How do we know what the Milky Way looks like?

You're right, it's really difficult to deduce the shape of the Milky Way, the main reason of course being that we're in it! On the other hand, astronomers can map the distribution of the stars and motion of them really easily. This research indicates that we're orbiting a 'central bulge' , this means the planets and stars are distributed along a thin plane. The most convincing evidence for the structure of the Milky Way comes from the studies of gas clouds - here they measure the velocities and positions of these clouds. So although we know we have spiral clouds - we have no idea how many spiral arms there are!

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