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 Glossary: Climate Change
 Glossary: Astronomy
 Glossary: Life in Universe

    Glossary of Terms - S to Z
    Index: A to F | G to L | M to R | S to Z

    saros cycle - (n.)

    An 18-year, 11-day repeating pattern of solar and lunar eclipses caused by a combination of the tilt of the lunar orbit with respect to the ecliptic and the precession of the plane of the moon's orbit.

    scattering - (n.)

    The random reflection of photons by particles such as atoms or ions in a gas, or dust particles in interstellar space.

    Schmidt camera - (n.)

    A type of telescope that uses a spherical mirror and a thin lens to provide photographs of a wide field.

    Schwarzschild radius - (n.)

    The radius that, according to Schwarzschild's solutions to Einstein's equations of the general theory of relativity, corresponds to the event horizon of a black hole. science : A branch of knowledge based on the laws of nature.

    science - (n.)

    A branch of knowledge based on the laws of nature.

    scientific law - (n.)

    A summary of experimental data; often expressed in the form of a mathematical equation.

    scientific model - (n.)

    A representation that serves to explain a scientific phenomenon.

    secondary cosmic rays - (n.)

    High energy particles that are generated in the earth's atmosphere by primary cosmic rays.

    secondary mirror - (n.)

    The second mirror in a reflecting telescope (after the primary mirror), usually either convex, to reflect the image out of a hole in the bottom of the telescope to the Cassegrain focus or along the telescope mount axis to the coudé focus; or flat, to reflect the image out of the side of the telescope to the Newtonian focus.

    second law of thermodynamics - (n.)

    The degree of randomness in the universe increases in any spontaneous process.

    seeing - (n.)

    The steadiness of the earth's atmosphere as it affects the resolution that can be obtained in astronomical observations. Good seeing corresponds to a steady atmosphere, and bad seeing corresponds to an unsteady atmosphere.

    seismic wave - (n.)

    A wave created in a planetary or satellite interior, usually caused by an earthquake.

    seismology - (n.)

    The study of waves propagating through a body and the resulting deduction of the internal properties of the body. The prefix "seismo-" comes from the Greek word for earthquake.

    selection effect - (n.)

    The tendency for a conclusion based on observations to be influenced by the method used to select the objects for observation. An example was the early belief that all quasars are radio sources, when the principal method used to discover quasars was to look for radio sources and then to find out whether they had other properties associated with quasars.

    semimajor axis - (n.)

    Half the major axis, that is, for an ellipse, half the longest diameter.

    Index: A to F | G to L | M to R | S to Z

    SETI - (n.)

    Search for Extra-Terrestrial Intelligence.

    Seyfert galaxy - (n.)

    A type of spiral galaxy that has a bright nucleus and whose spectrum shows emission lines. Historically, N galaxies and Seyfert galaxies were defined by different astronomers on the basis of different information, and the difference between them is not always clear cut.

    shear wave - (n.)

    A wave that consists of transverse motions; that is, motions perpendicular to the direction of wave travel.

    shock wave - (n.)

    discontinuity in the flow of a fluid (including a gas or plasma) marked by an abrupt increase in pressure, temperature, and flow velocity at the shock front.

    sidereal - (n.)

    time measured in relation to the fixed stars: the length of a sidereal day is 23 hr, 56 min, 4.09 sec of mean solar time.

    siderophile element - (n.)

    element with a weak affinity for oxygen and sulfur and readily soluble in molten iron (including iron, nickel, cobalt, platinum, gold, tin, and tantalum). Those measured digits that are known with certainty plus one uncertain digit.

    signal-to-noise ratio - (n.)

    Those measured digits that are known with certainty plus one uncertain digit.

    significant figures - (n.)

    Those measured digits that are known with certainty plus one uncertain digit.

    singularity - (n.)

    A point in space where quantities become exactly zero or become infinitely large; a singularity is present in a black hole.

    Sol - (n.)

    A Martian day, 24 hours and 37 minutes.

    solar activity cycle - (n.)

    The 11- or 22-year cycle with which such solar activity as sunspots, flares, and prominences varies

    solar cell - (n.)

    A device used for converting sunlight into electricity; a photoelectric cell.

    solar constant - (n.)

    rate at which radiant solar energy is received normally per unit area at the outer layer of Earth's atmosphere; its value is about 1.94 gram calories/cm2 /min.

    solar day - (n.)

