Why do galaxies look like they spread out in a single plane (ie, why do they look more like Frisbees than spheres)?
In: Physics
Everything in the universe is spinning, including galaxies. Galaxies start out as big spheres of gas. All those particles are orbiting around the center of the sphere. Some of them might be orbiting almost vertically, while others are orbiting more horizontally. Over a very, very long period of time, these particles crash into each other, and when they do, they cancel out the different directions that they were going in and start going in the same direction (conservation of angular momentum). So eventually, most of the things in the galaxy end up going in the same direction, because the stuff going in different directions crashes into each other.
There are many galaxies that aren’t shaped like frisbees, and some of them are really interesting. Most of the largest are more or less spherical, but the pretty ones (that get the most pictures) tend to be spirals.
Sane reason why pizza pies go flat when spun. Same reason why earth has a bulge at the equator.
As long as there is a force holding everything together and trying to pull it back to a central point (electrostatic, gravitational), things will tend to flatten perpendicularly to the axis of rotation.
That’s because things in that plane experience the most centrifugal force due to their rotation, most able to resist the pull inwards. So eventually, everything not in that plane either gets sucked to the centre or ends up on that plane.
Important note: Perpendicular to the axis of rotation. If there is not one axis of rotation, like for earth satellites orbiting whichever way on their own independent rotations, this doesn’t generally apply.
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