AstroGlossary - R
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radial velocity
The line-of-sight velocity of a star or other celestial object towards or away from an observer. Astronomers can calculate the radial velocity of a celestial objects by recording a spectrum with a spectrograph and measuring the Doppler shift of several spectral lines. Measurements of radial velocity have revealed how our Solar System moves through space and orbits the center of our galaxy. Astronomers have also used radial velocity measurements to observe that most objects beyond our galaxy travel away from us at speeds that increase with the object's distance. radio galaxy
A particular type of active galaxy that emits more light at radio wavelengths than at visible wavelengths, also known as a radio-luminous galaxy or radio-loud galaxy. Radio galaxies are driven by non-thermal emission. Radio telescopes show that some radio galaxies, called extended radio galaxies, have lobes of radio emission extending millions of light-years from their nuclei. Centaurus A is a nearby example of an extended radio galaxy that features two outer lobes 650,000 and 1,350,000 light-years in diameter. In contrast, compact radio galaxies emit radio lobes not much larger than the galactic nucleus. radio radiation
A very unenergetic wavelength, or frequency, of light. Radio waves are the least energetic form of radiation known. They have wavelengths longer than 10^-3 meters and frequencies less than 10^11 Hz. Radio waves are not harmful to life because they are not strong enough to ionize atoms or destroy cells. While the Earth's atmosphere shields us from some radio radiation, it does allow radio waves in the vicinity of VHF, UHF, and FM frequencies to pass through. Astronomers study waves that pass through this "radio window" with large radio telescopes or antennas, which resemble giant satellite dishes. Often, many radio antennas are coordinated together to synthesize even bigger telescopes, such as the Very Large Array in New Mexico. In addition to astronomy, radio waves have many useful applications on Earth, such as television and radio broadcasts and RADAR. radio telescope
A type of telescope that gathers and focuses radio wavelength light. Radio telescopes are huge dish-shaped antennae. Because of the long wavelengths of radio radiation, it is possible, and quite useful, to use many radio telescopes in different locations together in arrays. By combining telescopes in this way, astronomers are able to study celestial objects with much higher resolution than they could with a single antenna. An example of a radio telescope array is the Very Large Array, or VLA, which lies 80 kilometers west of Soccoro, New Mexico. VLA consists of 27 radio telescopes that are arranged in a "Y" pattern; each antenna has a parabolic dish shape and measures 25 meters across. Radio telescopes can probe the Galaxy and the universe where optical telescopes cannot. Astronomers have used radio telescopes to map the hydrogen content of our galaxy and to discover structures and processes in other galaxies that are invisible to optical telescopes. red giant
A state of stellar evolution beyond the main-sequence life of a star. A red giant core is degenerate ionized helium, surrounded by a shell of hydrogen fusion, that expands the outer atmosphere in response to higher core temperatures. The hydrogen fusing shell eats through the surrounding atmosphere and deposits helium onto the shrinking core. The ballooning atmosphere cools and glows red; hence red giant. The Sun will become a red giant the size of Earth's orbit in five to six billion years. Once the helium core reaches 100 million degrees, it explosively begins fusing helium. The birth of the active helium core is called the helium flash. The Sun as a red giant will fuse helium for about 2 billion years after the helium flash. red shift
A displacement of emission or absorption line patterns toward the red end of the spectrum as a result of the Doppler effect. As a star travels away from an observer, the wavelength of the star light increases. The observer sees the star as "redder" than the same star at rest, and the magnitude of the shift corresponds to the velocity of the source. Astronomers use red shifts, among many other things, to study the expansion of the universe. refraction
A change in the direction of light that occurs at the boundary between two different, transparent substances. The amount of directional change depends on the incident angle of the light, the wavelength (color) of the light, and the material of which the substance is made. For example, raindrops and glass prisms refract white light into rainbows. The many colors that make up white light are each a different wavelength, and they all refract at unique angles to form a band of color in order of wavelength: red, orange, yellow, green, blue, indigo, and violet. Relativity, General
"Space tells mass how to move" while "mass tells space how to curve" -- J.A. Wheeler. Einstein created this model, which describes gravity as curvature in space-time, the four-dimensional fabric of our universe. His theory is the best model for gravity so far, and has been confirmed in experiments and observations. According to the theory, regardless of one's point of view (as measured by speed and direction), physical law and the speed of light are unchanged. This implies that measurements made in time and space are not absolute, but relative to your particular point of view or reference frame. General relativity led to concepts and theories such as black hole, parallel universes, worm holes, and space-time. Relativity, Special
Einstein's rejection of the notion that space and time are absolute, based on the observation that the speed of light is independent of the motion of an observer. No matter how fast someone runs toward you with a flashlight, the speed of the light that flashlight emits will always remain the same. From this foundation, Einstein constructed a revolutionary model of gravity and a universe full of unexpected surprises like black hole, gravity waves, time dilation, and the equivalence of mass and energy: E=Mc^2. Astronomers and astrophysicists regularly use the theoretical tools of special relativity to interpret and analyze light. resolution
The ability of an instrument, such as a telescope, a pair of binoculars, or the human eye, to distinguish between separate objects that are closely spaced on the sky. Resolution is also the ability of a spectrograph to separate wavelengths of light. The higher the resolution of a piece of equipment, the greater its ability to distinguish between two closely spaced objects or between wavelengths of light. retrograde motion
The temporary apparent backward motion of a planet in the sky, from east to west, caused by the geometry between the Earth and planet. Due to this geometry, only planets that orbit outside the orbit of the Earth are observed to have retrograde motion. right ascension
The measure in hours, minutes, and seconds of arc of the position of a celestial object east of the vernal equinox. The 360 degrees along the celestial equator are divided into 24 hours of sidereal time, just like lines of longitude divide the 360 degrees of the equator into sections east or west of the prime meridian. One hour of right ascension equals 15 degrees. Right ascension and declination together comprise a coordinate system that allows astronomers to locate objects in the sky.
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