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Zero gravity in spiral galaxy center?

  1. Oct 29, 2014 #1
    In many vortex's, the center has low pressure, from the center of a stirring cup of tea to the centere of a hurricane or storm system. I'm trying to crudely model how this might apply to the center of a spiral galaxy. If the behavior is similar, and the center has a low pressure void, wouldn't an object in the middle experience zero net gravity as it would be equally acted upon by the surrounding mass?

    That being the case, how could the model fit in a blackhole which is super massive?
  2. jcsd
  3. Oct 29, 2014 #2


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    Vortices and hurricanes are both phenomena arising from fluid mechanics They have nothing in common with galaxies.

    Pressure does not apply to galaxies.

    You are correct inasmuch as a body at the centre of the galaxy would not be pulled in any direction, whereas a body on the edge of the galaxy would be pulled toward the centre, but that's about where the similarities end.
  4. Nov 2, 2014 #3
    Only the gas in a galaxy can be reasonably treated as a fluid. The stars are essentially collisionless, so the fluid model doesn't apply. In most models, dark matter is also a collisionless particle, so it isn't a fluid either.

    Mass segregation and multi-body interactions can drive more massive objects towards the center.


    For something to fall into the SMBH, it needs to first get rid of angular momentum. Three body interactions can accomplish this. Or, an accretion disk can form around the SMBH, and other processes can transfer the angular momentum outwards.
  5. Nov 2, 2014 #4
    I thought that whatever angular momentum gets eaten by the BH, is converted into an increased spin of the BH itself.
  6. Nov 2, 2014 #5
    That's true if the stuff makes it into the black hole. I was referring to stuff outside of the black hole. Since the stuff (gas, stars, etc) probably isn't falling straight towards the black hole, it will have some angular momentum and go into orbit around the black hole, rather than falling in. However, three body interactions (say, the SMBH, and two stars) can transfer angular momentum, leading to one object falling in.
  7. Nov 3, 2014 #6


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    It forms an accretion disk, yes, but it is not stable. The gas and dust will compact and collide, giving off huge amounts radiation, and causing the material to eventually fall into the SMBH.

    It is generally believed that most BHs are not naked. Most have an accretion disk of matter:
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