Peculiar velocity

As peculiar velocity (Latin peculiaris = peculiar ) the speed of the movement of a star is called relative to a group of similar objects in astronomy. As a reference motion while used, for example the average motion of all the stars of a star cluster or the rotation around the galactic center.

These processes, however, are actually doing under the term peculiar motion, and peculiar velocity as a special case of this is commonly referred to. And although the term usually refers to random velocity movement, or their component in the radial direction ( Star - observer ). Such motion causes a Doppler shift of the light ( and radiation in general), the ( general or a shift to longer wavelengths ) makes up, together with the effect caused by the Hubble redshift, the observed overall shift of the spectrum.

Since the shift due to the Hubble effect is a measure of distance to a radiation source, the displacement due to the peculiar velocity of the object a source of error in the distance measurement dar. this case, the relative error with increasing distance of the object decreases because according to the Hubble relation

( with: underlying rate of an equally large Doppler shift: Hubble constant and: Distance )

Is the displacement caused by the Hubble greater effect, whereas the left depending on the random velocity unchanged. However, If the peculiar velocity, and not the distance to be determined, it follows from this relation that only at relatively small distances reliable radial velocity measurements are possible.

In addition, you can improve the measurement accuracy by measuring the displacements of several mutually spatially close objects, and then forms an average of these measurements. Since the Pekuliarbewegungen of the object are caused by variations in the density of the surrounding space, such as other galaxies, assuming that the local density fluctuations are uniformly distributed on larger scales, to expect a trend towards abolition of the individual errors.

• Celestial mechanics