Spectroscopy and the doppler effect

[The Subject]

So the prof shows us how to tell if an object is red shifted, by comparing two spectra (i assume of the same object). The emission lines had similar gaps but were shifted right, therefore moving away from us.

How can you obtain two different spectra of the same object if it's constantly moving away from us? Wouldn't it only produce one red shifted spectra?

My understanding isn't clear.

 

[Buzz Bloom]

Hi Subject:

The red-shift is based on comparing the spectrum of a distant object with a spectrum that could be measured in a local laboratory. The line(s) that one compares are those that are present in both spectra. It is not necessary to compare all lines. For example., a spectrum from a distant star will likely include lines from hydrogen atoms. The pattern (ratios of wavelengths) of lines are recognizable as being from hydrogen. Their wavelengths can then be compared with those corresponding to lines from local hydrogen.

Hope this helps.

Regards,
Buzz

 

[|Glitch|]

Here is an example of what Buzz Bloom was talking about:

 

[websterling]

They wouldn't be different spectra of the same object. One would be of the object and the other would be of a lab standard. You would then look at the lines of, say hydrogen, in both the object's spectra and the lab standard to measure the redshift. You can then calculate the Doppler shift and relative velocity.

 

[The Subject]

That explains a lot actually! Thanks