The skeleton of the Milky Way

Our home galaxy, the Milky Way, is a typical barred spiral galaxy, a flattened disk of about a hundred billion stars, gas, and dust that is roughly one hundred thousand light-years in diameter. The galactic disk is surrounded by a large spherical, diffuse halo roughly about five hundred thousand light-years in diameter. Although it is our home, many fundamental questions remain about the Milky Way's structure. For instance, how many major spiral arms does it have, two or four? What are the preci