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Question of Galactic Proportions!

  1. Sep 4, 2009 #1
    Why are [most] galaxies flat? The milky-way is 2,000 ly tall and 100,000 ly long, for instance. Why do they flatten out? How about our Solar sytem? I can't find any model of the solar system that shows the planetary orbits in 3D so I can't tell if most planets are in the same "flat" orbit.
  2. jcsd
  3. Sep 6, 2009 #2
    Hi ranrod,

    Great questions. Lets start with our solar system. Think of it when it was in its infancy as just a collapsing cloud of gas, before the sun and planets had congealed. Virtually all astronomical things are noticeably spinning in some direction, and our infant cloud of gas is no different. As the cloud collapses the gas spins faster and faster (just as the ice skater spins faster when his arms are pulled in). Now the part of the cloud that is spinning the fastest starts dominating. In other words, the particles whose orbits don't align with the "spinning" plane collide with the particles that are in the "spinning" plane, and eventually virtually all the particles collapse into a spinning disc like structure. Then the planets form from the gas that is in this same plane. And sure enough, that's what our solar system looks like. This so called "nebular hypothesis" was put forth way back in the day (early 19th century) by Laplace and has since received observational validation.

    Galaxies are supposedly assembled in the same way. Except instead of a sun and planets congealing, it's stars that congeal from the gas.

  4. Sep 7, 2009 #3
    Thanks for the answer bombadil :) One more question. As the gas cloud collapses, what causes it to spin? I can imagine why water spins going into the drain on my tub - only so much water can fit through the drain at a time so as all the water molecules collide on their way to the drain, they spin. Is it like that?
  5. Sep 7, 2009 #4


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    Partially. Since the gas cloud collapses from an enormous size down to the size of a solar system (or galaxy), and since its rotation speeds up as it collapses, it only needs a very slow initial rotation to end up with an appreciable spin. Such a tiny rotation can be influenced by a lot of factors, like nearby supernova explosions and gravitational interactions with surrounding matter.
  6. Sep 8, 2009 #5


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    Spin is imparted at random during the collapse phase. Once enough molecules get aligned in a particular direction, the rest tend to join the crowd.
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