Glossary term: Radial Velocity

Description: When astronomers observe a distant object, the radial velocity is the part of the object's motion along the observer's line of sight – taking the object directly away from or directly towards the observer. Radial velocities are measured using the Doppler effect: For astronomical objects moving towards us, spectral lines are shifted towards smaller wavelengths (blueshift); for objects moving away from us, the lines are shifted towards larger wavelengths (redshift). Radial velocity is always relative to an observer. Precision measurements often give their results in terms of radial motion relative to the center of mass of the Solar System ("barycentric"), subtracting the influence of Earth's motion around the Sun. Objects that are orbiting each other have radial velocities that vary over time. The effect is strongest when we happen to see the orbital plane edge-on. In that case, orbital motion will periodically, and by turns, take each of the objects directly away from us and directly towards us. Radial velocity measurements can be used to deduce such orbital motions even when the objects themselves are not separately visible. Since higher masses induce higher velocities, such measurements can be used to estimate the orbiting objects' masses. This has been used to discover binary stars, or exoplanets orbiting stars, and for reconstructing the masses of star clusters and whole galaxies. A systematic analysis of galaxies' distances and radial velocities led to the discovery of cosmic expansion.

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