 In the early days, when knowledge of spiral arms around galaxies was relatively new, astronomers thought that they were made of material and orbited the center of the galaxy just like stars, dust, and gas. Our world pool galaxy is a good model for this. But as star motion within galaxies became better understood, we found that the stars near the center have a much shorter orbital period than the stars further away from the center. For example, in our Milky Way, stars near the central bulge can complete a revolution around the central bulge in about 90 million years. Out here, 26,000 light-years away from the center, it takes 230 million years. And in the outskirts, it can take over 400 million years. It followed that if the spiral arms were rotating with the stars, its structure would be lost in just a few rotations. You can see in this simulation that even after just one revolution, the arms would change significantly, with the inner galaxy areas being stretched more than the outer galaxy areas. In astronomical terms, we would see spiral arms' structure disappear in short order. But from all we see from spiral galaxies of all ages, the spiral arms' structure does persist over billions of years. This disconnect is called the winding problem. Clearly, the spiral arms do not rotate with the stars, dust, and gas. The density wave theory was proposed by C. C. Lin and Frank Xu in the mid-1960s to explain the spiral arms' structure of spiral galaxies. The theory holds that spiral arms are not material in nature, but instead are made up of areas of greater density. The stars, gas, and dust in the galaxy move around the center in elliptical orbits. If we simply assume that an orbit's major axis shifts slightly as the distance from the center goes up, we can reconstruct the spiral arms as we see them in real galaxies. Lin and Xu, examining magnetic fields, gas distributions, and star velocities, showed that this kind of grand design spiral pattern could persist indefinitely if the pattern also rotated, but with the outer portion of the arms rotating faster than the inner portions. By their calculations, a typical pattern velocity is around .004 km per second per light year. That is, the velocity of the pattern goes up, four one-thousandths of a kilometer per second, for each light year the pattern location is from the center. For our star, 26,000 light years from the center of the Milky Way, we have a pattern velocity for the Orion spur at around 100 km per second, or 65 miles per second. Our star's velocity around the center is more than twice that much. I expect we'll be leaving the Orion spur in a few million years. Here's a look at a gas cloud orbiting around a galaxy in the spiral pattern's frame of reference. As it approaches the spiral arm, it gets compressed as the orbiting molecules within the cloud move closer and closer together. This compression effect is thought to trigger cloud collapse. This in turn creates new stars. The most massive blue stars don't burn long enough to make it through the spiral arm before they run out of hydrogen fuel and supernova. That explains why we only see these giant blue stars in the trailing edge of spiral arms. Normal yellow and lower mass red stars exist for much longer and exit the spiral arm and orbit the galaxy for billions of years. It's important to note that just how these spiral arm structures form in the first place is still not understood.