Glosariusz - R

Radial velocity is the velocity of an object along the line of sight to the observer, which in astronomy is usually determined via Doppler spectroscopy. Positive values are used to indicate a receding object. An object such as a star can undergo changes in its radial velocity because of the gravitational perturbation of another body, or because of radial pulsations of its surface. The latter, for example, occurs with a Beta Cephei variable star.

Red-giant branch is a conspicuous trail of enlarged red stars found on the Hertzsprung–Russell diagram for a typical globular cluster. It begins at the main sequence turnoff point and extends toward the higher luminosity and lower temperature range until reaching the red-giant tip. This branch consists of older stars that have evolved away from the main sequence but have not yet initiated helium fusion in their core region.

Right ascension, in the equatorial coordinate system, is the celestial equivalent of terrestrial longitude. It divides the celestial equator into 24 hours, each of 60 minutes.

Roche limit is the distance from an astronomical object where the tidal force matches an orbiting body's gravitational self-attraction. Inside this limit, the tidal forces will cause the orbiting body to disintegrate, usually to disperse and form a ring. Outside this limit, loose material will tend to coalesce.

Rosseland optical depth (τR) is an extinction coefficient of an atmosphere, which describes the net opacity to radiation at a given depth. See optical depth.[14]

Rotational modulation causes the luminosity of a star to vary as rotation carries star spots or other localized activity across the line of sight. Examples include RS CVn and BY Dra variables.[15]