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Light from very distant objects

  1. May 1, 2012 #1

    I was just wondering, my understanding is that quasars are the most distant objects from us or are at least very distant. Now my question would be;

    How is visible light able to reach us from such distant objects?

    Wouldn't there be a lot of matter between us and the quasar, wouldn't visible light get absorbed by all that matter before it got to us?

  2. jcsd
  3. May 1, 2012 #2


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    Not really. Intergalactic space is probably the closest thing to a perfect vacuum as you can get. As long as our line of sight to the quasar isn't through another galaxy then it's pretty much a clear shot.

    There is dust and gas inside our own galaxy that might absorb absorb a little bit, but this depends on if we are looking up or down out of the disk, or through the disk itself. Looking through the disk and especially through the coreward parts of our galaxy will put lots of dust in our LOS.

    So, while some of the light is most certainly absorbed, as long as we have a decent line of sight to it almost all of it gets through.
  4. May 1, 2012 #3
    Thanks for the reply,

    I'm aware that the obviousness of us being able to see quasars sort of negates my question. But I just find it difficult to comprehend the vastness of intergalactic space, especially when I look at pics where they are teaming with hundreds(thousands) of galaxies (and that's only a small portion of the sky). I'm assuming it would be the same no matter what direction the lens is facing. So the chances of a clear LOS to a very distant object seems small to me.
  5. May 1, 2012 #4


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  6. May 1, 2012 #5


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    A cc of air [which is pretty easy to see through] contains about 10^19 atoms. This is about the same number of atoms in 33 light years of the interstellar medium and 3 million light years of the intergalactic medium. The density of the ISM is about 1 atom per cc and the density of the IGM is about 1 atom per 100,000 cc.
  7. May 1, 2012 #6
    There's not that much matter between us and quasars, and a lot of that matter is ionized into free protons and electrons that doesn't absorb that much light.

    However, there is enough of it so that you do see a absorption.

    The magic google terms are "Lyman-alpha forest" and "Gunn-Peterson trough"
  8. May 1, 2012 #7
    There are a number of techniques for making large numbers comprehensible. One is to focus on the exponent.

    A quasar is 10 billion light years away. One light year is 10^15 centimeters.

    So the number of centimeters to a quasar is 10^25. If you think "25" then that's a number that's comprehensible. You can then count the number of atoms that are between you and the quasar, and it's not a big number.

    The important thing in astronomy is to think about numbers that you can comprehend.

    First of all, galaxies tend to cluster so those pictures are usually of clusters. Second, you'll notice that most of the picture is black space.
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