What critical data point did Hubble seek by analyzing the spectral lines of distant targets?
The amount of shift ($z$)
Hubble’s investigation focused on transforming faint, fuzzy patches of light, the spiral nebulae, into quantifiable cosmological data points. The method involved passing the collected light through a spectrograph to separate it into a spectrum. Within this spectrum, astronomers look for known spectral lines, such as those produced by hydrogen. The critical measurement Hubble sought was precisely how far these known lines were displaced from their established resting positions—this measure of displacement is quantified as the redshift ($z$). Obtaining an accurate $z$ value was necessary because, via the principles of the Doppler effect, this shift directly provided the velocity at which the galaxy was receding relative to Earth.
