QSO: Quasi-stellar object, formally a radio-quiet version of a quasar, though now radio-loud quasars are also sometimes included.
quantized: Divided into discrete parts.
quantum: A bundle of energy.
quantum fluctuation: The spontaneous formation and disappearance of virtual particles; it does not violate the law of conservation of energy because of Heisenberg’s uncertainty Principle.
quantum mechanics: The branch of 20th-century physics that describes atoms and radiation.
quantum theory: The set of theories that evolved in the early 20th century to explain atoms and radiation as limited to discrete energy levels or quanta of energy, with quantum mechanics emerging as a particular version.
quark: One of the subatomic particles of which modern theoreticians believe such elementary particles as protons and neutrons are composed. The various kinds of quarks have positive or negative charges of 1/3e or 2/3e, where e is the unit of electric charges.
quasar: A very-large-redshift object that is almost stellar (point-like) in appearance, but has a very non-stellar spectrum consisting of broad emission lines thought to be the nucleus of a galaxy with an accreting supermassive black hole.
quiescent prominence: A long-lived and relatively stationary prominence.
quiet Sun: The collection of solar phenomena that do not vary with the solar activity cycle.
radar: The acronym for radio detection and ranging, an active rather than passive radio technique in which radio signals are transmitted and their reflections received and studied.
radial velocity: The velocity of an object along a line (the radius) joining the object and the observer; the component of velocity toward or away from the observer.
radiant: The point in the sky from which all meteors in a meteor shower appear to be coming.
radiation: Electromagnetic radiation. Sometimes also particles such as protons, electrons, and helium nuclei.
radiation belts: Belts of charged particles surrounding planets.
radioactive: Having the property of spontaneously charging into another isotope or element.
radio galaxy: A galaxy that emits radio radiation orders of magnitude stronger than those from normal galaxies.
radio telescope: An antenna or set of antennas, often together with a focusing, reflecting dish, that is used to detect radio radiation from space.
radio waves: Electromagnetic radiation with wavelengths longer than about one millimeter.
red giant: A post-main-sequence stage of the lifetime of a star, the star becomes relatively luminous and cool.
reddened: See reddening.
reddening: The phenomenon by which the extinction of blue light by interstellar matter is greater than the extinction of red light so that the redder part of the continuous spectrum is relatively enhanced.
redshifted: A shift of optical wavelengths towards the red, or in all cases toward longer wavelengths.
red supergiant: Extremely luminous, cool, and large stars; a post-main-sequence phase of evolution of stars of more than about 8 solar masses.
reflecting telescope: A type of telescope that uses a mirror or mirrors to form the primary image.
reflection nebula: Interstellar gas and dust that we see because it is reflecting light from a nearby star.
refracting telescope: A type of telescope in which the primary image is formed by a lens or lenses.
refraction: The bending of electromagnetic radiation as it passes from one medium to another or between parts of a medium that has varying properties.
refractory: Having a high melting point.
relativistic: Having a speed that is such a large fraction of the speed of light that the special theory of relativity must be applied.
resolution: The ability of an optical system to distinguish detail: also called angular resolution.
red mass: The mass an abject would have if it were not moving with respect to the observer.
rest wavelength: The wavelength radiation would have if its emitter were not moving with respect to the observer.
retrograde motion: The apparent motion of the planets when they appear to move backwards (from east to west with respect to the stars from the direction that they move ordinarily.
retrograde rotation: The rotation of a moon or planet opposite to the dominant direction in which the sun rotates and the planets orbit and rotate.
revolution: The orbiting of one body around another.
revolve: To move in an orbit around another body.
rich cluster: A cluster of many galaxies.
ridges: Raised surface features on the Moon, apparently volcanic, perhaps having developed on the crust of lava lakes, as volcanic vents, or from faulting; Mercury also has ridges.
right ascension: Celestial longitude, measured eastward along the celestial equator in hours of time from the vernal equinox.
rims: The raised edges of craters.
Roche limit: The sphere for each mass inside of which blobs of gas cannot agglomerate by gravitational interaction without being torn apart by tidal forces; normally about 2˝ times the radius of a planet.
rotate: To spin on one’s own axis.
rotation: Spin on an axis.
rotational curve: A graph of the speed of rotation vs. distance from the center of a rotating object like a galaxy.
RR Lyrae variable: A short-period variable star. All RR Lyrae stars have approximately equal luminosity and so are used to determine distances.