Radial Velocity and Doppler Measurement

Radial velocity measurement uses the Doppler shift of spectral lines to determine how fast an object moves toward or away from the observer along the line of sight.

Definition

Radial velocity is the component of an object's velocity along the observer's line of sight, measured from the Doppler shift of its spectral lines relative to their rest wavelengths.

Scope

This topic covers the measurement of line-of-sight velocities from spectral line shifts, including cross-correlation against template spectra, the precision techniques used to detect the tiny reflex motions induced by exoplanets, and the determination of galaxy and quasar redshifts. It addresses barycentric correction and the wavelength-reference methods that enable high precision.

Core questions

• How is a Doppler shift converted into a line-of-sight velocity?
• How does cross-correlation with a template spectrum yield a precise velocity?
• What instrumental and reference techniques enable metre-per-second precision for exoplanet detection?
• How are redshifts of galaxies and quasars measured and corrected to a standard frame?

Key theories

Cross-correlation velocity measurement

Shifting a template spectrum against an observed spectrum and finding the offset that maximizes their correlation gives a precise radial velocity even for noisy or composite spectra.

Doppler reflex method for exoplanets

A planet causes its host star to orbit their common center of mass, producing a periodic radial-velocity signal whose amplitude and period reveal the planet's mass and orbit.

Clinical relevance

Radial velocities reveal binary-star orbits and stellar masses, the reflex motions that have discovered hundreds of exoplanets, the dynamics of star clusters and galaxies, and the redshifts that map large-scale structure and cosmic expansion.

History

Doppler shifts were first used for stellar motions in the nineteenth century; cross-correlation methods systematized galaxy redshift surveys, and stabilized spectrographs with precise wavelength references later achieved the precision that detected the first exoplanet around a solar-type star.

Seminal works

• mayorQueloz1995
• tonryDavis1979
• gray2005

Frequently asked questions

Can radial velocity measure motion across the sky?

No; the Doppler shift only senses motion toward or away from the observer along the line of sight. Motion across the sky is measured separately by astrometry as proper motion.

Why is barycentric correction needed?

Earth's orbital and rotational motion adds its own Doppler shift; correcting to the solar-system barycenter removes this so the measured velocity reflects the object's true motion.