(Or any other galaxy, really. Or the Virgo Supercluster. Or any other really big object.)
More precisely, what is it about (most? all?) galaxies’ structures that they aren’t subject to the conditions of hydrostatic equilibrium; that is, why aren’t galaxies and other large objects spherical? Is it that they’re just so freaking diffuse? That must be it.
I mean, I thought the rule was that, roughly, the bigger (okay, more massive) some object was, the more vigorously physics insisted that object’s shape tend toward sphericality. But galaxies are really massive, and don’t. Are really massive objects not objects in the relevant sense? What about really massive objects exempts them from this rule, which maybe I just made up?
Because they are rotating.
It gets a bit more messy, but anything rotating will flatten out. The Earth is very slightly oblate, the Sun more so, and Jupiter quite obviously so.
Rotating dust clouds in star forming areas flatten out with a proto star building in the middle and aggregating dust and gas forming planets.
Same for galaxies.
However it is a bit more messy. There are globby closer to spherical galaxies. And more interestingly it is apparent that the dark matter that accounts for the majority of the mass is more of a sphere. This is inferred from the rotation of the visible matter of a galaxy, since we can’t, at least so far, see dark matter. But a sphere of dark matter seems to fit the requirements to explain the observed rotations. But why this should be so is not understood. At least as far as I understand it.
Galaxies are not solid objects so do not behave like solid objects. Indeed, galaxies are very diffuse.
If our sun was the size of a basketball and you placed it in downtown Chicago the next closest basketball would be in Indianapolis.
Galaxies flatten out because as the particles collide in the cloud forming them their momentum cancels each other out except in the direction of rotation.
Well stars aren’t solid objects yet they’re spherical, I assume, because of hydrostatic equilibrium.
And they spin, some of them pretty quickly, yet remain (I think?) pretty much spherical. Are neutron stars essentially disks? They whip around like crazy. Good ol’ Sol rotates once in a bit less than four weeks (albeit differentially by latitude), which seems pretty quick for something so big. Yet its equatorial diameter and polar diameter are only different by six miles (!!). Which is to say it’s very spherical.
Also — incidentally — some galaxies are indeed spherical. Or close to it. The roundest of elliptical galaxies, E0, are basically huge fuzzy spheres.
Define a solid object.
An object that hasn’t melted or boiled, and isn’t plasma…but all this may be tangential.
There are four known forces (weak nuclear, strong nuclear, electromagnetic and gravity).
Gravity is, by far, the weakest BUT it is long range so that is what works on galaxies.
The other three forces are much, much, much shorter range.
No, they’re not. The Sun’s oblateness has already been mentioned, but we know this holds for other stars, too. Dramatically so in some cases:
Would it be that the structural character of a solid object is electrostatic? The Earth would not be “solid” because its structure is governed by gravity. Last article I saw about white dwarfs described their constituency as “gaseous”, probably because of the high mobility of their atoms (due to heat).
Yes yes yes. I meant “spherical” in the sense that a billiard ball and an orange and a soap bubble are spherical. Not spherical = perfectly spherical. While not a sphere (in fact, there is no sphere), the Sun and I presume at least some other stars are astoundingly spherical.
Okay, but which of these four forces are doing most of the work to keep Ganymede and Pluto and Earth spherical? Gravity, right? Not so much the electromagnetic, puny nuclear, and mighty nuclear forces, right?
Dark energy may count here too.
Not counted among the fundamental forces it seems it really matters when it comes to galaxies (it seems it is a repulsive force).
Pedantically the electrostatic forces are what keep the planets at their current size. Otherwise gravity would have an easy time of crushing things down a great deal further.
This is part of the question about hydrostatic equilibrium. There need to be forces in equilibrium. The huge space between objects in galaxies means that there is no useful repulsive force. Unlike stars, which do behave as gasses bound by gravity.
Galaxy stars and gas clouds are bound by gravity and the only force holding them apart is their motion. There is an equilibrium, but it isn’t hydrostatic.
The equilibrium that arises is different because the forces are not symmetrical. Rotation of the constituents is about an axis, and that leads to different magnitude forces normal to the axis to along it. So the final equilibrium is asymmetric and you don’t get, nor should expect a sphere.
