Hi Pf, I've searched through the forums for an answer to this question and have found bits and pieces but not a full answer. Why are spiral galaxies planar (I know there are exceptions to the rule, I'm just trying to understand the why). From what I understand, it is a bit of a battle between gravity, centripetal acceleration and conservation of angular momentum. I understand these concepts but I can't figure out why this results in a planar galaxy. I was hoping someone might be able to provide a fuller explanation. Some of the more specific points I don't understand include: - if the gas is a spheroid originally, what brings the material down from the "top" of the sphere into the plane of rotation? - why does the mass start spinning in the first place? - do galaxies not collapse to a point or break apart because the gravitational and inertial forces are balanced? if not, what is happening to keep the general size constant? Thank you very much, I really appreciate any help!