Cika wannan jumla: Shapley ya shiga cikin "
Babban muhawara " tare da Heber D. Curtis a kan yanayin nebulae da galaxies da girman duniya. An yi muhawara a ranar 26 ga Afrilu, 1920, a zauren Cibiyar Kimiyya ta Kasa ta Amurka a Washington DC. Shapley ya ɗauki gefen cewa nebulae masu lanƙwasawa (wanda yanzu ake kira taurari) suna cikin Milky Way ɗinmu, yayin da Curtis ya ɗauki gefen cewa nebulae masu lanƙwasawa 'yan tsibiri ne' a waje da Milky Way ɗinmu kuma ana iya kwatanta su da girman da yanayin namu Milky Way. Wannan batun da muhawara sune farkon ilimin tauraron dan adam, yayin da cikakkun hujjoji da bayanai, sau da yawa tare da rashin fahimta, sun bayyana tare a 1921.