Dust obscures visible light from distant stars. Therefore, how can astronomers confidently measure the rotation curve of the Milky Way? Astronomers measure the motion of stars in globular clusters above the plane of the Galaxy. Astronomers use satellites to create an image of the plane of the Milky Way Galaxy face-on. Measurements are made of cold gas clouds using radio telescopes. Astronomers can measure the rotation curve of the Milky Way using only nearby visible stars. Using this technique, astronomers can create a galaxy rotation curve for the Milky Way. If visible stars were the only contributors to the mass of the Galaxy, the rotation curve would look like the figure on the left. The actual rotation curve of the Milky Way, however, looks like the figure on the right. Velocity Distance Expected Rotation Carve Velocity Distance Observed Rotation Curve What does this imply about the Milky Way? The objects used to measure the rotation curve are not part of the Milky Way. This implies that there is a significant amount of additional, unobserved matter besides the stars in the Galaxy. The measurements are unreliable.