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Doppler effect

  1. Oct 24, 2012 #1
    We can know about distant stars temperature,its elements etc by observing its spectra... But that star has a velocity related to us... The spectra will be shifted... Then how do we know the correct spectra?
    Another ques... Suppose, something emits a X ray... Then will its characteristic will change(gamma or visible light) for observers with relative velocity?
  2. jcsd
  3. Oct 24, 2012 #2
    The spectra of elements are very well known. They have very obvious patterns that can be recognised even if they are shifted a little. Just like you can recognise an old friend even if they take a few steps to one side :)
    Think of the sodium doublet for example or the Balmer series for Hydrogen.

    Your second question - yes.
  4. Oct 24, 2012 #3
    Thank you for your answer...

    But how can we know the temperature then?
  5. Oct 24, 2012 #4
    the temperature of a star is generally given as its "effective temperature"

    do you know what blackbody radiation is, and the Stefan-Boltzmann Law?
  6. Oct 24, 2012 #5
    I thought that sun's temperature was found out by using the concept of blackbody radiation... But as the spectra shifts we won't get the right temperature... However, stephen boltzman solved that problem I guess... I knew it but didn't came to my head while I was thinking...
  7. Oct 24, 2012 #6


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    As the spectrum shifts it maintains it's shape, so when we graph the new spectrum it still looks the same. Plus, since we know where various absorption lines and emission lines should be, we can match them back up with a non-shifted spectrum. And finally, the amount of shift is VERY small and has very little effect on temperature measurements in almost all cases.
  8. Oct 24, 2012 #7
    Ezio: the above explanations cover nearby star temperatures...for distant stars, which your post said was your interest, the situation becomes far more complex due to expansion of the universe.

    I'll see if I can get back tomorrow with a synopsis explanation....
  9. Oct 24, 2012 #8


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    Can we even measure single star spectrums at a distance where expansion starts to become noticeable?
  10. Oct 25, 2012 #9
    I thought I might have some notes based on postings here by experts....
    no such luck.....

    The question regarding distant stars is above my pay grade' so I'm not going much further. Here are three related articles that might be of interest, but I did not see much of a direct explanation:

    Apparently one of the most distant star clusters is about a billion light years distant:

    and one supernova story is told here:

    at about 5 billion light years distant.

    There is a list of star topics here:


    which I did not investigate....

    As I understand it Eddington luminosity [Eddington Limit] sets the maximum
    'steady state' energy [brightness] a star can generate...but whether one has ever been observed in such a transient state, for example, I have no idea.

    As Clint Eastwood said when not lecturing an empty chair:
    "A man has got to understand his limits" via his Dirty Harry character!
  11. Oct 25, 2012 #10
    I forgot to mention:

    Type 1a supernovas are the 'standard candle' measuring device.....like a fixed brightness light bulb from which distances can be determined...

    I did not read the article but I think an actual spectrum must be observed to insure an observation is in fact a type 1a....

    and here are ways astronomers determine large scale distances....

  12. Oct 25, 2012 #11
    Thank you very much for your answers Naty1 & Drakkith... And special thanks to Naty1 for your links... :)
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