### velocity dispersion

**(σ, dispersion)**
(statistical dispersion of velocities of a group of objects)

A **velocity dispersion** (σ) is the statistical dispersion
of velocities of a group of objects about the mean velocity.
It is a quality of interest in open clusters,
globular clusters, galaxies, galaxy clusters, and
superclusters. Radial velocity can be determined by
Doppler shift and the width of the effected spectral lines can
determine dispersion across a group, which, with the virial theorem,
can determine the group's mass.

**Statistical dispersion** characterizes the "spread".
The **standard deviation** of a sample set is one measure
of statistical dispersion.

The term **dispersion** (or **chromatic dispersion**) is also used in
optics and spectroscopy (or any type of wave such as sound waves)
to indicate waves' differences in velocity through a medium when
the velocity depends upon the frequency. This is used by
refractive spectroscopes and
also is the source of chromatic aberration in lenses.

(*measure,statistics*)
http://en.wikipedia.org/wiki/Velocity_dispersion

**Referenced by:**

broad emission line region

Faber-Jackson relation (FJR)

fast radio burst (FRB)

fundamental plane

protoplanetary disk (PPD)

Seyfert galaxy

velocity-metallicity relation

virial parameter

virial theorem

index