Pecular speed
In astronomy, the peculiar speed ( Latin peculiaris , peculiar ) is the speed at which a star moves relative to a group of similar objects. The mean movement of all stars in a star cluster or the rotation around the galactic center serve as a reference movement .
These processes actually come under the concept of peculiar movement , and a special case of peculiar speed is usually referred to as peculiar speed. The term peculiar speed mostly refers to a movement or its component in radial direction (direction star - observer). Such a movement causes a Doppler shift of the light (and radiation in general), which together with the redshift caused by the Hubble effect (or more generally a shift to larger wavelengths) makes up the observable overall shift of the spectrum.
Since the shift due to the Hubble effect is a measure of distance of a radiation source, the shift due to the random velocity of the object a source of error in the distance measurement is. In this case, the relative error with increasing distance decreases of the object, because after the Hubble relationship
(with : underlying speed of an equally large Doppler shift : Hubble constant and : Distance)
the displacement caused by the Hubble effect increases, whereas the one dependent on the peculiar velocity remains unchanged. However, if the peculiar speed, and not the distance, is to be determined, it follows from this relation that reliable radial speed measurements are only possible at relatively small distances.
In addition, the measurement accuracy can be improved by measuring the displacements of several objects that are spatially close to one another and then calculating an average value from these measurements. Since the peculiar movements of the object are caused by fluctuations in the density of the surrounding space, e.g. by other galaxies, assuming that the local density fluctuations are evenly distributed on larger length scales, a tendency to cancel out the individual errors can be expected.