Classification of Stellar Spectra: Some History

 (1) In the early 1800s Joseph Fraunhoffer discovered the dark lines in
the spectrum of the sun. He observed dark lines in the spectra of bright
stars and noted that different stars showed different patterns.

 (2) In 1866 Father P.A. Secchi noticed that the line patterns depended
on the colors of the stars. He assigned stars to four classes of color:

white, yellow, red, and deep red

Furthermore he noted :
white stars associated mostly with hydrogen absorption lines
yellow stars like the sun, many absorption lines, darkest calcium and
hydrogen
red stars numerous dark lines that formed bands

 (3) In late 1890s photography applied to the problem, more spectra and
better details. E.C. Pickering of Harvard College Observatory was able
to subdivide Secchi's color classes. Remember, color suggests
temperature.
A to D to Secchi's white class
E to L to the yellow class
M and N to the red class

 (4) In 1901 Annie Jump Cannon working on the classification, found a
more orderly sequence if the stars ordered by their temperatures. She
reordered Secchi's classes into the sequence

O B A F G K M 
O being the hottest stars, (blue) and 
M being the coolest (red)
(the spectral classes of today)
This is a morphological scheme; the spectra are ordered on their
appearance, especially upon the prominence of the absorption lines of
hydrogen. It lacked a physical basis. Ultimately Cannon classified
250,000 spectra!

 (5) In 1920s Cecilia Payne showed mathematically that a star's spectral
class is determined by its temperature and she made clear that stars are
composed mostly of hydrogen.