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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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher
The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".
