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- Quantum:
- A fixed packet, or quantity, of some physical property such as mass or energy.
- Quantum Chromodynamics (QCD):
- A theory that explains the strong force interactions between quarks in quantum terms.
- Quantum electrodynamics (QED):
- A theory that explains electromagnetic interactions between particles in quantum terms.
- Quantum gravity:
- A still-unrealized explanation of gravity in quantum mechanical terms, including its transmission by discrete particles called gravitons. Quantum gravity is crucial to the study of the Planck Era of the early universe.
- Quantum mechanics:
- A mathematical description of the rules by which subatomic particles interact, decay, and form atomic or nuclear objects.
- Quantum Tunneling:
- A quantum effect invoked to explain the movement of subatomic particles through otherwise impenetrable force barriers, such as into or out of an atomic nucleus.
- Quark:
- A fermion that responds to the strong force and so is never found in isolation. The six quark varieties are up, down, charm, strange, top, and bottom.
- Quasar:
- A shortened from "Quasi-stellar radio source"; an extremely powerful, bright source of energy, located in a very small region at the center of a distant galaxy, that outshines the whole galaxy around it.
- Radar
- A method of identifying the location or speed of a distant object by bouncing radio waves off its surface and measuring the interval before they return; also, an instrument used for this purpose. The term is an acronym for "radio detection and ranging
".
- Radial Velocity:
- The speed of a celestial body approaching or receding in an observer's line of sight, estimated from Doppler shifts in the body's spectrum.
- Radiation:
- Energy in the form of electromagnetic waves or subatomic particles.
- Radiation belt:
- a band of charged particles trapped in a planet's magnetic field; for example, the Earth's Van Allen belts.
- Radio:
- The least energetic form of electromagnetic radiation, having the lowest frequency and the longest wavelength.
- Radio altimeter:
- An instrument that determines altitude by bouncing radio waves off the surface below and measuring the interval before they return.
- Radio astronomy:
- The observation and study of raido waves produced by astronomical phenomena.
- Radio Burst:
- An abrupt, strong increase in radio emissions from an astronomical object.
- Radio Galaxy:
- A source of radio emissions associated with an optically identified galaxy; normally an active galaxy.
- Radio Lobe:
- A vast expanse of plasma highly luminous at radio or other wavelengths and emitted by the core of an extragalactic radio source.
- Radio telescope:
- An instrument for studying astronomical objects at radio wavelengths.
- Radioisotope thermoelectric generator (RTG):
- A type of nuclear power supply, carried on many deep-space probes, in which electricity is generated by the heat released in the decay of a radioactive isotope.
- Radiometer:
- Any device that meaures the intensity of radiation.
- Receiver:
- In radio astronomy, a device for detecting and measuring radio waves collected by an antenna.
- Red Dwarf:
- A dim, long-lived, low-mass, M-class star.
- Red giant:
- An aging, low-mass star that has greatly expanded and cooled after consuming most of its core hydrogen; usually of spectral class M.
- Red Shift:
- A stretching of the apparent wavelength of light, which shifts its spectral lines toward the red end of the spectrum.A Doppler red shift is caused by the motion of a light source; a cosmological red shift, by the expansion of space betwe
en the observer and the light source; and a gravitational red shift, by the time-distorting effects ofthe gravity of massive bodies.
- Relativistic Velocity:
- motion at a significant fraction of the speed of light, in which changes in time, length and mass become noticeable to observers who are stationary in relative terms.
- Relativity:
- A set of theories that show how measurements are affected by motion and gravity. See General Relativity and Special relativity.
- Renormalization:
- A mathematical procedure in quantum physics that redefines the mass and charge of elementary particles in order to avoid certain "infinite" predictions.
- Resolution:
- The degree to which details in an image can be separated, or resolved. The resolving power of a telescope is usually proportional to the diameter of its collector.
- Resonance:
- In astronomy, the enhanced gravitational effect on a smaller body when its orbital period is an exact fraction fo a larger neighbor's.
- Retrograde Motion:
- Real or apparent motion against the prevailing direction of movement. The apparent backward motion of planets as they are overtaken by the Earth is an illusion of retrograde motion; Triton's orbit against the direction of Neptune's rotation is real.
- Retrorocket:
- A small rocket used to decelrate a larger rocket or a sattelite.
- Ring galaxy:
- A peculiar galaxy with a central core surrounded by a ring in the same plane; thought to result from the collision of a disk galaxy and another, smaller galaxy.
