Does the Milky Way orbit around anything?
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:
- Moon orbits around Earth
- Earth orbits around Sun
- 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?
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.
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 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.
One of the best examples of the orbit around center of mass that I found online is http://labs.minutelabs.io/Chaotic-Planets/
@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.
"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 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.
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 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.
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 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.
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.
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.
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.
@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?