by Athena Basu
Many of you are probably familiar with the Harvard System of Stellar Classification. In this system, the O B A F G K M sequence groups stars in terms of apparent color and effective/surface temperature. O describes the bluest and hottest stars (> 30,000 K), while M includes the reddest and coolest stars (2400-3700 K). Each class is also subdivided by numeric digits – 0 indicating the hottest and 9 the coolest of each class (e.g. B8 like Rigel or G2 like our sun).
More recent classification systems – like the Morgan Keenan (MK) system – remain very similar to their precursor, but also add a Luminosity class to distinguish between stars of different sizes and total luminosities (for example, differentiates between a red dwarf vs a red giant) with our sun classified as G2V (the Roman numeral indicating that it is a main sequence star).
But how was the Harvard System created? For that, we will have to delve into the story of Annie Jump Cannon.
Who was Annie Jump Cannon
I first learned about Annie Jump Cannon – one of the most influential female astronomers in history – in Episode 8 of Cosmos: A Spacetime Odyssey. In 1896, she was hired by Harvard Observatory Director Edward C. Pickering to join Harvard’s female team of “Computers”; The “Harvard Computers” examined data taken by the observatory, aiming to map and catalogue every star to a photographic magnitude of 9. There, Cannon manually classified over 350,000 stars (more than anyone else in a lifetime) and revolutionized astronomy with the creation of the modern star classification scheme – on which stellar astronomy is based – all during a time where pursuit of careers in higher academia (or just careers in general) was not accepted for women.
Annie Jump Cannon – born on December 11, 1863 – was the eldest daughter of Wilson Cannon and Mary Jump. She developed her interest in astronomy in part due to her mother, who taught her the constellations and encouraged her to pursue the sciences at Wellesley College. There she studied physics and astronomy under the mentorship of Sarah Frances Whiting – who later inspired her to learn about spectroscopy – and became the school’s valedictorian. After graduating with a degree in physics in 1884, she traveled around Europe with a box camera and learned about the new art of photography, with her photos published by the camera manufacturer upon her return. Later she attended Radcliffe College to continue her studies in astronomy and was hired by Edward C. Pickering at the Harvard College Observatory. Cannon maintained a positive outlook on life in the face of adversity. She faced many challenges in her life and work – from sexist attitudes of the time challenging her success at every turn, to losing her hearing to scarlet fever even before starting her career, and the loss of her mother and greatest supporter at a young age – yet still managed to be a joyful and driven person through it all.
Annie Jump Cannon developed the Harvard system to fulfill the need for a robust yet straightforward classification scheme. She created the spectral classes O B A F G K M based on the strength of Balmer absorption lines (the specific wavelength emissions of hydrogen atoms) and sorted bright stars into these categories. Interestingly, it was not until after the Harvard system was created, that it was realized the spectral classes reflected star temperature. Her refinement of star classification processes was also incredible. At first progress was slow; from when she first joined the computers to the mid-point of her career, Cannon improved the cataloging technique from handling around 1 star per day, to 200 an hour. Using just a magnifying glass she could accurately classify stars down to 9th magnitude from their spectral pattern.
In 1901, Annie Jump Cannon published her first catalog of stellar spectra. For her work, she was later appointed the Curator of Astronomical Photographs at Harvard and in 1914 made an honorary member of the Royal Astronomical Society. In 1922 her stellar classification system was formally adopted by the International Astronomical Union and is still in use today with minor modifications. She collaborated with Cecilia Payne, who was completing her PhD in Astronomy at Harvard, to show that stars were mostly composed of hydrogen and helium – a radical idea at a time when scientists believed the earth and sun had the same ratio of elements but at different temperatures.
In 1919 she took over from Pickering to run the Harvard Computers and in 1925 Cannon became the first woman to be given an honorary doctorate from Oxford. She retired in 1940 but continued to work at the Harvard Observatory until she died a year later at the age of 77. She had a big influence on women gaining respect in science and particularly astronomy, and to this day there is the Annie Jump Cannon Award given to outstanding female astronomers in North America for their post-doctoral research, most recently won by Laura Kreidberg for her work on exoplanet atmospheres.
Annie Jump Cannon and her colleagues in the Harvard Computers, such as Henrietta Leavitt who observed variable stars that led to Edwin Hubble discovering galaxies and their distances, and Antonia Maury who figured out the relative sizes of stars from their spectra, had an incredible impact on the last century of astronomy and should be remembered long into future.
Athena Basu – some info about me
I joined AAAP over 3 years ago and became the youngest QO and Keyholder at age 14. I have found it to be a wonderful group of smart and helpful people that increased my interest in observational astronomy and motivated me to get my first decent telescope – a 10” Dobsonian. Currently I am a High School Senior in Bucks County PA and hoping to go to college in September to study Astrophysics.
Links shown in the table above: Effective Temperature Vega-relative chromaticity Chromaticity (D65) solar masses solar radii bolometric main-sequence stars