Do this thought experiment -
You have a bunch of stars, milling about, they will tend to fall toward each other, but because they have motion, they come close and miss each other, zip by for a return path…
So you have a random collection of stards swinging around each other - but also pulling the other stars along. (Multiple-body interactions are complex!) They will tend to settle into a series of orbits around the common center of mass. If there is a net angular momentum in one direction, the overall direction will sort out to that.
Obviously some stars will be going up and down out of the disk as they orbit - but as they pull away from the other stars upward, they are pulled back, same downward - pulled back to the middle. The net pull on a star is the sum of all the pulls from all the others. So a star further away than average from the “blob” of stars, will feel more gravity pulling it back; but when it gets into the middle of the pack, so to speak, it is pulled in all directions and so does not experience slowdown forces.
(For example, if you could rill a hole through the earth - ass you descend, more and more of the mass of earth is pulling you upward since it’s above you. In the center of the earth, mass pulls you equally in all directions and you would be weightless.
So essentially, this group gravitational force tends to “flatten” all the orbits to a common plane, and circularize the more extremely elliptical orbits. the net effect of this is a disk.
Of course.
But… why are they rotating?
I think it is because there is a slight imbalance. That little nudge one way adds up on a galactic scale. Once that slight imbalance gains steam it would snowball into a bigger and bigger effect.
It would be weird if it was all perfectly balanced.
A WAG on my part, but it’s because they have always been rotating. Conservation of momentum, and all that.
The question is perhaps more a matter of why are most galaxies rotating enough that that overall rotation dominates the equilibrium and thus shape.
A galaxy need not rotate enough for this to be, but most do.
The question is perhaps best addressed by looking at the very early universe. We tend to assume that the net rotational moment of the universe is zero. But even in the very young universe things were far from a stable expanding equilibrium. The favourite example of the fingerprint of this is the inhomogeneity of the cosmic microwave background. Even the tiny variations there cascade into further inhomogeneities.
Look at two streams of water merging and look at the vortices being created. Ask yourself “why do they rotate?” Same question. Those vortices themselves create even smaller vortices and they get shed out into the evolving system. Here again the net angular moment was zero, and remains so, but that is found by summing across all the individual whirling structures. So to we assume the universe evolved. Huge swirling clouds of matter filled an expanding universe. As things settled down and the universe grew, gravitational attraction would hold together lumps of swirling stuff, and slowly these would condense down into structures from which galaxies would form. At the large scale we see a universe filled with threads of matter, where each thread is itself comprised of chains of whirling clusters of galaxies. And within those clusters galaxies themselves were pulled together from the whirling gas clouds. So they started life as distinct entities with a net angular moment. Some with more than others. But it seems the dynamics of the systems tends to favour the mix we see.
Probably another aspect about what we see in galaxies is the distinction between ordinary matter and dark matter. Ordinary stuff interacts with itself, strong, weak, and electromagnetic forces. That means it clumps. This is a really big deal. If this was not so even stars would not form. Clouds get pulled together by gravity, but gravity isn’t strong enough to hold stuff together enough to allow really dense accretions of matter to occur. Clumping gets us bigger accretions of stuff, and from that we birth stars. They are born, burn, burn out, go out with a bang, and generate more useful matter, and the cycle goes on, with clouds of matter from which we get not just stars but planets. All this matter is swirling about, and any given accretion of stuff is likely as not to have a rotational moment. So we get plants that orbit their suns, rotating suns, all living within a rotating galaxy, that iself lives within a cluster of galaxies that is itself rotating. Add all the rotations up and you should sum to zero across the universe. But locally, things spin.
Dark matter doesn’t clump, and that probably provides the clue about why it doesn’t manage to spin down into a disk. As @md-2000 painted a neat picture of, clumped up lumps of matter will interact as separate bodies, all exchanging momentum with one another as they whip about. Eventually settling into some pattern dominated by the overall angular moment. I suspect that the lack of such activity in the clouds of dark matter is why they remain as a diffuse bubble around the galaxies.
There may well be a point where a proto-galaxy is rotating too much to be able to hold together, and this provides a cut off point beyond which you don’t get a galaxy, and thus why there is a favoured shape. Back when it was all whirling vortices there was a maximum vorticity and spin for a bubble of matter, and if that was exceeded it broke up into smaller stable vortices. Totally guessing here. But it seems reasonable. This is the sort of thing that the computational cosmologists like to play with. Then again, as I like to joke, cosmology is the only science where the error bars are on the exponent.