They’re like spirals right? But why? Why would gravity prefer a sphere for a planet but a disk for a galaxy?
For context, this image is what got me thinking…
[The needle galaxy](https://www.reddit.com/r/interestingasfuck/comments/hdzo5h/the_needle_galaxy_is_nearly_50_million_lightyears/?utm_medium=android_app&utm_source=share )
In: Physics
In addition to the other comments: planets *were* disks of gas and dust before they coalesced into planets. That’s why most planets in a star system orbit their star(s) in roughly the same plane, as well as any asteroid belts that may be present.
You know when you’re in the car with mommy on the way to school, and she gets on the freeway? All the cars go the same direction, so they don’t run into each other. If someone got on the freeway going the wrong way, they will eventually crash into one of the thousands of cars going the right way.
[Gravity is kind of like a heavy ball on a trampoline.](https://youtu.be/MTY1Kje0yLg) When there’s a really heavy object, all the smaller objects orbit around it. They may orbit in all kinds of different directions, but there will inevitably be an orbit with more stuff in it than all the other orbits. Things that orbit in the opposite direction or perpendicular to it eventually collide with the majority of objects all moving in the same direction.
For a more eli15 explanation, check out [this video by MinutePhysics.](https://youtu.be/tmNXKqeUtJM)
Just like the planets move around the sun, the stars in a galaxy move around each galactic core, usually a massive black hole.
A majority of galaxies are or were made up of mostly gases, which can be compressed while still maintaining and transferring momentum. Their momentum keeps them from falling into the black hole, but their own gravity pulls the gases together into a flat disk. This is what gives galaxies their flat shapes. The gases eventually form the stars, and thus the stars tend to be coplanar, or rotating on the same axis.
Some galaxies, specifically those with lower gas content, are not flat, and are more spherical in shape.
Because they rotate. It’s basically the same reason the earth bulges at the equator or pizza makers toss and spin dough balls to stretch them into actual flat pizza crusts.
As they spin there’s a force that pushes them out perpendicular to the axis they rotate around (it’s called the centripetal force). This basically pulls and stretches them out into more of a disk shape.
In addition to the explanation as to why spiral galaxies are flat, it’s worth mentioning that not all galaxies are spirals. There’s actually 4 classes- barred spiral (like the milky way), spiral , elliptical and irregular.
Elliptical galaxies are the largest type of galaxy and are as the name implies elliptical in shape.
Irregular galaxies have no defined shape at all, they tend to be the smallest of the 4 types.
Latest Answers