Bodies contained in these objects are analogized to cars moving through a traffic jam in a common explanation of a theory resolving their “winding problem.” Lin and Shu’s density wave theory describes these objects, which contain the majority of HII (“H two”) regions. The intrinsic luminosity of these objects is proportional to their asymptotic rotation velocity by the Tully-Fisher relation. Both prongs of Hubble’s “tuning fork” diagram contain these (-5[1])objects, some of which are barred. The central bulges of these objects contain (-5[1])supermassive black holes. (10[1])For 10 points, (10[1])name these disk-shaped systems of stars, gas, and dark matter, exemplified by Andromeda and the Milky Way. ■END■ (10[2])