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radar The technique of transmitting radio waves to an object and then detecting the radiation that the object reflects back to the transmitter; used to measure the distance to, and motion of, a target object.
radial velocity The component of relative velocity that lies in the fine of sight; motion toward or away from the observer.
radial velocity curve A plot of the variation of radial velocity with time for a binary or variable star.
radiant (of meteor shower) The point in the sky from which the meteors belonging to a shower seem to radiate.
radiation A mode of energy transport whereby energy is transmitted through a vacuum.
radiation pressure The transfer of momentum carried by electromagnetic radiation to a body that the radiation impinges upon.
radioactivity (radioactive decay) The process by which certain kinds of atomic nuclei naturally decompose, with the spontaneous emission of subatomic particles and gamma rays. 
radio galaxy A galaxy that emits greater amounts of radio radiation than average.
radio telescope A telescope designed to make observations in radio wavelengths.
reddening (interstellar) The reddening of starlight passing through interstellar dust, caused because dust scatters blue light more effectively than red.
red giant A large, cool star of high luminosity; a star occupying the upper right portion of the Hertzsprung-Russell diagram.
redshift A shift to longer wavelengths of light, typically a Doppler shift caused by the motion of the source away from the observer.
reflecting telescope A telescope in which the principal optical component (objective) is a concave mirror.
reflecfion nebula A relatively dense dust cloud in interstellar space that is illuminated by reflected starlight.
refracting telescope  A telescope in which the principal optical component (objective) is a lens or system of lenses. 
refraction The bending of light rays passing from one transparent medium (or a vacuum) to another.
relativistic particle (or electron) A particle (electron) moving at nearly the speed of light.
relativity A theory formulated by Einstein that describes the relations between measurements of physical phenomena by two different observers who are in relative motion at constant velocity (the special theory of relativity) or that describes how a gravitational field can be replaced by a curvature of space-time (the general theory of relativity).
resolution The degree to which fine details in an image are separated, or the smallest detail that can be discerned in an image.
resonance An orbital condition in which one object is subject to periodic gravitational perturbations by another, most commonly arising when two objects orbiting a third have periods of revolution that are simple multiples or fractions of each other.
retrograde (rotation or revolution)  Backward with respect to the common direction of motion in the solar system; counter-clock\vise as viewed from the north, and going from east to west rather than from west to east.
retrograde motion An apparent westward motion of a planet on the celestial sphere or with respect to the stars.
revolution  The motion of one body around another.
right ascension A coordinate for measuring the east-west positions of celestial bodies; the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through a body.
RR Lyrae variable One of a class of giant pulsating stars with periods less than one day.
runaway greenhouse effect A process whereby the heating of a planet leads to an increase in its atmospheric greenhouse effect and thus to further heating, thereby quickly altering the composition of its atmosphere and the temperature of its surface.
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