Jumbled up spectra of stars/glaxies

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Discussion Overview

The discussion revolves around how astronomers interpret the emission and absorption spectra of stars and galaxies, particularly in the context of identifying individual elements within a jumbled spectrum that may be affected by Doppler shifts.

Discussion Character

  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that the emission/absorption spectrum of a single element shows distinct lines, but the challenge arises when multiple elements contribute to the spectrum of stars and galaxies.
  • Another participant argues that Doppler shifts do not alter the relative fractions of wavelengths, and identifying dominant elements allows for the adjustment of the entire spectrum based on known ratios.
  • A different perspective emphasizes that the unique spectral lines of each element prevent confusion about their origins, asserting that Doppler shifts affect the entire spectrum uniformly, preserving line patterns.
  • An example is provided to illustrate how known absorption lines from laboratory measurements can be matched to observed lines in a spectrum, allowing for the determination of Doppler shifts.

Areas of Agreement / Disagreement

Participants generally agree on the uniqueness of spectral lines for each element and the uniform effect of Doppler shifts on the spectrum. However, the discussion does not resolve all nuances regarding the interpretation of complex spectra.

Contextual Notes

The discussion assumes familiarity with spectral analysis and does not address potential limitations in identifying elements in more complex or crowded spectra.

terahertz
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When you look at the emission/absorption spectrum of a single element, you can clearly see various emission/absorption lines, which are characteristic of that element. However, radiation from stars/galaxies contains spectra of many elements. How do astronomers make sense of this jumbled-up spectrum? In other words, how do they know that an absorption line, for instance, comes from this element and not that, especially since the spectra are doppler-shifted?
 
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Doppler-shift does not change the fractions of wavelengths. Typically a few elements are dominant in the spectrum, once you identify at least one of them (via those ratios) you know redshift and can adjust the whole spectrum. Afterwards you can look up all lines in tables.
 
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Since spectral lines of each element are unique, there is actually no room to mistake which line comes from which element. Similarly, Doppler shift doesn't pose a problem, since it always affects the whole spectrum, preserving the line patterns.

E.g.
Imagine we've determined absorption lines of elements A, B, C and D in a lab. Their lines have positions:
A = 2, 5, 9, 14
B = 4, 6, 11
C = 3, 12
D = 1, 7, 13

If you then observe a spectrum with the following lines:
2 4 5 7 8 12 13 14

There's only one way to fit the elements in there (and determine how Doppler shifted it is).
 
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Thanks a lot for your enlightening response. Your simple example clarified a lot of things!
 

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