How do we know that the Milky Way is a spiral galaxy and not some other kind of flat system of stars?
A. We know it is a kind of flat system of stars because when we look at the night sky, the Milky Way’s stars are arrayed along a rather thin band across the sky to form the familiar Milky Way. Telescopically, we can study other galaxies and we see that they come in three basic types: Spirals, Ellipticals and Irregulars. The Milky Way cannot be of the last two categories because no amount of juggling of position of the Sun in these types of galaxies would give us the kind of night sky that we see. So by the process of elimination, the Milky Way must be some kind of spiral galaxy. Further information about the Milky Way is available at the Web site below.
