Why aren't all galaxies spherical?

Question

Why do we see spiral shaped galaxies when gravity should pull everything into a sphere? You don't see any spiral shaped planets. Infact, what is stopping gravity from pulling everything in a galaxy into one massive ball of matter?

Answer 1

Galaxies come in a variety of shapes, and some are, in fact, roughly spherical.

The motion of matter is the key to understanding why there are spiral galaxies such as the Milky Way and the Andromeda Galaxy.

Without motion, the entire universe would be pulled into a spherical ball of matter which would collapse into a black hole.


Since everything in the universe is in motion, gravity is always in play, but the shapes that it forces are not always spheres.
Picture the Solar system. The individual planets, dwarf planets and sun are all relatively spherical. Each is made up of gasses and solids pulled into those shapes by gravity. Smaller bodies do not have the mass to generate the gravitional forces necessary to form spheres.
But looking at the solar system as a whole, you notice that most of the mass of the solar system exists in the same PLANE, similar to the shape of spiral shaped galaxies. This is due to the fact that somewhere before the formation of the solar system and the stars of the galaxies, the gasses and debris that were to form the stars and planets was already moving. As gravity pulled the clouds of matter into a smaller and smaller volume, the clouds flattened out into a broad disk, just as the rings of Saturn have done. As the cloud collapsed further, stars and planets began to form within that disk. Today, we see the sun at the middle of a mostly planar system, with occasional asteroids and comets in oddball orbits due to collisions, gravitational slingshotting by the planets, or due to the fact that they came from the Oort cloud, which is thought to be roughly spherical and made up of debris that is barely controlled by all the matter in the middle of the solar system (sun and planets), but was not pulled into the pre-system disk due to its distance from the center of mass of the proto-solar system.

Galaxies, on a much larger scale, formed in the same manner as the solar system.

If you're willing to wait about five billion years, though, the Andromeda Galaxy and the Milky Way will make several passes at each other as they eventually merge into one giant galaxy that may wind up in the shape of a sphere, just like many of the galaxies we can now see.

Answer 2

Rotation tends to flatten things. Galaxies are rotating, so they are flat and many of them have spiral structure. Planets are not totally spherical. All of them show a degree of flattening. The most noticeable one is Saturn, which is visibly flattened at the poles.

Gravity is balanced by motion. The Moon stays in orbit because it is moving at the right speed to stay up there despite Earth's gravity, but not fast enough to escape Earth's gravity. Most galaxies in the universe are racing away from each other faster than gravity can pull them back.

Answer 3

Rotation

Almost all galaxies wind up (pardon the pun) with some rotation. As they rotate, they flatten like spinning pizza dough. Those that don't have any spin today will likely merge with another galaxy with some spin and the resulting larger galaxy will also get an average spin out of the collision.

So, the typical life of a galaxy is spin into a flat disk, merge with another galaxy into great arcs of stars. Those stars fall into the round glob you were talking about. Residual spin pulls the new galaxy into a disk. Repeat.

Our milky way and the Andromeda galaxies are super giants that have merged with other galaxies many times. (My pet theory is all the globular clusters floating around our galaxy are the old nuclei of all the galaxies the milky way has eaten.) The Milky Way and the Andromeda galaxies are falling into each other even as I write this answer.