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Thread: If the Galaxy spins at the same rate...

  1. #1 If the Galaxy spins at the same rate... 
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    If the Galaxy spins at the same rate threw out itself because of dark matter why is there a spiral shape? Doesn't that shape require that the middle spins faster than the edges?


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    Its not really spinning. The stars on the outside are travelling at different speeds and they do not follow conventional laws of physics, unless there is an added mass within certain gravitational spheres i.e. dark matter (dark because some of it we can not see and also because astrophysicists expect there to be some form of matter which does not interact with electromagnetic radiation).

    Also, see density wave theory for more on why some galaxies have spiral arms. (...some say the arms are there because gravitational standing waves form but this is yet to be proven)

    Hope that helps!


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  4. #3 Re: If the Galaxy spins at the same rate... 
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    Quote Originally Posted by Wildstar
    If the Galaxy spins at the same rate threw out itself because of dark matter why is there a spiral shape? Doesn't that shape require that the middle spins faster than the edges?
    Good observation! This tells you that the spiral structure is not caused by the rotation of the Galaxy. The commonly found comparisons between a sink and matter swirling around a central Black Hole to explain the structure of a spiral galaxy are false. The spiral structure is the result of compression waves propagating through the interstellar matter within the Galaxy causing events of star formation. The newly formed stars that burn for millions and billions of years can be seen as the response that make the travelling wave visible.
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    Okay, So it is a gravitational density wave, probably of dark matter that causes the spiral structure. Cool. I've always wondered that ever since I heard about it back in the 90's. Thanks you guys.
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    Quote Originally Posted by Wildstar
    Okay, So it is a gravitational density wave, probably of dark matter that causes the spiral structure. Cool. I've always wondered that ever since I heard about it back in the 90's. Thanks you guys.
    The density wave does not need Dark Matter.
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    OH OK. Also in theory wouldn't dark matter also be clustered in our Sun and inside the earth?
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    Yeah and the Milky Way is a barred spiral too.
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  10. #9 Re: If the Galaxy spins at the same rate... 
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    Quote Originally Posted by Wildstar
    If the Galaxy spins at the same rate threw out itself because of dark matter why is there a spiral shape? Doesn't that shape require that the middle spins faster than the edges?
    The galaxy doesn't spin at the same rate throughout. The velocity curve is flat. Spinning at the same rate implies that a star at any given distance from the center would take the same time to orbit the galaxy, and since the closer star has shorter distance to travel in order to do so, its orbital velocity would have to be less than that of a more distant star.

    What we see however is that the stars orbit with the same orbital velocity, so nearer stars take less time to orbit the center than further ones.

    The reason the flat curve is considered a problem is that when you look at a galaxy, the vast majority of the mass is concentrated at the center bulge. In such a situation, orbital mechanics says that the closer stars should have higher orbital speeds and orbital speeds would decrease with distance. (this is what we see for stars close to the center of the galaxies.)

    If However, there is matter that we don't see, and that is distributed differently than the matter that we do see, the flat velocity curve can be explained. This is where Dark Matter comes in.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by Wildstar
    OH OK. Also in theory wouldn't dark matter also be clustered in our Sun and inside the earth?
    No, one of the properties of Dark Matter is that it doesn't interact except by gravity.

    The reason that regular matter tends to clump and form bodies such as the Sun and The Earth is that the matter interacts electromagnetically and tends to "stick" together. The Electromagnetic interaction also allows the matter to shed energy and speed in the form of radiation.

    Dark matter on the other hand, doesn't interact, it passes through normal matter and other dark matter as if it wasn't there. Thus a particle of dark matter drawn to the center of the Sun or Earth would reach it with all of its speed in tact, will keep going and pop out the other side of the Earth.

    Dark matter doesn't collect in clumps anywhere as easily as normal matter does.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    But I thought that dark matter was the reason galaxies clumped in the first place. Because of a large concentration of dark matter. Is not the density of dark matter less in interstellar space than it is in the galaxy. How can that be explained if dark matter does not clump?
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    Quote Originally Posted by Wildstar
    But I thought that dark matter was the reason galaxies clumped in the first place. Because of a large concentration of dark matter. Is not the density of dark matter less in interstellar space than it is in the galaxy. How can that be explained if dark matter does not clump?
    The idea of Dark Matter is that just by gravitational attraction it formed dense pockets that were the seeds for normal matter to accrete to form the galaxies. In this sense, Dark Matter tends to clump. The other way around, Dark Matter may also be attracted by normal matter. But since we know almost nothing about it, it is difficult to judge, if it can be trapped inside solid bodies. One has to consider conservation of momentum and energy. The exchange of momentum just by gravitational interaction is quite weak, so it may be difficult to describe local trapping just by gravitation. But this is just what I can imagine. As far as I know, observations suggest that - if it exists - Dark Matter is distributed monotonously and does not concentrate locally inside a galaxy.
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    Quote Originally Posted by Wildstar
    But I thought that dark matter was the reason galaxies clumped in the first place. Because of a large concentration of dark matter. Is not the density of dark matter less in interstellar space than it is in the galaxy. How can that be explained if dark matter does not clump?
    I didn't say that it didn't clump at all, only that it doesn't clump as readily as normal matter. The mechanisms that would cause dark matter to clump are much weaker. In the life time of the universe it has only had time to clump up to a certain degree forming the nodes around which galaxies can form. The dark matter of the our galaxy is spread out over a much larger volume than the galactic disk. So even though there is more of it, it is spread much more thinly.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Maybe I'm missing an important part of the argument, but wether it's dark matter or regular (bright ? ) matter, they both form the same gravitational wells, or curvature of space/time. That means any orbit, that of a planet around a star or a star around the galaxy, will follow Kepler's laws of orbital motion. So just like Mercury orbits faster because it is deeper in the gravitational well than Neptune ( sorry Pluto is no longer a planet ) does, so would stars closer to the galactic hub orbit faster than outlying stars at the arm's ends.
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    Quote Originally Posted by MigL
    Maybe I'm missing an important part of the argument, but wether it's dark matter or regular (bright ? ) matter, they both form the same gravitational wells, or curvature of space/time. That means any orbit, that of a planet around a star or a star around the galaxy, will follow Kepler's laws of orbital motion. So just like Mercury orbits faster because it is deeper in the gravitational well than Neptune ( sorry Pluto is no longer a planet ) does, so would stars closer to the galactic hub orbit faster than outlying stars at the arm's ends.
    Its a matter of the distribution of the primary mass. With the Sun the primary mass is concentrated in the body of the Sun. For the Galaxy, the visible mass is concentrated at the core. (the mass of the spiral arms themselves make only a small contribution.) In these cases, the mass of the object that you are orbiting doesn't change as you move closer or further from it, and you get the effect that orbital speeds increase as the size of the orbit decreases.

    Now however, assume that the primary mass has expanded out to a huge sphere that encompasses the orbits of the bodies that you are considering. The orbit of any body would only be influenced by the amount of mass inside of its orbit. Thus bodies in smaller orbits orbit a smaller mass than objects in larger orbits. This tends to flatten out the orbital speeds.

    Since what we see in galaxy rotation curves matches the second scenario, dark matter would be distributed as a spherical halo encompassing the galaxy and not distributed in a disk like the visible matter is and it this contribution to the total mass of the galaxy that flattens the rotation curve.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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