Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Very dumb question

  1. Oct 15, 2007 #1
    I have read that Edwin Hubble first noted the "reddening" of galaxies, and that this redshift was evidence for the idea that the universe is expanding.

    Now, why do none of the galaxies seen in pictures from the Hubble Space Telescope appear to be red?

    Does redshift simply mean that the light from the galaxies is reaching us at longer wavelengths, or does it literally mean that galaxies ought to look red when seen through a telescope?
     
  2. jcsd
  3. Oct 15, 2007 #2

    berkeman

    User Avatar

    Staff: Mentor

    I'm no expert, but the shift to longer wavelengths of the emission and absorption spectra is small. Certainly not enough to be noticed by the naked eye -- you need a sensitive spectrometer. You match up the spectra of far-away stars with the spectra of nearby stars, and see that the spectral lines of the distant star are shifted slightly in the direction of longer wavelengths. Like in this picture:

    http://en.wikipedia.org/wiki/Red_shift
     
  4. Oct 15, 2007 #3

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Depends on which galaxies you're referring to. The ones very nearby will only have very small redshifts that won't be noticable to the naked eye, as berkeman already said. However, if you look at the Hubble Deep Field or Ultradeep Field, the galaxies should look quite a bit redder than, say, M31, even without a spectrograph. This is because they are much more distant. Crudely speaking, the more distant the galaxy, the more its light is redshifted.
     
  5. Oct 16, 2007 #4

    Garth

    User Avatar
    Science Advisor
    Gold Member

    Holocene, if you look at very distant galaxies, such as those in the Hubble Ultra Deep Field, then you will find they are red.

    The most distant ones are those circled in green in that remarkable photograph.

    Garth
     
    Last edited: Oct 17, 2007
  6. Oct 16, 2007 #5

    berkeman

    User Avatar

    Staff: Mentor

    Wow! That's an incredible image. I just read on APOD this morning that a supernova out at 5 billion years has a red shift of 0.28, so yeah, I guess it really is visible!
     
  7. Oct 16, 2007 #6

    Garth

    User Avatar
    Science Advisor
    Gold Member

    In the HUDF we are talking about z > 6!

    Hubble's Deepest View Ever of the Universe Unveils Earliest Galaxies
    Garth
     
    Last edited: Oct 16, 2007
  8. Oct 25, 2007 #7
    I believe greater red shift also means greater speed for those distant objects.
     
  9. Oct 26, 2007 #8

    Chris Hillman

    User Avatar
    Science Advisor

    Well, "distance" and "speed" become tricky concepts at cosmological scales, but roughly speaking, that is probably a good way to think about it.
     
  10. Oct 27, 2007 #9
    If those distant galaxies illuminate only a narrow band of light then you will see it red or more exactly redder. But they emit long range of wavelength so some light become redder, but other in the UV range will become visible light. The only things you can see (only through a photospectrometer) are stripes that move to the Red region .
     
  11. Oct 27, 2007 #10

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Stars (and thus galaxies) actually do emit a a band light given by a blackbody curve, so we can see a change in colour, as our eyes are tuned by evolution to the band of a typical star, the Sun. Not all stars have peak intensity at the same colour, but the overall effect is visible.
     
  12. Oct 27, 2007 #11
    No we can not see the change in color which shifts to the red. The visible band of light is very narrow compared to the whole range of 'light' the stars emit. I do not talk about lights from plantary nebulae or something like that which have narrowbands of light reaching us.

    Possibly some stars may have peak intensity, but also the peak can be in UV region and for the redshift, we can see it very bright or more bright then.
     
  13. Oct 27, 2007 #12

    russ_watters

    User Avatar

    Staff: Mentor

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Very dumb question
  1. Very good question (Replies: 1)

  2. Very silly question (Replies: 5)

  3. Dumb Black Hole Question (Replies: 12)

Loading...