radar Acronym for RAdio Detection And Ranging. Radio waves are bounced off an object, and the time at which the echo is received indicates its distance.

radial motion Motion along a particular line of sight, which induces apparent changes in the wavelength (or frequency) of radiation received.

radiation A way in which energy is transferred from place to place in the form of a wave. Light is a form of electromagnetic radiation.

radiation darkening The effect of chemical reactions that result when high-energy particles strike the icy surfaces of objects in the outer solar system. The reactions lead to a build-up of a dark layer of material.

radiation-dominated universe Early epoch in the universe, when the density of radiation in the cosmos exceeded the density of matter.

radiation zone Region of the Sun's interior where extremely high temperatures guarantee that the gas is completely ionized. Photons are only occasionally diverted by electrons, and travel through this region with relative ease.

radio Region of the electromagnetic spectrum corresponding to radiation of the longest wavelengths.

radio galaxy Type of active galaxy that emits most of its energy in the form of long-wavelength radiation.

radio lobe Roundish region of radio-emitting gas, lying well beyond the center of a radio galaxy.

radio telescope Large instrument designed to detect radiation from space in radio wavelengths.

radioactivity The release of energy by rare, heavy elements when their nuclei decay into lighter nuclei.

radius-luminosity-temperature relation A mathematical proportionality, arising from simple geometry and Stefan's law, which allows astronomers to indirectly determine the radius of a star once its luminosity and temperature are known.

red dwarf Small, cool faint star at the lower-right end of the main sequence on the Hertzsprung–Russell diagram.

red giant A giant star whose surface temperature is relatively low, so that it glows with a red color.

red giant branch The section of the evolutionary track of a star corresponding to intense hydrogen shell burning, which drives a steady expansion and cooling of the outer envelope of the star. As the star gets larger in radius and its surface temperature cools, it becomes a red giant.

red giant region The upper-right-hand corner of the Hertzsprung–Russell diagram, where red-giant stars are found.

red shift Motion-induced change in the wavelength of light emitted from a source moving away from us. The relative recessional motion causes the wave to have an observed wavelength longer (and hence redder) than it would if it were not moving.

red supergiant An extremely luminous red star. Often found on the asymptotic giant branch of the H-R diagram.

reddening Dimming of starlight by interstellar matter, which tends to scatter higher-frequency (blue) components of the radiation more efficiently than the lower-frequency (red) components.

reflecting telescope A telescope which uses a mirror to gather and focus light from a distant object.

refracting telescope A telescope which uses a lens to gather and focus light from a distant object.

refraction The tendency of a wave to bend as it passes from one transparent medium to another.

residual cap Portion of Martian polar ice caps that remains permanently frozen, undergoing no seasonal variations.

retrograde motion Backward, westward loop traced out by a planet with respect to the fixed stars.

revolution Orbital motion of one body about another, such as the Earth about the Sun.

right ascension Celestial coordinate used to measure longitude on the celestial sphere. The zero point is the position of the Sun on the vernal equinox.

rille A ditch on the surface of the Moon where molten lava flowed in the past.

ringlet Narrow region in Saturn's planetary ring system where the density of ring particles is high. Voyager discovered that the rings visible from Earth are actually composed of tens of thousands of ringlets.

Roche limit Often called the tidal stability limit, the Roche limit gives the distance from a planet at which the tidal force, due to the planet, between adjacent objects exceeds their mutual attraction. Objects within this limit are unlikely to accumulate into larger objects. The rings of Saturn occupy the region within Saturn's Roche limit.

Roche lobes An imaginary surface around a star. Each star in a binary system can be pictured as being surrounded by a tear-shaped zone of gravitational influence, the Roche lobe. Any material within the Roche lobe of a star can be considered to be part of that star. During evolution, one member of the binary star can expand so that it overflows its own Roche lobe, and begins to transfer matter onto the other star.

rotation Spinning motion of a body about an axis.

rotation curve Plot of the orbital speed of disk material in a galaxy against its distance from the galactic center. Analysis of rotation curves of spiral galaxies indicates the existence of dark matter.

RR Lyrae star Variable star whose luminosity changes in a characteristic way. All RR Lyrae stars have more or less the same period.

runaway greenhouse effect A process in which the heating of a planet leads to an increase in its atmosphere's ability to retain heat and thus to further heating, quickly causing extreme changes in the temperature of the surface and the composition of the atmosphere.

runoff channel River-like surface feature on Mars, evidence that liquid water once existed there in great quantities. Runoff channels are found in the southern highlands, and are thought to have been formed by water that flowed nearly 4 billion years ago.