How accurate is our image (depiction) of the Milky Way?

  1. We have a good handle on what other galaxies look like, given that we're looking at them from a distance. But when I look at a depiction of our home galaxy, pointing out roughly where our solar system lies, do we really know with any degree of accuracy that this is what it looks like, or is it just a "best guess"?

    If all the arms in a pinwheel shaped galaxy are on the same plane, then I'd think that we'd be limited to seeing the edges of our own "arm" and perhaps the next one over - or are our telescopes in use today actually able to distinguish all of the arms?
  2. jcsd
  3. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    Food for thought - we didn't even know the milky way was a barred spiral until a few years ago.
  4. Drakkith

    Staff: Mentor

    Its definitely a "best guess", but it's still accurate to some degree. Just having distance measurements to stars gives us a means of understanding the 3d structure of our galaxy.
  5. marcus

    marcus 24,217
    Science Advisor
    Gold Member
    2014 Award

    21 cm line. Arms coincide roughly with gas clouds. Veryprecise dopper can map the clouds moving away, and those approaching ("catching up").
    This signal is not blocked the way starlight is by dust. It is like a "cat scan" for reconstructing structure. Still of course all inferential.

    At a level of DETAIL, I think we have no way to accurately depict what Milky would look like to someone on the outside. We simply cannot see most of the stars because of dust.

    But using the 21 cm "X-ray vision" we can determine (rough) STRUCTURE of a fair amount of Milky and then that is given an "artist's conception" dressing by spangling it all starry for purposes of illustration. I think that's more or less how it goes.
    In radio astronomy[edit]
    The spectral line appears within the radio spectrum (in the microwave window to be exact). Electromagnetic energy in this range can easily pass through the Earth's atmosphere and be observed from the Earth with little interference.

    Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a hydrogen line. The only difference between each of these lines is the doppler shift that each of these lines has. Hence, one can calculate the relative speed of each arm of our galaxy. The rotation curve of our galaxy has also been calculated using the 21-cm hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy.

    Hydrogen line observations have also been used indirectly to calculate the mass of galaxies, to put limits on any changes over time of the universal gravitational constant and to study dynamics of individual galaxies.
    Last edited: Apr 16, 2014
    2 people like this.
  6. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    Our galaxy is difficult to survey due to intervening dust, gas, stars, etc. You can't see the forest for the trees.
  7. Drakkith

    Staff: Mentor

    I take offense to that. My vision's just fine.

    *slams into a tree*

    ...when my glasses are on that is.
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: milky way