Astrophysics (Index)About

radial velocity

(RV, radial motion)
(component of velocity toward or away from observer)

Radial velocity (RV) is the component of velocity of a body on a line between the body and the observer, i.e., the movement toward or away from the observer. (The line between observer and body is a radius of the sphere surrounding the observer at the distance of the body.) The radial velocity of an astronomical object can be determined by measuring the Doppler shift of known spectral lines (yielding the spectral radial velocity), a measurement that can be carried out at any distance as long as the source is sufficiently bright. This offers a relatively easy clue to the peculiar velocity of the object, and within the Milky Way offers clues to the orbits of stars around the galactic center, and to distance (kinematic distance).

Radial velocity of stars can also be used to identify the presence of unseen binary companions or extra-solar planets, the radial velocity method (aka RV method) of exoplanet detection. The RV method is helpful for detecting planets with small (a day or few days) or medium (1-2 AU) orbits, but transits reveal more of the smaller orbits. Using the RV method requires HRS, which, in turn, requires a degree of brightness (apparent magnitude of 12 or brighter) and currently measures down to about one meter/second precision. 1/10 meter/second would be required to detect Earth. Observatories with instruments designed to do this are sometimes referred to as RV observatories (radial velocity observatories).

Radial velocity represents one component of a body's velocity relative to the observer, covering one of the three spatial dimensions. A two-dimensional velocity covering the other two dimensions (at right angles to the line of sight) is termed transverse velocity or tangential velocity. For nearby stars, proper motion offers a clue to the transverse velocity, which can be pinned down if the distance is known, e.g., using parallax. Transverse velocity does have a relativistic effect on Doppler shift (stemming from time dilation), which can significantly affect RV measurements if the magnitude of the transverse velocity is far greater than the radial velocity.

The term velocimetry is sometimes used for measurement of velocity, and radial velocimetry for the RV measurement.

Further reading:

Referenced by pages:
active galactic nucleus (AGN)
Automated Planet Finder (APF)
CfA digital speedometer
data cube
velocity dispersion (σ)
Doppler shift
double-line spectroscopic binary (SB2)
emission line
ESO 3.6m Telescope
extra-solar planet
55 Cancri e (55 Cnc e)
51 Pegasi b (51 Peg b)
galactic archaeology
Galactic All-sky Survey (GASS)
General Catalogue of Stellar Radial Velocities (GCRV)
gravitational redshift
Great Debate
H3 Survey (H3)
Hale Telescope
HD 114762
HD 80606 b
hot Jupiter (HJ)
Hubble constant (H0)
Hubble expansion
iron (Fe)
K correction
Keck Observatory
Kepler radius
kinematic distance
Keck Planet Finder (KPF)
line broadening
line tomography
Lick Observatory Calcium Line Survey (LkCl)
local standard of rest (LSR)
magma ocean
mass function
mass ratio (μ)
minimum mass (m sin i)
moving-cluster method
NGC 3201
OH/IR source
Oort constants
position-position-velocity space (PPV)
projected semi-major axis
proper motion (PM)
Proxima b
redshift (z)
radial velocity method
Scholz's Star
Sloan Digital Sky Survey (SDSS)
shell star (sh)
supernova light curve (SN light curve)
spectral line
spectral resolution
spectroscopic binary (SB)
stellar kinematics
stellar mass determination
stellar rotation
telluric line
Telescopio Nazionale Galileo (TNG)
ultra-fast outflow (UFO)
virial theorem
water-ice planet