Why do galaxies look like they spread out in a single plane (ie, why do they look more like frisbees than spheres)?
In: Physics
Imagine a spherical ball of gas and dust, where each individual particle is traveling on a circular orbit at some random angle relative to all others. Sometimes, the particles will collide or interact, and the result will be that they lose some energy along their path, or even stick together. If they collide head on, the motion of one can basically cancel out the motion of the other, and those particles functionally stop orbiting. They’ll fall to the centre of the cloud. If the cloud is dense enough, most of them will do this.
But what if the orbits aren’t totally random? What if the majority of the particles have some component of their motion spinning around in the same direction? Well, then when the particles collide, their orbits won’t totally cancel. The “up” and “down” parts of the orbits will, but the sideways components will add up… The result is, over time, the cloud will collapse down into a disk, spinning in the direction of the slight overall spin of the cloud.
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