A pinwheel and a whirlpool both spin away near the Big Dipper. They’re two of the closest and most beautiful spiral galaxies in the sky. There are billions of these “island universes.” Each of them is an agglomeration of billions of stars, along with vast clouds of gas and dust. This material forms a broad, flat disk, with a bright “bulge” of stars in the middle. A spiral galaxy gets its name, though, from its spiral arms, which wrap around the bulge like streamers of water from a rotating sprinkler. But while the streamer from a sprinkler consists of a fixed set of water droplets, the “droplets” in a spiral arm change. Stars constantly move into and out of the arm. The arm remains, but the stars inside it change. That’s because a spiral arm probably is created by a “wave” in the galaxy’s disk. This wave is slightly denser than the regions outside it. As stars and gas clouds pass through the wave, they pile up inside it like cars passing through a traffic jam. This dense environment ca
