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How Do We See the Milky Way

  1. Jan 27, 2006 #1
    As I've searched, (correct me if I'm wrong) it seems that the pioneer spacecrafts (10 or 11) are the farthest things away from the solar system, yet, not yet quite far enough out to be in interstellar space. How is it that there are pictures of our galaxy, or is it merely another galaxy that we think to represent the milky way?
  2. jcsd
  3. Jan 27, 2006 #2
    I don't understand your question.Do you mean if Pioneer 10 and 11 are seeing another galaxy that we think is ours or do you mean how are getting images of a galaxy form them?If it's that second one I think we lost cotact with them or it's very hard of getting commcations with them.
  4. Jan 28, 2006 #3
    It would be another galaxy which is similar to the Milky way - either that or an artists impression of what the Milky way would look like from outside.
  5. Jan 28, 2006 #4
    but what about the location of our solar system? how did they come to know about that if it's another galaxy which is similar or if it's an artist's impression?
  6. Jan 28, 2006 #5
    Maybe they used some sort of radio location thingy, kinda like sonar, but for space, and they can detect where the majority of stars stop and interstellar space begins, giving us a rough picture of the Milky Way.
  7. Jan 28, 2006 #6


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    The only pictures of our galaxy are from within it. We see it edge-on as a band of stars crossing the sky. Considering the scale and distances involved, the Pioneer craft see a virtually identical view of the galaxy as we do. Any pictures you see of galaxies are other galaxies.

    We have a fairly good idea of what our galaxy looks like by studying it from where we are within it. We've identified spiral arms and even named them.

    But we can't be completely sure. For example, there are some quite recent claims that the Milky Way is a particualr type of spiral called a barred spiral. Google "Milky Way barred spiral".

    So, as you can see, regarding what the Milky Way really looks like, the jury is still out on it.
  8. Jan 28, 2006 #7


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    You're actually on the right track. We can use something called parallax to measure the distances to stars. Rather than trying to bounce signals off stars (which would not only take thousands of years but untold billions of watts of power), we simply use geometry. As we move around the Sun in a year's orbit, the perspective from which we see nearby stars against the background of much more distant stars changes. We can use that change in perspective to measure distance.

    The most successful such project was called Hipparcos. It was a satellite in orbit around the Earth, and took extraordinarily precise measurements of distances to a very large number of stars.

    When you plot the distances on a graph, you can see that there are long, linear "clumps" of stars -- the arms of our spiral Galaxy.

    In this way, we can judge what our Galaxy would look like from far outside it. However, no spacecraft has even been outside our galaxy, and only a few have really ever escaped our solar system. (In fact, some people believe the early Pioneer spacecraft are actually still technically within our solar system.) We thus have no direct pictures of our Galaxy, though we do have a pretty good idea of what such a picture would look like.

    - Warren
  9. Jan 29, 2006 #8
    Thanks, that makes sense, but then it begs the question- Why would there be spiral arms on a galaxy? And how come they dont break apart?
  10. Jan 29, 2006 #9
  11. Jan 30, 2006 #10
    i don't think they do. as i have heard, there may be huge black holes at the center of every galaxy(correct me if i'm wrong) because present at the center are the oldest stars. well, just imagine a sink filled with water till the top(block the holes). then when you clear the holes, the water is sucked in, in a clockwise or an an anticlockwise direction. you'll see arms like thingies. i'm not telling that the black holes are sucking the galaxy! if it's the gravity of the black holes that give the galaxies the shape they have then i guess that might answer your question. somethings do puzzle me though, like these active galaxies. do only they have black holes at the center or every galaxy?

    well, i just told you what i think:smile:
  12. Jan 30, 2006 #11


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    Galaxies work very nicely without having black holes at their centres. The spiral arms are an effect of the overall sum of the gravity of the individual stars. The black holes account for only a small fraction of the gravity of a galaxy.

    As for why the arms don't break apart, well:
    1] The arms are being *formed* by the gravity and rotation. i.e. they're working to *make* them, not tear them apart.
    2] They do break apart. It's dynamic and evolving. You will find photos of galaxies in every shape and manner of dispersement and coalescence.
    Last edited: Jan 30, 2006
  13. Jan 31, 2006 #12
    so it's the gravity of the individual stars that are responsible for the spiral arms and not the black hole. ok...i wen't wrong there
  14. Jan 31, 2006 #13
  15. Jan 31, 2006 #14


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    BTW, just a further point of clarification: "density waves" are not a separate phenomenon from the stars - they are oscillations of stars - in the same way that "sound waves" are really just oscillations of air molecules - and for pretty much the same reason.

    It is the population density of stars (these stars are all close together, those stars are far apart) that is changing - like one of those desktop Newton's Cradle things.
    Last edited: Jan 31, 2006
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