Eli5 Why are galaxies flat instead of being spherical like stars and planets?
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Let’s start by assuming a galaxy starts off sphere-like where stars and planets are orbiting a black hole. Over time, there will be an imbalance as some planets and stars are clumped together in one region of this sphere as orbits go at different velocities. This means that a large of amount of mass is accumulated in that spot. What happens? Gravity happens. So even my miniscule amounts nearby planets and stars would be attracted to this mass. Over a long time this consolidates into a disk-like shape as this mass is concentrated in this equatorial region. Where it may deform is when there is another galaxy nearby, when it is in the process of merging, or post-merger.
The guy who mentioned pizza is fairly good making it easy to understand though.
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