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Stellar Classification

June 9, 2016

As everyone knows, the sciences have traditionally been male-dominated professions. Worse than that, scientists and engineers have a history of being downright misogynist. One glaring example of my generation is the mnemonic for electronic component color code. In those days before rapid printing of fine pitch numeric characters on odd-shaped objects, resistance, capacitance, and inductance values were specified by bands of color.

The colors, black, brown, red, orange, yellow, green, blue, violet, gray, and white, represented the numbers zero-nine. The mnemonic I learned in the dark corner of the lab was "Bad boys rape our young girls but violet gives willingly." Violet is still a common name today, ranking 77th as a girl's name in the current decade.[1]

Resistor color code
The electronic component color code, used for resistors, capacitors, and inductors. (Created using Inkscape.)

Stars do not have the same spectral characteristics, since the hotter stars are more blue, and the cooler stars are more red. These different spectral types are identified by a letter code. Astronomers were somewhat less crude in their mnemonic for stellar classification.

For historical reasons, the spectral star types were designated, O, B, A, F, G, K, M. Our Sun is a type G star; more specifically G2V. The astronomer's mnemonic for the stellar classification is, "Oh, be a fine girl, kiss me!" The one that I learned had three additional spectral types, R, N, S at the end, so the mnemonic I learned was "Oh, be a fine girl, kiss me right now, smack!"

Despite the fact that women were not really welcomed in the sciences, there have been quite a few prominent female astronomers, the earliest example being Caroline Herschel, the sister of the astronomer, William Herschel. I wrote about Caroline Herschel in an earlier article (Sexism in Science, September 28, 2012).

William Herschel, discoverer of the planet, Uranus, and his sister, CarolineWilliam Herschel, discoverer of the planet, Uranus, and his sister, Caroline.

Herschel is shown polishing a telescope mirror, assisted by Caroline.

Caroline Herschel was awarded a gold medal from the Royal Astronomical Society, and she received a small salary from King George III.

An 1896 lithograph from the Wellcome Library, via Wikimedia Commons.)

While Edwin Hubble is credited with discovering the expanding universe that's been a fundamental topic of modern astronomical observations, it was a discovery by American astronomer, Henrietta Swan Leavitt (1868-1921) that allowed Hubble's conjecture. Leavitt discovered the proportionality between the period and luminosity of the Cepheid variable stars.

These stars are named after the star, Delta Cephei, discovered to have variable luminosity in 1784. As Leavitt and her employer/colleague, Edward C. Pickering, wrote in volume 173 of the Harvard College Observatory Circular,
"A straight line can readily be drawn among each of the two series of points corresponding to the maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods... Since the variables are probably at nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light..."

Leavitt graduated from Radcliffe College, Harvard's female outpost, in 1893, and she joined the Harvard College Observatory. Her title there was "computer," and her work was to translate the images of stars on photographic plates to useful data. That's how her Cepheid variable discovery was enabled, a discovery that allowed Hubble to relate galactic redshift to their distance from the Earth, a relationship now known as Hubble's law.

Henrietta Swan Leavitt
"A woman's work is never done." American astronomer, Henrietta Swan Leavitt (1868-1921). (Photograph from the Emilio Segrè Visual Archives of the American Institute of Physics, via Wikimedia Commons.)

Joining Leavitt as a member of the Harvard College Observatory computers in 1896 was Annie Jump Cannon (1863-1941), who had received a physics degree from Wellesley College in 1884. She had been encouraged in her scientific studies by her mother, and she became a Wellesley College student of Sarah Frances Whiting (1847-1927), one of the earliest woman physicists in the United States. Cannon was the valedictorian of her class at Wellesley College.

Sarah Frances Whiting (1847-1927)A circa 1879 photograph of Sarah Frances Whiting (1847-1927).

When Wilhelm Röntgen discovered X-rays in 1895, Whiting configured an old Crookes tube to take some of the first radiographs. These were of coins in a purse, and bones through flesh.[3]

(A Wellesley College photograph, via Wikimedia Commons.)

After graduation, Cannon learned photography, but she didn't seek any professional position until her mother died in 1894. That year, Whiting hired her as a junior physics teacher at Wellesley. While teaching, Cannon learned spectroscopy, and she started studies for an MA degree that was finally completed in 1907. In 1896, physicist and astronomer, Edward C. Pickering, hired her as his assistant at the Harvard College Observatory.

Cannon's assigned task at Harvard was to catalog variable stars and classify the spectra of stars in the southern celestial hemisphere.[2] In 1911, her position was advanced to curator of observational photographs, and she embarked on a forty year project to spectrally classify stars down to ninth magnitude.[2]

The result of this study was the nine volumes of the Henry Draper Catalogue, named after an amateur astronomer in whose memory the project was funded.[2] This catalog, published from 1918-1924, lists nearly a quarter of a million stars, assigning them to the O, B, A, F, G, K and M letter classification. The catalog was extended over the years, classifying many more stars.

A 1922 portrait of Annie Jump CannonA 1922 portrait of Annie Jump Cannon (1863-1941).

(Portrait appearing in the New York World-Telegram and the Sun Newspaper, via Wikimedia Commons.)

People with disabilities have made important contributions to science. We have the example of Stephen Hawking. Annie Cannon was doubly disabled, being nearly deaf, and also (for that time) a woman. Her deafness is presumed to have been caused by her contracting scarlet fever. It wasn't until 1938, many years after the Draper Catalogue, that she was promoted from astronomical assistant to astronomer, at age 74.[2]

Aside from classifying stars, Cannon discovered 300 variable stars and five novas. Her system of stellar classification, with a few additional members (my R, N, and S stars) and other tweaks is still used by today's astronomers. Canon was the first woman officer of the American Astronomical Society. The Annie Jump Cannon Award is given annually to a distinguished North American woman astronomer.

Lunar crater, CannonThe 56 kilometer diameter Cannon Crater, named after Annie Jump Cannon, is located on the Moon at 19.9°N 81.4°E.

(A Lunar Orbiter 4 image, via Wikimedia Commons.)


  1. Baby Names for the 2010 Decade, United States Social Security Administration Web Site.
  2. Dan Malerbo, "Let's talk about science: development of star classification," Pittsburgh Post-Gazette, May 19, 2016.
  3. Annie J. Cannon, "Sarah Frances Whiting," Science, vol. 66, no. 1714 (November 4, 1927), pp. 417-418, DOI: 10.1126/science.66.1714.417.

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