Why are Galaxies Disk Shaped?

Andromeda Galaxy
This is M31 or most commonly known as the Andromeda Galaxy, Credit: unsplash.com/Bryan Goff

We’ve all seen a picture of a galaxy. It’s a big, spinning disk with spiraling arms that wrap around a bright core.

Why are galaxies so flat and disk-shaped?

You would be right to think that all galaxies emerge from the same basic physical laws.


To explain how galaxies become flat, let’s start at the beginning, when they are formed, and move forward from there.


Before you can make a galaxy, you need a big cloud of gas and rocks. All of this matter begins to cluster together due to gravity, until a supermassive black hole forms in the center of the galaxy.


At first, the particles in this cloud will move all over the place, but over time, they will start to move in the same direction as their black hole. As smaller groups of gas and rock start to collide, stars and star systems are formed.


If you use the center of rotation as a point of reference, some of this matter will gather around it, and in these early stages, some of it will still be moving “up” and “down.”


As millions of years go by, the matter will keep crashing into each other and forming bigger objects whose gravity will pull in more and more pieces.


When all the “up” and “down” collisions happen, the forces that make them happen will cancel each other out. This means that all the newly formed objects from the collisions will end up perfectly aligned around the center of the galaxy’s rotation.


These new objects will keep moving in the same direction as the rest of the galaxy, but they won’t move much in other directions. This happens due to the conservation of angular momentum.


Conservation of angular momentum is simply a rule for spinning objects, which basically says this object will remain spinning unless an outside force acts on it.


If gravity was left to do its thing without angular momentum, it would keep pulling the gas into a smaller and smaller area until it formed a star or planet.


But the gas clouds are so big that as the gas condenses, its spin gets fast enough to overcome gravity’s pull. Gravity is trying to pull the gas in, but centrifugal forces from the spin are trying to push it out.


In the plane of rotation, the forces of gravity and centrifugal force reach a balance. Because of this balance, the familiar disk shapes are able to form.


Centrifugal force: The outward force on an object when it is rotated. Think of a ball on the end of a string that is being spun around, or the outward movement you feel when turning a corner in a car.

Flesch Kincaid Grade Level: 8.6


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