In an inadvertently amusing press release this week, the Harvard-Smithsonian Center for Astrophysics released a study calling for a four-armed model for the Milky Way, but used an illustration created by Robert Hurt six months ago to support a Spitzer study concluding that our galaxy has two spiral arms!
Just goes to show that decent illustrations of the Milky Way (and good scientific illustrators) are still in very short supply.
But is it that surprising that the two studies draw different conclusions? No.
Like the proverbial blind men describing an elephant, the two groups of scientists are examining very different parts of the Milky Way. The Spitzer study detected hot objects visible in infrared. The CfA study used radio telescopes which can also detect colder objects such as supernova remnants, very young star formation regions and huge clouds of hydrogen gas. So it seems as though the older established star formation regions are mostly concentrated in two spiral arms, but that the very new star formation regions and the hydrogen clouds from which they form are also developing in two additional arms.
If you were hovering above the Milky Way in a spacecraft using binoculars or a regular optical telescope, you would see two main arms. But if you also had a radio telescope with you, it would detect two more.
This pattern of different spiral structures at different frequencies is actually quite common in other galaxies as well. For example, see this story about M106.