What Creates the Spiral Arms?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by TruthSeeker, Dec 3, 2007.

  1. TruthSeeker Fancy Virtual Reality Monkey Valued Senior Member

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    So what does? Have we reached a conclusion yet? How many hypothesis are there?

    How about using the new E8 thingy to find out? Anyone dares?

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    Seriously. Let's list the current hypothesis and think about it. :shrug:
     
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  3. cosmictraveler Be kind to yourself always. Valued Senior Member

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    Simple Newtonian gravitation is sufficient to explain the entire behavior of Galaxy Spiral Arms. The well-known Keplerian effect certainly dominates, but a Spiral Arm also has internal mutual gravitation, which has surprisingly strong effects!
    The stability of galaxy spiral arms has long troubled the Astrophysics community. It has been assumed that ONLY the Keplerian central force was acting, as suggested in the drawing at the right. The orange dot represents the Sun. If we assume that the Sun is revolving in the Milky Way Galaxy with a velocity of 250 km/sec, and that the Sun is around 28,000 light years from the center of the Galaxy, it is easy to calculate that a centripetal (Keplerian) acceleration must exist of a = v2/r or 2.36 * 10-10 meters/second2. This acceleration must also equal G * Mgalaxy/r2 which tells us that the effective Keplerian mass of the Galaxy is around 125 billion solar masses. This is ONLY true if ONLY a Keplerian central force is causing the Sun to have its revolution motion.

    Now, think of a galaxy spiral arm as a very large and massive "open cluster" of stars. We all accept that open clusters have a structural integrity due to mutual gravitation, even while revolving with the galaxy as a whole. If we consider a galaxy to primarily have two (symmetric) main spiral arms, it seems reasonable to estimate that the total mass of each of those arms is around 1/10 of the mass of the entire galaxy (leaving 80% of the mass as being in the Core). In the case of the Milky Way Galaxy, that would be around 12.5 billion solar masses in each of two major Arms. Given the known dimensions of the Galaxy, reasonable estimates of the dimensions of an Arm are 15,000 light years wide at the start, with uniform taper to end 60,000 light years away. If the Spiral Arm is estimated to have a uniform thickness of 1000 light years, this results in credible mass density figures, as discussed below.

    http://mb-soft.com/public/galaxy.html

    http://mb-soft.com/public/galaxyzz.html
     
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  5. zephir Banned Banned

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    Galactic arms are the result of positive feedback during conversion of matter into radiation from interstelar gas. It's induced by the interference of elliptic star paths accross gallaxy.


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  7. blobrana Registered Senior Member

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    Hum,
    The spiral arms are only highlighted because they contain young bright stars.
    So it is thought that moving density waves (ie shock waves) induce star formation in the galactic dust clouds.
    The orbiting traffic jam model is probably what initially starts this process (the young stars by their nature blow up too quickly to have any galactic `path`; and so it is probably better applied to the orbital motion of the dust clouds.
     

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