The sun is located inside one of the spiral arms of the Milky Way galaxy, roughly two-thirds of the way from the galactic center to the outer regions. Because we are inside the galaxy, obscuration by dust and the confusion of sources along our lines-of-sight make mapping the galaxy a difficult task. Astronomers think that the galaxy is a symmetric spiral, and about 10 years ago, CfA astronomers Tom Dame and Pat Thaddeus, using millimeter observations of the gas carbon monoxide, discovered symmetric components to the spiral arms deep in the inner galaxy that lent support to this model.