In space, there are a lot of disc-shaped congolmerations, like:
solar system(s?)
spiral galaxies
black hole accretion disks
Why the prevalance of so many disk-shaped things? Is it because disks result when things bound tighly together, spinning, lose their “glue” and fly out, and the disks are just remnants of that planar distribution?
Why is our solar system in a plane – is it the same reason that our galaxy is spiral? (Is it even aligned to the galactic plane, or nearly so?)
The round part, I get. But I’d like more insight into the planar aspect.
Spiral is only one shape of many that galaxies form, though most resemble a planar shape. The reason is because the elements of galaxies and even more of solar systems (and pretty much everything else) are formed from tiny bits of matter that over eons have congealed into masses that are all rotating together around a common gravimetric center of mass.
When stuff spins around an axis, angular momentum distorts spheres (and gravitationally bound objects) into a more oblate shape, as all points, on and within the sphere, want to fly off at their relative velocities at any given time. Since the velocities are greater around the equator/ecliptic than it is at the poles, there’s more force there that’s trying to pull away from the gravity.
So what you’re both saying is that, any eccentricity in the original orbital shape of the mass of objects will get amplified over time to flatten out the oblate shape?
If there’s no angular momentum to begin with, how does it arise? That is, take a bunch of objects with some random dispersion and masses and hit “start”. There’s no angular momentum to begin with. Would it develop? Seems to me that total angular momentum would have to be conserved. If one galaxy spins one way is it only because nearby another one spun another way?
It’s actually collisions that create disks, along with angular momentum.
Things like condensing gas clouds, a cloud of fragments around a planet, etc may start out moving in random directions. But over time, these pieces (or atoms) collide with each other, and their random motion gets slowed down. But the overall, average rotation doesn’t die down - the angular momentum of the whole group is preserved. Eventually everything ends up orbiting in the same direction, in the same plane - i.e. a disk.
Not necessarily. As far as orbiting bodies go, they will either get ejected, crash and burn into another body, or find some relative stability in the system.
But in reality, things like galaxies are an impossibly complex and chaotic gravitational system. But, overall, everything wants to find equilibrium; yet it’s never perfectly balanced, and entropy always increases.
Even in our own solar system, things are winding down, or getting micro gravitational speed bumps and boosts that will just keep evolving our solar system in ways we can’t really predict over eons, because we can’t account for every object pulling on every other object out there.
I was specifically singling out an idealized spherical body (if you can imagine a galaxy that has an overall spherical gravitational attraction), in that when you spin something around an axis, angular momentum will pull on every point. The faster you spin something (let’s say a bunch of gummy bears held together by toothpicks stuck together like the EPCOT center), the momentum is stronger on the gummybears around the equator than it is nearer the poles. Faster and faster, it’s going to flatten out as the gummybears at the equator are pulling harder on the one’s a higher/lower latitudes. Eventually, spin it fast enough to overcome the grip on the toothpicks, and every gummybear will fly apart from each other at their respective velocities.
A very limited metaphor, but that’s the gist. If you can extrapolate from there that every piece of candy might have different sizes, masses and are all attracted to each other, over enough time, the noisy chaotic motions will fall into some sort of equilibrium and usually form a disk-like system.
If there is zero angular momentum to begin with, everything will fall down to the center and form a planet, star or black hole, depending on the mass. But this happens only if there is exactly ZERO angular momentum. It only takes a tiny bit of asymmetric motion (rotation) to create a disk, because as things fall down towards the center, the rotational motion is amplified by conservation of angular momentum.
I guess the answer for accretion disks isn’t simple, except in cases which started as a binary star where one partner evolved to a neutron star or black hole, and the accretion is from the other.
I was surprised to learn that viscosity is an important factor in many cases, being more used to thinking of stars in a vacuum.
I know it’s common to think viscosity mainly applies to fluids or liquids, but gasses and other particulate matter follow the same sort of laws in fluid dynamics.
Stars form in nebula nurseries dozens of lightyears across in interstellar space, and accretion discs are tremendous rivers of matter flowing toward and into a massive gravitational body.
Heck, our planet is blanketed but a layer of fairly unmassive gaseous molecules, and yet, after 4.5 billion years suspended in the vacuum of space, it’s still pretty thick and viscous-y! A vacuum compared to gravity and magnetism at these magnitudes are pretty puny (however, we are losing helium faster than anything else, but that’s another story).