1913 Harlow Shapley doctorate at Princeton University under Henry Norris Russell with the work The orbits of Eighty -Seven Eclipsing Binaries, A Summary. From 1914 he worked at the Mount Wilson Observatory, from 1921 to 1952 he was director of the Harvard College Observatory.
Shapley determined by the study of variable stars on the type of Cepheids in globular clusters for the first time the size of the Milky Way.
In 1915 he formulated the " Big Galaxy " hypothesis, according to which the Milky Way is the only galaxy in the universe and the previously known galaxies (or "fog" after damaligem parlance ) are within the Milky Way. The antithesis, the "world island " theory, according to which the Milky Way is just one of many galaxies, was formulated by Heber Doust Curtis. This debate is known in astronomy as the "great debate" and it was decided in 1923 by Edwin Hubble in favor of the world island theory.
Shapley proceeded from an erroneous normalization of the period-luminosity relation of Cepheids, making it the size of the Milky Way far overestimated ( 300,000 light-years instead of 100,000 light years). Furthermore, the age differentiation of the stellar populations it was not known, so that he underestimated the distance to the Small Magellanic Cloud and the Andromeda Galaxy. First normalizations of the period - luminosity relation derived from the Danish astronomer Ejnar Hertzsprung.
Shapley also led studies of galaxy clusters and distance determinations of stars by means of photometry.