S Band	A radiofrequency band at a wavelength of 11.1 cm. [H76]
s-Electron	An orbital electron whose l quantum number is zero. [H76]
S-Factor	A nuclear cross-section factor measured in keV-barns. [H76]
S-Matrix	Scattering Matrix: A matrix representing the transitions from some initial to some final state in a given interaction. The transitions may involve changes in the number of particles in the system. [H76]
S-Process	Slow Neutron Capture: A process in which heavy, stable, neutron-rich nuclei are synthesized from iron-peak elements by successive captures of free neutrons in a weak neutron flux, so there is time for -decay before another neutron is captured (cf. r-process). This a slow but sure process of nucleosynthesis which is assumed to take place in the intershell regions during the red-giant phase of evolution, at densities up to 105 g cm-3 and temperatures of about 3 × 108 K (neutron densities assumed are 1010 cm-3). The s-process slowly builds stable nuclear species up to A = 208 (time between captures about 10-100 years). It ends there, because any further capture of neutrons leads immediately to -decay back to lead or thallium. The most likely source of neutrons for the s-process is linked to thermal instabilities in the helium shell during double shell burning after core He exhaustion. The s-process probably occurs in stars where M < 9 M. [H76]
S Stars	Red-giant stars of spectral type S are similar to M stars except that the dominant oxides are those of the metals of the fifth period (Zr, Y, etc.) instead of the third (Ti, Sc, V). They also have strong CN bands and contain spectral lines of lithium and technetium. Pure S stars are those in which ZrO bands are very strong and TiO bands are either absent or only barely detectable. Almost all S stars are LPVs. (S1,0. The number following the comma is an abundance parameter.) [H76]
S-State/S-Level	The state of an atom in which the orbital angular momentum L (the vector sum of the orbital angular momenta l of the individual electrons) is zero. [H76]
S-Wave	Secondary Wave: A seismic shear wave that moves transversely through Earth. The s-waves cannot penetrate the core of the Earth, being totally reflected by the 2900-km discontinuity. [H76]
SAA	South Atlantic Anomaly
SAO	Smithsonian Astronomical Observatory [LLM96]
SC Stars	Stars which appear to be intermediate in type between S stars and carbon stars (C/O ratio near unity). [H76]
sdB	Subdwarf B-type stars with very broad and shallow Balmer lines; fewer lines of the Balmer series are visible than for normal dwarfs. [JJ95]
sdO	Subdwarf O stars showing few very broad and shallow Balmer lines and a very strong He II 4686 line. [JJ95]
SEC	Secondary Electron Conduction [LLM96]
SFR	Star Formation Rate
SFH	Star Formation History
SI Units	see International System of Units. [H76]
SIN	Superconductor Insulator Normal [LLM96]
SIS	Solid-state Imaging Spectrometer (ASCA X-ray satellite).
SIS	Superconductor Insulator Superconductor [LLM96]
SIT	Silicon Intensified Target [LLM96]
Sachs-Wolfe Effect
FG Sagittae	A supergiant whose spectral type has changed from B4 Ia in 1955 to A5 Ia in 1967 to F6 Ia in 1972. It ejected a planetary nebula some 6000 years ago. It showed s-process elements in its surface layer in 1972 that did not exist in 1965 - an indication of deep mixing. [H76]
WZ Sagittae	A recurrent DAe old nova (1913 and 1946) with the shortest known orbital period (about 80 minutes). It is almost certainly a close binary system in which mass is being transferred onto a white-dwarf primary. [H76]
Sagittarius A	A radio source (the galactic center) about 12 pc in diameter. (Sgr A West is a thermal source; Sgr A East is a nonthermal source.) [H76]
Sagittarius A*	The very center of the Milky Way, Sagittarius A* is a strong source of radio waves and probably a massive black hole. [C95]
Sagittarius Arm	One of the spiral arms of the Milky Way, lying between us and the center of the Galaxy in the direction of Sagittarius. It includes the Scutum arm, the 3-kpc arm , and the Norma arm. It is about 1.5 kpc from the Sun and about 8.7 kpc from the galactic center. Density of H I and H II in Sagittarius arm is about 1.2 atoms cm-3. [H76]
Sagittarius B2	A massive (3 × 106 M), dense (up to 108 particles per cm3) H II region and molecular cloud complex - the richest molecular source in the Galaxy. It is in the galactic plane about 10 kpc distant, near the galactic center. [H76]
Saha Equation	An equation that determines the number of atoms of a given species in various stages of ionization that exist in a gas in thermal equilibrium at some specified temperature and total density. [Silk90]
Sakharov Oscillations
Salpeter Function	A simple functional interpolation for the distribution by mass of newly formed stars. Also referred to as the initial mass function of stars, the Salpeter function (the number of stars formed per unit mass range) is proportional to m-2.35, where m is the mass of a star. [Silk90]
Samarium	A silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoids. Samarium is used in the metallurgical, glass, and nuclear industries. Symbol: Sm; m.p. 1077°C; b.p. 1791°C; r.d. 7.52 (20°C); p.n. 62; r.a.m. 150.36. [DC99]
Saros	(a) A particular cycle of similar eclipses (lunar or solar) known to the Babylonians, that recur at intervals of 6585 days (about 18 tropical years). The interval contains 223 synodic months (6585.32 days) and 19 ecliptic years (6585.78 days). (It also contains 242 nodical months.) The difference of a fraction of a day causes each eclipse to fall about 120° west of the previous eclipse. [H76] (b) A Babylonian lunar cycle of 6585.32 days, or 18 years 11.33 days, or 223 lunations, at the end of which the centers of the Sun and Moon return so nearly to the relative positions of the beginning that all the eclipses (see eclipse) of the period recur approximately as before, but in longitudes (see longitude, terrestrial) approximately 120° to the west. (See lunar phases.) [S92]
Satellite	Body orbiting a planet. Since 1957 the term has also been applied to man-made (artificial) satellites; many astronomers make the distinction by calling natural satellites moons (and the Earth's natural satellite the Moon). [A84]
Satellite Galaxy	A galaxy that orbits a larger one. The Milky Way has at least ten satellite galaxies: the Large Magellanic Cloud, the Small Magellanic Cloud, Ursa Minor, Draco, Sculptor, Sextans, Carina, Fornax, Leo II, and Leo I. [C95]
Satellite Lines	Of an OH source: The lines arising from transitions at 1612 and 1730 MHz. [H76]
Saturn	(a) Sixth major planet out from the Sun. The most spectacular of the Solar System, it is circled by a series of concentric rings. [A84] (b) Sixth planet from the Sun. Mean distance from Sun 9.540 AU; e = 0.056, i = 2°29'33". Sidereal period 29.458 years; synodic period 378 days. Equatorial diameter 116,340 km. Oblateness 0.1. Mass 5.7 × 1029 g = 95.2 ME; mean density 0.7 g cm-3; surface gravity 11 m s-2; Vesc 33.1 km s-1. Rotation period at equator 10h14m; at poles 10h38m. Obliquity 26°44'. Teff about 160 K. Vorb 9.65 km s-1. Albedo 0.50. Atmosphere hydrogen and helium. Ten satellites, all of which are locked in synchronous rotation. [H76]
Saturn's Rings	A system of four concentric rings, only about 2-4 km thick. The outermost ring is ring A, then comes Cassini's division, then ring B (also called the bright ring), then Lyot's division, then ring C (the crepe ring), then ring D (discovered in 1969). The rings are a swarm of solid particles, probably [1973] jagged rocks about 1 meter to 1 km across, not ice as previously had been assumed, inside the Roche limit. Bobrov (1969) estimates the total mass of the rings to be about 0.01 the lunar mass. [H76]
Saturn Nebula	A double-ring planetary about 700 pc distant (NGC 7009). [H76]
Sawtooth Wave	A waveform generated electronically (such as the variation of voltage with time), having a uniform increase that regularly and rapidly drops to the initial value. A sawtooth wave is used as the time base for scanning circuits in a cathode-ray tube. [DC99]
Scalar Field	A field of energy generated by scalar particles. These hypothesized particles have no intrinsic spin. All known elementary particles have some intrinsic spin; thus scalar particles and scalar fields are theoretical to date. (See quantum field.) [LB90]
Scalar-Tensor Theory	A class of theories of gravity more complex than Einstein's theory, general relativity. The best known scalar-tensor theory is the Brans-Dicke theory. In some scalar-tensor theories, the gravitational constant is not constant, as it is in general relativity. (See Brans-Dicke theory; general relativity.) [LB90]
Scale Factor	A measure of changing distances in cosmology. The distance between any two galaxies, for example, is proportional to the scale factor, which is always increasing in an expanding Universe. If the scale factor doubles in size, then the distance between any two galaxies doubles. [LB90]
Scale Height	(a) The mean distance of a group of stars from the Galactic plane. In general, old stars have larger scale heights than young ones. [C95] (b) The height at which a given parameter changes by a factor e. For example, an atmospheric scale height of 100 km means that the value at 100 km is 1/e the value at the surface. [H76]
Scale Invariance	S physical system is said to exhibit scale-invariance if its appearance remains unchanged (in a statistical sense, and to within simple readjustments of the units of measurements) by a coarse-graining operation. [D89]
Scale-Invariant	Most inflationary models predict that the spectrum of density perturbations is nearly scale-invariant, meaning essentially that each wavelength has the same strength. This spectrum is also called the Harrison-Zeldovich spectrum, named for two astrophysicists who proposed the spectrum a decade before inflation was invented. [G97]
Scale Length	A measure of the size of a physical system or region of space. [LB90]
Scaling	(a) The phenomenon observed in deep inelastic scattering, and predicted by James Bjorken, whereby the structure functions which describe the shape of the nucleon depend not on the energy or momentum involved in the reaction, but on some dimensionless ratio of the two. The structure functions are hence independent of any dimensional scale. [CD99] (b) In scattering experiments, the property whereby the likelihood of a reaction depends not so much on the amount of energy transferred to the target as on the ratio between energy transferred and momentum transferred. [H76]
Scandium	A lightweight silvery element. It is found in minute amounts in over 800 minerals, often associated with lanthanoids. Scandium is used in high-intensity lights and in electronic devices. Symbol: Sc; m.p. 1541°C; b.p. 2831°C; r.d. 2.989 (0°C); p.n. 21; r.a.m. 44.955910. [DC99]
Scattering	(a) The `spreading out' of a beam of radiation as it passes through matter, reducing the energy moving in the original direction. Depending on the circumstances, scattering can follow any combination of three processes as the radiation interacts with matter particles - reflection (elastic scattering), absorption followed by re-radiation (inelastic or Compton scattering), and diffraction. Thus sunlight is scattered (or diffused) as it passes through cloud and dust in the atmosphere. However, even perfectly clear air scatters sunlight, making the sky color blue - high frequencies are scattered more than low frequencies. [DC99] (b) The process whereby light is absorbed and reemitted in all directions, with essentially no change in frequency. Scattering by free electrons was the dominant source of opacity in the early Universe. [Silk90]
Scattering Matrix	The S-matrix relates the incoming and out-going states of elementary particles during interactions and scattering experiments. The mathematical structure and properties of the S-matrix has received considerable attention. [P88]
Schmidt Camera	Telescopic camera incorporating an internal corrective lens or plate that compensates for optical defects and chromatic faults in the main mirror. The system was invented by Bernhard Schmidt. [A84]
Schmidt Plates	Photographic plates obtained with a Schmidt telescope, which is a type of telescope with a particularly large field of view. [LB90]
Schmidt Telescope	(a) A telescope with a spherical primary mirror and a thin refractive corrector plate with a complex, non-spherical shape. Very wide-field performance for surveys. [McL97] (b) A type of reflecting telescope (more accurately, a large camera) in which the coma produced by a spherical concave mirror is compensated for by a thin correcting lens placed at the opening of the telescope tube. The Schmidt has a usable field of 0°.6. [H76]
Scholastics	Adherents to the philosophy and cosmology of Aristotle. Their dominance in the universities, which had been founded largely to study Aristotle, constituted an obstacle to acceptance of the Copernican system advocated by Kepler and Galileo. [F88]
Schottky Barrier	A metal to semiconductor interface without any insulation layer produces an energy barrier in the semiconductor which can be used like a diode. [McL97]
Schrödinger Equation	(a) Equation governing the evolution of probability waves in quantum mechanics. [G99] (b) A quantum-mechanical wave equation describing the nonrelativistic motion of a particle or system of particles under the influence of forces. The solutions to Schrödinger's equation yield the wave function describing the system (particle, atom, molecule). This is the fundamental equation in nonrelativistic quantum mechanics. [H76] (c) The equation that describes the propagation of the waves associated with subatomic particles. In a more general context it describes the time evolution of the state of a quantum system. [D89] (d) The equation from quantum theory that tells how to calculate the effects of the forces on the particles, It is the quantum theory equivalent of Newton's second law.
