What is the origin of the stellar classification system "OBAFGKM"? (Intermediate)

I'm an eighth grader at Anchorage Christin Schools, and we've been studying about the solor system and things, but when we were talking about the star spectral charts (O,B,A,F,G,K...) and I was wondering who developed it, and how they came up with the system. My science teacher, Mr. Duncun, didn't know either, and suggested that I try to find out. I've been searching on the internet for what seems like forever, and it would would be great if you could answer my question.

Just like many other names for objects and phenomena in astronomy, the origin of the classification scheme for stars, OBAFGKM, lies in the history of research in this subject. The story revolves around one of the most famous female astronomers of all time: Annie Jump Cannon.

In the 1890s, many scientists were interested in developing a classification scheme for the stars. Edward C. Pickering at Harvard University, together with his assistant Williamina P. Fleming, assigned stars a letter according to how much Hydrogen could be observed in their spectra: stars labeled A had the most Hydrogen, B the next most, and so on through the alphabet. There were 22 types in all. This scheme was rather cumbersome, and it wasn't clear what its physical significance was.

In 1901, another of Pickering's assistants, Annie Jump Cannon, also began to work on the classification sequence. Her meticulous observations led her to simplify the 22-type scheme into a sequence of *temperature*: OBAFGKM. Not only was the scheme more straightforward than the previous, but it related the amount of hydrogen observed to a physical property of the stars for the first time.

Annie Jump Cannon was also a pioneer of women in science. Back in those days, women were hired to do jobs like stellar classification because they could be paid less than men (if at all). Cannon showed the American astronomy community that women are as competent at astronomy as men, and deserved equal rights and opportunities.

Of course, the stellar classification scheme she developed is still in use today, as you know from your class!

For more information about Annie Jump Cannon and her work, check out this biography.

This page was last updated July 18, 2015. 

About the Author

Kristine Spekkens

Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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