    The synodic rotation period of the earth with respect to the sun; that is, the length of time from one local noon, when the sun is on the meridian, to the next local noon.

    Index: A to F | G to L | M to R | S to Z

    solar flare - (n.)

    An explosive outburst of ionized gas from the sun, us ally accompanied by X-ray emission and the injection of large quantities of charged particles into the solar wind.

    solar mass (Mg) - (n.)

    mass of the Sun, 2x1030 kg, used commonly as a unit to measure the masses of stars.

    solar motion - (n.)

    The deviation of the sun's velocity from perfect circular motion about the center of the galaxy; that is, the sun's peculiar velocity.

    solar nebula - (n.)

    cloud of gas and dust out of which a star condenses. The primordial gas and dust cloud from which the sun and planets condensed.

    solar (stellar) wind - (n.)

    radial outflow of hot plasma from a star's corona, carrying both mass, angular momentum, and energy away from the star

    solar time - (n.)

    A system of time-keeping with respect to the sun such that the sun is overhead of a given location at noon.

    solar wind - (n.)

    The stream of charged subatomic particles flowing steadily outward from the sun.

    solid angle - (n.)

    A three-dimensional angle.

    solstice - (n.)

    The occasion when the sun, as viewed from the earth, reaches its farthest northern point (the summer solstice) or its farthest southern point (the winter solstice).

    space velocity - (n.)

    The velocity of a star with respect to the sun.

    specific heat - (n.)

    The amount of heat required to raise the temperature of 1 g of the substance by 1 °C.

    speckle interferometry - (n.)

    method of using short-exposure photographs to recover information down to the diffraction limit of large optical telescopes.

    spectral analysis - (n.)

    study of the distribution by wavelength or frequency of the radiation emitted by an object of interest.

    spectral binary - (n.)

    : A binary system recognized as a binary because its spectrum contains lines of two stars of different spectral types.

    Index: A to F | G to L | M to R | S to Z

    spectral type/class - (n.)

    classification used to sort stars by photospheric temperature and intrinsic brightness. The seven spectral classes O-B-A-F-G-K-M, listed in order of decreasing temperature, include 99% of all known stars. Each spectral type is divided into a variable number of subtypes.

    spectrogram - (n.)

    A photograph of a spectrum.

    spectrograph - (n.)

    An instrument for recording the spectra of astronomical bodies or other sources of light.

    spectroscope - (n.)

    An instrument allowing an observer to view the spectrum of a source of light.

    spectroscopic binary - (n.)

    binary star that can be distinguished from a single star only through analysis of the Doppler shift of the spectral lines of one or both stars as they revolve about their common center of mass.

    spectroscopic parallax - (n.)

    The distance to a star derived from comparison of its apparent magnitude with its absolute magnitude deduced from study of its position on an H-R diagram determined by observation of its spectrum (spectral type and luminosity class).

    spectroscopy - (n.)

    The science of analyzing the spectra of stars or other sources of light.

    spectrum - (n.)

    An arrangement of electromagnetic radiation according to wavelength.

    spin-orbit coupling - (n.)

    A simple relationship between the orbital and spin periods of a satellite or planet, caused by tidal forces that have slowed the rate of rotation of the orbiting body. Synchronous rotation is the simplest and most common form of spin-orbit coupling.

    spiral density wave - (n.)

    A spiral wave pattern in a rotating, thin disk, such as the rings of Saturn or the plane of a spiral galaxy like the Milky Way.

    spiral galaxy - (n.)

    Any of a large class of galaxies exhibiting a disk with spiral arms.

    sporadic meteor - (n.)

    A meteor that is not associated with a shower.

    standard candle - (n.)

    A general term for any astronomical object whose absolute magnitude can be inferred from its other observed characteristics, and which is therefore useful as a distance indicator.

    star - (n.)

    : A self-luminous ball of gas that shines or has shone because of nuclear reactions in its interior.

    Index: A to F | G to L | M to R | S to Z

    steady-state theory - (n.)

    The theory of cosmology' in which the universe is thought to have had no beginning and is postulated not to change with time.

    Stefan-Boltzmann Law - (n.)

    The radiation law that states that the energy emitted by a black body varies with the fourth power of the temperature.

