Doppler spectroscopy

Doppler spectroscopy infers an object's motion along the line of sight by measuring shifts in the wavelengths of its spectral lines.

Doppler effect: A change in observed wave frequency or wavelength caused by relative motion between a source and an observer. Spectral-line shifts are the wave-based signature that the technique converts into radial velocity.
Radial velocity method: An exoplanet detection method that infers a planet's gravitational tug from periodic changes in its star's line-of-sight velocity. It uses repeated Doppler measurements of a star to reveal an unseen orbiting planet.
Electromagnetic spectrum: The full range of electromagnetic radiation, ordered by frequency or wavelength. Doppler spectroscopy analyzes the wavelength portion of this broader radiation range.
Astrometry: The precise measurement of celestial positions and their changes on the sky. Astrometry detects transverse motion, complementary to Doppler spectroscopy's line-of-sight motion.
Radial velocity: The component of an object's velocity directed toward or away from an observer. Doppler spectroscopy measures this line-of-sight component rather than transverse motion.
Exoplanet: A planet orbiting a star other than the Sun. Periodic stellar line shifts provide one of the principal ways to detect these planets.
Atomic spectrum: The characteristic set of wavelengths emitted or absorbed by atoms as their electrons change energy states. Stable atomic features provide identifiable reference lines for measuring shifts.
Transit photometry: An exoplanet detection method that measures periodic dips in a star's brightness as a planet crosses its disk. It detects planets through blocked light rather than stellar spectral shifts.
Spectral line: A narrow wavelength feature produced when matter absorbs or emits radiation at particular energies. Its measured displacement provides the wavelength change used to infer motion.
Binary star: A system of two stars bound by their mutual gravitational attraction. Alternating spectral shifts reveal orbital motion and can expose unseen companions.

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