Radialvelocity

Radial velocity is the component of an object's motion that is directed along the line of sight between the object and the observer. In astronomy, it is typically expressed as a velocity, with positive values indicating recession (moving away) and negative values indicating approach (moving closer). This direction-dependent speed is inferred from the Doppler shift of spectral features.

Measurement relies on high-resolution spectroscopy. When an object moves along the line of sight, its spectral lines shift in wavelength: toward longer wavelengths (redshift) if it is receding, and toward shorter wavelengths (blueshift) if it is approaching. Modern measurements use stable spectrographs and precise wavelength calibrations, sometimes employing reference sources such as an iodine absorption cell or a laser frequency comb. Observations are further corrected for the Earth's motion around the Sun to recover the intrinsic radial velocity of the target.

The basic, non-relativistic relation between observed wavelength shift and radial velocity is Δλ/λ ≈ v_r/c, where z ≈ v_r/c for small velocities. A full relativistic treatment uses 1+z = sqrt((1+β)/(1−β)) with β = v_r/c. In practice, radial velocities for stars and planets are extracted by modeling many spectral lines and tracking their shifts over time.

Applications are wide. In exoplanet science, periodic radial-velocity signals reveal orbiting planets and allow mass estimates when combined with inclination information. Radial velocity also informs studies of binary stars, stellar and galactic kinematics, and the dynamics of star clusters and galaxies. Limitations include stellar activity, pulsations, and instrumental systematics, which can masquerade as or obscure true Doppler signals, necessitating careful calibration and long-term monitoring.