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r2k-in-the-vortex t1_j80tezj wrote

First, Oort cloud is a bit hypothetical, those objects are far enough away that they are not catalogued. At those distances, Sun looks like just a bright star, how do you observe a object so dimly lit?

That said, assuming they are there, they wouldn't really have had time to form a accretion disk. The objects are too sparce to interact with each other much and the periods are too long. A object 2000au away(which is considered lower bound of Oort cloud) on circular orbit around Sun has a period of 90 000 years. Outer Oort cloud objects can have periods in millions of years

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r2k-in-the-vortex t1_j811pds wrote

Replying to Realeron (#1,773,970)

It's not a bad hypothesis at all, there is solid basis to proposing that it exists, its just that how would you go about confirming it?

The logic is that comets are inherently not a stable phenomena, a single comet can't keep going for geological time periods. So somehow you need to have a mechanism for new comets to appear, there needs to be some sort of reservoir of potential comets yet to become comets. If you do a bit of statistics based on that idea you end up with the model of Oort cloud that we have now.

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_MagnumDong t1_j812o5l wrote

The Sun formed from a collapsing molecular gas cloud, which itself had some net angular momentum. As the rotational velocity of this collapsing gas cloud increased due to conservation of angular momentum, it flattened out into the circumsolar disk.

It was in 1950 that Jan Oort first suggested that comets originated from a distant, sparse, spherical cloud. He proposed that comets were born in the circumsolar disk, in plane with the planets, before being perturbed by the gas giants and ejected into their spherical distribution. Subsequent studies of the orbits of comets and potential perturbations from passing stars strongly supported the existence of the Oort cloud, which was necessary because it is practically impossible to observe. J.G. Hills advanced the theory in 1981, suggesting that there exists an inner Oort cloud, now also dubbed the Hills cloud, that explains the mix of showers of short-axis comets and individual long-axis comets. The Hills cloud would be significantly denser than the Oort cloud, and works like this one used simulations to show first that it is indeed possible to get an Oort cloud from comets formed in the outer planetary region of the circumsolar disk due to combined perturbations by the galactic tide and gas giants, and second that the inclination of orbits only becomes random (spherical) beyond 5,000 AU due to the influence of passing stars and gas clouds.

So the much denser Hills cloud, which starts around 3,000 AU, is actually roughly torus-shaped like you expect of the Oort cloud. However the outer Oort cloud, which did likely start out in the orbital plane of the solar system, has probably been scattered by the combined effect of initial scattering by gas giants, and then further perturbation by extrasolar objects.

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thomasxin t1_j813hgr wrote

Replying to r2k-in-the-vortex (#1,773,648)

On the topic of spherical vs disk... do elliptical galaxies eventually collapse into spiral disks? It's a question that's always had confusion about it since spiral galaxies form by themselves but ellipticals form from collisions; if undisturbed, would spherical orbits always eventually collapse into disks (albeit perhaps taking vast amounts of time), or is it actually physically possible and statistically likely to have perfectly stable spheres that never decay?

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Player_KK t1_j81edoc wrote

Replying to thomasxin (#1,774,140)

Originally, it was believed that elliptical galaxies would turn into spiral galaxies, but we have since found this to not be true. Elliptical galaxies..... in short, are not active. They are characterized by a distinct lack of star forming gases, and little to no new star formation. Existing stars seldom interact with one another, so much of the shape comes from those star forming gases, via angular momentum.

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At least this is how I understand it.

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thomasxin t1_j81gnu6 wrote

Replying to Player_KK (#1,774,655)

Gravity does interact over long distances over long periods of time though, is there a mechanism that acts against that compared to smaller accretion disks? What prevents an elliptical galaxy collapsing into a spiral, and how do we know that won't happen even if given a trillion years?

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Cheshire-Kate t1_j81i8j2 wrote

Replying to thomasxin (#1,774,776)

If the only thing acting on stars is gravity, then they will stay in their elliptical orbits undisturbed and the overall shape of the galaxy won't change.

It's only once you start adding drag and friction into the mix from gas clouds and nebulas that the forces necessary to collapse the shape into a disc are present. With those gases, a galaxy will eventually collapse into a disc, but without them, there's no reason for them to do so.

