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Thread: Why are galaxies disc shaped?

  1. #1
    Registered Senior Member
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    Why are galaxies disc shaped?

    Today, one of my friends and I got into a conversation based on astronomy. Our conversation eventually led to galaxies and there is one question he brought up that I just can't seem to answer. The question is "Why are galaxies disc shaped?" We found out that this does not only apply to galaxies, but solar systems as well. When solar systems are only in their infancy, the dust that is left over from the formation of the star gathers into a disc as well. This eventually leads to the formation of the planets. Also, rings that form around a planet are very thin and in a near perfect disc shape. It seems like the only way to get a "sphere" of multiple objects (such as an Elliptical Galaxy) is with some sort of gravitational disturbance.
    So my question is to you guys, how do orbits of multiple objects usually end up to create a disc and not a sphere or some thing related to a sphere?

  2. #2
    Curmudgeon of Lucidity Grumpy's Avatar
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    Electro522

    It has to do with the angular momentum and gravitational influence of the mass that gathers to form the systems. The angular momentum causes spin or rotation of the whole mass and the mutual gravity soon causes outliers to conform(more or less) to that rotation at roughly 90 degrees from the poles of that rotation. Any orbit not in that plane is influenced by the mass of the plane as it passes through, losing a bit of it's excentricity with each pass until it too is orbiting in the plane. Our own solar system has a bit of excentricity, causing us to oscillate up and down through the plane as we rotate around the galactic core. Solar systems are formed as part of the rotating cloud of gas that forms the star, starting as a fat torus within which the planets form. Planetary discs are probably formed by the collision or outgassing of satelites(moons) and stay on the same plane as the original moon's orbit. Eliptical galaxies are probably the result of many mergers(they tend to be the largest galaxies) and may eventually become more spiral in nature if such disturbances are not continued(and maybe not, there are effects of scale at both the small end(globular clusters)and the large(eliptical galaxies)). Hope that helps.

    Grumpy

  3. #3
    flat Earth skeptic Aqueous Id's Avatar
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    This figure shows a mosaic of 80 bright galaxies from the GEMS survey, and illustrates the diversity of different galaxy shapes, sizes and types: watermelon-shaped elliptical galaxies, majestic spiral galaxies, some with elongated bars in their centers, and spectacular galaxy mergers. (Image: the GEMS collaboration)


    http://www.sciencedaily.com/releases...0120034341.htm


    http://articles.adsabs.harvard.edu/c...;filetype=.pdf

  4. #4
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    Quote Originally Posted by Grumpy View Post
    Electro522

    It has to do with the angular momentum and gravitational influence of the mass that gathers to form the systems. The angular momentum causes spin or rotation of the whole mass and the mutual gravity soon causes outliers to conform(more or less) to that rotation at roughly 90 degrees from the poles of that rotation. Any orbit not in that plane is influenced by the mass of the plane as it passes through, losing a bit of it's excentricity with each pass until it too is orbiting in the plane. Our own solar system has a bit of excentricity, causing us to oscillate up and down through the plane as we rotate around the galactic core. Solar systems are formed as part of the rotating cloud of gas that forms the star, starting as a fat torus within which the planets form. Planetary discs are probably formed by the collision or outgassing of satelites(moons) and stay on the same plane as the original moon's orbit. Eliptical galaxies are probably the result of many mergers(they tend to be the largest galaxies) and may eventually become more spiral in nature if such disturbances are not continued(and maybe not, there are effects of scale at both the small end(globular clusters)and the large(eliptical galaxies)). Hope that helps.

    Grumpy
    That was a good description, Grumpy. I started a reply earlier then quit as I tend to get carried away, with unimportant detail sometimes. I was hoping someone would come up with something a bit clearer.., and you did.

  5. #5
    Registered Senior Member
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    Thank you Grumpy. I didn't expect that good of an answer.

  6. #6
    Curmudgeon of Lucidity Grumpy's Avatar
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    Electro522

    Glad I could help.

    Grumpy

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