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!!