Harvard Computers: The Brilliant Women Who Classified Stars


By Emily Levesque, University of Washington

Edward Pickering, director of the Harvard College Observatory, was responsible for the creation of the Harvard Computers. At the turn of the 20th century, the Harvard Computers were an undeniable force of scientific analysis, classification, and discovery. However, the engine of discovery didn’t come in the form of computational or electronic machines: the Harvard Computers were a group of women.

Photo of a group of women while working.
Members of the Harvard Computers were extremely influential in the advance of astronomy in the 20th century. (Image: Harvard College Observatory/Public domain)

Williamina Fleming’s Contributions to Science

One of the first Harvard Computers was Williamina Fleming. She proved to be an exceptionally talented analyst. Working with imaging data, she discovered dozens of variable stars and nebulae, including the famous Horsehead Nebula, and was the first to observe a star known as a white dwarf—the tiny, incredibly dense core remnant of a star like our Sun that’s left behind after its death.

Fleming also worked extensively with Harvard’s spectroscopic data. She began developing a method for classifying the hundreds of thousands of stars in their dataset according to their spectra, identifying stars based on how strong the dark lines from hydrogen atoms absorbing light appeared to be, and sorting the stars into classes denoted by letters. 

The strongest hydrogen absorption lines belong to the A spectral type, the second-strongest to the B type, and so on, down to type O stars with visible helium lines but very weak hydrogen lines.

Her research laid an important foundation for the groundbreaking work of another Harvard Computer: Annie Jump Cannon.

Annie Jump Cannon  

Annie Jump Cannon was born in Delaware in 1863, with an early interest in astronomy encouraged by her mother, Mary. Cannon attended Wellesley College to study physics and astronomy, and also developed a strong interest in photography. While in college, she survived scarlet fever, an illness that left her almost entirely deaf.

Cannon joined the Harvard Computers in 1896 and quickly became one of the fastest classifiers of stellar spectra on the team; in the end, she classified around 350,000 stars during her career. She also quickly got involved in the ongoing question of how to classify stellar spectra.

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Cannon’s Spectral Type Sequence

Other Computers had identified many dark absorption lines in the stellar spectra and speculated about devising new and complex classifications schemes, but Cannon combined these new line identifications with the clear and simple system that Williamina Fleming had devised, classifying stars according to their spectral line strengths.

Image of all the members of the Harvard Computers, including Williamina Fleming and Annie Cannon
Williamina Fleming and Annie Cannon were just two of the Harvard Computers. (Image: Unknown/Public domain)

Cannon’s key insight was to rearrange the classification system’s order, and to base it on a broader set of lines from multiple elements. At the time, astronomers still weren’t quite sure what caused the variations in stellar spectra—speculations included age, temperature, or their fundamental chemical composition. 

Cannon’s classification system moved the O stars, with their clear helium lines, to the top of the list, followed by B and then A stars as the hydrogen lines got stronger and stronger. These were followed by four more categories in roughly alphabetical order, some of which contained composites of several of Fleming’s classes. 

The order of the classes was also based on another discovery of Cannon’s: the stellar types appeared to follow a continuous progression, with some stars appearing halfway between the O and B types, or between the K and M types. In the end, Annie Jump Cannon’s spectral type sequence fell into order as O, B, A, F, G, K, M, with subtypes to indicate exactly where in the sequence a star might fall. Our Sun, for example, would be considered a G2 type star.

Organizing Stars Today

The arrangement of these spectral classes may seem a bit haphazard, but in truth, it’s anything but. Today, astronomers organize stars on what’s known as the Hertzsprung-Russell, or H-R, diagram, named for Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell. 

On the H-R diagram, we plot a star’s effective temperature—the temperature it appears to have in its atmosphere, or outermost layers—against its luminosity, a measure of how much energy the star is emitting, which can be determined based on its relative brightness and distance. If we plot a large collection of stars from all over the sky on this diagram, they fall into a consistent and clear distribution, with the majority of stars tracing along a diagonal. 

We know today that these stars comprise what’s called the main sequence, marking the brightness and temperature of stars born with different initial masses and fusing hydrogen in their cores. 

Portrait of Annie Jump Cannon
Annie Cannon was extremely fast in classifying stellar spectra. (Image: Schlesinger Library, RIAS, Harvard University/Public domain)

Later in their lives, stars like our Sun, with relatively low masses, will cool off and expand after about 10 billion years on the main sequence, becoming red giants and drifting up and to the right on the Hertzsprung-Russell diagram as they switch from fusing hydrogen to fusing helium. Higher-mass stars, marked at the top of the diagram, also cool off and expand after only 10 million years, exhausting their supply of hydrogen much faster and moving straight to the right on the diagram.

Cannon’s Work Stood the Test of Time

None of this was known when Annie Jump Cannon first created her classification scheme, but her method of identifying spectral lines along a continuum in these stars proved brilliant. If we add Cannon’s spectral types—O, B, A, F, G, K, and M—to the Hertzsprung-Russell diagram, we can see where they fall along the horizontal axis: they fall perfectly into order. 

Cannon’s system, years before the surface temperatures of stars were studied in detail, had perfectly traced this elusive stellar property. Cannon’s classification scheme was adopted by the International Astronomical Union in 1922, and it is still in use today. 

Cannon also became the first woman to receive an honorary doctorate of science from Oxford. In 1933, she created the Annie Jump Cannon Prize, awarded annually by the American Astronomical Society to recognize outstanding research and promise by a young female astronomer. The Cannon Prize is still awarded today to young women astronomers.

Common Questions about the Harvard Computers

Q: What were some of Williamina Fleming’s contributions to science?

Williamina Fleming was one of the first Harvard Computers. Working with imaging data, she discovered dozens of variable stars and nebulae. She also developed a method for classifying the stars according to their spectra.

Q: How do astronomers organize stars today?

Today, astronomers organize stars on what’s known as the Hertzsprung-Russell, or H-R, diagram.

Q: Who is awarded the Annie Jump Cannon Prize?

The Annie Jump Cannon Prize is awarded annually by the American Astronomical Society to recognize outstanding research and promise by a young female astronomer.

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