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B How do stars orbit the center of the Milky Way?

  1. Sep 3, 2016 #1
    I've been wondering how all the stars of the Milky Way orbit the center of the Milky Way almost like it orbits Sagittarius A*. It is possible that there is a common center of mass that happens to be in the center of the Milky Way that also happens to be in the center, ultimately giving it its spiral shape.
  2. jcsd
  3. Sep 3, 2016 #2
    I always figured that the super-massive black hole(s) provided an axis around which material spins, with layers of gravitational spreading out until the very edge of galaxy. Basically, large amounts of stuff orbit the black hole, which in turn pulls in other stuff around the ring it forms, and so on until there was no more substantial matter in the galaxy's vicinity to pull in, or the gravitational field became too dispersed and weak at the edges. I could of course be completely wrong on this.
  4. Sep 3, 2016 #3


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    The modestly-massed black hole at the center of the galaxy plays almost no role in galactic dynamics other than for the 20 or so stars that orbit it. Instead, the dynamics of the galaxy are nearly completely dictated by the enormous dark matter halo in which it is embedded.
  5. Sep 4, 2016 #4


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    Which is also true of most galaxies we observe. The dark matter mass component of most galaxies greatly exceeds the mass of visible matter in most cases..
  6. Sep 4, 2016 #5
    Agreed but why is a 4 million solar mass object at the centre of rotation?
  7. Sep 4, 2016 #6
    Could there possibly be a concentration of dark matter around the center and the rest of the galaxy has a more tenuous concentration?

    That may be true, but the visible matters' gravitational pull would eventually dissipate with great distance like the ones in our galaxy. Maybe that's where dark matter comes into play and why the galaxies mostly have a stable configuration.
  8. Sep 6, 2016 #7

    Ken G

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    It's not well known how supermassive black holes form. In some galaxies, it is thought you might actually have a binary black hole in there, orbiting each other. The gravity of the black hole is not very important, as pointed out above, but the energy released in the formation of that black hole is the main energy release that has occurred in the history of the entire galaxy, so that energy release might be playing an important role in galactic dynamics in ways we don't fully understand. But note that it is natural for big self-gravitating systems to orbit around the center of mass of the system, and when the visible matter in the system has a lot of angular momentum so is all in a plane going around the same way, there are forces at play in such systems that tend to circularize the orbits even if there is no central body.
  9. Sep 6, 2016 #8
    Ask it this way: would a binary of a black hole and a star cluster be stable in a long term?
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