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Thread: Question about gaalaxies and solar systems.

  1. #1 Question about gaalaxies and solar systems. 
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    Why are galaxies and solar systems a disk shape. Shouldn't the gravity keeping the planets around the sun be more like a sphere then a disk.
    And if the gravity was like a sphere then wouldn't a galaxy or a solar system resemble an atom.

    The only solution I can think of is that the power of gravity is stronger on the horizontal axis.


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  3. #2  
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    Firstly, gravity exists as a field, so it does not favor any particular axes. Secondly, you cannot compare an atom and a galaxy, since there is no localised central body of mass. And lastly, if you want to understand the shapes of galaxies, look more into dark matter and dark energy for your explanation


    Beyond Equations,

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    The disc shape is a result of rotation. It produces a counter force called centrifugal force. It partly compensates the gravitational force pulling the bodies inward. There is no centrifugal force toward the direction of the poles. The disc shape is already introduced during the formation of a star (in case of a planetary system), that forms out of a slowly rotating cloud of gas and dust. This material collapses due to gravitation leading to higher rotation velocities just like a figure skater increases its rotation during a pirouette by pulling in his arms. As a result, the centrifugal force increases even more the more the gas/dust clump collapses to finally form a star. So, the gravitational collapse is quicker at the poles of the rotation. Friction inside the disc causes breaking that leads to a stream of material inward toward the future star. This is called accretion.
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  5. #4  
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    Thanks Dishmaster that was excellent!
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    If you could slice the sun into cross-sections from top to bottom, then you would have a series of disks. The top disk has a smaller diameter than the disk below. The center disk has the biggest diameter and hence the most mass. That is why objects orbit the center ‘plane’, gravity is stronger there than anywhere else around the Sun. Farther out than Pluto’s orbit, where the Sun’s gravity is weaker, the Oort cloud does create a shell of comet like objects around our solar system.
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    Quote Originally Posted by Arch2008
    If you could slice the sun into cross-sections from top to bottom, then you would have a series of disks. The top disk has a smaller diameter than the disk below. The center disk has the biggest diameter and hence the most mass. That is why objects orbit the center ‘plane’, gravity is stronger there than anywhere else around the Sun.
    No. You could also try to slice the sun parallel to its rotation axis. Then you get a larger slice at the poles. So, why aren't the planets orbiting the sun around the poles? You see, this does not work. In addition, the size of the sun is tiny in comparison to the size of the solar system. For the planets, in particular the ones far out, the sun and its gravitational force is point-like.

    By the way, the Oort cloud you mentioned is not concentrated toward the planetary plane. Although it has never been actually proven that it exists. It was just deduced from the orbits of the long-period comets that seem to originate from there.
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  8. #7 Wie geht's 
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    Yes it spins in relation to the sun’s rotational axis, but the reason that the gas cloud flattens to a plane around the equator and not at the poles of the sun is its gravity is strongest there. The equator also has the greatest angular momentum.

    I’ll let NASA in on your scoop about the Oort cloud.
    http://solarsystem.nasa.gov/planets/...y=OverviewLong
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  9. #8 Re: Wie geht's 
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    Quote Originally Posted by Arch2008
    Yes it spins in relation to the sun’s rotational axis, but the reason that the gas cloud flattens to a plane around the equator and not at the poles of the sun is its gravity is strongest there. The equator also has the greatest angular momentum.

    I’ll let NASA in on your scoop about the Oort cloud.
    http://solarsystem.nasa.gov/planets/...y=OverviewLong
    Concerning your first point: The disc already forms before a central object like the sun is built. The reason for the flattening is clearly the rotation of the cloud itself. This would happen even without the sun. In fact it is the other way around: The sun inherited its rotation from the parental cloud it emerged from. I don't see, how a spherical symmetric body can produce a gravitational field that is confined to an angular range.

    To your second point: I am aware that NASA and many other public sources tend to sell working hypotheses as facts. Let me assure you that in the professional astronomical community the Oort cloud is accepted as nothing more than this. Here is a reference:
    http://www.solstation.com/stars/oort.htm
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  10. #9 Wie steht's? 
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    Angular momentum causes the spin, however gravity causes the angular momentum. Gravity is a force, not an effect and its strongest at the equator of the Sun.

    From your link:
    “In 1950, Jan Hendrik Oort (1900-1992) inferred the existence of the Oort Cloud from the physical evidence of long-period comets entering the planetary system. These comets are observed to come into the Solar System from all directions, which implies an immense spherical cloud of trillions of small icy, planetary objects -- all potentially active but currently dormant comets -- that extend as much as two light-years outward from Sol. In contrast, the Edgeworth-Kuiper Belt is roughly 100 times closer to Earth than this hypothesized Oort Cloud. Estimates of the total mass of this Oort Cloud range from about 40 times that of Earth to greater than that of Jupiter”

    So this supports what you posted and refutes what I posted?
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  11. #10  
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    First about the link: I just wanted to point out that the Oort cloud is a hypothesis, not a proven fact. Although I am convinced that something like such a cloud exists. You might notice the slight difference in phrasing used on these websites.

    About the other discussion: I am not sure, if we really differ in our argumentation. This is what I mean:

    1. Rotation flattens a spherical structure due to centrifugal force regardless of gravitational effects. This is, how the disc forms.

    2. Gravity indeed is a force. It is purely attractive. It acts as a counter force to the centrifugal force (or vice versa). This combination is the reason for stable orbits of the planets (or other objects). These orbits are defined as the ranges, where gravitational and centrifugal forces cancel out.

    3. The gravitational field of a spherical symmetric and homogeneous body is isotropic. This means, it is the same in all directions. But: Seen from another body, e.g. a planet, the equator region of the sun contributes (slightly) more to the attractive force than other parts simply because it is closer than the poles. However, this difference is neglegible and has no influence on the shape or the stability of the plane of planets. You would not notice a difference, if you replaced the sun with a point-like object of the same mass. Otherwise, one could not explain comets having orbits extending far above it. The direction of the rotation axis of the sun has no influence on the gravitational force it produces. In fact, the sun's equator is slightly tilted with regard to the plane of planets.
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  12. #11  
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    I'm aware that no one has ever actually seen the Oort cloud and that is hypothesized as the area from which long period comets come from. Could there not actually be an Oort cloud and the long period comets that we see the orbits as coming from the Oort cloud be merly space rocks floating with great velocity through space that are then captured by the Sun? I'm aware that long period comets are made up of different material thna both short period comets (?) as well as the asteroids and that rocky belt floating between Saturn and Uranus (I think it's called the plaedies or something). I'm sure that the passage through non gravitational space with temperatures of near absolute zero would have something to do with it.
    "The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt" - Bertrand Russell
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  13. #12 Sweet! 
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    Ich habe mein Herz in Heidelberg verloren. Willkommen, und auf Wiederhoeren!
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  14. #13  
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    thanks I understand now.
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