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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 belts Zones or belts of charged particles that are trapped in magnetic fields around the Earth. [More Info]

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

radiation pressure The transfer of momentum carried by electromagnetic radiation to a body that the radiation impinges upon.

radio Region of the electromagnetic spectrum corresponding to radiation of the longest wavelengths. [More Info]

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 Stefan's Law, which allows astronomers to indirectly determine the radius of a star once its luminosity and temperature are known.

reaction wheel Wheels on the spacecraft which change the spacecraft attitude.

red dwarfs Small, cool faint stars at the lower-right end of the main sequence on the H-R diagram, whose color and size give them their name.

red giant star An evolved star that has exhausted the hydrogen fuel in its core and is powered by nuclear reactions in a hot shell around the stellar core. The diameter of a red giant is much larger than that of the Sun, and its surface temperature is relatively low, so that it glows with a red color. [More Info: Field Guide]

red-giant branch The section of the evolutionary track of a star that corresponds to continued heating from rapid 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 shift 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. The cosmological red shift is caused by the stretching of space as the universe expands. [More Info: Photo Album]

red supergiant An extremely luminous and large red star.

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

reflecting telescope A telescope which uses a carefully designed 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.

relativistic particle A particle moving at nearly the speed of light.

relativity, general theory A theory formulated by Einstein that describes how a gravitational field can by replaced by a curvature of space-time.

relativity, special theory 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.

resolution In astronomy, "resolution" or "resolving power" refers to the ability of a telescope to distinguish details. "Angular resolution" refers to the ability to distinguish details in an image. For example, Chandra can distinguish details that are only half an arc second apart. If your eyes had similar resolving power, you could read the letters on a stop sign at a distance of 12 miles! "energy resolution" refers to the ability to distinguish the energies or wavelengths of photons. In visible light, this amounts to the ability to distinguish different colors. When Chandra makes an observation with the transmission gratings in place, it can distinguish thousands of different X-ray energies or colors.

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. [More Info: Field Guide]

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

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