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We know most of the objects in the Universe have a spherical or elliptical shape. The object which has less mass and gravitational pull orbits around the nearest object with more mass and gravitational pull. For example:

  1. Moon orbits around Earth
  2. Earth orbits around Sun
  3. Sun orbits around Sagittarius A* which is the center of Milky Way.

Thus, is the Milky Way orbiting around some object or perhaps Black Hole?

I know that the Milky Way is going towards Andromeda as they are attracting each other and they will collide with each other after 3 billion years to 6 billion years. But it is possible that the Milky way is orbiting around some object at the same time? Perhaps both galaxies are present in a group of galaxies which is orbiting around some object.

If the Milky Way is not orbiting around some object then is there any proof found by the scientists for that?

Chappo Hasn't Forgotten
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Zeeshan Ahmad Khalil
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3 Answers3

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The object which has less mass and gravitational pull orbits around the nearest object with more mass and gravitational pull.

Actually, both the heavier and the lighter object orbit around their common center of mass. It's just that the heavier object doesn't move much (has a tiny orbit), while the lighter object moves a lot (has a wide orbit).

E.g. our Sun actually orbits the center of mass of the whole solar system, but that motion is tiny, it barely budges.

In the case of a double star, where both partners have about the same mass, you can clearly see how both are making similar orbits around their common mass center.

Sun Orbits Around Sagittarius A* which us center of Milky Way.

With galaxies, including ours, it's a little different.

There is no super-heavy thing at the center, around which everything else is orbiting. Not even the very large black hole at the center of our galaxy is heavy enough for that.

Rather, galaxies are clumps of matter that create a common gravitational field. Stars, and everything else, are trapped in this common field and orbit around the common center of mass.

So the question is that is that is Milky Way is orbiting around some object or perhaps Black Hole.

Same idea. There is no single point-object nearby massive enough for our galaxy to "orbit" around it.

Our galaxy, along with Andromeda, and a handful of other galaxies, are bound together in what is known as the Local Group. Each galaxy is moving within the common gravitational field of the whole group. The Local Group has a diameter of about 10 million light-years.

The Local Group is part of a larger structure, the Virgo Supercluster, which is about 100 million light-years in diameter and has at least 100 galaxies. However, the Virgo Supercluster is more "loose" - it is not gravitationally bound together.