    Stellar epoch - (n.)

    sixth epoch in the history of the Universe, lasting perhaps 1010 yr from the galactic era to the present, dominated by the formation of stars.

    Stellar parallax - (n.)

    The apparent annual shifting of position of a nearby star with respect to more distant background stars. The term stellar parallax is often assumed to mean the parallax angle, which is one- half of the total angular motion a star undergoes. ~ See also parallax, parsec.

    Stellar wind - (n.)

    Any stream of gas flowing outward from a star, including the very rapid winds from hot, luminous stars; the intermediate- velocity, rarefied winds from stars like the sun; and the slow, dense winds from cool supergiant stars

    Stones - (n.)

    A stony type of meteorite, including the chondrites.

    STP - (n.)

    Standard temperature (0 °C) and pressure (1 atm).

    Stradian - (n.)

    The unit of solid angular measure, defined as the ratio of the surface area of that section of a sphere intercepted by a solid angle to the square of the radius. A full sphere subtends 4 P steradians from its center.

    stratosphere - (n.)

    One of the upper layers of the atmosphere of a planet, above the weather. The earth's stratosphere ranges from about 20 to 50 km in altitude.

    strong force - (n.)

    The nuclear force, the strongest of the four fundamental forces of nature.

    sublimation - (n.)

    The process of passing from gas to solid state (or vice versa) without becoming a liquid.

    subtend - (n.)

    The angle that an object appears to take up in your field of view; actually, the angle between lines drawn from opposite sides of the object to your eye. For example, the full moon subtends 1/2°.

    subtypes - (n.)

    designated by Arabic numerals. Further, stars are sorted by intrinsic brightness into luminosity classes designated by the first five Roman numerals. In turn, these are subdivided into a small number of subclasses designated by the first few letters of the lower case English alphabet; for example, the Sun is a G2 V star (also sometimes denoted as a dwarf G2 star) and Betelgeuse (a Orionis) is classified as M2 lab (i.e., intermediate between la and lb).

    sunspot - (n.)

    A relatively dark area of the solar surface. Sunspots appear dark because they are relatively cool; they represent regions of extremely high magnetic field.

    Index: A to F | G to L | M to R | S to Z

    sunspot cycle - (n.)

    The roughly 11-year cycle of variation of the number of sunspots visible on the sun.

    supercluster - (n.)

    A cluster of clusters of galaxies.

    supergiant - (n.)

    A post-main-sequence phase of evolution of stars of more than about 4 solar masses. Supergiants fall in the extreme upper right of the H-R diagram.

    supergranulation - (n.)

    The pattern of large cells seen in the sun's chromosphere, when viewed in the light of the strong emission line of ionized hydrogen.

    superior planet - (n.)

    Any planet whose orbit lies beyond the earth's orbit around the sun.

    supernova - (n.)

    An explosion of a massive star (with mass greater than 8 to 10 times the mass of the Sun) which ejects, at speeds about a tenth the speed of light, most of the original mass into space. The shell of material left behind may form a white dwarf, neutron star, or black hole, a region where gravity is so strong that no radiation or matter can escape. There is a short billionfold increase in luminosity.

    S wave - (n.)

    A type of seismic wave that is a transverse, or shear, wave, and which can travel only through rigid materials.

    synchotron self-absorption - (n.)

    re-absorption of radiation from accelerated electrons by other nearby electrons; this is a possible source of low frequency turnovers observed in the radio spectra of compact sources.

    synchotron emission - (n.)

    : radiation from electrons constantly accelerated in a magnetic field at a rate great enough for relativistic effects to be important. Predicted long ago, this radiation was first encountered in the particle accelerator called the synchrotron. Much of the radiation observed by radio astronomers originates in this fashion.

    synchotron rotation - (n.)

    A situation in which the rotational and orbital periods of an orbiting body are equal, so that the same side is always facing the companion object.

    synergistic effect - (n.)

    An effect much greater than the sum of the expected effects.

    synodic - (n.)

    Measured with respect to an alignment of astronomical bodies other than or in addition to the sun or the stars (usually the moon or a planet). For example, a synodic month depends on the positions of the sun, earth, and moon.

    synodic period - (n.)

