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Orbits in the universe

  1. Jan 29, 2016 #1
    Dear PF Forum,
    Lunar Satelite orbits the moon,
    The moon orbits the earth,
    The earth orbits the sun,
    I know that some of you know about this picture
    You might want to tell me. "No Steven, the Sun also orbits the earth"
    But for all practical purpose, we'd say that the earth orbits the sun. So does the moon, the moon satelite, etc

    Okay, so I'l continue.
    The sun orbits the SMBH in the centre of our galaxy.
    And in turn, what Milky Way orbits to?

    Considering Hubble Law, that every galaxy is moving away each other.
    Does Milky way somehow orbits a bigger galaxy,
    And if it's true how many galaxies orbit that thing?
    And what thing that that thing orbits?
    And in that sense what is the last thing that doesn't orbit anything, if there is such last thing.

    Thank you very much
  2. jcsd
  3. Jan 29, 2016 #2


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    This is not very precise nor correct. The Sun is not on an orbit atound the central black hole. There are many more astronomical bodies which influence the solar orbit in the galaxy. More correct would be to say the sun has an orbit in the galaxy, which it is part of as well. It is not as simple to describe this as having the sun orbit what essentially is a point mass.

    This becomes even worse on the galaxy cluster scale and as you increase the distances you are looking at the universe becomes essentially homogeneous.
  4. Jan 29, 2016 #3


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    To add a little bit of perspective, the black hole at the center of our galaxy (Sagittarius A*) is roughly 0.0006% of the total mass of the Milky Way galaxy, and much less than 1% of the mass that significantly influences the Sun's orbit (only the mass that is closer to the center of the Milky Way than the Sun has a big influence on the Sun's orbit).

    There are some dwarf galaxies that are orbiting the Milky Way as well (the large and small Magellanic clouds are the largest of these). I'm not so sure that the Milky Way could really be described as orbiting anything else, however. It is currently falling towards the Great Attractor:
  5. Jan 29, 2016 #4
    ... and Hubble said that every galaxy are moving away from us, aside from Andromeda :smile:
  6. Jan 29, 2016 #5


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    Not every. That is the overall, average motion on large scales. In the next few billion years, for example, the Milky Way will collide with Andromeda.

    I'm not sure that the Milky Way will ever reach the Great Attractor, though: we are moving towards the Great Attractor relative to the Hubble flow, but the Hubble flow is large enough that the Great Attractor is still getting further away at this time. So we're falling in that direction, but will probably never get there.
  7. Jan 29, 2016 #6
    Thanks for the enlightment
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