Why is our galaxy a disc and not a sphere?

[p h i l]

I hope this is the right place to ask this.

for a long time I wondered why our galaxy, or all galaxies for that matter, is more or less shaped like a disc. I assume this has something to do with the way gravity works. Yet I also assume that the simplest shape gravity would form is a sphere?

I know that there is a limit of mass needed to form a sphere (for stone,for example something like the earth, its a minimum diameter of 1000 km)

so is there a limit of mass needed to form a disc? is a disc just the next logical step in geometry as you increase the mass of a given object?

along with the problem posted above, the objects of our solar system also move more or less within a disc. why is that? why are the orbits of our planets aligned? why not set within a sphere?

I hope not too many questions at once ;) thanks a lot in advance!

 

[mgb_phys]

for a long time I wondered why our galaxy...is more or less shaped like a disc

Any big lump of gas and dust is likely to be rotating slightly.
As it collapses under gravity the angular momentum is conserved - this means a large thing spinning slowly becoming a smaller thing must spin more quickly.
As it spins more quickly it starts to flatten out into a disk - because stuff at the edges is pushed out.
Then it gets complicated - but basically anything that isn't in the main plane of the spin as it turns from a sphere into a disk is captured by the disk and added to it.
So whichever part of the sphere first lengthens out to become more like a disk will gradually attract all the other material and grow into a full disk.

Yet I also assume that the simplest shape gravity would form is a sphere?

There are spherical galaxies, they tend to be smaller ones and are just a group of stars and gas. Too widely distributed to pull themselves together into a smaller sphere but without enough angular momentum to spin into a disk.

I know that there is a limit of mass needed to form a sphere (for stone,for example something like the earth, its a minimum diameter of 1000 km)

That's slightly different - that's pretty much the maximum size somethign solid made out of eg. rocks can be before it's own gravity pulls it into a sphere. As the rocks are pulled together there is enough energy to melt them and you end up with a solid sphere of rock.
The density of a galaxy is so low (billions of stars but spread out into a space 100,000 lyr across) that the stars really don't feel each others gravity in any noticable amount

the objects of our solar system also move more or less within a disc. why is that? why are the orbits of our planets aligned? why not set within a sphere?

Again angular momentum, the rough sphere of gas they formed from shrank and speeded up - as the disk gets denser anything not in the plane of the disk (so anything orbiting the sun pole-pole) is likely to get hit or captured gravitationally by something in the disk and so all the other material gradually gets swept up. The same process captured all the little rocks around Saturn into it's rings for example

 

[p h i l]

ah, I see. thanks for that quick and clear answer!