 The number of stars in the Milky Way is very difficult to determine, but based on detailed analysis of star distances, star motions, hydrogen radiation from spiral arms, galaxy rotation curves and mass, including dark matter, astronomers currently believe that the galaxy has a relatively flat rotating disk, 100 to 120,000 light-years wide and 1,000 light-years deep, with some 100 to 400 billion stars. This image, out of the Spitzer Science Center and the University of Wisconsin, represents an attempt to synthesize over a half century of work on the galactic disk structure. Based on data obtained from the literature, had radio, infrared, and visible light wavelengths. The galactic center itself, with the supermassive black hole that we discussed earlier, is shaped like a bar. Although most parts of the Milky Way galaxy are relatively uncrowded, roughly 10 million stars are known to orbit within just a single light-year of the galactic center in a region known as the central bold. These surveys discovered the two three kiloparsec arms named for their length. They are now generally thought to be associated with gas flow roughly parallel to the central bar. Using infrared images from Spitzer, scientists have discovered that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of the central bar. One is named Scutum Centaurus and the other is named Perseus. Each of these major arms consist of billions of young and old stars. Three thinner arms spiral out between the two giant arms. These are called Sagittarius, Norma, and the outer arm. These are primarily filled with gas and pockets of star-forming activity. There is also a spur off the Sagittarius arm called the Orion spur. There's 3,500 light-years across and approximately 10,000 light-years long. We are located on the inner edge, halfway along this spur, around 26,000 light-years from the galactic center. When we fill the space between the arms, we get the full picture. It's interesting to note that the number of stars per unit volume of space in the region between arms is the same as the number in the arms themselves. What distinguishes the arms is that they have a far greater number of younger stars. In fact, all the known H2 star-forming regions in the galaxy exist inside the arms. We don't see any in the area between the arms.