    The orbital or rotational period of an object as seen by an observer on the earth. For the moon or a planet, the synodic period is the interval between repetitions of the same phase or configuration. The synodic period of Mars depends on the relative positions of the earth and Mars as they orbit the sun.

    systems analysis - (n.)

    analysis of the response to inputs of a set of interconnected units whose individual characteristics are known.

    Index: A to F | G to L | M to R | S to Z

    syzygy - (n.)

    An alignment of three celestial bodies. Sometimes applied more specifically to an alignment of the sun, earth, and moon.

    T association - (n.)

    A grouping of several T Tauri stars, presumably formed out of the same cloud of interstellar dust and gas.

    technology - (n.)

    The sum total of processes by which humans modify the materials of nature to better satisfy their needs and wants.

    tektites - (n.)

    Small glassy objects found scattered around the southern part of the southern hemisphere of the earth.

    temperature - (n.)

    A measure of heat intensity, or how energetic the particles of a sample are.

    terminator - (n.)

    The line between nighttime and daytime on a moon or planet; the edge of the part of a moon or planet that is lighted by the sun.

    terrestrial planets - (n.)

    Mercury, Venus, Earth, and Mars.

    theories - (n.)

    Detailed explanations of the behavior of matter based on experiments; may be revised if new data warrant.

    thermonuclear reactions - (n.)

    Radiation whose distribution of intensity over wavelength can be characterized by a single number (the temperature). Black-body radiation, which follows Planck's law, is an example of thermal radiation.

    total eclipse - (n.)

    Any eclipse in which the eclipsed body is totally blocked from view or totally immersed in shadow.

    transit - (n.)

    The passage of one celestial body in front of another celestial body. When a planet is in transit, we understand that it is passing in front of the sun.

    transition elements - (n.)

    Metallic elements situated in the center portion of the periodic table in the B groups.

    transit telescope - (n.)

    : A telescope designed to point straight overhead and accurately measure the times at which stars cross the meridian

    triple-alpha process - (n.)

    : A chain of fusion processes by which three helium nuclei (alpha particles) combine to form a carbon nucleus.

    Index: A to F | G to L | M to R | S to Z

    tritium - (n.)

    : A rare radioactive isotope of hydrogen with two neutrons and one proton in the nucleus (a mass of 3 atomic mass units).

    Trojan asteroids - (n.)

    A group of asteroids that precede or follow Jupiter in its orbit by 60°.

    tropical year - (n.)

    The length of time between two successive vernal equinoxes.

    T-Tauri stars - (n.)

    luminous variable stars associated with interstellar clouds and found in very young clusters; they are believed to be still in the process of gravitational contraction from their protostellar phase and have not yet arrived at the Main Sequence and begun to burn hydrogen.

    UBV system - (n.)

    A system of photometry that uses three standard filters to define wavelength regions in the ultraviolet, blue, and green-yellow (visual) regions of the spectrum.

    ULE - (n.)

    Ultra-Low Expansion, Corning Glass's successor to Pyrex as a material out of which telescope mirrors are made. U.L.E. expands or contracts very little when the temperature changes, and thus a mirror made out of it holds its shape as the temperature varies without distorting the image.

    ultraviolet - (n.)

    The region of the spectrum between about 100 and 4000 angstroms; also used in the restricted sense of ultraviolet radiation that reaches the ground, namely, that between about 3000 and 4000 angstroms.

    umbra - (n.)

    : (a) The dark inner portion of a shadow, such as the part of the earth's shadow which the moon is in total eclipse during a lunar eclipse; (b) the dark central portion of a sunspot.

    universe - (n.)

    everything that came into existence at the moment of the Big Bang, and everything that evolved from that initial mass of energy, or everything we can in principle, observe.

    UVBY - (n.)

    A system of photometry that uses four standard filters to define wavelength regions in the ultraviolet, blue, and yellow regions of the spectrum.

    UV light - (n.)

    ultraviolet (UV) light.

    van Allen belts - (n.)

    Zones in the earth's magnetosphere where charged particles are confined by the earth's magnetic field. There are two main belts, one centered at an altitude of roughly 1.5 times the earth' radius, and the other between 4.5 and 6.0 times the earth's radius.

    vaporization - (n.)

    The process in which a substance changes from the liquid to the gaseous (vapor) state.

    variable star - (n.)

    A star whose brightness changes over time.

    Index: A to F | G to L | M to R | S to Z

    velocity - (n.)

