2 ms·

I wonder why galaxies tend to be shaped like a disc rather than a ball. Doesn't really make sense to me.

mgobl 2y ago
It sort of does to me - disks made up of concentric orbits are have no intersections, and so collisions are less likely. In a ball, debris would have different directional vectors that would eventually collide with one another. The disk is the stable evolution of that state.
- penteract 2y ago
  I believe it's not that collisions (and gravitational interactions) don't happen when it's in a disk, but the galaxy as a whole has an overall rotational momentum about some axis, and collisions within the galaxy can't change that. This means that it can flatten along the axis, but it can't shrink in the perpendicular plane. If everything were twice as close to the axis (as gravity would like it to be), it would need to be moving twice as fast to conserve rotational momentum, but that's faster than the velocity of a circular orbit at the new radius (assuming everything was in a circular orbits to begin with). The same principle is why almost everything in the solar system approximately lies within a plane.
oofbey 2y ago
Because galaxies spin. A spinning ball will compress into a disc after a while. The parts which are close to the axis just fall into the middle and some amount of friction slows them down until they stay close to the plane.
TaurenHunter 2y ago
If you started with a galaxy that is a nearly uniform ball, the gas and bodies near the equator would attract the gas and bodies that are further out. Those further out would tend to collapse towards the equator and eventually that initial ball would become more disk shaped. Then there is this explanation refering to the conservation of angular momentum: https://van.physics.illinois.edu/ask/listing/18318 https://van.physics.illinois.edu/ask/listing/18318
arunix 2y ago
Elliptical galaxies are ball-shaped. https://en.wikipedia.org/wiki/Elliptical_galaxy https://en.wikipedia.org/wiki/Elliptical_galaxy

It sort of does to me - disks made up of concentric orbits are have no intersections, and so collisions are less likely. In a ball, debris would have different directional vectors that would eventually collide with one another. The disk is the stable evolution of that state.

penteract 2y ago
I believe it's not that collisions (and gravitational interactions) don't happen when it's in a disk, but the galaxy as a whole has an overall rotational momentum about some axis, and collisions within the galaxy can't change that. This means that it can flatten along the axis, but it can't shrink in the perpendicular plane. If everything were twice as close to the axis (as gravity would like it to be), it would need to be moving twice as fast to conserve rotational momentum, but that's faster than the velocity of a circular orbit at the new radius (assuming everything was in a circular orbits to begin with). The same principle is why almost everything in the solar system approximately lies within a plane.

oofbey 2y ago

Because galaxies spin. A spinning ball will compress into a disc after a while. The parts which are close to the axis just fall into the middle and some amount of friction slows them down until they stay close to the plane.

TaurenHunter 2y ago

If you started with a galaxy that is a nearly uniform ball, the gas and bodies near the equator would attract the gas and bodies that are further out. Those further out would tend to collapse towards the equator and eventually that initial ball would become more disk shaped. Then there is this explanation refering to the conservation of angular momentum: https://van.physics.illinois.edu/ask/listing/18318 https://van.physics.illinois.edu/ask/listing/18318

arunix 2y ago

Elliptical galaxies are ball-shaped. https://en.wikipedia.org/wiki/Elliptical_galaxy https://en.wikipedia.org/wiki/Elliptical_galaxy