Schuster Mechanism	A scattering mechanism in the continuum, which under certain conditions can yield emission lines in the spectrum even under the assumption of LTE. It is the modifica- tion of the emergent radiation, for a given temperature distribu- tion. by variations in the ratio of pure absorption to scattering opacity. [H76]
Schwarzschild Black Hole	A nonrotating, spherically symmetric black hole derived from Karl Schwarzchild's 1916 exact solution to Einstein's vacuum field equations. [H76]
Schwarzschild Filling Factor	Ratio of the actual density to the limiting value for a system. [H76]
Schwarzschild Radius	(a) The critical radius, according to the general theory of relativity, at which a massive body becomes a black hole, i.e., at which light is unable to escape to infinity. Rs = 2GM / c2; Rs for Sun, 2.5 km; Rs for Earth, 0.9 cm. [H76] (b) The event horizon of a spherical black hole; the critical radius from which light is unable to escape to infinity. The Schwarzschild radius of a star of solar mass is 2.5 km. [Silk90] (c) The "surface" of a black hole, within which the strength of gravity is so strong that light cannot escape. The Schwarzschild radius is proportional to the mass of the black hole and would be about 2 miles for a black hole of the mass of our sun. Black holes were first "theoretically" discovered by Karl Schwarzschild in 1917. (see Black Hole.) [LB90] (d) The effective radius of a spherically symmetric black hole, rg: (2GM/c2)1/2: 3(M/M) km. The gravitational redshift of radiation emitted at this radius is infinite to an external observer. Photons cannot escape from a black hole to the outside world from within this radius. The Schwarzschild radius is therefore the smallest physical size which an astronomical object of mass M can have. Matter falling within the radius rg inevitably collapses into the singularity. [D89]
Schwarzschild Singularity	The center of a black hole. According to Einstein's theory of general relativity, the entire mass of a black hole is concentrated at a point at its center, the "singularity". It is believed that quantum mechanical effects, not included in the theory, would cause the mass to spread out over a tiny but nonzero region, thus preventing an infinite density of matter and doing away with the singularity. [LB90]
Schwarzschild Solution	Solution to the equations of general relativity for a spherical distribution of matter; one implication of this solution is the possible existence of black holes. [G99]
Science	(a) Systematic study of Nature, based upon the presumption that the Universe is based upon rationally intelligible principles and that its behavior can therefore be predicted by subjecting observational data to logical analysis. [F88] (b) Science can be defined as a self-correcting way to get knowledge about the natural universe, plus the body of knowledge obtained that way. It is both a method and the resulting understanding and knowledge. The method requires making models to explain phenomena, testing them experimentally, and revising them until they work. The goal of science is understanding. Once part of the natural world is understood, it may be possible to develop applications of the new knowledge. The process of developing such applications is properly called technology, not science. Although scientific knowledge may, and usually does, lead to technology, science is not necessary for technology, and technological developments have led to new science as much as the opposite. Before the time of Galileo, many technological developments occurred that had no scientific connection. Since the time of Maxwell and his formulation of the electromagnetic theory, nearly all technological developments have depended on earlier science. In recent years the words science and technology have been frequently misused, as though they were interchangeable. Because science and technology are really different, it is better to distinguish carefully between them.
Scintillation	(a) In radio astronomy, a rapid oscillation in the detected intensity of radiation emitted by stellar radio sources, caused by disturbances in ionized gas at some point between the source and the Earth's surface (usually in the Earth's own upper atmosphere). [A84] (b) Variations in the brightness of starlight (i.e., "twinkling") caused by turbulent strata very high in Earth's atmosphere. Scintillation increases with distance from the turbulent zone (cf. seeing). [H76]
Scintillation Counter	A device used with a photomultiplier tube to detect or count charged particles or gamma rays. [H76]
Scintillator	A detector for high-energy photons such as gamma-rays. The impact of a gamma-ray causes a burst of light which can be observed with a PMT. [McL97]
Sco	see Antares. [H76]
Sco	A system with at least five components which during the 1970s is undergoing a series of occultations by the Moon and by Jupiter. Component A is a spectroscopic binary (B0.5 V, B V). In 1971 component C was occulted by Jo. [H76]
Sco-Cen Association	An association of very young stars about 200 pc distant in the Gould Belt. The most luminous member is a B star of Mv = - 4.9. [H76]
Scorpius OB1	An extremely young association of OB stars in Scorpius about 2 kpc distant. [H76]
Scorpius X-1	A compact eclipsing X-ray source about 250-500 pc distant. It is the brightest X-ray source in the sky (besides the Sun) and was discovered in 1962. It has day-to-day variations (period about 0.78 days?) of as much as 1 mag; it also has optical and radio counterparts but no correlation has been found among the flares observed at the three different wavelengths. It is a thermal X-ray source, probably associated with a rotating collapsed star surrounded by an extensive envelope. Tentative optical identification with the 13th mag blue variable V818 Sco. The spectrum of Sco X-1 is similar to that of an old nova. (3U 1617-15) [H76]
Scott Effect	A selection effect in the study of the magnitude-redshift relation in cosmology. It was pointed out by Elizabeth Scott in 1957 that at great distances only the most luminous clusters of galaxies would be visible, and this fact would introduce a bias into the data. [H76]
Sculptor	1. A faint constellation in the southern sky. 2. A dwarf galaxy that orbits the Milky Way in the constellation Sculptor. It is 255,000 light-years from the Galactic center. [C95]
Sculptor Group	The nearest group of galaxies to the Local Group, 4 to 10 million light-years away. Its brightest member is the beautiful edge-on spiral NGC 253. [C95]
Sculptor Dwarf	A dwarf elliptical galaxy (Mv = - 11.28 mag, mass about 3 × 106 M), about 85 kpc distant, in the Local Group. Discovered in 1938. [H76]
Scuti Stars	A group of pulsating variable stars of spectral class A-F with regular periods of 1-3 hours and with small variations in amplitude. They lie in the lower part of the Cepheid instability strip. (also called dwarf Cepheids or ultrashort-period Cepheids) [H76]
Second	A unit of time defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. In 1967 the General Conference of Weights and Measures (CGPM) adopted this as the tentative definition of the second in SI units, replacing the ephemeris second, which remains in the IAU system of astronomical constants. [H76]
Second Law of Thermodynamics	(a) A physical law formulated in the nineteenth century and stating that any isolated system becomes more disordered in time. [LB90] (b) Law stating that total entropy always increases. [G99]
Second Parameter	The color of a globular cluster's horizontal branch is determined largely by its metallicity: all other things being equal, the more metal-poor a cluster, the bluer its horizontal branch. However, all other things are not always equal, because globulars with similar metallicities sometimes have different horizontal-branch colors, so a second parameter must be responsible. Searle and Zinn speculated that the second parameter was age and said that all globulars had not formed at the same time. [C95]
Second Quantization	This goes beyond the quantum theory of Heisenberg and Schrödinger by applying the act of quantization a second time. In this way, matter and energy fields can themselves become quantized. The quantum excitations of these fields are the elementary particles. [P88]
Second Superstring Revolution	Period in the development of string theory beginning around 1995 in which some nonperturbative aspects of the theory began to be understood. [G99]
Secondary Cosmic Rays	Atomic fragments - mainly muons - produced by collisions between primary cosmic rays and the molecules in Earth's atmosphere. [H76]
Secondary Mirror	The second reflecting surface encountered by the light in a telescope. The secondary is usually suspended in the beam and therefore obstructs part of the primary. [McL97]
Secular	In astronomy, gradual, taking aeons to accomplish. [A84]
Secular Acceleration	Apparent acceleration of the Moon and Sun across the sky, caused by extremely gradual reduction in speed of the Earth's rotation (one 50-millionth of a second per day). [A84]
Secular Change	A continuous, nonperiodic change in one of the attributes of the states of a system. Often, a change in an orbit due to dissipation of energy (cf. canonical change). [H76]
Secular Instability	Instability caused by the dissipation of energy. [H76]
Secular Parallax	A parallax based on Solar motion; i.e., the baseline is the distance the Sun moves in a given interval of time with respect to the Local Standard of Rest (4.09 AU per year). [H76]
Secular Stability	The condition in which the equilibrium configuration of a system is stable over long periods of time. [H76]
Seed Nuclei	Nuclei from which other nuclei are synthesized. [H76]
Seeing	Describes the blurring of a stellar (point-like) image due to turbulence in the Earth's atmosphere, both at high altitudes and within the telescope dome. Seeing estimates are often given in terms of the full-width in arcseconds of the image at the points where the intensity has fallen to half its peak value. The typical value at a good site is a little better than 1 arcsecond. [McL97]
Segmented Mirrors	A large mirror construction technique in which many smaller elements are built and then actively controlled to conform to the shape of the required large mirror. [McL97]
Selected Areas	262 small (75' square) regions of the sky in which magnitudes, spectral types, and luminosity classes of stars have been accurately measured and which have served as standards for magnitude systems. [H76]
Selection Rule	A rule whereby changes in quantum numbers can take only certain allowed values: e.g., l = ± 1 or 0 for dipole transitions. [H76]
Selective Absorption	The reddening of starlight in passing through fine particles of interstellar dust. [H76]
Selectron	The supersymmetric partner of the electron.
