Cannon expanded on the classification used by Edward Pickering (director of the Harvard Observatory from 1877-1919) and Williamina Fleming, adding the O, B, A, F, G, K, M spectral classes.

The Harvard Classification system, adopted by the International Astronomical Union in 1922, is still in use today.

Annie Jump Cannon learned the basics of astronomy from her mother Mary, who taught her to recognize constellations. Together they built a little observatory in the attic of their home.

Her father encouraged her to attend a new college for women in Massachusetts –– Wellesley. She liked the school but not the weather. She was often sick during her first year there. These illnesses may have contributed to the eventual loss of her hearing.

At Wellesley she studied Physics and Astronomy, working with Sarah Frances Whiting.

https://en.wikipedia.org/wiki/Sarah_Frances_Whiting

Cannon excelled and graduated in 1884 as the valedictorian of her class. But she didn't pursue a career in science. Instead, she moved home to Delaware where she led an active social life.

In 1892, after traveling through Europe and publishing a collection of her photography, Cannon contracted scarlet fever and soon lost her hearing. Then in 1894 her mother died.

Annie and her mother had been very close. She decided she had to leave Delaware, so she wrote to Whiting who offered her a position as a physics instructor at Wellesley.

Cannon returned to Massachusetts and enrolled at Radcliffe for graduate study in physics and astronomy. She began to work at the Harvard observatory under the direction of Edward Pickering, and was surprised to find several other women working there.

Pickering put Cannon to work classifying stars according to their spectra. He had a ten year backlog of spectrograms that needed to be sorted according to the system he had developed with Williamina Fleming.

But there was a disagreement over how exactly the stars should be classified, with Antonia Maury advocating for a more complex system than the simple scheme used by Fleming.

Annie Jump Cannon proposed a third system, a refinement of the Fleming-Pickering system that gathered stars into seven groups – O, B, A, F, G, K, M – according to the strength of the Balmer lines in their spectra.

Later, Cecelia Payne would show that the OBAFGKM classification corresponds to descending temperature, with type O stars the hottest and type M the coolest.