Why do galaxies and solarsystems form disks?

In summary, the reason disks form is that angular momentum is conserved, and when a system contracts due to gravity, the average speed increases like the inverse square root of the size of the system. Disk fraction increases as the size of the system contracts.
  • #1
Nova
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In galaxy formations/collisions and planetary dust from newborn solar systems, they begin with an irregularly shaped cloud of matter. But why, over their lifetime, do they form a disk shape? In the case of spiral galaxies and planet orbits.
 
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  • #2
This very question was answer here recently and has been answered several times in the past as well. I suggest a forum search.

For many such basic questions a forum search is usually a good place to start
 
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  • #4
phinds said:
For many such basic questions a forum search is usually a good place to start

And, you can check out the list of "Similar Threads" at the bottom of the page... :-)
 
  • #5
Yet the answer is relatively simple, and is probably easier to just state here than to try to find in those other threads. The reason contracting systems form disks is that if you conserve angular momentum as you lose heat, you will always create a more disk-like shape. The reason for that is that a contracting system obeys what is called "the virial theorem", which essentially means the kinetic energy of its particles remains proportional to the gravitational energy of the whole system. That tells you something important about the average speed of the particles, they have to scale with the inverse square root of the characteristic size of the system (because the kinetic energy scales with the square of the average speed, and the gravitational energy scales with the inverse of the size, so make them proportional and you get it). Now imagine that some fraction f of the particles are responsible for carrying the orbital angular momentum (and there starts out with some conserved value for this), and the rest have orbits in all different directions, so have no net orbital angular momentum. As the whole system contracts due to gravity and the slow loss of heat, the average speed increases like the inverse square root of the size R of the system, but the characteristic angular momentum is f*M*v*R, where M is the total mass, v is the average speed, R is the size scale, and f is the fraction that are all going around the same way (sort of like a "disk fraction", if you will). If mass and angular momentum are conserved, then f*M*v*R and M both stay the same as R drops, and we have v going like the inverse square root of R, so this all requires that f must also scale with the inverse square root of R, so f must increase as R drops. That's why you get a disk-- if angular momentum and mass were conserved, you would always get down to some R where f reaches unity, and everything has to be in a disk because it all has to be going around the same way to be the fraction responsible for holding the angular momentum.

The above simplified argument treated the system in terms of characteristic numbers, which requires the system be "all one thing." But actual star systems are not like that, they have stars and companion stars and debris disks, so are more complicated. Also, real stellar systems don't conserve mass and angular momentum, they spit both of those out for various reasons. So you don't only get a disk, you get a disk and also one or more stars, and the places to put angular momentum are more complicated. But still, the bottom line is, you get disks because that is a good place to satisfy the need to put angular momentum somewhere, when the characteristic speeds of the particles that you get from the energy considerations are not keeping pace with the speeds you need from the angular momentum considerations.
 

1. Why do galaxies and solar systems form disks?

Galaxies and solar systems form disks due to the conservation of angular momentum. As a cloud of gas and dust collapses under its own gravity, it begins to rotate faster and faster. This rotation causes the cloud to flatten into a disk shape, with the majority of the mass concentrated in the center. This is similar to how a pizza dough flattens and spins faster as a chef tosses it in the air.

2. How do disks form in galaxies and solar systems?

Disks form in galaxies and solar systems through a process known as accretion. As the cloud of gas and dust collapses, it begins to form clumps and protostars. These clumps continue to grow and merge with one another, eventually forming a disk-shaped structure. This process is also aided by the rotation of the cloud, which helps to flatten it into a disk.

3. Why do disks have a flat shape?

Disks have a flat shape due to the conservation of angular momentum, as mentioned before. As the cloud collapses and rotates faster, the centrifugal force begins to balance out the force of gravity pulling the cloud inward. This results in a disk shape, with the majority of the mass concentrated in the center.

4. Are all galaxies and solar systems flat disks?

While most galaxies and solar systems do have disk shapes, not all of them are perfectly flat. Some galaxies, known as irregular galaxies, have a more chaotic and irregular shape. This can be due to interactions with other galaxies or the presence of a strong gravitational pull from a nearby object. However, the majority of galaxies and solar systems do have a disk shape.

5. How do disks influence the formation of planets?

Disks play a crucial role in the formation of planets. As protostars and clumps within the disk continue to accrete material, they begin to form planets. The disk provides a steady supply of material and acts as a gravitational nursery for these young planets to grow. Additionally, the flat shape of the disk allows for the formation of stable orbits, which is essential for the development of a planetary system.

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