Selenium	A metalloid element existing in several allotropic forms. The common gray metallic allotrope is very light-sensitive and is used in photocells, solar cells, some glasses, and in xerography. The red allotrope is unstable and reverts to the gray form under normal conditions. Symbol: Se; m.p. 217°C (gray); b.p. 684.9°C (gray); r.d. 4.79 (gray); p.n. 34; r.a.m. 78.96. [DC99]
Selenocentric	With reference to, or pertaining to, the center of the Moon. [S92]
Self-Absorption	Reduction in relative intensity in the central portion of spectral lines resulting from selective absorption by a cooler shell surrounding the hot source. [H76]
Self-Consistent Field Approach	An approach in which the density distribution and state of motion in a system are determined so as to be self-consistent with the force field (e.g., gravitational or electromagnetic) arising from the system itself. [H76]
Self-Organisation	spontaneous emergence of order, arising when certain parameters built in a system reach critical values. [D89]
Semi-Conductor	A material like silicon or germanium in which the valence band and the conduction band are separated by a small (forbidden) energy gap. Such materials have some of the properties of a good electrical conductor - in which the energy gap is zero - and some of the properties of an insulator - in which the gap is very large. [McL97]
Semi-Convection	The partial convective mixing that takes place in a convectively unstable region where stability can be attained by the results of the mixing before the region is completely mixed. [H76]
Semi-Diameter	The angle at the observer subtended by the equatorial radius of the Sun, Moon, or a planet. [S92]
Semi-Forbidden Lines	Spectral lines from "semiforbidden" transitions, i.e., those whose transition probabilities are perhaps 1 in 106 instead of about 1 in 109 for forbidden transitions. One bracket - e.g., [C III] - is used to indicate semiforbidden lines. [H76]
Semi-Major Axis	Half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit. [S92]
Semi-Minor Axis	Half the length of the minor axis of an ellipse; a standard element used to describe an elliptical orbit. [S92]
Semi-Regular Variable	A class of giant and supergiant pulsating stars of spectral class M, K, N, R, or S with a periodic (or semiperiodic) light curve of varying amplitude. Betelgeuse is one. [H76]
Sense	One of two opposite directions describable by the motion of a point, line, or surface. [H76]
Sequencer	That part of an electronic system responsible for the accurate phasing of time-critical events such as CCD clocking and readout. [McL97]
Serial Register	The final (horizontal) row of a CCD in which the controlling electrodes are arranged at right angles to those on the rest of the CCD. This enables charges coupled onto this row to be transferred in single-file through the CCD output amplifier. [McL97]
CV Serpentis	A sometimes-eclipsing binary composed of a Wolf-Rayet star and a B0 star with a period of 29.6 days. [H76]
Serpentis	A G0 V star almost identical to the Sun in its energy distribution. [H76]
SETI	The Search for Extraterrestrial Intelligence, by using radiotelescopes to listen for signals transmitted by intelligent alien beings. [F88]
Sextans Dwarf	A dwarf companion to the Milky Way. Discovered by computer in 1990, Sextans lies about 300 kpc from the Galactic center. [C95]
Sextant	Instrument employed to measure the elevation of astronomical objects above the horizon. Based upon an arc equal to a sixth of a circle, sextants are more compact and easier to use than are the quadrants that preceded them. [F88]
Seyfert Galaxy	(a) A type of spiral galaxy first discovered by Karl Seyfert in the 1940s. The central region of a Seyfert galaxy is distinguished by powerful radiation, much of it focused into narrow frequencies. [LB90] (b) One of a small class of galaxies (many of which are spirals) of very high luminosity and very blue continuum radiation with small, intensely bright nuclei whose spectra show strong, broad, high-excitation emission lines probably caused by discrete clouds moving at velocities that are higher than the escape velocity. Seyferts possess many of the properties of QSOs, such as the ultraviolet excess of the continuum, the wide emission lines, and the strong infrared luminosity. The energy sources in their nuclei are unexplained; presumably the energy input can be associated with some process that liberates gravitational binding energy to accelerate relativistic particles. Seyferts comprise about 1 percent of the bright galaxies. The brightest Seyfert known is NGC 1068. Weedman-Khachikian classification: class 1 Seyferts have broad Balmer line wings; class 2 have no obvious Balmer line wings. [H76]
Seyfert's Sextet	A compact group of galaxies surrounding NGC 6027. It has both spiral and irregular members. [H76]
Shadow Matter	Theoretical classes of particles, their existence intimated by supersymmetry, theory, that participate in few if any of the four known fundamental forces. Planets, stars, and galaxies made of shadow matter could conceivably exist in the same space and time we occupy without our sensing their presence. [F88]
Shane-Wirtanen Catalogue	A catalogue of all galaxies brighter than seventeenth magnitude (a measure of brightness). There are about a million galaxies in the Shane-Wirtanen catalogue. [LB90]
Shapley-Ames Catalogue	A catalogue of galaxies brighter than thirteenth magnitude, completed in 1932. There are about 1200 galaxies in this catalogue. [LB90]
Shear	A stress applied to a body in the plane of one of its faces. [H76]
Sheath	The boundary layer of charged particles between a plasma and its surrounding material. [H76]
Shell Star	A hot main-sequence star, usually of spectral class B-F, whose spectrum shows bright emission lines presumed to be due to a gaseous ring or shell surrounding the star. [H76]
Shock Wave	A sharp change in the pressure, temperature, and density of a fluid which develops when the velocity of the fluid begins to exceed the velocity of sound. [H76]
Shooting Star	The streak of light in the sky produced by the firey entry of a meteoroid into the Earth's atmosphere; also the glowing meteoroid itself. The term "fireball" is sometimes used for a meteor approaching the brightness of Venus; the term "bolide" for one approaching the brightness of the full Moon. (same as meteor) [A84]
Short Scale	The cosmological distance scale which uses a Hubble constant of approximately 100 km/s/Mpc. [C97]
Shot Noise	Noise, or fluctuations in the current of a detector, due to the fact that the current is carried not by a smooth fluid, but by a large number of individual electrons (cf. wave noise; correlator). [H76]
Side Lobe	In radio astronomy, a component of the reception pattern of an antenna away from the main beam, representing a direction in which the antenna is sensitive when it should be insensitive. [H76]
Sideband	A range of frequencies contained in a modulated carrier wave, either above or below the unmodulated frequency (hence upper and lower sidebands). The existence of sidebands is a consequence of the modulation process. For instance, in an amplitude modulated wave, if the carrier frequency is fc and the modulating signal frequency is fs, then the modulated wave has three components of frequency fc - fs, fc, and fc + fs. [DC99]
Sidereal	In astronomy, relating to the period of time based on the apparent rotation of the stars, and therefore equivalent to the rotation of the body from which the observation is made. Thus on Earth a sidereal year is 365.256 times the sidereal day of 23 hours, 56 minutes and 4 seconds. [A84]
Sidereal Day	The length of time (23h56m4s.091) between two successive meridian transits of the vernal equinox (cf. mean solar day). Because of precession the sidereal day is about 0.0084 second shorter than the period of rotation of Earth relative to a fixed direction (23h56m4s.099). [H76]
Sidereal Hour Angle	Angular distance on the celestial sphere measured westward along the celestial equator from the catalog equinox to the hour circle passing through the celestial object. It is equal to 360° minus right ascension in degrees. [S92]
Sidereal Period	The time it takes for a planet or satellite to make one complete circuit of its orbit (360°) relative to the stars. Earth's sidereal period (or sidereal year) is equal to 365.2564 mean solar days. [H76]
Sidereal Time	(ST) The measure of time defined by the apparent diurnal motion of the catalog equinox; hence a measure of the rotation of the Earth with respect to the stars rather than the Sun. [S92]
Siderite	An iron (or iron and nickel) meteorite. Siderites comprise about 6 percent of known falls. (lit.: "star stone") [H76]
Siderolite	A stony iron meteorite. Siderolites comprise less than 2 percent of known falls. (lit.: "sky stone")[H76]
Siemens (S)	The SI unit of electrical conduction (reciprocal ohm). [H76]
Sigma	In astronomy, a quantitative measure of the random speeds of stars in a collection of stars. If the stars were molecules of gas, darting this way and that, then sigma would be directly related to the temperature of the system. A high sigma is analogous to a high temperature. Sigma is also called the velocity dispersion. [LB90]
Signal Band	The wavelength interval within which a feature (e.g., the 21-cm line) is measured (cf. comparison band). [H76]
Signal-To-Noise Ratio	The ratio of the amount of intelligible meaning in a signal to the amount of background noise. [H76]
Signature	A new particle will have some characteristic behavior in a detector that allows it to be recognized. Particles that decay into others do so in a unique way that is different for every kind of particle. Knowing the properties of the particle allows us to calculate how it will decay. The features that allow a new particle to be identified in a detector are called its signature.
Silicon	(a)Element with atomic number fourteen and the sixth most common metal in the Universe. It is produced by high-mass stars that explode. [C95] (b) A hard brittle gray metalloid element. It has the electronic configuration of neon with four additional outer electrons; i.e., [Ne]3s23p2. Silicon accounts for 27.7% of the mass of the Earth's crust and occurs in a wide variety of silicates with other metals, clays, micas, and sand, which is largely SiO2. It is used widely in semiconductor applications. [DC99]
Silicon Burning	The end of the line for a high-mass star, silicon burning creates iron and other elements of similar mass and presages a supernova. [C95]
Silk Damping
Silver	(a) Element with atomic number 47. It is produced by both the r-process and the s-process, but more by the former. [C95] (b) A transition metal that occurs native and as the sulfide and chloride. It is extracted as a by-product in refining copper and lead ores. Silver darkens in air due to the formation of silver sulfide. It is used in coinage alloys, tableware, and jewelry. Silver compounds are used in photography. Symbol: Ag; m.p. 961.93°C; b.p. 2212°C; r.d. 10.5 (20°C); p.n. 47; r.a.m. 107.8682. [DC99]
Simulations	In science, simulations of physical systems with a computer. (see N-body Simulations.) [LB90]
Singularity	(a) Anomaly in space-time at which a state not in accord with the classical laws of physics obtains. An example is a black hole; another is the moment of the big bang. [A84] (b) A point of infinite curvature of space where the equations of general relativity break down. A black hole represents a singularity; so, perhaps, did the Universe at the first moment of time. [F88] (c) A place, either in space or in time at which some quantity, such as density, becomes infinite. The laws of physics cannot describe infinite quantities and, in fact, physicists believe that infinities do not exist in nature. All singularities, such as the Schwarzschild singularity, are therefore probably the artifacts of inadequate theories rather than real properties of nature. According to Einstein's theory of general relativity, the Universe began in a singularity of infinite density, the big bang. Physicists believe that an improved and yet-to-be discovered modification of general relativity, incorporating quantum mechanics, will show that the Universe did not begin as a singularity. (see Schwarzschild Singularity.) [LB90] (d) A point in space-time at which the space-time curvature and other physical quantities become infinite and the laws of physics break down. [CD99] (e) If the standard big bang theory is extrapolated all the way back to time zero, one reaches an instant of infinite density, infinite pressure, and infinite temperature - an instant that is frequently called the initial singularity. This singularity is sometimes said to mark the beginning of time, but it is more realistic to recognize that an extrapolation to infinite density cannot be trusted. [G97] (f) A region of space-time where physical variables become infinite, such as density, tidal forces, pressure, and world line of observer terminates; a bad place to be. [D89]
Singularity Theorems	In astronomy and cosmology, mathematical proofs that show the conditions under which a mass will gravitationally collapse to form a singularity. The singularity theorems of cosmology, proved in the 1960s, indicate that the current behavior of the Universe, together with the laws of general relativity without quantum mechanical corrections, require that at some definite time in the past the Universe was compressed to a state of zero size and infinite density, called a singularity. The laws of physics break down at a singularity and cannot be used to predict anything during or before the singularity occurred. (see Singularity) [LB90]
Sink	In general, a region where energy is given up, in contrast to a source, where energy is released. [H76]
Sirius ( CMa)	(a) The brightest star in the night sky. It is a white, A-type star that lies just 8.6 light-years from Earth in the constellation Canis Major. Orbiting the main star (officially called Sirius A) is a faint white dwarf, Sirius B. Sirius A is the nearest A-type main-sequence star to Earth; Sirius B is the nearest white dwarf to Earth. [C95] (b) Also called Dog Star. An A1 V star 2.7 pc distant - the (apparently) brightest star in the sky. Its companion (Sirius B) is a white dwarf of about 0.96 M but only about 0.03 R. Period 49.9 years. [H76]
SIS Junction	Superconductor-Insulator-Superconductor Junction. Can be used as the mixer in a radio receiver system. [McL97]
61 Cygni	The first star other than the Sun to have its parallax, and hence distance, measured. The star is a double orange dwarf that lies in the constellation Cygnus 11.4 light-years away. [C95]
Skylight	The faint, diffuse glow of the night sky. It comes from four main sources : airglow, diffuse Galactic light, Zodiacal light, and the light from these sources scattered by the troposphere. [H76] See night-sky light. [H76]
SLAC	The acronym for the Stanford Linear Accelerator Center at Stanford University in California, USA. It is distinguished by having a 2-mile-long linear accelerator in which electrons and positrons can be accelerated for subsequent injection into storage rings such as PEP, an e+e- collider which was commissioned in 1980. It was in the SPEAR rings at SLAC that the J / (psi) meson and the (tau) lepton were first observed in the mid-1970s. However, the most fascinating of SLAC's facilities is the novel SLC (Stanford Linear Collider), consisting of the old linear accelerator together with two new collider arcs. [CD99]
Slepton	The supersymmetric partner of any of the leptons.
