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MickeG
2011-Sep-25, 02:48 AM
We are using spectroscopy both to verify distance to other stars and to discern what materials they consist of

How do we decide if the light from a star is red or blue shifted
and how does red or blue shift interfere with deciding what kind of material the star is made of?

Shaula
2011-Sep-25, 06:14 AM
The spectra of material consist of very many lines arranged in complex patterns that depend strongly on what it is made of. Red and blue shift just move this pattern in frequency. Since each pattern is pretty much unique it is just a case of matching the pattern rather than the specific frequency you see on Earth. Think of it like there being a fingerprint that can be moved around spatially but still keeps the same shape. So you just slide your patterns left and right on the spectral plot until you get a match!

So to first order red/blue shift doesn't really hurt you with material ID. There are some complicating factors like the fact that you rather get just one material floating around so you can get several complex patterns overlaid, and since there can be line of sight effects you have to untangle all of these factors to explain a spectrum. There is also the issue of the atmosphere - we look out through frequency windows in it and frequency shifts can move otherwise observable lines outside these windows.

StupendousMan
2011-Sep-25, 03:34 PM
Some of the lectures in astronomy courses I teach might be relevant here.

http://spiff.rit.edu/classes/phys301/lectures/doppler/doppler.html
http://spiff.rit.edu/classes/phys240/lectures/expand/expand.html