Hi,

We can get some idea of the possibilities of galaxy shapes by looking at other galaxies, although getting to that point was not easy - In 1920 people were still debating about the nature of those funny little things in the sky we now call galaxies. Were they a part of our galaxy, or something outside it? Eventually that was resolved we now know other galaxies are very, very far away (as opposed to stars in our galaxy which are merely very far away :-))

As for observations of our galaxy, the first thing we can tell is that the visible Milky Way is mostly flat, because it forms a relatively narrow band in the sky. So that suggests it is disk shape rather than an irregular cloud. Next, astronomers can measure the distances and velocities of stars and dust in our galaxy. It turns out that these are consistant with motions around a center and that they seem to fall into "arms" . That suggests we have a spiral galaxy like some of the other spiral galaxies we can see in the sky. There is more about that at the Astronomy Cafe: http://www.astronomycafe.net/qadir/q2213.html

Terry