Slew	The relatively rapid motion of a telescope (under computer control) as it moves to point at a new position in the sky. Once at the new position the motion of the telescope returns to that required to cancel the effect of the Earth's rotation relative to the stars - the sidereal rate. [McL97]
Slow Nova	A nova whose light curve shows a much more gradual development - i.e., rise time of several days, maximum of several weeks, slower decline, amplitude only about 10 mag. [H76]
Small Magellanic Cloud	SMC: The second largest, and the second nearest, of the galaxies that orbit the Milky Way. It lies in the southern sky, 190,000 light-years away. [C95]
S-Matrix	The S-matrix relates the incoming and out-going states of elementary particles during interactions and scattering experiments. The mathematical structure and properties of the S-matrix has received considerable attention (also called the scattering matrix). [P88]
Smatter	The superpartners of the Standard Model particles. This book argues th at the experimental discovery of smatter will provide us with information that will be essential for gaining insights into the ultimate laws of nature, the primary theory.
SMC X-1	An X-ray source in the Small Magellanic Cloud. It is a binary system with a 3.89-day period. Identified with Sanduleak No. 160, a B0 I supergiant (mv = + 13.6). Because no radial-velocity variations are apparent in Sk 160, the mass of the X-ray emitter must be small relative to Sk 160 (about 2 M if Sk 160 is 20 M), unlike the compact member of CygX-1. (SMC X-1 also called 2U 0115-73) [H76]
Smooth Space	A spatial region in which the fabric of space is flat or gently curved, with no pinches, ruptures, or creases of any kind. [G99]
Snell's Law	For a refracted light beam, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. (also called the Law of Refraction) [H76]
Snickers	A putative satellite galaxy of the Milky Way, reported in 1975. Its reality is in dispute. [BFM2002]
Snowplow Model	A sunspot model in which the expanding current sheath scoops up material like a snowplow, but discards all the accumulated matter when the magnetic field reverses. [H76]
SNR	(a) Supernova Remnant [LLM96] (b) Signal-to-Noise Ratio
SNU	Solar Neutrino Unit [LLM96]
Sobieski	Former name of the southern constellation Scutum. [H76]
Sodium	A soft reactive metal. It has the electronic configuration of a neon structure plus an additional outer 3s electron. Electronic excitation in flames or the familiar sodium lamps gives a distinctive yellow color arising from intense emission at the so called `Sodium-D' line pair. The metal has a body-centered structure. Symbol: Na; m.p. 97.81°C; b.p. 883°C; r.d. 0.971 (20°C); p.n. 11; r.a.m. 22.989768. [DC99]
SOFIA	Stratospheric Observatory for Far-Infrared Astronomy. [LLM96]
Solar	Of the Sun. [A84]
Solar Apex	A point on the celestial sphere lying in the constellation Hercules toward which the Sun and the solar system are moving with respect to the Local Standard of Rest at a rate of about 19.4 km per second (about 4.09 AU per year). [H76]
Solar Burst	see Burst [H76]
Solar Constant	(a) Mean radiation received from the Sun at the top level of Earth's atmosphere: 1.95 cal cm-2 min-1. [A84] (b) Energy received per cm2 per second by a planet at r astronomical units from the Sun (equal to 1.39 × 106 ergs cm-2 s-1 / r2). [H76]
Solar Cycle	The 11-year period between maxima (or minima) of solar activity. Every 11 years the magnetic field of the Sun reverses polarity; hence the more basic period may be 22 years. [H76]
Solar Energy	Is produced by nuclear fusion and comprises almost entirely electromagnetic radiation (particularly in the form of light and heat); particles are also radiated forming the solar wind. [A84]
Solar Flare	Sudden and dramatic release of a huge burst of solar energy through a break in the Sun's chromosphere in the region of a sunspot. Effects on Earth include aurorae, magnetic storms and radio interference. [A84]
Solar Mass	The amount of mass in the Sun, and the unit in which stellar and galactic masses are expressed. [C95]
Solar Motion	The velocity of the Sun through space, relative to the Local Standard of Rest. The solar motion is U = -9 kilometers per second, V = +12 kilometers per second, and W = +7 kilometers per second. [C95]
Solar Neutrinos	The reactions that fuel the sun lead to the emission of photons, which reach the earth as sunlight, and of neutrinos, which we do not see with our eyes but which can be detected in special neutrino detectors. At present there is great interest in these neutrinos, because the number being detected is fewer than expected, and this may be a signal that neutrinos have mass, in which case we could account for the lesser number detected. If they have mass, the experiments to detect them will allow the value of their mass to be measured. [K2000]
Solar Neutrino Unit (SNU)	1 SNU = 10-36 solar-neutrino captures per second per target atom. [H76]
Solar Parallax	(a) The parallax of the Sun, now measured as 8.794". [A84] (b) Angle subtended (8".79) by the equatorial radius of the Earth at a distance of 1 AU. [H76]
Solar Phase Angle ()	Angular distance at the planet between the Earth and the Sun. [H76]
Solar Prominence	Mass of hot, hydrogen rising from the Sun's chromosphere, best observed indirectly during a total eclipse. Eruptive prominences are violent in force and may reach heights of 2 million km; quiescent prominences are relatively pacific but may last for months. [A84]
Solar Rotation	Is differential, the equatorial rotation taking less time than the polar by up to 9.4 Earth-days. [A84]
Solar System	The Sun and all objects gravitationally bound to it. The solar system is roughly a sphere with a radius greater than 100,000 AU, with the Sun at the center. The Sun is overwhelmingly the dominant object. Planets, satellites, and all interplanetary material together comprise only about 1/750 of the total mass. Geochemical dating methods show that the solar system chemically isolated itself from the rest of the Galaxy (4.7 ± 0.1) × 109years ago. [H76]
Solar Velocity	Velocity of the Sun (19.4 km sin the direction lII = 51°, bII = 23°) with respect to the local standard of rest. [H76]
Solar Wind	(a) Stream of charged particles flowing from the Sun at a speed of about 600 km sec-1. It is the effects of the solar wind that produce aurorae in the Earth's upper atmosphere, that cause the tails of comets to stream back from the Sun, and that distort the symmetry of planetary magnetospheres. [A84] (b) A radial outflow of energetic charged particles from the solar corona, carrying mass and angular momentum away from the Sun. Mean number density of solar wind (1971), 5 per cm3; mean velocity at Earth 400 km s-1; mean magnetic field 5 × 10-5 gauss; mean electron temperature 20,000 K; mean ion temperature 10,000 K. The Sun ejects about 10-13 M per year via the solar wind. [H76] (c) A radial outflow of hot plasma from the solar corona. The sun ejects about 10-13 of its mass per year in the solar wind, which carries both mass and angular momentum away from the sun. [Silk90]
Solid Angle	(a) A measure of the angular size of an extended object, equal to the area it subtends on the surface of a sphere of unit radius. [Silk90] (b) Symbol: The three-dimensional analog of angle; it is subtended at a point by a surface (rather than by a line). The unit is the steradian (sr), which is defined analogously to the radian - the solid angle subtending unit area at unit distance. As the area of a sphere is 4 r2, the solid angle corresponding to the revolution (2 radians) is 4 steradians.[DC99]
Solid-State	Usually implies crystalline semiconductor materials used in the electronics industry. [McL97]
Soliton	A finite-amplitude hydrodynamic disturbance which is propagated through a fluid without any change of shape. MHD solitons are also known. [H76]
Solstice	One of the two points on the ecliptic at which the Sun appears to be farthest away from the celestial equator (representing therefore mid-summer or mid-winter). [A84]
Sombrero Galaxy	A spiral galaxy in the constellation Virgo. It was the first galaxy whose rotation was detected. (M104, NGC 4594) [C95]
Sonde	A rocket or balloon carrying instruments to probe conditions in the upper atmosphere. [H76]
Source	Anything which is emitting electromagnetic radiation. [C97]
Source Function	The amount of radiant energy per unit mass per unit solid angle emitted in a specified direction. For the case of LTE, it is equal to the Planck function; for pure, isotropic scattering, it is equal to the mean intensity. [H76]
South Atlantic Anomaly	A disturbance in the geomagnetic field (a region of intense charged-particle fluxes) over the south part of the Atlantic Ocean. It was discovered in early OAO (Orbiting Atronomical Observatory) flights that when the detector passed over that area, the data it collected were not valid. [H76]
South Galactic Pole	A point in the constellation Sculptor toward which our line of sight is perpendicular to and below the Galactic disk. [C95]
Space	Traditionally the three-dimensional theater within which events transpire, explicable by means of Euclidean geometry. In relativity, space is depicted in terms of Non-Euclidean geometries as well. In quantum physics, space may be constructed out of any of a variety of abstractions, such as a "charge space" employed in dealing with electrically charged particles or the "color space" in which quarks can for convenience be plotted. [F88]
Space Charge Wave	An electrostatic wave brought about by oscillations of the charges. [H76]
Space Curvature	see Curvature. [LB90]
Space-Like Path	A trajectory along which U · U > 0. [H76]
Space Motion	Velocity of a star with respect to the Sun; hypotenuse of the right triangle formed by its radial and tangential velocities (cf. peculiar velocity). Space motion vectors are U (in the direction of the galactic anticenter), V (in the direction of galactic rotation), and W (in the direction of the galactic north pole). [H76]
Space-Time	(a) Arena in which events are depicted in the theory of relativity. The orbit of a planet for instance, can be described as a "world line" in a four-dimensional space-time continuum. [F88] (b) The three physical dimensions of space are combined with time, treated as a fourth dimension, to constitute the space-time continuum that is used as the fundamental framework of the theory of relativity. [Silk90] (b) A union of space and time originally emerging from special relativity. Can be viewed as the "fabric" out of which the Universe is fashioned; it constitutes the dynamical arena within which the events of the Universe take place. [G99] (c) In both the special and general theories of relativity it is necessary to treat space and time on an equal footing. The ensuing mathematical space is called `space-time'. [D89]
Space-Time Continuum	(a) A four-dimensional framework in which events take place. [C97] (b) Einsteinian concept of the Universe in accordance with his theories of relativity; Four-dimensional actuality, in which any anomaly is known as a singularity. [A84]
Space-Time Foam	Frothy, writhing, tumultuous character of the spacetime fabric on ultramicroscopic scales, according to a conventional point-particle perspective. An essential reason for the incompatibility of quantum mechanics and general relativity prior to string theory. [G99]
Space Velocity	A star's total velocity with respect to the local standard of rest. This is the combination of the star's U, V, and W velocities: space velocity = sqrt (U2 + V2 + W2). For example, the Sun (U = -9, V = +12, W = +7) has a space velocity of 17 kilometers per second. [C95]
Spallation	The process in which an incoming beam of particles or energy collides with a substance, reacts with it, and knocks off pieces of it. [LB90]
SPAN	Space Physics Analysis Network [LLM96]
Spark Chamber	A means of detecting high energy particles by the trail of ionizations left as they pass through a chamber containing many charge plates. [McL97]
