So have you ever wondered why planets and stars are spherical and galaxies are disks?

Well maybe or maybe not. Hopefully if you haven't and I've asked the question now you're wondering?

So the good news is, I wasn't the first person to ask this question. Someone far smarter than me has already figured it out and this question came to me when I was in school and I never did get a clear answer. It jumped into my head the other day and I went looking for the answer.

So as with all things, it comes down to physics, 

2 Key principles.

  1. Equilibrium
  2. Symmetry

Let's start with Equilibrium. Everything tries to reach this, even building a house or a bridge. The forces in play ultimately reach it, gravity up and down etc. The idea being all the forces in play all cancel each other out at this point a shape remains fixed. Its the symmetry of the forces at play that decides the final shape.

Let's talk about Symmetry. So for things like Planets and Stars you have spherical symmetry. Ie you can turn them any which way and the shape from all angels is the same. Things like Galaxies and Solar Systems have circular symmetry, so you can rotate them on the same Axis and it remains the same shape but try tilting or flipping them and the shape changes.

But how does a force have symmetry? This honestly baffled me for a bit.

Ok so in the case of all the really big space stuff... the main force is gravity. So lets start with that. 

(here comes the maths bit)

Newton's law of universal gravitation tells us that 


where: 

  • F is the force between the masses;
  • G is the gravitational constant (6.674×10−11 N · (m/kg)2);
  • m1 is the first mass;
  • m2 is the second mass;
  • r is the distance between the centers of the masses.

Or in layman's terms the further away from an object you are the weaker the force of gravity. But here is the critical bit. It drops of equally in all directions. So Gravity has spherical symmetry. 

Ok so Gravity will in theory make a sphere, but that's not the only force at play.

So lets take Earth.

Its roughly spherical (i'm getting to why it isn't a perfect ball soon), it's held together by its own gravitational field, which is handy as it holds all of us down as well.

So the closer you are to the Earth's core the more you will feel the effect of gravity.

Lets play lego earth

Imagine each of these blocks are like lego. the top block is the crust, the next 3 are in the mantle, then the outer core and finally the inner core. The closer each of these blocks are to the middle the stronger the gravity.

Now also Imagine each of these blocks are being squeezed by gravity this will create  lot of pressure and this resists gravity and actually provides an up force as its very hard to compress depending on the material beyond a certain point.

 

This balances out with Gravity and reaches an equilibrium. so a lego block in the middle of earth would have equal force going both up and down. So you can kind of think of the planet as a bit like a web of lego blocks

Ok cool but then why do these blocks form a sphere as they could easy make a disk shape?

Well pressure does the same thing as gravity but the opposite direction. Gravity pulls towards itself equally in all directions so pressure pushes away equally in all directions

So all though out the lego bloc's are all push against each other all with equal for and cancelling each other out, we have equilibrium.

Now earth is not completely spherical due to the fact that the pressures and gravity are not the only forces acting on it. The earth spins and this spin creates a centrifugal force that tries to flatten out earth.

Infact the equator of the earth is 20Km further away from the core than the poles. This accounts as to why earth is not a perfect sphere. 

Now these rules do seem to have a minimum size requirement of about 1000km radius. Thus why Asteroids are all misshapen and moons are not.

Stars follow the same rules as planets just on a much much larger scale!

So this begged the question to me. With galaxies being so massive and the force of gravity being so huge. Why are galaxies and solar systems not round?

So galaxies have supermassive black holes in their centre. These spin very very fast and everything orbits these massively dense objects in their galaxy.

The centrifugal force of the rotation of the galaxy and the gravity work together to both spin and squash everything into a disc

Solar systems are the same with the parent star spinning rapidly before the planets are even formed.

So to summarise.

Spheres happen when pressure is the dominant force over gravity. Pressure seeks equilibrium spherically. Disks happen when Orbital motion or centrifugal force is the dominant force over gravity. It seeks equilibrium in the form of a disk.

Well done if you made it to the bottom!

Comments welcome