What measurement technique uses the Doppler effect to trace distant gas clouds?

Radial velocity

To determine the three-dimensional positions of gas clouds and stellar components for mapping the Milky Way, astronomers must measure not only their direction from Earth but also their speed relative to us. The velocity of gas clouds moving either toward or away from the observer is specifically termed radial velocity. This measurement is derived using the Doppler effect, which manifests as a shift in the observed wavelength of the emitted radiation (like the 21-centimeter radio wave from hydrogen). By knowing the velocity component (radial velocity) alongside the geometric position derived from other means, scientists can accurately plot the cloud's location relative to the Sun, revealing the underlying organized structure of the galaxy, such as the spiral arms.