- Rotation:
- The turning of a celestial body about its axis.
- Roche Limit:
- The boundary within which a planet's tidal forces both prevent the formation of moons and destroy orbiting moons that stray inside the limit; named after its discoverer, French astronomer Edouard Roche.
- Sattellite:
- Any body, natural or artificial, in orbit around a planet; used most often to describe moons and spacecraft.
- Scan platform:
- On a space probe, the movable component holding cameras and other scientific equipment that needs to be aimed.
- Scarp:
- A steep, clifflike ridge, formed by faulting or by erosion of the softer rock layers of tilted formations.
- Sediment:
- Mineral particles deposited by wind or water action. Layers of sediment may consolidate to form rock.
- Seismic Waves:
- A vibration through solid rock, triggered by an earthquake or artificial means, that extends in all directions from the point of the initial disturbance.
- Seismograph
- A device for recording the strength and frequency of seismic wavs.
- Shepherd Moons:
- Small moons, sometimes paired, that graviationally influence the orbits of particles in some planetary rings.
- Shock wave:
- In astronomy, a sudden discontnuity in the flow of a gas, liquid, or plasma characterized by abrupt increases in temperature, pressure, and velocity.
- Sidereal day:
- The period of time it takes a planet to rotate once on its axis, measured from one appearance of a chosen fixed star on the meridian to the next such appearance.
- Silicate:
- A mineral based on silicon and oxygen, usually containing one or more other elements. Silicates are the primary components of most rocks.
- Singularity:
- The infitely condensed mass at the center of a black hole that has no dimensions in the physical universe.
- Skyfert galaxy:
- An active disk galaxy with a very bright, starlike nucleus; first identified by American astronomer Carl Seyfert in 1943.
- Sol
- see apparent day.
- Solar eclipse:
- The obscuring of the Sun's disk as the Moon passes directly between the Earth and the Sun.
- Solar mass:
- A stellar mass unit equal to the Sun's mass, about two thousand trillion trillion grams.
- Solar nebula:
- The disk of spinning gas and dust from which the Sun and planets formed.
- Solar System:
- The sun and its associated family of planets, asteroids, and other orbiting bodies; more generally, any star and the objects that orbit it.
- Solar wind:
- A current of charged particles that streams outward from the Sun.
- Space-time continuum:
- A four-dimensional system that incorporates theree spatial dimensions plus time.
- Space-time event:
- An object's spatial position at an assigned instant in time.
- Space time interval:
- A mathematical expression fo the distance in space and time between two points (events) on a worldline.
- Special relativity:
- A theory postulating that observers in uniform motion cannot perceive their motion and that all obervers in such motion obtain the same value for the speed of light. From these two principle the theory concludes that measures of distance, time, and ma
ss will vary depending on the motion of an obsrver moving uniformly in relation to the thing being measured.
- Speckle Interferometry:
- A technique for obtaining a high-resolution image of a planet or star by using a computer to analyze the speckled patterns of light produced by atmospheric distortion of multiple high-speed photographs of the body.
- Spectral Class:
- A star's classifictation, based on its spectrum, according to an established system.
- Spectral line:
- A bright or dark band in an astronomical spectum, produced by atoms as they emit or absorb light.
- Spectral Type:
- A star's classification, based on its spectrum, according to an established system organized by surface temperature and mass.
- Spectrogram:
- A photographic image of an astronomical spectrum.
- Spectrograph:
- An instrument that splits light or other electromagnetic radiation into its individual wavelengths-producing a spectrum-and records the result.
- Spectroscopy:
- The study of spectra, including the position and intensity of spectral lines.
- Spectrum:
- The array of colors or frequencies obtained by dispersing light from a star or other source, as through a prism; often banded with spectral lines.
- Spin:
- An abstract property of subatomic particles analogous to the angular momentum of a spinning top. Spin can be fractional , and it may be postive or negative.
- Spin Axis:
- The line around which an object rotates; in planets and stars, it extends through the center of the planet between the north and south geographic poles.
- Spiral arm:
- A dense region in the disk of a spiral galaxy, containing very young stars and ionized gas clouds that form a pattern of two or more spiral arms curving out from the galaxy's center.
- Spiral Galaxy:
- A disk galaxy with bright stars and ionized gas clouds that form a pattern of two or more spiral arms curving out from the galaxy's center.
