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Why do galaxies and solarsystems form disks?

  1. Oct 24, 2014 #1
    In galaxy formations/collisions and planetary dust from newborn solar systems, they begin with an irregularly shaped cloud of matter. But why, over thier lifetime, do they form a disk shape? In the case of spiral galaxies and planet orbits.
  2. jcsd
  3. Oct 24, 2014 #2


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    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
    Last edited: Oct 24, 2014
  4. Oct 24, 2014 #3


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  5. Oct 24, 2014 #4


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    And, you can check out the list of "Similar Threads" at the bottom of the page... :-)
  6. Nov 2, 2014 #5

    Ken G

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    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.
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