    The measure of the speed and direction of an object; the distance traveled by an object per unit of time.

    velocity curve - (n.)

    A plot showing the orbital velocity of stars in a spiral galaxy versus distance from the galactic center.

    velocity dispersion - (n.)

    A measure of the average velocity of stars in a group or cluster with random internal motions. In globular clusters and elliptical galaxies, the velocity dispersion can be used to infer the central mass.

    vernal equinox - (n.)

    The intersection of the ecliptic and the celestial equator that the sun passes each year when moving from southern to northern inclinations.

    visible light - (n.)

    That part of the electromagnetic spectrum visible to humans and lying between the infrared and ultraviolet parts of the spectrum.

    Visual Binary - (n.)

    binary star system whose components can be identified with an optical telescope.

    VLA - (n.)

    The Very Large Array, a set of radio telescopes being built by the National Radio Astronomy Observatory in New Mexico to make aperture synthesis measurements.

    VLBI - (n.)

    Very-long-baseline interferometry, the technique of using simultaneous measurements made with radio telescopes at widely separated locations to obtain extremely high resolution.

    volatile - (n.)

    The property of being easily vaporized. Volatile elements stay in gaseous form except at very low temperatures; they did not condense into solid form during the formation of the solar system.

    volatile organic compounds - (n.)

    class of organics that is easily vaporizable at low temperatures and pressures.

    wave guide - (n.)

    special transmission medium resembling a pipe and often having a rectangular cross section, inside of which radio waves may be propagated.

    wavelength - (n.)

    The distance between wave crests in any type of wave.

    weak force - (n.)

    One of the four fundamental forces of nature, weaker than the strong force and the electromagnetic force. It is important only in the decay of certain elementary particles.

    weight - (n.)

    The force determined by the gravitational pull on a mass.

    Index: A to F | G to L | M to R | S to Z

    whistler - (n.)

    electromagnetic ultra-low frequency radiation observed in planetary magnetospheres; energized by lightning and other discharges.

    white dwarf - (n.)

    The final stage of the evolution of a star of between 0.07 and 1.4 solar masses; a star supported by electron degeneracy. white dwarfs are found to the lower left of the main sequence of the H-R diagram.

    Wien’s law - (n.)

    An experimentally discovered law applicable to thermal continuum radiation, which states that the wavelength of maximum emission intensity is inversely proportional to the absolute temperature.

    Wm-2 (or W/m2 ) - (n.)

    watts per square meter of incident signal flux per whatever resolution bandwidth is in use. It is the total signal flux if the receiving bandwidth equals or is greater than the bandwidth of the signal.

    Wm-2 Hz-1 (or W/m2 Hz) - (n.)

    spectral flux density (see Jansky).

    Wolf-Rayet star - (n.)

    A type of O star whose spectrum shows very broad emission lines.

    W Virginis star - (n.)

    A type of II Cepheid, one of the fainter class of Cepheid variable stars characteristic of Cepheids in globular clusters.

    x radiation - (n.)

    electromagnetic radiation in the range of approximately 0.05-100 A.

    X-ray - (n.)

    A photon of electromagnetic radiation in the wavelength interval between about 1 Angstrom and 100 Angstroms.

    year - (n.)

    The period of revolution of a planet around its central star; more particularly, the earth's period of revolution around the sun.

    Zeeman effect - (n.)

    The broadening or splitting of spectral lines caused by the presence of a (strong) magnetic field in the gas where the lines are formed.

    zero-age main sequence (ZAMS) - (n.)

    The main sequence in the H-R diagram formed by stars that have just begun their hydrogen-burning life- times, and have not yet converted any significant fraction of their core mass into helium. The zero-age main sequence forms the lower left boundary of the broader band representing the general main sequence.

    zircon - (n.)

    a silicate mineral, ZrSiO4, diamondlike in appearance, that contains silica silicon dioxide, and the element zirconium. Some zircons in meteorites contain material from the period before the Solar System formed.

    zodiac - (n.)

    A band circling the celestial sphere along the ecliptic, broad enough to encompass the paths of all the planets visible to the naked eye. .In some usages, the sequence of constellations lying along the ecliptic.

    zodiacal light - (n.)

    A diffuse band of light visible along the ecliptic near sunrise and sunset, created by sunlight scattered off of interplanetary dust.

    Index: A to F | G to L | M to R | S to Z


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