The component of velocity along the line of sight to the observer. Objects with a negative radial velocity are travelling towards the observer whereas those with a positive radial velocity are moving away.

In astronomy, radial velocities can be determined by examining the redshift of spectral lines in a star or galaxy’s spectrum. This allows astronomers to compute the distance to galaxies using the Hubble expansion law and also study the orbits of stars in binaries.

The breakdown of a star’s velocity v into the radial v_r and transverse v_T components.

Because of the expansion of the Universe almost all galaxies (except some of the very nearest) have positive radial velocities.