Most elliptical galaxies are very old, and no longer have any new star formation. This means far less drag and friction, meaning no forces to flatten the orbits of stars within the galaxy, meaning it will remain an elliptical galaxy forever

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willun t1_j81up2b wrote

Replying to Cheshire-Kate (#1,774,851)

Would the dark matter provide drag? I wonder what the movements of dark matter would be in galaxies like that. I assume for the Milky Way it just rotates in line with the movement of stars (or more accurately, the stars move in line with the dark matter)

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gladfelter t1_j82x2uy wrote

Replying to willun (#1,775,446)

The electrical field of an atom is many orders of magnitude larger than the radius of a theoretical WIMP. So interactions and friction would be orders of magnitude less for dark matter.

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_MagnumDong t1_j830366 wrote

Replying to Pandagineer (#1,775,399)

As the other commenter said, rotation of clouds is just the net effect of the orbits of their components. This is just speculation on my part, but I suspect the Hills cloud shows more rotation, because the objects that make it up are closer to their original orbits. It would also be rotating faster because it is more compact. In the outer cloud, I suspect there’s some small net rotation, but largely the velocities and orbits are randomly distributed so there’s less bulk movement.

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DeepDuh t1_j835j1d wrote

Replying to _MagnumDong (#1,774,093)

Is it correct to think that the Oort Cloud is roughly where the sun‘s gravity is equal to the galactic gravity (i.e the sum of all the galactic influences) and is thus spherical?

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Client_Hello t1_j836l12 wrote

Replying to thomasxin (#1,774,140)

You having it backwards, disks expand to ellipticals as they use up their gas.

Forming a disk requires collisions, and stars are too far apart. The gas in young galaxies does collide, which bleeds off angular momentum, allowing the gas to form a disc, then stars form in the gas.

Those blue stars you see in spiral galaxy arms will not survive a single trip around the core. They light up the region of compressed gas from density waves.

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thomasxin t1_j839eyv wrote

Replying to Client_Hello (#1,777,714)

There are smaller blue stars that can still last a billion years or so, right? That would be at least 4 orbits around the milky way for instance. Though larger ones would obviously not last as long

This is an interesting take though, I've not seen sources explaining spiral galaxies evolving into elliptical ones even without disturbance. What would be the cause of the opposite for galaxies, when normal cloud/sphere orbits collapse into rings?

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_MagnumDong t1_j839spe wrote

Replying to DeepDuh (#1,777,680)

The Oort Cloud is continuous and its density falls off with distance, so it can’t be said to be at a specific distance. However there are estimates of an outer edge which place it around and occasionally beyond the Sun’s sphere of influence.

The Sun’s gravity is spherically symmetric, so it itself doesn’t exert any force to keep objects in a plane. Self-interaction of objects in the disk provides this force, through collisions or gravity. What the Sun does do is draw objects in, so the density of objects is greater nearer to the Sun, meaning there’s more self-interaction in the disk in these regions. The Oort cloud, though it likely formed in the disk, is far from the Sun and therefore underdense, so when orbits in the Hills cloud are perturbed and moved into the Oort cloud, there’s not enough self interaction to correct their inclination.

So, to answer your question: kinda, but maybe not in the way I interpreted your question to imply.

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Outside_Sprinkles_72 t1_j89hl2l wrote

The Oort cloud is thought to be spherical because it is believed to be composed of icy objects that were gravitationally drawn together by the Sun's gravity when the solar system was forming. Its spherical shape is also likely due to the fact that it is composed of particles in random orbits, which generally tend to form a spherical shape due to their chaotic orbits.

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Any-Broccoli-3911 t1_j8irfcr wrote

Gravitationally bound objects are elleptical if they have low friction (due to extremely low gas density between compact objects) like elliptic galaxies, oort clouds, and many other things, or are supported by pressure like stars, planets, and big satellites. They get more spherical if the angular momenta of each of their constituents are more equally distributed (only for low friction objects. High friction objects supported by pressure have an angular momentum in a specific direction) and if they are less disturbed by the gravitation of nearby objects.

They are flat disk if they have high friction (due to high gas density between compact objects) and are supported by angular momentum., like spiral galaxy, planet disks and accretion disks. They remain flat until perturbed by a collision even if the gas density went down enough that the friction is now low. That's the case of many planet disk and spiral galaxies. After a collision, they will get first irregular and then elliptical.

The accretion disk around the sun used to be a high density region while the gas was falling into the sun so it became a disk. Most of the gas and dust fell into the sun, but the remains are the planets and asteroids. The oort cloud is the part of the cloud that formed the sun that never got enough density to become a disk.

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