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Our galaxy is one of the biggest

  1. Nov 14, 2014 #1
    Heard that line on one of those science channel shows. Forget who they were interviewing. But my question is what is the mathematics behind discovering this fact? How could we possibly know that our galaxy is one of the biggest in the Universe unless we counted all or most of them, how could we form an average consensus? And isn't it still up for debate whether or not our Universe is limitless/infinite. How could a claim like this be made and furthermore verified?

    Thank you
  2. jcsd
  3. Nov 14, 2014 #2


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    There are about a hundred billion galaxies in the visible universe. We've observed many tens of thousands of those galaxies (at least). So we've got a pretty good idea about how many galaxies of each size there are. And yes, there are galaxies that are much, much larger than our own (the biggest of these are found at the centers of galaxy clusters). I think by "one of the biggest" they mean "bigger than most galaxies". Which is true. I don't know exactly where in the distribution ours falls. It might be 80th percentile (meaning 80% of galaxies are smaller), 90th percentile, or 99th percentile. But either way, our galaxy is pretty big compared to most.
  4. Nov 14, 2014 #3


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    On broadcast media there simply isn't time (or interest) for careful rigorous qualification of every statement.

    The speaker may have been talking about the sizes of galaxies that we can see. Also when they say universe they often mean the observable portion of it. Also we observe a lot of uniformity. the farther out you look the more it seems that matter is more or less evenly distributed. The galaxies are in earlier stages of development because we look back in time, but what we see of them then is consistent with what we see closer to us in distance and time. So there is a lot of observed sameness.

    It's customary to assume (what Einstein called the Cosmological Principle) that kind of uniformity, and experience over many decades has born it out.
    Last edited: Nov 14, 2014
  5. Nov 14, 2014 #4
    Wonderful replies very informative
    Chanlnoth, that's what I thought as well. I know we scan the night skies constantly so it would make sense. The observable Universe is always something that I find hard to grapple with. There is just so much more out there to discover....
  6. Nov 15, 2014 #5


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    Gaging the size of distant galaxies is a challenge, even the size of our own Milky Way remains debatable.
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