NOVA: Do we know what the Milky Way looks like?
Reid: The answer really is no. At this point, we do know it is a spiral galaxy, we know it has spiral patterns, but we don't really know where these spiral arms lie. There's even debate between astronomers about whether there are two or four arms. I would say within the next few years when we've been able to map the positions of young stars that trace out these arms, then we'll know exactly where the arms are, and how many there are. We have good preliminary evidence to show there are four arms. And we know a little bit more about how tightly bound or open the arms are, there's been some debate about that, and it looks like it's a fairly loosely bound spiral, but to go beyond that it'll take a lot more observations and data.
http://www.pbs.org/wgbh/nova/insidenova/2012/06/mapping-the-milky-way-from-the-inside-out.html