external galaxy

Before astronomers could establish the existence of galaxies, they had to develop a way to measure their distances. In an earlier section, it was explained how astronomers first accomplished this exceedingly difficult task for the nearby galaxies during the…

…the outstanding puzzles about the external galaxies (or extragalactic nebulae, as Hubble always called them): Why did the vast majority seem to be moving away from Earth (if the redshifts in their spectra are interpreted as the result of Doppler shifts)? To this end, Hubble was aided by another Mount…