Why do galaxies look like they spread out in a single plane (ie, why do they look more like frisbees than spheres)?
In: Physics
Take a ball of dough and spin it. What happens? It turns into the shape of a pizza. Why wouldn’t that also happen in space?
This fun short video answers your question perfectly. It is about solar systems, but it applies to galaxies as well.
Great question. It is one of the effects of gravity in a 3D space. Before they are galaxies, they are giant masses of gas spinning around some axis, with time gravity pulls objects towards each other making it look more like frisbee and less like a sphere.
They might look that way and may galaxy typers are flat.
On common perhaps most common galaxy type is “dwarf spheroidal galaxies” that are sphere.
The will look quite flat if you see them but that is because you only see them from one direction.
You tell a sphere and disc apart by looking how ligh illuminte it, how it cahgnes when it moves. But if you buld a sphere or disk with lightsources like a galaxy you can exacty tell them apart.
https://physics.stackexchange.com/questions/148418/why-arent-there-spherical-galaxies/148533
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.
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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Try stiring a cup of tea really fast then dropping a bit of milk in. The milk will fall to the bottom initially but the continues spinning of the stirred tea will draw it in so the milk eventually spins with the tea
Everything is pulled equally towards the center of mass by gravity but that force is only resisted by spinning. The spinning can only occur along a single plane because more than one plane will cause the objects to collide with each other. Everything averages out in the end and you get a disk.
For the same reason why if you toss a ball of dough, you can make a flat pizza. When things spin really fast, they flatten around the rotation axis. Galaxies do the same
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