Spark Spectra	The spectra of ions often produced by a spark discharge (cf. arc spectra). [H76]
Sparticles	Hypothetical particles which are predicted by some Grand Unified Theories. [C97]
Special Relativity	(a) Einstein's theory of time and space, formulated in 1905, which shows how measurements of length and time differ for observers in relative motion. [LB90] (b) Theory formulated by Albert Einstein comprising two basic yet very original propositions: that a spaceship (or other enclosed vessel) traveling at uniform speed through space contains its own space-time continuum, and that a ray of light passes an observer at the speed of light no matter how (uniformly) fast nor in what direction the observer is travelling. One consequence of this theory was the equation of mass (m) with energy (E), formulated as E = mc2 (where c is the speed of light). Ten years later, Einstein produced his General Theory of Relativity. [A84] (c) A theory of space, time and motion formulated by Einstein in 1905. The theory was generalized in 1915 to include gravity (see General Theory of Relativity). [D89] (d) The constraints of special relativity are two conditions that Einstein pointed out should be satisfied by any acceptable physical theory. Somewhat oversimplified, these conditions are, first, that light moves at the same speed in vacuum regardless of how it is emitted and, second, that scientists working in different labs moving with different relative speeds should formulate the same natural laws. The constraints imposed by these conditions have surprising implications for the structure of acceptable theories. For example, the Schrodinger equation of quantum theory does not satisfy these conditions. But when it was generalized by Dirac to do so, the resulting equation led to the prediction of antiparticles, which need not have existed from the point of view of quantum theory alone. [K2000]
Specific Gravity	Ratio of the mass of a given volume of a substance to that of an equal volume of water. [H76]
Specific Heat	Ratio of the quantity of heat required to raise the temperature of a unit mass of a substance by a unit amount to that required to raise the temperature of an equal mass of water by the same amount. [H76]
Specific Intensity	see Intensity [H76]
Speckle Interferometry	The technique of recovering the diffraction-limited angular resolution of a telescope by analysis of images obtained using a very high speed camera system to "freeze" the blurring due to atmospheric turbulence. [McL97]
Spectra	Atoms can exist in a number of discreet energy levels. They emit or absorb photons when they make transitions from one level to another. The energies of the photons emitted or absorbed by one atom are different from those of all other atoms. The photon energies are directly related to their frequencies, which set their colors in the spectrum, so by observing the colors of the photons, it is possible to determine which atoms are being observed. This can be done in a laboratory, and it can also be done with the light reaching us from stars, near or distant, which enables us to identify the atoms that stars are made of. Only the same ninety-two elements we find on earth are seen throughout the universe. [K2000]
Spectral Bandwidth	The wavelength, or frequency range over which photons are detected at any one time; some detectors can operate in one or more bands placed within a broader range of spectral response.
Spectral Classification	Commonly, the system devised by Annie Cannon combining the perceived colour of a star with its spectral characteristics. Very generally, of the overall sequence O B A F G K M R N S, stars in the group O B A are white or blue and display increasing characteristics of the presence of hydrogen; in F G are yellow and show increasing calcium; in K are orange and strongly metallic; and in M R N S are red and indicate titanium oxide through carbon to zirconium oxide bands. The groups are numerically subdivided, according to other characteristics, and there are further small classes for very unusual categories of star. Different methods of classification exist but are not in such common use. [A84]
Spectral Energy Distribution	SED: The distribution of a star's light among various wavelengths. [H76]
Spectral Index	The power of the frequency to which the intensity at that frequency is proportional. It is positive for thermal radiation, negative for nonthermal radiation. [H76]
Spectral Lines	(a) Dark lines visible in an absorption spectrum, or bright lines that make up an emission spectrum. They are caused by the transference of an electron in an atom from one energy level to another; strong lines are produced at levels at which such transference occurs easily, weak where it occurs with difficulty. Ionization of certain elements can affect such transferences and cause problems in spectral analysis. [A84] (b) Discrete emissions (or absorptions) in frequency, usually formed by atomic transitions. The essential difference between optical line spectra and X-ray spectra is that the former correspond to energy changes in the outer electrons in an atom, and the latter to energy changes in the inner electron orbitals. Gamma rays usually correspond to energy changes in the nucleus. Infrared radiation is produced by high-n transitions of atoms or by the vibration or rotation of molecules. Thermal radio emission is usually produced by still higher-n transitions (the notation 109 corresponds to a transition in a hydrogenic atom between the principal quantum number n = 109 and n' = n = n + 1 = 110; similarly, a -transition indicates n = 2. etc.). [H76] (c) Emission or absorption at a discrete wavelength or frequency, caused by atomic or molecular transitions. In the case of atoms, the transitions involve the jump of an electron from one orbit to another; a quantum of light is emitted if the electron jumps toward the nucleus and absorbed if it jumps outward. [Silk90]
Spectral Ratio	The ratio of electromagnetic wavelengths from different cosmic epochs. This gives the expansion factor of the Universe. [C97]
Spectral Bandwidth	The total wavelength or frequency range over which photons can be detected with reasonable efficiency.
Spectral Series	(a) All spectral lines of a given atom arising from transitions with a common lower energy level. [H76] (b) A group of related lines in the absorption or emission spectrum of a substance. The lines in a spectral series occur when the transitions are all between the same energy level and a set of different levels. [DC99]
Spectral Type	Classification of a star's spectrum, which correlates with the star's temperature and color. There are seven main spectral types. From hot and blue to cool and red, they are O, B, A, F, G, K, and M. For further precision, astronomers divide each spectral type. For example, from warmest to coolest, spectral type G is G0, G1, G2, G3, and so on to G9. The Sun is spectra] type G2. [C95]
Spectrograph	(a) A device, usually based on a finely etched grate that performs the function of a prism, for breaking up light into its constituent parts and making a photographic or electronic record of the resulting spectrum. When lacking a means for recording the spectrum, the device is called a spectroscope. [F88] (b) An instrument that records the amount of light in each range of wavelength, that is, in each range of color. In general, each type of astronomical object, such as a star or a galaxy, will emit a characteristic spectrum of light. (see Spectrum.) [LB90]
Spectroheliograph	Device with which spectra of the various regions of the Sun are obtained and photographed. [A84]
Spectrometer	A spectroscope fitted with a device such as a photoelectric cell for measuring the spectra observed with it. [H76]
Spectroscopic Binaries	Stars whose binary nature can be detected from the periodic Doppler shifts of their spectra, owing to their varying velocities in the line of sight. Double-lined spectroscopic binaries have two sets of spectral features, oscillating with opposite phases. Single-lined spectroscopic binaries have only one set of oscillating spectral lines, owing to the dimness of the secondary component. Spectroscopic binaries are typically of spectral type B, with almost circular orbits (whereas long-period M-type binaries have highly eccentric orbits). [H76]
Spectroscopic Parallax	Parallax for a group of stars based on the magnitudes and spectral types of the member stars. Spectroscopic parallax is by far the most common method of determining stellar distances. [H76]
Spectroscopy	(a) Study of spectra; in astronomy, the investigation of the composition of celestial bodies using information derived from spectral lines. [A84] (b) The quantitative study of the spectral or energy content in a beam of light. Usually achieved by spreading or dispersing the light into its constituent wavelengths by means of a diffraction grating and recording the image with a CCD or other electronic imaging detector. The patterns, wavelengths, intensities and shapes of the spectral features yield considerable physical information. [McL97]
Spectroscopy	1. The production and analysis of spectra. There are many spectroscopic techniques designed for investigating the electromagnetic radiation emitted or absorbed by substances. Spectroscopy, in various forms, is used for analysis of mixtures, for identifying and determining the structures of chemical compounds, and for investigating energy levels in atoms, ions, and molecules. In astronomy it is used for determining the composition of celestial objects and for measuring red shifts. 2. Any of various techniques for analyzing the energy spectra of beams of particles or for determining mass spectra. [DC99]
Spectrum	(a) The breakdown of light into a rainbow of colors. A good spectrum reveals a star's spectral type, radial velocity (from the spectrum's Doppler shift), and metallicity. (plural: spectra) [C95] (b)A record of the distribution of matter or energy (e.g., light) by wavelength. Spectra can be studied to learn the chemical composition and motion of stars and galaxies. [F88] (c) The amount of light in each range of wavelength, that is, in each range of color. The term spectrum can also be applied more generally to the intensity of something at each length scale. An object that emits radiation in a continuous range of colors is said to have a continuous spectrum. An object that emits radiation only at certain wavelengths is said to have emission lines; objects that absorb radiation only at certain wavelengths are said to have absorption lines. [LB90]
Spectrum Variables	Main-sequence Am or Ap stars whose spectra show anomalously strong lines of metals and rare earths which vary in intensity by about 0.1 mag over periods of about 1-25 days. They are characterized by large magnetic fields (103-104 gauss) at the surface, small variations in light and color, and small projected rotational velocities. These peculiarities are sometimes interpreted in terms of an oblique rotator. (2 CVn stars) [H76]
Speed of Light	(a) c = 299,792 km sec-1 (186,180 miles sec-1). [A84] (b) Light and all other massless particles travel in vacuum with a speed, usually labeled c, whose value is about three hundred million meters a second. Special relativity implies that no particle or signal can move faster than the speed of light and that photons always have this speed, regardless of the speed of their source. [K2000]
Speed-of-Light Circle	see Velocity-of-Light Radius. [H76]
Sphere	The outer surface of a ball. The surface of a familiar three-dimensional ball has two dimensions (which can be labeled by two numbers such as "latitude" and "longitude," as on the surface of the earth). The concept of a sphere, though, applies more generally to balls and hence their surfaces, in any number of dimensions. A one-dimensional sphere is a fancy name for a circle; a zero-dimensional sphere is two points (as explained in the text). A three-dimensional sphere is harder to picture; it is the surface of a four-dimensional ball. [G99]
