The Milky Way Galaxy is a collection of gas, dust, and about 200 or so billion stars. It’s got three main parts: the central bulge (sometimes called the “hub”), the disk, and a spherical halo of stars. Our Galaxy is a spiral, because the disk appears to have great sweeping spiral arms.
Surprisingly, we know a lot about the disk. It’s hard to map it out because we’re in it; imagine being deep in the woods and trying to figure out what shape the forest boundary is. Still, using various different methods, astronomers have been pretty successful getting a feel for it.
The inner part is different, though. There’s 25,000 light years worth of dust, gas, stars, and assorted junk between us and it, so even seeing those stars gets to be pretty hard.
Enter the Spitzer Space Telescope. This is an observatory designed to see infrared light, invisible to the eye. You can think of the light as being redder than red, if that helps. While thick clouds of gas and dust block our view of the Galaxy’s core in visible light, infrared can pass through. So stars near the center can be seen by Spitzer.
We’ve known for a long time that our Galaxy has a bar in the center, a rectangular collection of stars. What’s different now is that this new data from Spitzer clearly show the bar, and also show it to be about 27,000 light years long, which is pretty big. It’s also very well-defined. They put together an artist’s illustration:
But what forms the bar? As the stars orbit the center, they feel a pull from the rest of the Galaxy. This can distort their orbits from a circle into truly weird shapes, including ellipses, curved diamonds (like a kite outline), and other odd paths. This effect gets amplified, and the gravitational physics works out to produce this long rectangular feature.
Most spiral galaxies have one, though it’s more pronounced in some galaxies than others. I suppose, if you care to phrase it this way, that if galaxies are cities in space, than ours just so happens to have a pretty good bar downtown.