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"Amount" red light in red shifted light

  1. Feb 7, 2015 #1
    Please can someone help confirm this

    White light is a mixture of light with different wavelengths. A percentage of the white light is light with long wavelengths and a percentage of white light is light with short wavelengths.

    When scientists look at the light from distant galaxies using a device that disperses the light (spectroscope), they see that the spectral lines on the absorption spectra have shifted towards the red end of the spectrum and this tells them about red shift and galaxies moving away etc.

    Does it also mean that the light coming from distant galaxies has a greater percentage of longer wavelength light?
  2. jcsd
  3. Feb 7, 2015 #2


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    yes, and when they look at the CMB it's ALL long waves (microwaves to be a bit more precise) totally out of the visible range
  4. Feb 7, 2015 #3
    Thanks phinds.

    If I were to be sitting on the Hubble Telescope looking at these distant galaxies with a telescope (which doesn't disperse the light)...would these galaxies appear red in colour? because they have more long wavelength visible light?

    Also, can someone point me to a graph that shows that the light from distant galaxies has a higher percentage of red light than blue (when compared to light from a nearby star).
  5. Feb 7, 2015 #4


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    I think that might depend on what color light they are emitting locally combined with how far away they are (because recession velocity is proportional to distance) but in any case they would certainly be redder than they are locally.

    Perhaps a google of "spectral shift of distant galaxies" would help. It would be to your advantage to learn how to word searches so you can find out things like that on your own.
  6. Feb 7, 2015 #5


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    Keep in mind the word shift in red-shift. The EM spectrum is continuous; it doesn't simply end at red, or even IR. A distant galaxy that is red-shifted will have the same spectrum as it would without red-shifting, it will just be moved farther down the spectrum. That starts to move it out of range of our eyes (which lose sensitivity as you go into the infra-red range), but it's certainly not out of range of telescopes. It's all the same to them. In fact, there is FAR more information available in the huge swaths of the microwave and radio parts of the spectrum than in the tiny visible light parts of the spectrum.

    Last edited: Feb 7, 2015
  7. Feb 7, 2015 #6
    But if the ratio of red light intensity to blue light intensity was higher in the light from a distant galaxy...
    when compared to the ratio of red light intensity to blue light intensity in the light from a nearby galaxy...

    why do we need to bother about the spectral lines.

    Surely the intensity of red light being higher in the light from distant galaxies tells us that the wavelength appears to have stretched and there is more red light than blue light in the mix
  8. Feb 7, 2015 #7


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    Have a look at the Hubble Deep Field images. Distant galaxies do indeed look red and as said, they can even be shifted completely out of the visual spectrum.
  9. Feb 7, 2015 #8


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    Spectral lines are both more precise and more specific than the whole spectrum.
    You never know exactly how the continuous spectrum of a particular galaxy should look like. Maybe there's more dust in it, maybe there's more lower metallicity stars, maybe there's an intra-galactic (in the Milky Way) cloud in the line of sight, maybe there's something you can't even account for changing the spectrum. It's not like all galaxies are exactly the same.

    I suppose you could do some statistical analysis to find the 'average galactic spectrum', and then try to subtract all factors that you can think of to get an observed spectrum that you could more or less confidently declare as being redshifted with respect to the average, but that's a lot of hard work for an uncertain result.

    On the other hand, you can always tell where spectral lines ought to be, and to a very high precision at that. And there's nothing else that can affect their positions so they're a very specific indicator.

    It's simple, accurate, and works fine. If it ain't broken don't fix it, and all that. Or, in other words, why would we ever bother about the total spectrum if we've got spectral lines?
  10. Feb 8, 2015 #9


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    What if a given galaxy had a huge preponderance of red giants? It would tend toward the red, yet that would misinform us about its overall velocity.
    And how would you determine how red? Would you just approximate? Considering you're sampling billions of stars, you're going to get a very approximate shift. Spectral lines are exact.
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