Kammala wannan magana: A shekara ta 1917, Heber Curtis ya lura da nova S Andromedae a cikin Babban Andromeda Nebula (Messier abu 31). Sa'ad da yake bincika hoton, ya ga wasu taurari 11 da suka fito daga sama. Curtis ya lura cewa waɗannan novae sun kasance, a matsakaici, 10 masu girma fiye da waɗanda suka faru a cikin Milky Way. A sakamakon haka, ya sami damar zuwa kimanin nesa na 150,000 parsecs. Ya zama mai ba da shawara game da "tsibirin duniya", wanda ya yi imanin cewa nebulae masu jujjuyawar taurari ne masu zaman kansu. A cikin 1920
Babban muhawara ya faru tsakanin Harlow Shapley da Heber Curtis, game da yanayin Milky Way, da kuma ma'aunin sararin samaniya. Don tallafawa da'awarsa cewa Babban Andromeda Nebula wani galaxy ne na waje, Curtis ya lura da bayyanar layuka masu duhu waɗanda suke kama da gajimare na ƙura a cikin Milky Way, da kuma mahimmancin Doppler.