Astronomy and Spectroscopy

Work with Spectra

AJC Work Method

Annie Jump Cannon is most well known for her work with stellar spectra. Using glass plate photographs on which starlight had been spread out by a prism at the telescope focal point, she was able to identify similarities and differences in the physical properties of stars (Fig. 2: Plate i38727, March 24, 1914). This art of classification, also called spectroscopy, takes cues from stark spectral lines that interrupt the stellar smear. With a lightbox and magnifying glass and a few assistants (Fig. 1 & 3), Cannon could classify stars at a rate of one every two to three minutes. With the technology available to her, these spectra appeared in greyscale with transparent line; decades later, our technology can present these spectra in the full rainbow with clear, dark lines (Fig. 8)

AJC Stellar Classification System Spectra

Cannon’s classification system modified an earlier one developed by Williamina Fleming, rearranging her groupings and omitting, adding, and subdividing others based on the spectral lines and their intensity -- a combination of their width and opacity. Figure 8 presents her classification scheme with spectra sorted broadly into alphabetical buckets (OBAFGKM), with numerical digits added to clarify detail among stars. Using this method, she published her first catalog of stars in the Annals of the Astronomical Observatory of Harvard College Observatory, as part of the Henry Draper Memorial, in 1901. One important contribution she added to Fleming's method was the correlation between the star’s surface temperature and spectral class -- her colleagues had fiercely debated this relationship at the observatory. Meanwhile, the wider astronomical community debated each others classification systems, until the reach, success, and sense of the Henry Draper classification system (due wholly to Cannon's work ethic and the Observatory's material mastery of the profession) convinced them to adopt a standard method. The International Astronomical Union officially adopted this system in 1922 and it is still used today.

Page from AJC notebook
Transcript by Project PHaEDRA "volunpeers"

After classifying with breakneck speeds, Cannon focused on particular peculiar spectra. In her logbooks, she records notes and observations with which she could refine her system (Fig. 5 & 6). Cannon worked by hand, but today's astronomy demands machine readable text (and her notes are indeed relevant to this science both in historical genealogy and possibility for future refinement). To this end, the archival project PHaEDRA (Preserving Harvard's Early Data and Research in Astronomy) has been crowdsourcing transcriptions, such as Fig. 6 from the Smithsonian Transcription Service. While the clear typeface is better on the eyes, the formatting and special tags for astronomers still hinder the general legibility of these details.

Astronomy at the Observatory

AJC in college at telescope

Unlike the other computers whom Edward Pickering hired for daytime calculations, Cannon also performed night work at the telescope. It was unusual for women of the late 19th century to be allowed at such work (in Cannon's case is was more a paternalistic concern for safety than a patronizing dismissal of ability), but Pickering recognized that Cannon had come to the observatory with a solid education in both theoretical and practical astronomy. She had learned these techniques during her school years at Wellesley College, where she studied under Sarah Francis Whitting, a previous student of Pickering (Fig. 7). And she carried them all the way to Peru, where she could photograph the stars from the massive Bruce telescope herself, participating in the making of the "glass universe" from which she could decode the stars (Fig. 9)

AJC Observing in Peru

To read more about Cannon, her mentorship of the computers, and her enduring legacy, read on!

Image Sources

2. Wargelin, M. (2018). Photograph of Plate i38727(1914). Center for Astrophysics | Harvard & Smithsonian, Photographic Glass Plate Collection, Cambridge, MA.

3. Wargelin, M. (2018). Photograph of Plate i38727(1914) [Image]. Center for Astrophysics | Harvard & Smithsonian, Photographic Glass Plate Collection, Cambridge, MA.

4. Garrett, E. (2018, May 2). Star luminosity classes [Blog post]. Retrieved from

5. Cannon, A. J. (1899). Notebook, Project PHaEDRA (Sequence 7), Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA.

6. Harvard-Smithsonian Center for Astrophysics. (2017). Harvard-Smithsonian Women Computers Project - Annie Jump Cannon 07. Retrieved from

7. (1880-1884). Annie Jump Cannon, '84 [Image]. Retrieved from

8. Cell Code. (n.d.). Star spectral type chart [Image]. Retrieved from