Soon after graduating from Harvard, Pickering taught physics at the Massachusetts Institute of Technology. Later, he served as director of Harvard College Observatory from 1877 to his death in 1919, where he made great leaps forward in the gathering of stellar spectra through the use of photography. , standing in front of Building C at the Harvard College Observatory, 13 May 1913 At Harvard, he recruited over 80 women to work for him, including Annie Jump Cannon, Henrietta Swan Leavitt, Antonia Maury, and Florence Cushman. These women, the Harvard Computers, made several important discoveries at HCO. Leavitt's discovery of the period-luminosity relationship for Cepheids, published by Pickering, would prove the foundation for the modern understanding of cosmological distances. In 1876 he co-founded the Appalachian Mountain Club. In 1882, Pickering developed a method to photograph the spectra of multiple stars simultaneously by putting a large prism in front of the photographic plate. He also, along with Williamina Fleming and Annie Jump Cannon designed a stellar classification system based on an alphabetic system for spectral classes that was first known as the Harvard Stellar Classification and became the basis for the Henry Draper Catalog. In 1896, Pickering published observations of previously unknown lines in the spectra of the star ζ-Puppis. These lines became known as the Pickering series and Pickering attributed them to hydrogen in 1897. Alfred Fowler gave the same attribution to similar lines that he observed in a hydrogen-helium mixture in 1912. Analysis by Niels Bohr included in his 'trilogy' on atomic structure argued that the spectral lines arose from ionised helium, He+, and not from hydrogen. Fowler was initially-skeptical but was ultimately convinced that Bohr was correct, and by 1915 "spectroscopists had transferred definitively to helium." Bohr's theoretical work on the Pickering series had demonstrated the need for "a re-examination of problems that seemed already to have been solved within classical theories" and provided important confirmation for his atomic theory. Pickering is credited for making the Harvard College Observatory known and respected around the world, and it continues today to be a well-respected observatory and program.
