Radial velocity

In vectorial representation of any movement in polar or spherical coordinates, the radial velocity referred to (from Latin radius " staff ", " wheel spoke " ) is the velocity component along the beam ( radius), which points from the coordinate origin to the object being viewed.

The other velocity components are time-variable angle, ie rotations, while it is in the radial speed of a translation.

In observations, the radial velocity can be inferred directly from the frequency shift of the carrier signal used for the observation (Doppler effect). Here, one observes either the change of the object reflected wave (sonar, radar ) or transformation of emitted by the object wave ( observational astronomy ), provided the initial frequency is known. A special feature of the Doppler frequency occurs when a reflected wave on the back and on the way back, so has the double value as an emitted by the object wave.

Radar and sonar

In the radar and sonar technique, the radial velocity is the measure of the transmit signal imposed on the Doppler frequency. The evaluation of the Doppler frequency is used to distinguish between moving and stationary objects ( moving target indication ) the property " move " can be recognized when a radial velocity is present; an object is moving with a tangential velocity, so it is only recognized as an idle object.

When a radar of a moving platform is operated, so both flow rates in the radial velocity, it must be taken into account both directions of movement by a cosine relationship. As a radial velocity is measured at approximately the opposite direction of movement:

Wherein V1 and V2 are the respective velocities of the radar and the reflecting object, the angles θ1 and θ2, the angle deviations of the course of the direct line of sight with each other. Have approximately the same platform and reflecting object direction of movement so changes in the expression of at least one sign, as the angle a is greater than 90 ° and therefore the cosine is negative.


Analogous to the general definition is in astronomy, the radial velocity, the velocity component of a celestial body in the direction of the line of sight of an observer. The light an object with significant radial velocity is governed by the Doppler effect:

The radial velocity of a star or any other bright, the remote object can be measured accurately by a high-resolution spectrum is received by him, and its spectral lines are compared with the spectral lines are known from laboratory tests. The derived therefrom, red or blue shift can be converted directly into a speed. In this way it is only possible to determine the speed component in the direction to the sun, not the component perpendicular to the line of sight ( tangential ).

Examples of radial velocities:

  • Andromeda galaxy M 31: -301 km / s, so it moves towards us.
  • Star Arcturus ( α Boo ): -5 km / s
  • Wild Duck Cluster M11, an open star cluster: 22 km / s

The above radial velocity for M 31 is a heliocentric radial velocity as 31 results for example from spectroscopic measurements of the blue shift of the spectral lines of M during the year. Since the sun (and thus the terrestrial observation), however, revolves at high speed around the galactic center of the Milky Way, corresponds to the heliocentric radial velocity, not the speed at which get around M31 and the Milky Way total of one another; this value is significantly lower at -114 km / s.

In many binary star systems, the orbital motion of the stars involved causes a continuous change in their radial velocities. This can be also in systems which are not optically resolvable in a telescope, find that it is a double star is ( spectroscopic binaries ). Through detailed analysis of the radial velocities estimates the ratio of the stellar masses and some orbital elements, such as eccentricity and semi-major axis can be made. The same method has been very successfully used to detect otherwise invisible planets around stars. The rotational velocities larger astronomical objects such as galaxies and galaxy clusters, can be determined by means of the different radial velocities of the material contained.