Florin Andrei
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    It's probably worth pointing out that the barycenter of the solar system, due to the usual mass distribution of the planets and everything else, is typically inside the sun. – chepner Jul 02 '19 at 23:34
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    @chepner Wikipedia has diagrams of the solar system barycentre relative to the Sun here, for 1945-1995 and 2000-2050. It's hard to tell from those diagrams, but I think the barycentre is outside of the Sun at least 50% of the time. It's been outside since mid 2016 and will remain so until early 2027. – PM 2Ring Jul 03 '19 at 00:19
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    One of the best examples of the orbit around center of mass that I found online is http://labs.minutelabs.io/Chaotic-Planets/ – Ferrybig Jul 03 '19 at 07:54
  • @PM2Ring: I believe that the diagram linked is the inferred barycenter, not the measured barycenter. Discovering e.g. a large planet at 80+ AU may alter that diagram. – dotancohen Jul 03 '19 at 08:53
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    "There is no super-heavy thing at the center" Is that something we know (measured) or do we simply have no reason to assume there is? – Mast Jul 03 '19 at 10:11
  • @PM2Ring Thanks, happy to be corrected. – chepner Jul 03 '19 at 14:03
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    @Mast Saittarius A*, the black hole at the centre of the Milky Way, has a mass close to 4 million solar masses, which can be calculated from the orbits of the stars that are close to it. But that's only about 0.25% of the mass of the whole galaxy. – PM 2Ring Jul 03 '19 at 14:24
  • If the Virgo Supercluster isn't gravitationally bound together, then why is it where it is? I'd tried to answer with 'dark [stuff]' but failed miserably.... isn't there something about that stuff being a scaffold for the distribution of matter in the universe? – Mazura Jul 03 '19 at 15:47
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    @Mazura I think much of the large-scale structure of the universe is remnants of fluctuations in matter/energy density in the nascent universe, which then turned into these clusters when the universe expanded. – Barmar Jul 04 '19 at 00:02
  • Great answer, however However, the Virgo Supercluster is more "loose" - it is not gravitationally bound together. is like a hard TV series cliff-hanger. Why is it not bound? Why does it stick together anyway (does it in fact stick together, or is everything just there coincidentally? – AnoE Jul 04 '19 at 08:19
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    @AnoE Put simply - it's literally too loose, not compact enough; it would be bound gravitationally if its components were located closer together. As to its origin, I do not know the answer. – Florin Andrei Jul 04 '19 at 09:13
  • So it's basically the ratio of mass to distance that's too small here, @FlorinAndrei? Would be nice if you added that to the answer, it would round it up nicely. – AnoE Jul 04 '19 at 09:42
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    Hmm. I interpreted the question differently. You seem to be answering the question of "is every object in the Milky Way orbiting around an object in the centre of the Milky Way?", whereas I read the question as "is the Milky Way itself orbiting around the centre of mass of some larger object, in the same way that the Earth-Moon system orbits around the barycentre of the Solar system?" That's actually quite an interesting question, because it depends on the scale at which the expansion of the universe takes over from local gravitational interactions. – N. Virgo Jul 04 '19 at 13:11
  • From this simulation of the Milky Way-Andromeda collision, I would guess that the two objects are moving slowly enough relative to each other that they would orbit if they didn't collide. It certainly looks like the Triangulum galaxy ends up in an orbit around the new galaxy that forms during the collision. – N. Virgo Jul 04 '19 at 13:13
  • @FlorinAndrei given the vast majority of Local Group galaxies are simply satellites of the 2 large spirals (ours and Andromeda), at a simplistic level one would expect the 2 large galaxies to orbit around their common barycentre, similar to binary stars - but instead, the 2 spirals are on a collision course. Is this because the evolution of galaxies is fundamentally different from the evolution of binary stars? I'm thinking (much handwaving): stars form out of swirling molecular clouds with angular momentum, but galaxies independently "condense" around lumpy distributions of dark matter? – Chappo Hasn't Forgotten Jul 05 '19 at 00:28
  • This answer suggests, but does not explicitly state, that the Milky Way and any other galaxy orbit (perhaps as part of a group or supergroup) around the center of mass of all the galaxies in the universe. – WBT Jul 08 '19 at 18:50
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The Milky Way would likely orbit the Great Attractor, the gravitational center of our local supergroup, but the metric expansion of space overwhelms gravitational attraction at that scale. The metric expansion of space only produces significant effects at the intergalactic scale so anything inside a galaxy is not affected much. In between galaxies however, and ever moreso with increasing scale, the metric expansion of space rips apart gravitational attraction so we see a foamy appearance rather than orbits.

KevinRethwisch
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    This. barycenters> local group> >super cluster > [?????] > [!!!!?] ... "The Milky Way is part of the Local Group galaxy group (which contains more than 54 galaxies), which in turn is part of the Virgo Cluster, which is part of the Laniakea Supercluster." which in turn is (orbiting) what (or exists where it does, because of why)? – Mazura Jul 03 '19 at 15:56
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    @Mazura It's complicated. "Because the laws of physics", in short. Gravity plays a role. Quantum fluctuations shortly after the Big Bang play another role. Random thermal fluctuations in early matter also shape the current distribution. There is no one single WHY in a strong sense. It's distribution of matter plus the interactions of this matter. The universe is what it is because of its entire evolution up to this point. You'd pretty much have to run a full simulation since the Big Bang to account for the current landscape. – Florin Andrei Jul 03 '19 at 18:13
  • @FlorinAndrei if space were not expanding there would be orbital motion in the universe even at the largest scales. The "foamy" appearance of the universe is because the expanding universe prevents the structures that would normally form from occurring. The expansion of space is the only factor relevant to the original question. – KevinRethwisch Jul 03 '19 at 21:02
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For a visual representation of our home supercluster (mentioned above) consider this image,

enter image description here

and this video, Laniakea: Our home supercluster might be helpful showing the local group in relation to the supercluster and the respective movements. I didn't see orbiting motion depicted in the video.

It says "the entire universe can be seen as an intricate network of galaxies, a cosmic web..."

Bob516
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