Comparing Spectra of Distant Galaxies

[Ellio]

Homework Statement

none

Relevant Equations

see the screenshot

So tell me if I'm wrong, the left spectrum (white) and the right one (red) show spectra of two different galaxies (with different distances), right ?
Why is it possible to know the velocity of the red-shifted spectrum by comparing it to the left one ??

 

[Orodruin]

Homework Statement:: none
Relevant Equations:: see the screenshot

View attachment 258817
So tell me if I'm wrong, the left spectrum (white) and the right one (red) show spectra of two different galaxies (with different distances), right ?
Why is it possible to know the velocity of the red-shifted spectrum by comparing it to the left one ??

The spectra have the same shape so you can just look at what the wavelength shift is for one of the peaks or absorption lines.