Pickering published a more detailed study of the spectra of bright stars in the northern hemisphere in 1897. Maury used classifications numbered from I to XXII; groups I to XX corresponded to subdivisions of the Draper Catalogue types B, A, F, G, K, and M, while XXI and XXII corresponded to the Draper Catalogue types N and O. She was the first to place B stars in their current position, prior to A stars, in the spectral classification.

In 1890, the Harvard College Observatory constructed the Boyden Observatory