Classifying the Fingerprints of Stars

The spectrum from a star is comparable to a bar-code 
on a grocery store item.  For a bar-code, the thickness 
and location of the black lines allows a grocery store 
scanner to interpret important details about the item, 
such as brand, item, weight, quantity, or price.

In the early 1900's, astronomers began photographing 
the spectra of a vast number of stars.  At first, the 
diversity of spectral features was too confusing to 
explain, so they simply grouped similar appearing spectra 
together in classes designated by the letters of the 
alphabet (A,B,C,D, etc.).  When a group of Harvard 
astronomers, led by Annie Cannon, discovered a smooth 
sequence of types of spectra, the already assigned 
letters fell into the sequence O,B,A,F,G,K,M.  

For finer discrimination, the classes are further 
divided into subclasses from 0-9.  Within each class 
the spectra are similar, yet the various classes merge 
smoothly into one another.  Therefore, a F9 star and 
a G0 star are more similar to each other than 
a G0 and G9. 

Read more about the Harvard Women and the history of Stellar Spectral Classification.

Early Astronomical Uses of Photography - Henry Draper

The Harvard Observatory Program to Classify Stellar Spectra - Edward Pickering & Williamina Fleming

Annie Cannon and the 'Computers'

The Spectral Classification Sequence is a Temperature Sequence - Cecilia Payne-Gaposchkin

Other Spectral Types: R, N, S, L and T

Click here to start the on-line exercise