Halkaasi ka sii wad 1920-kii dood ayaa dhex martay Harlow Shapley iyo Heber Curtis (The Great Debate
), oo ku saabsan dabeecadda Wadada Caanaha, nebula spiral, iyo cabirka caalamka. Si uu u taageero sheegashadiisa ah in Nebula weyn ee Andromeda uu yahay galaxy dibadeed, Curtis wuxuu xusay muuqaalka wadooyinka mugdiga ah ee u eg daruuraha boodhka ee Wadada Caanaha, iyo sidoo kale wareejinta Doppler ee muhiimka ah.