Quasar;Object in the universe that emits huge quantities of radiation and appears very small when seen from the Earth, due to its distance from us.
Radar;Emitted radio waves that are reflected by a solid object. RADAR is an acronym for RAdio Detection And Ranging.
Radial velocity;Velocity at which a celestial body moves towards or away from the Sun.
Radian;Measurement for plane angles.
Radiant;Point in the celestial sphere from where the meteors of a meteor shower appear to come from.
Radiation;Energy in the form of waves or particles that escapes into space.
Radiative zone;Zone where energy is emitted in the form of radiation or particles that escape into space.
Radio astronomy;Comparatively young discipline of astronomy, studying the radio waves emitted by objects in the universe.
Radio telescope;Instrument for the observation of radio waves emitted by celestial objects.
Radio waves;Long-wave electromagnetic radiation.
Recoil;Force exercised on a body when it expels a mass.
Red dwarf;General term used to describe stars that have a smaller mass than our Sun.
Red giant;Expanding star.
Red shift;Phenomenon of a shift towards longer wavelengths observed in the lines of spectra of celestial objects.
Reflecting nebula;Huge cloud of gas and dust that reflects the light of nearby stars.
Reflecting telescope;Telescope design where the incoming light is initially focused by a concave primary mirror and then reflected to the eyepiece by a secondary mirror located at the focal point.
Reflector;Telescope where light is focused by means of a mirror.
Refractor;Telescope where light is focused by means of a lens system.
Relativity;Theory in physics, first proposed by Albert Einstein.
Resolution;Ability of an optical system to represent two points located near each other as two separate points.
Right ascension;Projection of the geographical longitudes onto the celestial sphere.
Rockets;Spacecraft propelled by the expulsion of combustion gases.
Rotation;Spinning motion of a celestial body about its own axis.
Satellite;Small body orbiting a planet.
Saturn V rocket;Larger carrier rocket of the Apollo missions to the Moon.
Schiaparelli;Schiaparelli, Giovanni Virginio, 1835-1910, Italian astronomer.
Schmidt telescope;Reflecting telescope designed to take pictures of large stellar fields correcting the optical aberration of the coma.
Scintillation;Twinkling of a small source of light, caused by the refraction of light in turbulent air currents of the atmosphere.
Seasons;The four equal periods of the tropical year.
Seyfert galaxies;Galaxy with extremely small but brilliant nucleus and inconspicuous spiral arms.
Signs of the zodiac;Term used to describe the constellations of the zodiac, each extending over 30░.
Silicate rock;Large group of minerals containing silicon and oxygen, normally combined with metals
Silicates;Large group of minerals containing silicon and oxygen, normally combined with metals.
Singularity;Point in a physical field where the field strength is unlimited.
Solar eclipse;When the Moon is located directly between the Sun and Earth, it covers the disc of the Sun, due to the fact that both objects have the same diameter when seen from the Earth. Eclipses can be total, partial or annular.
Solar spectrum;The spectrum of the light emitted by the Sun corresponds to that of the spectral class G2V.
Solar system;Term used to describe all objects that orbit the Sun, including the Sun itself.
Solar wind;Jet of charged particles emitted by the Sun.
Solstice;Most northern or southern position of the Sun in the celestial sphere, marking the beginning of Winter and Summer respectively.
Spacecraft;Craft designed to travel through space.
Spectroscope;Instrument for the measuring of the wavelength of electromagnetic radiation.
Spectrum;When light is split up into its individual colors, dark lines called Fraunhofer lines appear among bands of colored light.
Speed of light;Speed of light in a vacuum: approx. 300,000 km/s.
Spicules;Typical phenomena at the centers of activity in the atmosphere of the Sun.
Spiral galaxy;Spiral shaped galaxy with a high density of stars in its middle.
Standard candle;Term referring to objects of which the exact brightness is known.
Star;Object consisting of hydrogen and helium. Compacting of these elements eventually results in nuclear fusion reactions.
Star motion;Slow apparent movement of the stars in the night sky.
Star rotation; Slow apparent movement of the stars in the night sky.
Static mass;Mass of a body that is not accelerated.
Sumerians;Ancient people of Sumer in Mesopotamia during the 3rd and 4th millennium BC.
Sun;Star around which the Earth orbits. The Sun is a hot, luminous gaseous sphere.
Sunspot;Dark spot in the photosphere of the Sun. Dark spots have extremely strong magnetic fields.
Sunspot cycle;Sunspots are areas on the surface of the Sun that appear darker than their immediate surroundings because of their lower temperature.
Supergiant;Large, massive and extremely luminous star of low density.
Supernova;Stellar explosion, during which the star reaches a brightness that is several thousand times greater than that of a nova.
Synodic period;Interval for an outer planet to return to the same position as seen from Earth.