Spheres	Concept probably older than the ancient Greeks, in which the Sun, Moon, planets and the stars were thought to orbit the Earth travelling on their own crystalline but - except for that of the stars - transparent spheres. [A84]
Spherical Aberration	see Aberration. [A84]
Spherical Collapse	Initial stage in the collapse of a star, followed by gravitational collapse and finally singularity. [A84]
Spherical Space	A three-dimensional space whose geometry resembles that of the surface of a sphere and is said to have positive curvature. [Silk90]
Spica	(a) ( Vir): The brightest star in the constellation Virgo, Spica consists of two blue B-type stars about 220 light-years from Earth. [C95] (b) A double-lined spectroscopic binary (B1.5 V, late B) (period 4.01452 days, e = 0.146, i = 65°) about 80 pc distant. Component A (10.9 M) is a Cephei star which seems to be near core hydrogen exhaustion. [H76]
Spicule	A short-lived (about 5 minutes), narrow jet of gas spouting out of the solar chromosphere. Spicules tend to cluster at the edges of supergranulation cells. [H76]
Spin	(a) A quantum property of all particles which denotes the intrinsic angular momentum of the particle. [C97] (b) The intrinsic angular momentum possessed by many particles. It can be thought of as resulting from the particles spinning about an axis through their centers. In contrast to orbital angular momentum, spin is quantized in integer and half-integer units of . Fundamentally, spin describes how quantum fields transform under the transformations of special relativity. [CD99] (c) A quantum-mechanical version of the familiar notion of the same name; particles have an intrinsic amount of spin that is either a whole number or half a whole number (in multiples of Plancks constant), and which never changes. [G99] (d) The intrinsic angular momentum of an elementary particle, as by the particle's spinning on its axis. Spin is quantized in units of Planck's constant of action, h, so that, e.g., "spin 1," means spin = 1h. Particles with integral spin (0, 1) are called bosons; those with half spin are fermions. [F88] (e) Electrons, protons and neutrons have an intrinsic angular momentum, known as spin, of 1/2 and a magnetic moment parallel (or antiparallel) to that angular momentum. When electrons are combined together to form an atom or ion there is a resultant angular momentum which is a combination of the intrinsic spin of the electrons and the angular momentum due to their motion about the nucleus, which is called the spin of the atom or ion; there is also a magnetic moment associated with this angular momentum (unless it is zero). Such atoms or ions with nonzero spin are therefore magnetic atoms or ions. The protons and neutrons in a nucleus also combine together in the same way to give a nuclear spin, but the magnetic moments associated with the nuclei are much smaller, and are only important for magnetic materials at exceedingly low temperatures. [D89] (f) Spin is a property that all particles have. It is as though particles were always spinning at a fixed rate (which could be zero), which can be different according to the type of particle. It is not quite right to think of them actually spinning, because the particles do not have to have spatial extension to have spin; calling this property spin is an analogy. The amount of spin is required by the quantum theory to come in definite amounts; if the unit is chosen to be Planck's constant, h, divided by 2, then particles can have zero spin, half a unit of spin, one unit of spin, etc. [K2000]
Spin-Flip Collisions	Collisions between particles in which the direction of the spin angular momentum changes. Since the total angular momentum is conserved, the orbital angular momentum must be changed in magnitude or direction or both. (see 21-cm Radiation) [H76]
Spin Network	A term used by Roger Penrose to denote collections or networks of quantum mechanical spinors. Although they were not created within any background space, Penrose discovered that these spin networks had properties that were similar to those of Euclidian angles in a three-dimensional space. One of Penrose's early goals was to extend the spin network idea by employing twistors and in this way derive the properties of the space-time quantum mechanically. [P88]
Spinor	A mathematical object that reverses sign after a rotation by 360 degrees and returns to itself only after a rotation by 720 degrees. (More familiar mathematical and physical objects return to themselves after a rotation by 360 degrees.) A physical example showing spinor behavior is the following: Paint each face of a cube a different color and connect each of the eight corners of the cube to the corresponding corners of the room with threads. Now rotate the cube by 360 degrees. The threads are hopelessly tangled up, even though the cube has returned to its original position. Rotation of the cube by another 360 degrees, however, allows one to untangle the threads. Spinors involve complex numbers. (see Complex Numbers) [LB90]
Spin-Up	A discontinuous increase in the pulse frequency of a pulsar. [H76]
Spiral Density Wave	A wave, due to a local increase in the gravitational field, that produces a series of alternate compressions and rarefactions as it propagates with fixed angular velocity in a rotating galaxy. The compression also acts on interstellar gas in the galaxy, which is triggered to form stars on the leading edges of the spiral arms. The large-scale structure of spiral galaxies can be understood in this way. [Silk90]
Spiral Galaxy	A galaxy with a prominent nuclear bulge and luminous spiral arms of gas, dust, and young stars that wind out from the nucleus. Masses span the range from 1010 to 1012 M. [Silk90]
Spiral Nebula	A spiral galaxy - not really a nebula at all (although many do appear nebulous). [A84]
Spiritualism	Belief that material interactions alone cannot account for all phenomena, and that some - e.g., thought - are due to the fundamentally insensible actions of intangibles. [F88]
Spitzer-Oort Hypothesis	A hypothesis which explains the mass motion of the interstellar gas in terms of the gas pressure gradients existing between H I and H II regions. [H76]
Spitzer-Schwarzschild Scattering Mechanism	The process by which stars in the Milky Way's disk encounter interstellar clouds and are accelerated by them. Over time, this perturbs the stars, so that older disk stars have more elliptical orbits, larger velocity dispersions, and greater scale heights than younger disk stars. This mechanism cannot, however, explain the motions of halo stars. [C95]
Spörer's Law of Zones	The equatorward drift of average sunspot latitudes. [H76]
Spontaneous Emission	(a) Radiation emitted by an isolated body. [LB90] (b) An excited atom can shed its excitation by radiating a photon in a spontaneous emission. This emission is independent of external radiation and is entirely random and uncontrolled. [D89]
Spontaneous Symmetry Breaking	(a) The breaking of an exact symmetry of the underlying laws of physics by the random formation of some object. For example, the rotational in variance of the laws of physics can be broken by the randomly chosen orientation of an orthorhombic crystal that condenses as the material is cooled. In the standard model of particle physics, the symmetry between electrons and neutrinos is spontaneously broken by the values that are randomly chosen by the Higgs fields. In grand unified theories, the symmetry between electrons, neutrinos, and quarks is spontaneously broken by the values chosen randomly by the Higgs fields. [G97] (b) Any situation in physics in which the ground state (i.e. the state of minimum energy) of a system has less symmetry than the system itself. For example, the state of minimum energy for an iron magnet is that in which the atomic spins are all aligned in the same direction, giving rise to a net macroscopic magnetic field. By selecting a particular direction in space. the magnetic field has broken the rotational symmetry of the system. However, if the energy of the system is raised, the symmetry may be restored (e.g. the application of heat to an iron magnet destroys the magnetic field and restores rotational symmetry). [CD99] (c) In many physical systems the actual state of the system does not reflect the underlying symmetries of the dynamics because the manifestly symmetric state is unstable. The system then trades stability for asymmetry. The symmetry breaking in this case is said to be spontaneous. [D89] (d) Often the equations of a theory may have certain symmetries, though their solutions may not; the symmetries are hidden, or broken. For example, the equations may describe several particles in identical ways, so the equations are unchanged if the particles are interchanged, but the solutions may give the particles different properties. This phenomenon is called spontaneous symmetry breaking. [K2000]
SPOT	Satellite Probatoire d'Observation de Ia Terre [LLM96]
S-Process	The process by which elements heavier than copper are formed through a slow flux of neutrons. The s-process operates in red giant stars; prominent s-process elements include barium, zirconium, yttrium, and lanthanum. [C95]
Sputnik 1	First artificial Earth satellite, launched by the Soviet Union on 4 October 1957. (lit.: companion) [A84]
Squark	The supersymmetric partner of any of the quarks. [K2000]
SSB	Single Side Band [LLM96]
SSPM	Solid State Photomultiplier [LLM96]
Stable Particle	Particles that do not decay into others. (dee also Decay) [K2000]
Stability	A measure of how hard it is to displace an object or system from equilibrium. Three cases are met in statics (see equilibrium). They differ in the effect on the center of mass of a small displacement. An object's stability is improved by: (a) lowering the center of mass; or (b) increasing the area of support; or by both. (see also Equilibrium) [DC99]
Standard Big Bang Model	The Friedmann - Lemaître cosmological models of an isotropic and homogeneous Universe composed of expanding matter and radiation. There are three possible choices for the geometry of space in a standard Big Bang model: space can be positively curved, like the surface of a sphere, in which case the Universe is finite, closed, and will eventually recollapse; or, space can either be Euclidean or have negative curvature (like a saddle-shaped surface), in which case the Universe is infinite, open, and will expand forever. In all three models, space is unbounded. [Silk90]
Standard Candle	An object - usually a star or a galaxy of known intrinsic brightness. Measuring the apparent brightness of a standard candle yields its distance. [C95]
Standard Deviation ()	The root mean square deviation from the arithmetic mean. [H76]
Standard Epoch	A date and time that specifies the reference system to which celestial coordinates are referred. Prior to 1984 coordinates of star catalogs were commonly referred to the mean equator and equinox of the beginning of a Besselian year. Beginning with 1984 the Julian year has been used, as denoted by the prefix J, e.g., J2000.0. [S92]
Standard Error (s.e.)	The standard deviation of a distribution of means or any other statistical measure computed from samples. It is equal to 1.4826 times the probable error. [H76]
Standard Model of Cosmology	Big bang theory together with an understanding of the three nongravitational forces as summarized by the standard model of particle physics. [G99]
Standard Model of Particle Physics	(a) A theory of particle interactions, developed in the early 1970's, which successfully describes electromagnetism, the weak interactions, and the strong interactions. The theory consists of two parts, quantum chromodynamics to describe the strong interactions, and the unified electroweak theory to describe the electromagnetic and weak interactions. [G97] (b) The very successful theory of quarks and leptons and their interactions is called the Standard Model by particle physicists. The name arose historically as the theory developed and then was difficult to change because it is widely used. The Standard Model is the most complete mathematical theory of the natural world ever developed and is well tested experimentally. [K2000]
