After his success with Bunsen in finding nearly 30 elements in the spectrum of the Sun,  Kirchhoff and fellow physicist A Secchi started a detailed study of the Fraunhofer and looked at the spectra of other stars.  In 1866 Secchi divided stars into 3 spectral classes.

In 1872  the wealthy amateur astronomer, Henry Draper, was the first person to manage to photograph a star's absorption lines.   He financed a team at the Harvard Observatory to record the spectra of as many stars as possible.   The team including Annie Jump Cannon, Willamina Fleming, Antonia Maury, Henrietta Swan Leavitt and Edward Pickering  analysed the spectra of 250,000 stars and produced the Draper catalogue of stars.

The Harvard Observatory Team

Annie Jump Cannon lead the team and developed the basis for the star classification system that we still use today.  TO B A F G K M .   Generations of astronomers have learnt this sequence with the pneumonic 'Oh, be a fine girl, kiss me!'

Annie Jump Cannon and the spectra plate which she used to start her system of star classification

Samples of star spectra as published by the Harvard group

The contribution of this project to astronomy  was so great that three of the researchers ;Cannon, Maury  and Leavitt (who also discovered Cepheid variable stars) have had craters on the Moon named after them.  

In the 1920s the work of the English astronomer Cecilia Payne and the Indian physicist Meghnad Saha, showed that the sequence of OBAFGKM is in fact a sequence in temperature, with O being the hottest.  Payne realised that stars are made mostly of Hydrogen and helium (10%) with only trace amounts of heavier elements.