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Astronomical spectroscopy and doppler shift.

  1. Nov 28, 2011 #1

    I have two (stupid) questions:

    1) We can determine what elements are present in distant stars by studying their spectrum, right? But if the spectrum of some star is redshifted, wouldn't we be fooled into believing some elements to be present there that might not really be there, due to the shift in the spectrum?

    2) How can we tell whether an inherently red object is redshifted?

    Many thanks!
  2. jcsd
  3. Nov 28, 2011 #2


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    Hey ViolentCorpse, these are excellent questions.

    1) The spectrum of a specific element is like a "fingerprint" -- it includes a bunch of individual lines with specific spacings between them. The spectra for different elements appear as distinct patterns. Even if they are redshifted (or blueshifted), the patterns remain disctint and recognizable.

    2) What's an "inherently red object?" The redshift phenomenon was discovered by observing that certain patterns of lines (like the Hydrogen series) were systematically shifted toward the red end of the spectrum. The amount of redshift turned out to be correlated to distance, leading to the modern understanding of the universe's evolution.

    - Warren
  4. Nov 29, 2011 #3
    Thank you very much for the answers, chroot!

    And for my second question, I meant to say "intrinsically red", as in something that is red by it's very nature. Will the spectrum obtained by such an object be the same as one that is not intrinsically red but rather redshifted?
    I'm sorry for not being able to explain things lucidly.

    Thank you again, chroot!
  5. Nov 29, 2011 #4
    As Warren said, elements and simple compounds each have a bunch of characteristic lines.

    These 'handprints' may appear multiple times, each set with different shifts, as light from eg a quasar passes through foreground clouds.

    Even if the source object is red, there will be emission and absorption lines. The problem, IIRC, is that some carbon compounds have so many closely spaced lines that they form a 'fog' which masks all but the strongest individual signals...

    A rough analogy is 'tree-ring' dating, where 'handprints' of 'long & short' gaps can be matched across different trees and cut timbers, the overlaps thus spanning many centuries. (This also helps to calibrate carbon dating results. ;- )
  6. Nov 29, 2011 #5
    Thanks a lot, Nik2213! You guys are awesome! :D
  7. Dec 2, 2011 #6

    Ken G

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    Another useful analogy might be the Doppler shift of sound waves, like the whine of a racecar and how the tone drops as it goes past, changing from moving toward us and then moving away. Now imagine the racecar was blasting out your favorite song-- you would be able to tell if the car was moving toward or away from you becuase you could tell if the notes you normally recognize were shifted to higher or lower tones. The pattern of notes is like the pattern of spectral lines-- you recognize them well enough to know when they have been shifted. It could be a different song that just happens to use all the same notes, only shifted, but that's very unlikely.
  8. Dec 6, 2011 #7
    Are there any 'fingerprint' examples of at least a couple elements IE Titanium on the moon ?
  9. Dec 6, 2011 #8


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    I don't think we can use the same spectroscopy on the moon as we can on stars. The moon only reflects sunlight, it does not emit light on it's own. However I do believe we still can use spectroscopy, just differently. I'm not sure on the details of all of it.
  10. Dec 7, 2011 #9

    Ken G

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    The lines of neutral hydrogen atoms are one commonly used fingerprint in astronomy, because hydrogen is abundant and the H atom has a simple structure (at low density) that is easily calculated. You get spectral fingerprints called "series", like the Lyman series or the Balmer series, and the shifts between the lines in the series are very characteristic of hydrogen. Then if all the lines are shifted by the same factor (as for Doppler shifts and cosmological redshifts), it is easy to know that you have hydrogen lines, and you can also know by what factor they are shifted. This is hugely important in astronomy, from detecting extrasolar planets (by looking at shifts in lines of the star, not the planet) to doing cosmology.
  11. Dec 7, 2011 #10


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    The moon has no emission spectrum.
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