https://www.reddit.com/r/askscience/comments/vx3x5w/why_are_galaxies_not_round/
created by Nedpet on 12/07/2022 at 05:57 UTC
12 upvotes, 2 top-level comments (showing 2)
Observing all the round things in the universe, why are galaxies different?
Comment by d0meson at 12/07/2022 at 20:26 UTC*
18 upvotes, 1 direct replies
Things are round when their angular momentum is small compared to the attractive forces holding particles together. When angular momentum gets large compared to the attractive forces, things become disk-shaped.
Planets are round because they're not rotating fast enough to rip themselves apart into a disk. Solar systems are disk-shaped because the angular momentum of the original gas cloud that condensed into them is large.
But nothing is perfectly round or perfectly disk-shaped. The Earth bulges out at its middle due to its rotation. The Milky Way is not a flat disk, but bulges near the core, and it's also surrounded by a round "galactic halo" of stars not in the main disk.
Comment by Aseyhe at 12/07/2022 at 20:42 UTC
5 upvotes, 0 direct replies
There are two possible questions here:
1. Many galaxies are disk-shaped. Why don't they have three-dimensional extent as spheres?
2. The galaxies that have significant three-dimensional extent can still be very ellipsoidal, rather than spherical. Why is that?
1.
The disk shape arises because material loses orbital energy through inelastic processes. For example, energy is lost as emitted radiation. Energy is also "sequestered" into compact objects or orbital subsystems (like binary stars), which behave as a single object within the larger galactic system.
Why does such energy loss ("cooling") produce disks? Because while the material loses energy, it does not also lose angular momentum. Any initial angular momentum that the system had must be preserved. Therefore the system settles into a configuration that -- compared to a sphere -- has a higher ratio of angular momentum to energy: a disk.
This is the same reason planetary systems tend to be planar.
2.
Ellipsoidal galaxies tend to be galaxies that recently had a large influx of energy, such as through accretion of a lot of new material. Per the above, the injection of energy explains why they are not disks. But why are they ellipsoidal and not necessarily spherical? (They certainly can be spherical.)
It's more insightful to think about why things tend to be spherical in the first place. Spheres are energetically favorable from a gravitational standpoint: they minimize the gravitational energy for any given volume. Planets and stars are (nearly) spherical because they are supported by pressure while being held together by gravity, and these competing effects favor a spherical shape.
Galaxies are not pressure supported. Ellipsoidal galaxies specifically are *velocity dispersion*-supported, which means that large random velocities maintain the galaxy's shape. In particular, there are very few actual collisions between objects. Without collisions, there is little to drive the galaxy toward a spherical shape.
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Also, there is an dynamical instability (the "radial orbit instability") that suppresses the abundance of highly spherical cosmic structures. If a galaxy forms by pulling in material from a long distance away that is otherwise relatively undisturbed, then the material ends up on highly eccentric orbits. Unfortunately, a spherical system with only highly eccentric orbits is unstable: the orbits tend to cluster in angles to produce a more ellipsoidal object. This effect doesn't mean galaxies can't be spherical: if a galaxy accretes material with high angular momenta -- which settles into more circular orbits -- then a spherical configuration can be stable. But it makes spherical configurations less likely.