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Do we see galaxies the way they really are ?

  1. Jul 6, 2011 #1
    I don't know how to put my question into words so I'll give a brief example of what I want to say.

    The andromeda galaxy is a spiral galaxy 2.5 million lightyears from earth. When we observe it, we see it nearly on its plane. Light travels at a finite speed, so we see the andromeda galaxy as it was 2.5 million years ago. Right.
    Now here's the weird part.
    Since it is approximately 220,000 lightyears in diameter, when we look at it, we must see it's closer side as it was say 2.5 million years ago, but it's far side as it was 2.72 million years ago.
    This number is even more exaggerated for a supergiant galaxy like M87, which has a diameter of a little under 1 million lightyears.
    This all must mean than when we observe a galaxy, not only do we see it in the past, but we also see it in an a shape it was never really in. We see a combination of it in different times.

    Am I right? And if so, do astronomers correct pictures to know what the whole galaxy looks like FOR REAL at a given moment?

    Thanks!
     
  2. jcsd
  3. Jul 6, 2011 #2
    You're right, but "For Real" isn't meaningful. What you see is what you get.
     
  4. Jul 6, 2011 #3
    or to put it another way, you don't see it as it "is" but rather as it was.
     
  5. Jul 6, 2011 #4

    Vanadium 50

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    Note that you don't see anything as it "is". Look at a tree. You are seeing the front side perhaps 50ns earlier than the back side.
     
  6. Jul 6, 2011 #5
    Right, but this delay is greatly amplified at supergalactic distances. So, to get back to my original question.. Do astronomers correct pictures or take into account the small discrepancies in their observations? After all, shouldn't this 200,000+ year difference between the near and far side of a galaxy in a picture affect their understanding of galaxy formation?
     
  7. Jul 6, 2011 #6

    Drakkith

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    200,000 years isn't THAT long on a cosmological scale. Sure there are differences, but things don't change that much in that short of time. The andromeda galaxy is still shaped pretty much the same between the ends.

    And no, when pictures are taken they are not corrected in such a manner, as that is impossible. We cannot see what will happen before the light hits us, so there is no way to compensate. (At least not that I can see)

    Also, remember that even if you break the galaxy into small parts, adjacent parts look exactly the same as they would if you were much much closer.
     
  8. Jul 10, 2011 #7

    Chronos

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    Every other galaxy we observe is as it appeared when the photons we now observer were emitted. Even the sun is several minutes older than it currently appears from earth.
     
  9. Jul 10, 2011 #8

    Janus

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    Consider this:

    A star 100,000 ly from the center of Andromeda will take ~900,000,000 yrs to make 1 orbit around the galaxy. In 200,000 yrs it will have made 1/4500 of an orbit, or traveled 0.08 degrees. So the difference in positions of stars on the near edge compared to stars on the far edge due to the light crossing the diameter of the galaxy is insignificant .
     
  10. Jul 18, 2011 #9
    Hi Janus,

    Sure. Stuff moves only at a dynamical time. Your example, nevertheless is highly underestimating the effect though. Stuff at 100.000 ly from the center is VERY far. The exponential fall of of the disk has a scale length of roughly 10.000 ly, so your object is at a position where the density is more than 10.000 times lower than in the center (and for seeing stuff that far from the center we need pretty decent telescopes).

    Taking the hypothetical distance of the objects 10.00 ly would be much more realistic. I also don't know how you got to the number 900 million years for a complete orbit. Typical numbers for the dynamical times of such galaxies are a few hundred million yrs (say, 200 million), but that number probably results from the large distance from the center.

    Of course, the amount of the orbit that IS actually traversed is still very small, but I think it would be good to educate people with numbers that represent reality and what we see of it a bit better.

    Cheers, Harcel
     
  11. Jul 18, 2011 #10

    Drakkith

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    I think the point is that the further the distances between two points, the greater the difference in "real time" if you will. However this is countered by the greater amount of time needed for large scale changes in the first place. Does every point in a galaxy look the same as it did when it's light was emitted in the past? Yes.
     
  12. Jul 25, 2011 #11
    Right, but I'm talking about the galaxy as a whole, not just arbitrary points. The galaxy does NOT look like it did when it's light was emitted in the past, because we see a mix of it in different times in the past. Wouldn't this affect our knowledge on supernova rates, the age of a galaxy etc..
    Wouldn't astronomers somehow "photoshop" the pictures to correct this 1/4500th of an orbit and therefore better understand how the galaxy behaves?
     
  13. Jul 25, 2011 #12

    Janus

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    For many galaxies, that 1/4500 of an orbit results in an apparent shift in position that is smaller than the resolution of the image itself. Secondly, since we see galaxies at all angles, we are just as likely to view a galaxy from "the top" as from "edge on".
     
  14. Jul 25, 2011 #13

    russ_watters

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    Third, there is no reason for the noted distortion to affect the frequency of noted supernovae.
     
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