Standard Ruler	Any extended celestial object which is more or less constant diameter. It can be used to gauge distances, because the further away it is, the smaller it will appear. [C97]
Standing Wave	A pattern of oscillations in space in which the regions of maximum displacement and of zero displacement (the nodes) remain fixed in position. [H76]
Standstill	An interval in the cycle of a variable star during which the brightness temporarily stops changing. [H76]
STP	Standard Temperature and Pressure [LLM96]
Star	A celestial object that generates energy by means of nuclear fusion at its core. To do this it must have more than about 0.08 the sun's mass. If, for instance, the planet Jupiter were some fifty to one hundred times more massive than it is, fusion reactions would transpire in its core and it would be a star. See planet. [F88]
Star Cluster	A gravitationally bound aggregation of stars, smaller and less massive than galaxies. "Globular" clusters are the largest category; they are old, and may harbor hundreds of thousands to millions of stars, and are found both within and well away from the galactic disk. "Open" clusters are smaller, have a wide range of ages, and reside within the disk. [F88]
Star Counts	Determination of the number of stars in a region of the sky as a function of apparent magnitude and sometimes color. [C95]
Star Stream	Discovered by Kapteyn in 1902, a star stream is a group of stars traveling in more or less the same direction. Kapteyn found what he thought were two oppositely directed star streams, but astronomers now recognize that these simply reflect the tendency of stars to have their largest velocities in the U direction. [C95]
Star Streaming	A phenomenon that arises because the mean random speeds of the stars are different in different directions. The direction of star streaming is the direction along which the mean random speed has a maximum value. The phenomenon is caused by the rotation of the Galaxy. [H76]
Star System	A few stars that orbit each other. For example, a double star system consists of two stars; a triple star system consists of three stars; and so on. [C95]
Starburst Galaxy	Any galaxy in which an anomalously large rate of star formation is taking place. [C97]
Stark Effect	Broadening or splitting of a spectral line caused when a radiating atom or ion is influenced by an electric field, which slightly changes the energy level of the atom. Stark broadening is proportional to the ion and electron density in a plasma and is a good indicator of atmospheric pressure in a stellar atmosphere and hence of the star's luminosity. [H76]
Starlight	Energy (seen as light) produced by a star through nuclear fusion. [A84]
STARLINK	A software environment and suite of programs for astronomical data analysis developed in the UK and supported by the Rutherford-Appleton Labs. [McL97]
Stat-Coulomb	The unit of charge in the cgs electrostatic system. 1 stat-coulomb = 3.3 × 10-10 coulombs. [H76]
State Space	the mathematical space whose points represent the states of a physical system. [D89]
Static Limit	In the Kerr solution to Einstein's equations, a surface on which a particle would have to travel at the local light velocity in order to appear stationary to an observer at infinity, and just inside which no particle can remain stationary as viewed from infinity. The stationary limit lies outside the event horizon, touching it only at the poles. (In the Schwarzschild solution, the stationary limit coincides with the event horizon.) (also called stationary limit) [H76]
Static Universe	A Universe whose radius of curvature is constant and independent of time, as in the Einstein Universe. [H76]
Stationary Limit	In the Kerr solution to Einstein's equations, a surface on which a particle would have to travel at the local light velocity in order to appear stationary to an observer at infinity, and just inside which no particle can remain stationary as viewed from infinity. The stationary limit lies outside the event horizon, touching it only at the poles. (In the Schwarzschild solution, the stationary limit coincides with the event horizon.) (also called Static Limit) [H76]
stationary nonequilibrium state	time-independent state of a system subjected to fixed constraints. [D89]
Stationary Point	(Of a planet): The position at which the rate of change of the apparent right ascension (see Apparent Place) of a planet is momentarily zero. [S92]
Stationary Wave	A standing wave; the pattern formed when two waves of the same amplitude and frequency move simultaneously through a medium in opposite directions. [H76]
Statistical Distribution	The range of variation of some quantity in a population, obtained by sampling many members of the population. For example, the statistical distribution of the height of American males could be obtained by sampling 10,000 randomly chosen males and counting the number of them within each range of heights. In cosmology, the distance between pairs of galaxies, averaged over a large number of galaxies, would constitute a statistical distribution. [LB90]
Statistical Equilibrium	A state in which the average density of atoms per cubic centimeter in any atomic state does not change with time and in which, statistically, energy is equally divided among all degrees of freedom if classical concepts prevail. [H76]
Statistical Error	The uncertainty resulting from a measurement of purely random events. Such an uncertainty is defined as bracketing a range of values within which the correct value has a 66% chance of lying. For example, a value of (100 ± 10) obtained from a given measurement means that the true value has a 66% chance of lying between 90 and 110, and a 34% chance of being either above or below this range. [H76]
Statistical Mechanics	The area of physics that analyzes the behavior of a system with very many members, such as a gas with many individual molecules. In such a situation, the behavior of the whole system is obtained by averaging over the behavior of individual members. [LB90]
Statistical Parallax	The mean parallax for a group of stars which are all at approximately the same distance, determined from their radial velocities and from the tau components of their proper motion. [H76]
Statistical Weight	g: The probability that the state will appear under a given set of conditions. Usually, the number of ordinarily degenerate substates contained in the state; e.g., the (2l + 1)m states of an atom in the absence of a magnetic field. [H76]
Steady State Theory	(a) Theory that the expanding Universe was never in a state of appreciably higher density - i.e., that there was no "big bang" - and that matter is constantly being created out of empty space in order to maintain the cosmic matter density. [F88] (b) A cosmological theory propounded by Bondi, Gold, and Hoyle in which the Universe has no beginning and no end and maintains the same mean density, in the face of its observed expansion, by the continuous creation of matter at the current rate of 2.8 × 10-46 g cm-3 s-1 (or roughly one nucleon per cubic kilometer per year). Discovery of the microwave background has persuaded most astronomers to reject the steady-state theory. [H76]
Stefan-Boltzmann Constant	(a) : The constant of proportionality relating the luminosity of a star to its absolute temperature: = 5.67 × 10-5 ergs cm-2 (deg-K)-4 s-1. [H76] (b) () A constant relating the energy radiated by a black body to its absolute temperature and incorporated in the Stefan-Boltzmann radiation law which states that the energy radiated per unit time is given by (T4 - T40), where is the Stefan-Boltzmann constant and T and T0 are the absolute temperatures of the body and its surroundings expressed in kelvins. [JM92]
Stefan's Law	The flux of radiation from a blackbody is proportional to the fourth power of its absolute temperature: L = 4R2T4. [H76]
Stellar	Of a star, of the stars. [A84]
Stellarator	A type of plasma machine. It has a twisted-field configuration in the form of a figure 8 to fold the plasma back on itself; therefore, unlike a pinch machine. it has no ends where the plasma can leak out. Stellarators and tokomaks resemble each other in that both are toroidal devices that attain equilibrium and MHD stability through rotational transform and shear; they differ mainly in the way they attain these properties. [H76]
Stellar Evolution	(a) How a star changes with time. [C95] (b) The building of complex atomic nuclei from simpler nuclei in stars, with the result that succeeding generations of stars and planets contain a greater variety of chemical elements than did their predecessors. [F88]
Stellar Halo	see Halo [C95]
Stellar Parallax	see Parallax [C95]
Stellar Population	A Galaxy-wide group of stars of all types that have similar ages, locations, kinematics, and metallicities. As astronomers presently know the Milky Way, they recognize four stellar populations: the thin disk; the thick disk; the stellar halo; and the bulge. [C95]
Stellar Wind	a steady or unsteady outflow of material from the surface of a star. In many classes of star hot coronae are observed and these are believed to be due to heating by waves generated in the upper layers of the star. This results in the outflow of mass in the form of a stellar wind. For a star like the Sun, the mass outflow in the solar wind amounts to only about 10-13 M y-1 but in massive blue supergiant stars the mass loss in the form of stellar winds can amount to as much as 10-4 to 10-5 M y-1. [D89]
Stephan's Quintet	A highly disturbed cluster of five peculiar galaxies (NGC 7317, NGC 7318A, NGC 7318B, NGC 7319, NGC 7320) in Pegasus which seem to exhibit gaseous connecting bridges. Four have large redshifts (of the order of 5700-6700 km s-1), but the fifth member (NGC 7320) has a much smaller redshift (800 km s-1). Discovered in 1877 by M. E. Stephan. [H76]
Steradian (sr, 0)	A unit of solid (three-dimensional) angular measure. One steradian is equal to the angle subtended at the centre of a sphere by an area of surface equal to the square of the radius. The name for the unit seems to have come into use about 1880 and was comparatively common by the turn of the century. The surface of a sphere subtends an angle of 4 steradians at its centre. [JM92]
Sticking Coefficient	The fraction of all atoms (e.g., hydrogen) incident on an interstellar dust grain that become adsorbed. [H76]
Stilb (sb)	1 stilb = 1 candela per square centimeter. [H76]
Stimulated Emission	(a) Radiation emitted by a body, such as an atom, when it is bombarded by radiation. The stimulated radiation has the same wavelength and direction as the bombarding radiation. [LB90] (b) Incident radiation can induce an atom to radiate by stimulated emission at a rate which depends on the intensity of the incident light. Stimulated emission has a definite phase relationship with the incident light and is the driving force behind the laser, it was discovered by Einstein. [D89]
Stochastic Cooling	The gathering(i.e., focusing) of clouds of subatomic particles in an accelerator by monitoring their scattering vectors and altering the magnetic environment in an accelerator storage ring to keep them close together. First employed in storing particles of antimatter, which are expensive to manufacture and ought not to be wasted. [F88]
Stokes	1 St = 1 cm2 s-1. [H76]
Stokes Parameters	(a) A way of characterizing the polarization state of light which is closely related to actual measurements. [McL97] (b) Four parameters (I, Q, U, V) which must be evaluated to describe fully a beam of polarized light. They involve the maximum and minimum intensity, the ellipticity, and the direction of polarization. The nonvanishing of V indicates the presence of elliptical polarization.
