ASTR 103 - Astronomy

Glossary - R

radar

Technique of bouncing pulsed radio waves from a body and observing their echoes. Derived from radio detection and ranging.

radar astronomy

Field in which radio signals, pulsed at high power, are transmitted from Earth and reflected from the object of interest.

radial velocity

Component of an object's motion that lies in the line of sight, producing the Doppler shift in the spectral lines of the body.

radiant (of a meteor shower)

Place in the sky from which meteors diverge during a meteor shower. The shower is named after the constellations in which the radiant appears.

radiation belt

Zone of high intensity radiation within the magnetosphere of a planet.

radiation pressure

Pressure exerted on a body by electromagnetic radiation.

radioactive

Describing the spontaneous breakdown of atomic nuclei (normally the heaviest atoms) into lighter nuclei with the ejection of alpha particles, electrons, and gamma photons.

radioactive half-life

(see also half-life).

radio astronomy

branch of astronomy in which radio telescopes are used to observe electromagnetic radiations longer than about 1 millimeter in wavelength from celestial sources.

radio galaxy

galaxy that emits strong radio radiation.

radio telescope

highly sensitive receiving system employing a parabolic reflector or other highly directional antenna to locate and study radio sources.

ray (lunar)

bright elongated streaks radiating from certain craters on the Moon and Mercury.

reddening

see interstellar reddening.

red giant

Large cool star of high luminosity.

red-giant branch

Nearly vertical string of red giants extending from the middle to upper part of the right-hand side of the Hertzsprung-Russell diagram.

redshift

displacement of spectral lines toward the longer wavelengths arising from the Doppler shift.

reflecting telescope

 

reflection nebula

dusty gas cloud that reflects the light of nearby stars.

refracting telescope

 

refraction

change in the direction of light rays passing from one medium into another of different density.

regolith

pulverized debris on the surface of a planet produced by meteoritic bombardment of the surface material.

relativistic

pertains to an object moving at an appreciable fraction of the speed of light.

relativity

principle that postulates the equivalence of the description of the Universe, in terms of physical laws, by observers in their respective frames of reference, whether in uniform or nonuniform motion relative to each other.

resolution

 

resolving power

ability of a telescope to separate fine details in an image.

retrograde motion

apparent westerly motion of a plane as viewed from Earth, contrary to its usual easterly movement among the stars.

revolution

motion of a body around an axis external to the body.

right ascension

coordinate in the equatorial system of measurement. It measures the arc along the celestial equator from the vernal equinox eastward to the position of the celestial body. It is similar to the measurement of longitude on Earth.

rille

canyon or gorge found in the surface of the Moon or Mars.

Roche limit

the distance within which tidal forces can disrupt particles trying to adhere to each other.

rotation

turning of a body around an axis inside the body.

r process

 

RR Lyrae star

pulsating variable of the Population II cepheid group with a period less than one day.