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