Figure 13-18 The Galaxy’s Rotation Curve The blue curve shows the orbital speeds of stars and gas in the disk of the Galaxy out to a distance of 60,000 ly (shown as 18,000 pc) from the galactic center. (Very few stars are found beyond this distance.) The dashed purple curve indicates how this orbital speed should decline beyond the confines of most of the Galaxy’s visible mass. Because there is no such decline, there must be an abundance of invisible dark matter that extends to great distances from the galactic center.