How can we measure a galaxy's velocity via a star?

[Phys12]

In this video (), we first took the spectrum of a star and then of a galaxy which look like the following:

However, what I don't get is: if we take the spectrum of a star and then of the galaxy the star is in, shouldn't it give me the same spectrum as they're moving in the same speed? And if we want to determine the redshift/blueshift, shouldn't we look at the absorption lines and compare them to the ones we get in our laboratories rather than a star itself?

Thanks!

 

[phyzguy]

The star is in our galaxy, not in the galaxy whose redshift you are measuring. So the star is nearby and is basically "the laboratory".

 

[Phys12]

The star is in our galaxy, not in the galaxy whose redshift you are measuring. So the star is nearby and is basically "the laboratory".

Oh, got it, thanks! :)