How do astronomers use Doppler shifts to measure the Milky Way Galaxy’s rotation?

Astronomers measure the rotation of the Milky Way Galaxy by observing Doppler shifts in electromagnetic radiation emitted by interstellar matter.1

Because the Galaxy rotates at different rates depending on the distance from the galactic center, astronomers analyze the radial velocities of objects to map these motions. Specifically, they measure the Doppler shifts—the change in frequency of waves caused by relative motion—in the 21-cm emission line of neutral hydrogen and in millimeter-wavelength spectral lines from molecules such as carbon monoxide.231

By interpreting these shifts, astronomers can determine the orbital velocities of gas and stars at various distances from the center. Historically, this method was pioneered by Jan H. Oort, who used the radial velocities of stars to demonstrate systematic variations in motion, confirming the Galaxy's differential rotation. These measurements are critical for understanding galactic dynamics and have provided evidence for the existence of dark matter, as rotational velocities at great distances from the center do not decrease as expected based on visible mass alone.21