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Earth/solar position vs spiral arms

  1. Mar 16, 2008 #1
    in a post on global warming linking to a PDF http://www.spacecenter.dk/research/...blications/svensmark_2007cosmoClimatology.pdf
    there is a figure #9 showing the earth+ solar position over the last 200 million years
    as our sun moves in and out of the spiral arms of the Milkyway

    WHY does this happen
    as if the other stars moved at a near speed
    would not the solar position stay with in a given spiral arm
    or the whole arm no long be in place as the stars move out
    is our sun at a special orbit or speed
    or what is going on?????
     
  2. jcsd
  3. Mar 16, 2008 #2

    Wallace

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    Spiral arms are 'density waves' rather than a permanent structure that rotates with the stars. This means that stars are sometimes in and sometimes out of the spiral arm.
     
  4. Mar 16, 2008 #3

    Janus

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    The spiral arms are not due to star movement around the center of the galaxy. They are compression waves moving through the galaxy.
     
  5. Mar 16, 2008 #4
    how much variation in speeds is there in our current local group of stars

    I thought the whole need for "DARK MATTER" was to limit the speed of the outer
    stars as they move at nearly the same speed in orbits around the galaxy

    so how do compression waves move a star or group of stars
    and how can someone say our sun was in a give arm at a given date 100 or 300 million years ago?
     
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