The mesure of distances using the redshift

[houhou.trad]

Hi all,
in order to determine the recessional velocity of same galaxy we use the redshift law:
[(observed wavelength) -(source wavelength) ]/source wavelength=v/c
The problem I have found here is how we can know the source wavelength.
thinks.

 

[marcus]

Different elements have distinctive lines in their spectra. These lines have known wavelengths you can measure in the lab. Hot hydrogen gas and plasma have distinctive patterns of spectral lines that you can recognize, and see how much it has been shifted towards longer wavelengths.

Other elements also play a role.

 

[houhou.trad]

Different elements have distinctive lines in their spectra. These lines have known wavelengths you can measure in the lab. Hot hydrogen gas and plasma have distinctive patterns of spectral lines that you can recognize, and see how much it has been shifted towards longer wavelengths.

Other elements also play a role.

Thus, want you say that we know the source wavelength of these galaxies because we know already the elements that constituting them?

 

[marcus]

It is hard NOT to know, most of the universe is made of hydrogen and helium and elements have distinctive PATTERNs of lines, like 3 close together with the longer one twice as far from the middle, and relative brightnesses of the lines within a pattern

It is like a face or fingerprint, you just see "oh! those are the hydrogen lines, but just shifted over!" or Oh I see the bright yellow sodium line but it is shifted over into the red. And the shifts are all consistent, by the same ratio.

So it is hard to mistake the light of a particular abundant element, at least with nearby galaxies where we get a good sample of the light.