Peculiar velocity
Encyclopedia
Peculiar motion or peculiar velocity refers to the true velocity of an object, relative to a rest frame.
, the term peculiar motion (or peculiar velocity) refers to the motion of an object (usually a star) through space.
Local objects are usually related in terms of proper motion
and radial velocity
, that is the amount an object appears to move across the sphere of the heavens, and the measured speed at which it travels towards or away from us, respectively. Its actual motion is the peculiar motion.
, the term peculiar velocity (or peculiar motion) refers to the components of a receding galaxy
's velocity that cannot be explained by Hubble's law
.
According to Hubble, and as verified by many astronomers, a galaxy is receding from us at a speed proportional to its distance. The relationship between speed and distance would be exact in the absence of other effects.
Galaxies are not distributed evenly throughout observable space, but typically found in groups or clusters
, ranging in size from fewer than a dozen to several thousands. All these nearby galaxies have a gravitational effect, to the extent that the original galaxy can have a velocity of over 1,000 km/s
in an apparently random direction. This velocity will therefore add, or subtract, from the radial velocity that one would expect from Hubble's law.
The main consequence is that, in determining the distance of a single galaxy, a possible error must be assumed. This error becomes smaller, relative to the total speed, as the distance increases.
A more accurate estimate can be made by taking the average velocity of a group of galaxies: the peculiar velocities, assumed to be essentially random, will cancel each other, leaving a much more accurate measurement.
Galactic astronomy
In galactic astronomyGalactic astronomy
Galactic astronomy is the study of our own Milky Way galaxy and all its contents. This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies....
, the term peculiar motion (or peculiar velocity) refers to the motion of an object (usually a star) through space.
Local objects are usually related in terms of proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...
and radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
, that is the amount an object appears to move across the sphere of the heavens, and the measured speed at which it travels towards or away from us, respectively. Its actual motion is the peculiar motion.
Cosmology
In physical cosmologyPhysical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
, the term peculiar velocity (or peculiar motion) refers to the components of a receding galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
's velocity that cannot be explained by Hubble's law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
.
According to Hubble, and as verified by many astronomers, a galaxy is receding from us at a speed proportional to its distance. The relationship between speed and distance would be exact in the absence of other effects.
Galaxies are not distributed evenly throughout observable space, but typically found in groups or clusters
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
, ranging in size from fewer than a dozen to several thousands. All these nearby galaxies have a gravitational effect, to the extent that the original galaxy can have a velocity of over 1,000 km/s
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
in an apparently random direction. This velocity will therefore add, or subtract, from the radial velocity that one would expect from Hubble's law.
The main consequence is that, in determining the distance of a single galaxy, a possible error must be assumed. This error becomes smaller, relative to the total speed, as the distance increases.
A more accurate estimate can be made by taking the average velocity of a group of galaxies: the peculiar velocities, assumed to be essentially random, will cancel each other, leaving a much more accurate measurement.