Figure 13-10Our Galaxy As seen from the side, three major visible components of our Galaxy are a thin disk, a central bulge, and a two-part halo system. As noted earlier, there is also a central bar. The visible Galaxy’s diameter is about 100,000 ly, and the Sun is about 26,000 ly from the galactic center. The disk contains gas and dust along with Population I (young, metal-rich) stars. The halo is composed almost exclusively of Population II (old, metal-poor) stars. Inset: The visible matter in our Galaxy fills only a small volume compared to the distribution of dark matter, whose composition is presently unknown. Dark matter’s presence is felt by its gravitational effect on visible matter.