Briefly describe how we can use the orbital characteristics of stars at many distances from the galactic center to determine the distribution of mass in the Milky Way

Question

What will be an ideal response?

Answer 1

Using the orbital velocity law, related to Newton's version of Kepler's third law, we can determine the mass of the galaxy that lies within a particular orbit (and thus distance from the center) if we know the average orbital velocity of stars or clouds at that distance. Therefore, by applying the orbital velocity law to the orbits of stars at many distances from the galactic center, we can determine how much mass lies within each radius. We often use the 21-cm line from atomic hydrogen to measure velocities of gas clouds because light at radio wavelengths can penetrate the dust that would normally obscure our vision in other wavelengths. Thus, we can measure orbital velocities of gas clouds wherever the gas is located in the galaxy.