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12.10 Spectral Classification

One hundred years ago, dozens of women labored in the basement of the Harvard College Observatory, paving the way for the classification and understanding of stars. These women examined photographic plates on which many stellar images had been smeared out into tiny spectra. They painstakingly measured hundreds of thousands of stellar spectra, noting the wavelength of each of the prominent lines. Known as "computers," these women were paid 25 cents an hour. This was far less than half of a man's rate of pay for similar work. Not only did these women analyze data, but they also carried out many tedious calculations that had to be done manually in the days before electronic calculators. At the time, women could not be staff members at the Observatory, nor could they take classes or earn a degree at Harvard University.

The photographic plates taken at Harvard Observatory a century ago were part of a large survey of the properties of stars. Spectroscopic pioneers of the 19th century noticed that few stars had spectra that exactly matched that of the Sun. Some had more powerful lines of hydrogen; in others the hydrogen lines were extremely weak. The chemical composition revealed by the "fingerprint" of spectral lines varied from star to star. Astronomers wondered if the chemical composition related to any other property of a star. One of the Harvard "computers" answered this question.

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Representative stellar spectra from a range of spectral classes. Click here for original source URL.

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Annie Jump Cannon sitting at her desk at Harvard College Observatory. Click here for original source URL.

Annie Jump Cannon was one of the most important members of the classification project. Born in 1863, Cannon was one of the first women from her state to attend a university. After graduating from Wellesley and Radcliffe, she took a job at Harvard College Observatory as a lowly computer. She worked with incredible speed and precision, and she steadily began to make bigger and bigger contributions to the project. Cannon proposed a spectral classification system that is still used in astronomy today. Cannon arranged stars by temperature in alphabetical categories. Her sequence runs from white-hot A stars, through yellowish G stars, such as the Sun, to cool red M stars. Cannon personally classified 225,300 stars, which was a heroic contribution to her subject. Later in her career, Cannon received many honors and awards. She was the first woman to be given an honorary doctorate by Oxford University. She used the cash portion of one award to establish a prize that is still given to honor the achievement of young women in astronomy.

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Cecilia Payne-Gaposchkin. Click here for original source URL.

In the 1920s, Cecilia Payne-Gaposchkin took the data from the classification project and extended it. Payne-Gaposchkin was born in England and educated at Cambridge University. She was the first person to receive a Ph.D. in astronomy at Harvard University and eventually she became a professor there and its first female department chair. Payne-Gaposchkin realized that the spectral variations among stars were mostly a reflection of temperature in the outer stellar atmosphere. Her unifying idea allowed the apparent diversity of stars to be understood in terms of a temperature sequence.

Science is an incremental process — it progresses in small steps. Fundamental advances in science often begin with the apparently tedious act of classification. Observations are compiled and sorted in the hope that patterns will emerge in the data. The patterns suggest a hypothesis, which in turn leads to a theory based on well-tested physical ideas. Much the same process was followed in the subject of paleontology, where the patterns in the fossil record were eventually understood in terms of the theory of evolution and the mechanism of natural selection. Likewise, the classification of spectral smudges on photographs was just as essential as the insights of theorists to the understanding of how stars work. 

Recognition for women in science came slowly; it was 100 years after the work of the female "computers" that the first woman became a full professor of physics at Harvard University. Thanks to pioneers like Cannon and Gaposchkin and others, we now know that the stars are hot balls of gas like the Sun. They are made of hydrogen and helium with small traces of heavier elements, just like the Sun. They get their energy from the fusion process. Our ideas of how stars work can be tested against observations of the full range of stellar types. We have made the night sky our laboratory.

Even today, women are a minority in astronomy, as in almost all branches of science. Only in a few biological fields do they have equal representation. In astronomy, women are 30% of the members in the professional organizations. In chemistry, the proportion is under 10%. Also, there are subtle and not-so-subtle pressures and biases in academic that mean that women make up a small fraction of scientists at the highest levels. Naturally, men and women may choose to make different career choices. But there is ample evidence that women face a harsh "climate" in science and leave the field in disproportionate numbers. This is a great loss for science, and for the benefits it can bring our society.