How do astronomers measure the rotation of the Milky Way Galaxy?

Astronomers measure the rotation of the Milky Way Galaxy by analyzing the motions of stars and neutral hydrogen gas within the galactic disk.

For stellar populations, researchers study the radial velocities and proper motions of stars. In the 1920s, Jan H. Oort demonstrated that the differential rotation of the disk causes a systematic variation in these velocities based on galactic longitude, which can be calculated using specific mathematical expressions.1

To map the rotation of the broader galaxy, astronomers observe neutral hydrogen gas. By measuring the Doppler effect on the 21-cm spectral line of atomic hydrogen, they determine the velocities of gas clouds at various distances from the galactic centre. These measurements allow for the construction of a rotation curve, which relates circular velocity to distance. This data is essential for understanding the galaxy's dynamics and provides evidence for the presence of dark matter, as the observed velocities remain high in the outer regions of the galaxy.23