Astrophysics (Index)About

radial velocity

(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.) A star's radial velocity can be determined by measuring the Doppler shift of known spectral lines (spectral radial velocity), which can be used to identify the presence of extra-solar planets and/or binary stars. Observatories with instruments designed to determine radial velocities are sometimes referred to as RV observatories (radial velocity observatories).

Measuring changes in radial velocity over time is helpful for detecting planets with small (a day or few days) or medium (1-2 AU) orbits (the radial velocity method or RV method of exoplanet detection), 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.

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, peculiar velocity 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)
data cube
velocity dispersion (σ)
Doppler shift
double-line spectroscopic binary (SB2)
emission line
extra-solar planet
55 Cancri e (55 Cnc e)
51 Pegasi b (51 Peg b)
Galactic All-sky Survey (GASS)
General Catalogue of Stellar Radial Velocities (GCRV)
gravitational redshift
great debate
HD 80606 b
hot Jupiter (HJ)
K correction
Keck Observatory
Kepler radius
kinematic distance
Keck Planet Finder (KPF)
line broadening
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)
proper motion (PM)
Proxima b
redshift (z)
radial velocity method
Scholz's Star
Sloan Digital Sky Survey (SDSS)
spectral line
spectroscopic binary (SB)
stellar mass determination
stellar rotation
telluric line
Telescopio Nazionale Galileo (TNG)
virial theorem
water-ice planet