The spread of velocities of stars or galaxies in a cluster. Galaxies in a cluster have individual orbital velocities about the cluster's centre of mass, which depend on the total cluster mass. By measuring the radial velocities of selected members, the velocity dispersion of the cluster can be estimated, and used to derive the cluster's mass from the virial theorem. The velocity dispersion of individual stars in a star cluster can be measured in the same way, but if the stars are unresolved (as in a distant galaxy) the velocity dispersion can be determined from Doppler broadening in the spectrum of the integrated starlight.
Subjects: Astronomy and Astrophysics.