Mezue ahịrịokwu na-esonụ: N'afọ 1920, arụmụka weere ọnọdụ n'etiti Harlow Shapley na Heber Curtis (onye
Nnukwu arụmụka), gbasara ọdịdị nke Milky Way, nebulae spiral, na akụkụ nke eluigwe na ala. Iji kwado ihe o kwuru nke bụ́ na Nnukwu Igwe ojii Andromeda bụ ụyọkọ kpakpando dị n'èzí, Curtis kwuru na e nwere ụzọ ndị gbara ọchịchịrị yiri ígwé ojii ájá nke Milky Way, nakwa na e nwere nnukwu mgbanwe Doppler.