How do we know what shape the Milky Way is?
Dear Straight Dope:
Every illustration of our galaxy that I've ever seen (even those novelty "You Are Here" shirts, complete with helpful arrow) depicts pretty much the same swirling mass of stars. What I want to know is: how do they know that? It's been only recently that any human vessel has left our solar system, let alone the galaxy, so they certainly couldn't have taken any accurate pictures of it. Even with a good telescope, I can't see enough stars to form that recognizable "Milky Way" shape, either. When I asked my astronomy professor, he mumbled something about radio telescopy, then tried to drop some numbers with a bunch of exponents on me; his darting eyes, cold sweat and speedy exit led me to believe that he really didn't know either. So international conspiracies aside, why does everyone agree that our galaxy has that galactic shape?
We can see the Milky Way directly, but as you realize, we're not even close to being able to see it from the outside, so we can't just take a snapshot to find the overall shape. Most of the pictures you see on T-shirts, posters and the like are just artists' conceptions.
That's all very well, but where did the artists get those conceptions? In other words, how does one find the shape of a galaxy, from the inside? The most straightforward way would seem to be to find the distances and directions to various parts of the galaxy, and build a three-dimensional model from that. Directions, of course, are easy, and there are a variety of methods one can use to find the distance (see my earlier Staff Report on stellar distances). Of course, if it were that simple, I wouldn't be bothering to write this. The catch is that there's a lot of dust in our galaxy, which tends to obscure the more distant parts, making it hard to measure anything about them. So what can we do?
Well, for starters, we can look up. Even without a telescope, on a clear, dark night you can see a pale band of light across the sky, formed by the billions of unresolved stars of our galaxy. Since it's shaped like a band (rather than covering the whole sky), we can conclude that the Milky Way is more or less flat. Further, if you look in the direction of the constellation Sagittarius, you'll notice that the band is thicker in that area, so we know that the galaxy has some sort of bulge. If it's a good enough night, you can also see darker streaks through the light band. Those are the dust lanes mentioned earlier.
That's all simple enough, but how do we go from "flat with a bulge" to a spiral? Again, we look up, but this time, at other galaxies, not our own. With any decent-sized telescope, such as one an amateur might use, you can see a great variety of galaxy shapes. Some are spirals, some are elliptical clumps, and some don't seem to have any particular shape whatever. What's significant for our purposes is that all the galaxies that appear to be flat with a bulge are spirals--a good example is our nearest major neighbor, the Andromeda galaxy. Furthermore, spirals also all have dust, which isn't common to most of the other types. It's pretty safe to assume, then, that our galaxy, flattened with a bulge and full of dust, is also a spiral. We can improve on that a bit by seeing which spiral galaxies match ours in terms of things that we can measure, such as density of stars.
The story's not over yet, though. As recently as 1991, Leo Blitz and David N. Spergel, using infrared observations, were able to detect a straight bar of stars across the center of the Milky Way (infrared isn't as strongly affected by dust as is visible light). This isn't uncommon among spiral galaxies (you can see a nice bar in the galaxy NGC 1365, for example), but prior to 1991, nobody knew that we had one, too. Since then, similar observations have been made of other parts of the galaxy, but we're far from knowing the shape completely.
L. Blitz and D. N. Spergel, ApJ 379, 631-631, 1991