- Static Limit:
- The outer boundary of the ergosphere of a black hole, within which objects must remain in motion to avoid entering the singularity.
- Stationary Line:
- In stellar spectra, an absorbtion line that exhibits a Doppler shift different from other lines in the same spectrum, indicating that the object producing the line intervenes along the line of sight between Earth and the background stars. Stationary l
ines of calcium and sodium were the first evidence of interstellar gas.
- Steady state model:
- An alternate theory to the Big Bang, not widely accepted, stipulating that the universe has always existed, and will always exist, in a state similar to the present.
- Stellar association:
- A loose cluster of stars, believed to have formed together.
- Stellar Population:
- A broad classification of stars based on their age, galactic location, chemical composition, spectra, and motions.
- Strong Force:
- The force that binds quarks together into composite particles and holds protons and neutrons together to form atomic nuclei.
- Subatomic Particle:
- Any particle smaller than an atom, from atomic components such as protons to the constituents of protons, quarks.
- Sulfuric Acid:
- A corrosive compound of sulfur, hydrogen, and oxygen. A major component of the Venusian atmosphere, it occurs in a dilute form on Earth as acid rain.
- Sunspot:
- A temporary, cool feature on the Sun's surface that appears dark by contrast with its surroundings.
- Supercluster:
- A large association of clusters of galaxies.
- Supercooling:
- A phenomenon in whifch a substance cools so rapidly that there is not enough time for a phase transition such as freezing to occur at the expected temperature.
- Super Giant:
- An old, high-mass star greatly expanded from its original size; larger and brighter than a giant star.
- Superior planet:
- A planet whose orbit is farther from the Sun than Earth's.
- Supermassive Black Hole:
- A hypothetical object at the center of some galaxies, with a mass equivalent ot several hundred million stars.
- Supernova:
- A stellar explosion that expels all or most of the star's mass and is extremely luminous.
- Supernova remnant:
- An expanding nebula, consisting of the stellar matter ejected by a supernova.
- Superstring:
- A hypothetical constituent of fundamental particles in the form of a closed loop of energy. Superstrings would vibrate and manifest properties in many dimensions of space.
- Supersymmetry:
- A theory in particle physics proposing that every type of fermion or boson has a matching partner particle differing from it onlyh in spin.
- Synchronous rotation:
- A phenomenon in which a moon spins on its axis exactly once per orbit, thus keeping the same face toward its planet at all times; seen in most moons in the Solar System.
- Synchroton radiation:
- A type of nonthermal emission generated by electrons and other charged particles spiraling around magnetic field lines at near light-speed.
- Tail:
- A curved filament of stars extending from a galaxy on the side opposite to its interaction with another galaxy.
- Tau:
- A negatively charged lepton, similar to an electron or a muon but much more massive and very short-lived.
- Tectonics
- The study of a planet's crust, including its structure and processes.
- Telemetry:
- The data, usually measurements, transmitted from a remote sensor to a recording receiver.
- Thermal emission:
- An electromagnetic radiation pattern that decreases in intensity as frequency increases; produced by heat-related processes.
- Third Cambridge Catalogue (3C):
- A catalog of radio sources published in 1959.
- Thought experiment:
- A hypothetical series of simplified events, often imporactical to replicate physically, in which logic is used to predict the outcome.
- Tidal Force:
- The force generated in a body by variations in the fravitational attraction exerted by another body.
- Tidal slowing:
- The deceleration of rotation in a moon or planet caused by the gravitational pull of other bodies, primarily the Sun.
- Tide:
- A change in the shape of one body resulting from the gravitational pull of another.
- Time dilation:
- A relativistic effect in which the passage of time in a moving body slows from the viewpoint of a stationary observer.
- Trajectory:
- The curved path of a moving body.
- Transit:
- The passage of a small clestial body across the visible face of another, larger body; also, the movement of any celestial body across a meridian.
- Triple-alpha process:
- A fusion reaction, characteristic of red giants and other highly evolved stars, in which three helium nuclei, also called alpha particles, fuse to carbon.
- Tritium:
- An isotope of hydrogen, with a nucleus containing one proton and two neutrons.
- T Tauri object:
- A very young star characterized by extensive and violent ejections of its mass; named after the first known star of this type.
- 21-centimeter line:
- A spectral line produced by neutral hydrogen at a radio wavelength of just over 21 centimeters; the first radio spectral line to be detected.