Figure 13-8 A Map of the Galaxy (a) This map, based on radio telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in a face-on view of the Galaxy. This view just hints at spiral structure. Details in the large, blank, wedge-shaped region toward the upper left of the map are unknown because gas in this part of the sky is moving perpendicular to our line of sight and thus does not exhibit a detectable Doppler shift. Inset: This drawing, based on visible-light data, shows that our solar system lies between the two major arms of the Milky Way Galaxy. (b) This drawing labels the spiral arms in the Milky Way.