were tremendous achievements by women working against enormous odds and pervasive prejudice. I think all three are pretty awe-inspiring.