How do know if something is red-shifted without prior knowledge?

[Saoist]

sorry if this sounds a tad retarded but astronomical topics arent my forte...how do know if something is red-shifted without prior knowledge?

say you've got light emitted from galaxy far far away at 500nm, and it ends up on Earth at 510nm. how can you differentiate between 510nm actual radiation and 500nm radiation red shifted?

it sounds like circular reasoning to me: by the amount of red shift, you can figure out the source velocity, except you don't know the original wavelength, so we can figure it out from the shift, except to know the original wavelength you need to know the shift already and hence the velocity...aaand now I'm confused.

i thought you could do it by recognising familiar groupings of line spectra...but surely the spectra will vary according to what the source is composed of etc.

 

[EL]

i thought you could do it by recognising familiar groupings of line spectra...

Yes, this is how it's done.
Most sources hardly consist of anything but hydrogene and helium, so there's no problem with identifying these lines.

 

[russ_watters]

i thought you could do it by recognising familiar groupings of line spectra...but surely the spectra will vary according to what the source is composed of etc.

In addition to what EL said, those spectra vary so much based on the chemical composition that elements really can't be mistaken for other elements, redshifted.

 

[SpaceTiger]

In addition to what EL said, those spectra vary so much based on the chemical composition that elements really can't be mistaken for other elements, redshifted.

Well, they were initially, but once it was pointed out, it was pretty unambiguous.

It's amusing some of the elements they used to identify with quasar absorption and emission lines.

 

[matt.o]

forbidden lines would have been a great source of confusion!