Ka cika wannan magana: Kafin a fahimci cewa akwai taurari masu kama da dunƙule a waje da damin taurari na Milky Way, sau da yawa ana kiransu "spiral nebulae". Tambayar ko irin waɗannan abubuwa sun kasance galaxies daban-daban masu zaman kansu na Milky Way, ko wani nau'i na nebula da ke cikin galaxy namu, shine batun batun.
Babban muhawara na 1920, tsakanin Heber Curtis na Lick Observatory da Harlow Shapley na Mt. Gidan kallo na Wilson. Farawa a 1923, Edwin Hubble ya lura da masu canjin Cepheid a cikin wasu nebulae masu juyawa, gami da abin da ake kira "Andromeda Nebula", yana tabbatar da cewa, a zahiri, dukkanin taurari ne a waje da namu. Maganar "ƙuƙwalwar ƙuƙwalwa" tun lokacin da aka yi amfani da shi.