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Can the mass center and the geometrical center of fx. the milky way be different?

  1. Nov 3, 2011 #1
    My understanding is that if for example one object is orbiting another, then the mass center of the two masses is located somewhere in between the two objects. And if we have many object (for example stars), then the mass center of the total system can be located another place than were any star is located.

    My question is then whether it is possible that in the milky way that the total center of all the masses is located another place than where the geometrical center is located. And if so then isn't it possible that the center of the milky-way is not Sagittarius A, but for example some visible star?

    And wouldn't it be possible to explain the apparent existence of black holes such as Sagittarius A, as the phenomena that the total center of masses and therefore the center to where (smaller) masses are attracted, can be located at another place than were any physical visible stars are located?

    And if so wouldn't that mean that a small object could be attracted to different non physical centers depending on where in fx. the milky way it is? And wouldn't that explain why there appears to be black holes attracting surrounding masses around the universe?
    Last edited: Nov 3, 2011
  2. jcsd
  3. Nov 4, 2011 #2


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    Yes, this is known as the Barycenter.

    Sure. I can almost guarantee you that the *exact* geometric center of the Milky Way is NOT in the middle of the black hole at Sagitarius A. It should move around the center of the galaxy as time passes and objects move in their orbits.

    I don't know what you mean by this. The black hole in the middle of the galaxy is able to be inferred to exist due to extremely high velocity's of many stars around a central point. Note that in order for this to happen there MUST be a concentrated amount of mass on the order of a million + solar masses providing the gravity to keep these stars in there orbits. Also, the barycenter of the Galaxy can move around outside these orbits, however the barycenter of the black hole and the stars can NOT, it must stay inside near the black hole.

    No, the barycenter itself is not a physical object and exerts no force on anything. Objects are only attracted to other objects.
    See the last picture here: http://en.wikipedia.org/wiki/Barycentric_coordinates_(astronomy)
    Notice that if the 2 stars were orbiting the barycenter their orbits would NOT be shaped like that, they would be circular instead. Each star is attracted to the other star.
  4. Nov 4, 2011 #3
    What is meant by "geometric center" anyway? How do you determine that?
  5. Nov 4, 2011 #4


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    I assume it's the middle of the galaxy if you give it set boundaries. It's based on the size and shape of the galaxy, not the mass.
  6. Nov 4, 2011 #5
    Has anyone tried to calculate the relative position of the geometrical center?

    But couldn't it be that the milky way is constructed in such a way that position of the barycenter appears to fixed and that the idea of a black hole thereby is an illusion?

    And even though the barycenter is the result of all the attracting forces of all the masses in the milky way, then it is still the center of the resulting attracting force in the milky way isn't it?

    So couldn't that mean that the barycenter of the milky way is located in the same position as Sagitarius A?
  7. Nov 4, 2011 #6
    I suppose that you can say that the geometrical center of our solar system is the sun even though the barycenter do not align exactly with the sun!

    If a lot of the total masses in our solar system were located far away from sun, then I suppose that the sun would still be the geometrical center even though it would rotate around the barycenter or would it?

    Then I'm thinking the same about the milky way as I am wondering where the geometrical center is located!

    Or is the geometrical center the same as the barycenter?
  8. Nov 4, 2011 #7


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    Not really. It's almost impossible since we can't observe a huge portion of our galaxy thanks to dust and stuff in the way. Since we don't know it's exact shape and size, there's no real way to accurately determine the "center". I'd say just call the center somewhere near Sagittarius A for now.

    Edit: It's possible for our galaxy to look similar to this one: http://en.wikipedia.org/wiki/Pinwheel_Galaxy
    Notice how it's a little lopsided? Where would you put the center of it?

    No, as I said previously things do not orbit Barycenters, they orbit objects with mass. There MUST be something at Sagittarius A with a huge mass in order for the stars to orbit around it as such a high velocity. Take a binary system with 2 equal mass stars as an example. Let's put them at 100 AU from each other and in a near circular orbit around their barycenter. Any planets, asteroids, comets, ETC, will NOT be orbiting the barycenter of the star system, they will either be orbiting one star or the other, or being perturbed into chaotic orbits and possible ejected or swallowed into one of the stars.

    Only from a great distance out from our galaxy, and even then only to an approximation.

    It is possible that the barycenter passes through it, but it does not stay still. The distribution of mass changes over time as our galaxy rotates, and as such the barycenter changes. This is exactly like how the barycenter of our own solar system changes as the planets orbit around the Sun.
  9. Nov 4, 2011 #8
    But according to fx. the animation from


    then obviously the masses are orbiting around the barycenters!

    But if some mass is somewhere in the fx. the milky way and no heavy masses are near, then shouldn't it be possible to have a rotation around a barycenter?
  10. Nov 4, 2011 #9


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    Perhaps I have been explaining it badly. The stars are attracted to each other. Their combined gravitation causes their orbits to have a "barycenter". The barycenter is NOT attracting anything. Throw a planet in there and it does NOT orbit around the barycenter, it orbits around one of the stars. However, the new star-planet system DOES orbit around the barycenter, which has now shifted towards the star-planet system since it now has more mass. Even if I put a planet 1 mile from the barycenter of the 2 stars it would immediately start moving toward the closer star, not orbit the barycenter. Does that make sense?

    I'm not sure what you mean. You need to specify a system in order to have a barycenter. If we look at the solar system itself, the barycenter is always near the sun. However if we look at the galaxy as a whole it is no longer the case.

    A lone star in our galaxy will orbit the galaxy due to the combined gravity from everything in the milky way. The closer it is to a massive object, the more tug it feels from that object and vice-versa. The stars near Sagittarius A exhibit a motion around a massive object just like the Earth orbiting the Sun. Take away the black hole and instead of all these stars whizzing about at extremely high velocities, they would have to slow to a crawl in order to not fly away. Similarly, if the sun suddenly disappeared, the Earth and the rest of the planets would simply fly off into interstellar space due to their orbital velocities. The stars in the middle of the galaxy are simply traveling too fast for there NOT to be a massive object nearby.
  11. Nov 4, 2011 #10
    Are you sure that if we have a n-body system with n being very high, that it is not possible to distribute the masses, rotations, and distances in such a way that some mass/body somewhere in the system will be orbiting around a barycenter? I think that I read somewhere that when a galaxy is created, then at some points, some masses are orbiting barycenters!

    But let's say for the sake of argument, that the distance to Sagittarius A for some reason was calculated wrong so that it is nearer than expected and therefore the stars orbiting Sagittarius A is having a lot slower velocity. Then in theory would it be possible that it is all orbiting directly around the barycenter (= Sagittarius A) of the milky way?
    Last edited: Nov 4, 2011
  12. Nov 4, 2011 #11


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    I can't be sure, but I don't think so. At least not a stable orbit at high velocities like the stars near Sagittarius A.

    There's practically no way we could be so far off. And even then, I don't think so. I'm not a professional astronomer, so I can't be 100% sure on any of this, but I've never heard of this happening before and I don't know how it could possible work except under the most carefully set up circumstances if at all.
  13. Nov 5, 2011 #12
    2 objects in orbit around each other orbit the barycenter of the 2 objects. The foci of the orbits of the objects will be at the barycenter. The gravity comes from the 2 objects but the net result of the combined gravity of the objects is to make it seem as though there's gravity coming from the barycenter. If you introduce a 3rd object at a great distance from the first 2 it will orbit the barycenter even though there is nothing there.

    http://en.wikipedia.org/wiki/Barycentric_coordinates_(astronomy [Broken])
    Last edited by a moderator: May 5, 2017
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