Storage Ring	A ring in which particles are kept in a circular motion, suspended in a magnetic field, until they can be injected into the larger ring of an accelerator. [F88]
Strain	The fractional change in dimension produced by a stress applied to a body. Tensile strain applies to the stretching of a body. It is the change in length divided by the original length (l/l). Bulk strain occurs when a body is subjected to a change of pressure. It is the change in volume divided by the original volume. Shear strain occurs when an angular deformation occurs, and is equal to the angular displacement produced. (see also Stress) [DC99]
Strange	A flavor of quark. (see Flavor) [G97]
Strange Attractor	(a) A path in phase space that is not closed. Strange attractors are characteristic of chaotic behavior. (see Attractor; Phase Space) [DC99] (b) An attractor in the phase space of some dynamical system having fractional dimensionality. Strange attractors are associated with chaotic dynamics. [D89]
Strange Particles	The collective name for a group of strongly interacting particles possessing the property of strangeness. According to one theory, the strange particles are regarded as the higher quantum states of the nucleus. [H76]
Strangeness	(a) A property of hadrons which may have a zero or non-zero value, depending on their rate of decay. [DC99] (b) A property ascribed to certain hyperons whose lifetimes before decay are abnormally long (about 10-8 to 10-10 seconds) relative to their rates of production (about one every 10-23 seconds). Like parity, strangeness is conserved in strong interactions but not in weak ones. [H76] (c) A quantum number associated with the strange quark. Strangeness is conserved by the strong nuclear force. [CD99] (d) A property possessed by all matter containing a strange quark. This quark has charge -1/3 and partners the charmed quark in the second generation of particles. [D89]
Stratosphere	The region of Earth's atmosphere immediately above the troposphere. It starts at a height of about 15 km and goes to a height of about 50 km. The temperature increases from about 240 K to about 270 K. [H76]
Streamline	(a) A line following the direction of the fluid in laminar or streamline flow. Where the speed increases, as it does in a narrower section of a pipe, the streamlines are closer together. (see also Laminar Flow) [DC99] (b) Path followed by a moving particle in a fluid when the flow is laminar - i.e., nonturbulent. It is a line in a fluid such that the tangent to it at every point is in the direction of the velocity flow. [H76]
Strehl (Intensity) Ratio	(a) The ratio of the peak intensity in the point spread function of an optical system to that of the equivalent diffraction-limited system. [McL97] (b) A criterion of image quality used in lens design: it quantifies aberration. It is defined as the ratio of the luminous intensity in the central maximum of the actual image (i.e. the airy disc for a circular source) to that of an aberration-free image. In Germany it is called the definitionshelligkeit. [JM92]
Stress	When a system of opposing forces acts on a body the material is subject to some form of stress. This is expressed in terms of one of the forces divided by the area on which it acts. Bulk stress is a change of pressure applied to a fluid, or applied by a fluid to a solid. Tensile stress is a stress that stretches a body. Shear stress causes a deformation without any change of volume. (see also Strain) [DC99]
String	(a) Fundamental one-dimensional object that is the essential ingredient in string theory. [G99] (b) Nambu's original idea that the elementary particles could be described as extended, one-dimensional objects was called string theory. Since the ends of Nambu's strings whipped around at the speed of light they were also called light strings. Later attempts to include the spin half fermions within a string theory led to the term spinning strings. Strings that possess supersymmetry are called superstrings. Heterotic strings combine spaces of two different dimensionalities. The term string is used in a generic way to describe all these different variations, including superstrings. [P88] (c) The hypothesized, basic constituents of matter, according to new theories of physics. In earlier theories of physics, the basic constituents of matter were point-like particles, such as electrons, which interacted with other particles at a point. According to the string theory, the basic constituents are 1-dimensional structures called strings. There are completely different strings, called cosmic strings, which can form according to some theories and which may extend for great distances in space. Postulated to have formed as a result of processes in the early Universe, cosmic strings are 1-dimensional structures of enormous energy, extending for perhaps thousands or millions of light years in space. There is no good observational evidence that either kind of strings exist. (See superstring theory.) [LB90]
String Coupling Constant	A (positive) number that governs how likely it is for a given string to split apart into two strings or for two strings to join together into one-the basic processes in string theory. Each string theory has its own string coupling constant, the value of which should be determined by an equation; currently such equations are not understood well enough to yield any useful information. Coupling constants less than 1 imply that perturbative methods are valid. [G99]
String Mode	A possible configuration (vibrational pattern, winding configuration) that a string can assume. [G99]
String Theory	(a) A theory in which the fundamental constituents of matter are not particles but tiny one-dimensional objects, which we can think of as strings. These strings are so minute (only 10-33 cm long) that, even at current experimental energies, they seem to behave just like particles. So, according to string theory, what we call "elementary particles" are actually tiny strings. each of which is vibrating in a way characteristic of the particular "elementary particle". [CD99] (b) Unified theory of the Universe postulating that fundamental ingredients of nature are not zero-dimensional point particles but tiny one-dimensional filaments called strings. String theory harmoniously unites quantum mechanics and general relativity the previously known laws of the small and the large, that are otherwise incompatible. Often short for superstring theory. [G99] (c) Theory that subatomic particles actually have extension along one axis, and that their properties are determined by the arrangement and vibration of the strings. [F88] (d) The latest theory of fundamental physics in which the basic entity is a one-dimensional object rather than the `zero-dimensional' point of conventional elementary particle physics. [D89] (e) String theory is a theory that aims to unify all of the forces and particles of nature and explain why they are as they are. In string theory, there is only one force (gravity), in ten space-time dimensions, but when looked at from our four-dimensional world, the extra dimensions imply the other forces we observe. Particles are strings that vibrate in different ways to account for their various properties. String theories appear to allow the construction of a quantum theory of gravity. String theory is very much research in progress. [K2000].
Strömgren Spheres	Zones of ionized hydrogen gas surrounding hot stars embedded in interstellar gas clouds; they are called additionally H II zones. [A84]
Strong Equivalence Principle	(a) A generalization of the Einstein equivalence principle, stating that all bodies, including those with self-gravitational binding, fall with the same acceleration, and that physics in freely falling reference frames, including local gravitational physics, is independent of the velocity and location of the frame. [D89] (b) In a freely falling and non-rotating laboratory the laws of physics, including their numerical content, are the same everywhere including gravity-free space. [H76]
Strong Force	(a) Strongest of the four fundamental forces, responsible for keeping quarks locked inside protons and neutrons and for keeping protons and neutrons crammed inside of atomic nuclei. [G99] (b) The dominant force which acts between hadrons, for example, the force which binds neutrons and protons in nuclei. The inter-hadron force is now known to be a remnant of the more powerful force that acts between the quarks which make up the hadrons. This force, which is conveyed by the exchange of gluons, is described by the theory of quantum chromodynamics (QCD). [D89]
Strong Force Symmetry	Gauge symmetry underlying the strong force, associated with invariance of a physical system under shifts in the color charges of quarks. [G99]
Strong Interaction	(a) The short-range nuclear force which is assumed to be responsible for binding the nucleus together. Strong interactions are so called because they occur in the extremely short time of about 10-23 seconds. Strong interactions can occur only when the particles involved are less than 3 fermis apart. [H76] (b) The short-range nuclear interactions responsible for holding nuclei together. The characteristic range of the strong interaction is 10-13 cm, and the time scale over which it operates is 10-33 second. [Silk90] (c) The interactions which bind quarks together to form rotons, neutrons, and other particles. The residual effects of these forces are responsible for the forces between protons and neutrons. (see Yang-Mills Theories) [G97]
Strong Nuclear Force	(a) One of the four fundamental forces of nature. It governs the interaction between particles in atomic nuclei. [C97] (b) Fundamental force of nature that binds quarks together, and holds nucleons (which are comprised of quarks) together as the nuclei of atoms. Portrayed in quantum chromodynamics as conveyed by quanta called gluons. [F88]
Strongly Coupled	Theory whose string coupling constant is larger than 1. [G99]
Strong-Weak Duality	Situation in which a strongly coupled theory is dual-physically identical-to a different, weakly coupled theory. [G99]
Strontium	A soft low-melting reactive metal. The electronic configuration is that of krypton with two additional outer 5s electrons. Symbol: Sr; m.p. 769°C; b.p. 1384°C; r.d. 2.54 (20°C); p.n. 38; r.a.m. 87.62. [DC99]
Structure	Objects have structure if they have parts - that is, if they are made of other things. Whether objects have structure can be learned from experiments that probe them with projectiles. Over the past century, each stage of matter that was found as it became possible to search for ever-smaller things turned out to have structure. Quarks and leptons appear not to have structure, so perhaps the search for the basic constituents has finally ended. There are also theoretical arguments that quarks and leptons are the basic constituents. [K2000]
Structure Formation
SU(N)	mathematical structure known as a `group' that describes operations on N objects. Examples include SU(2) applied to the two quarks or two leptons in a generation and SU(3) applied to the three colors of quark. The three colors and two flavors have recently been combined to yield a set of live entities that can be described by a grand unified theory exploiting SU(5). [D89]
SU(3)	Symmetrical Unitary of Order 3: A symmetry found in sub-nuclear spectra. It is a concept in group theory, by which Gell-Mann and others, using eight quantum numbers, have been able to combine particles into family groups or supermultiplets, as the lowest-lying eightfold group of the nucleon doublet, the singlet, the triplet, and the doublet. The SU(3) theory applies only to the strongly interacting particles. [H76]
SU(5)	Symmetrical Unitary of Order 5: The simplest type of grand unified theory, proposed in the 1970s. (see Grand Unified Theories) [LB90]
Sub-Atomic	Of a scale smaller than that of an atom. [F88]
Sub-Atomic Particle	Any particle that is contained in an atom, or any particle that can be created in collisions of such particles, is loosely called subatomic, whether it is composite like a proton or elementary like a quark or electron. [K2000]
Subcritical	Describing an arrangement of fissile material that does not permit a sustained chain reaction because too many neutrons are absorbed without causing fission or otherwise lost. [DC99]
Subdwarf (sd)	(a) A metal-poor main-sequence star. On the H-R diagram, subdwarfs lie slightly below the metal-rich Main Sequence, because they are fainter than metal-rich main-sequence stars of the same color. [C95] (b) A star whose luminosity is 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. Subdwarfs are primarily Population II and lie just below the Main Sequence on the H-R diagram. [H76] (c) Late-type object whose observed color and absolute magnitude place it below the Main Sequence. [JJ95]
Subgiant	A star whose position on the H-R diagram is intermediate between that of main-sequence stars and normal giants of the same spectral type. [H76]
Subgiant CH Stars	Hot Ba stars (spectral type < G5). [JJ95]
Sublattice Magnetization	in an antiferromagnet the magnetic atoms can be divided into two equivalent classes, each magnetized in opposite directions. The total magnetization of one of these classes is the sublattice magnetization. [D89]
Sublimation	A direct change of state from solid to vapor without melting. [DC99]
Sub-Luminous Stars	Stars fainter than those on the main sequence. Subluminous stars are stars whose age divided by their life span is close to unity. [H76]
Sub-Pulse	The weaker component of the pulse of a pulsar. [H76]
Subsonic	Describing a speed that is less than the speed of sound in the medium concerned. See supersonic. [DC99]
Subspace	a subset of a vector space which is closed under the operations of vector addition and scalar multiplication. [D89]
Sulfur	(a) Element with atomic number sixteen and the eighth most common metal in the Universe. It was produced by oxygen burning in high-mass stars that exploded. [C95] (b) A low melting non-metallic solid, yellow colored in its common forms. It has the electronic configuration [Ne]3s23p4. The element exhibits allotropy and its structure in all phases is quite complex. The common crystalline modification, rhombic sulfur, is in equilibrium with a triclinic modification above 96°C. Both have structures based on S8-rings but the crystals are quite different. If molten sulfur is poured into water a dark red `plastic' form is obtained in a semielastic form. The structure appears to be a helical chain of S atoms. (var. Sulphur) Symbol: S; m.p. 112.8°C; b.p. 444.6°C; r.d. 2.07; p.n. 16; r.a.m. 32.066. [DC99]
Sum-Over-Histories	Probabilistic interpretation of a system's past, in which quantum indeterminacy is taken into account and the history is reconstructed in terms of each possible path and its relative likelihood.
Sum-Over-Paths	Formulation of quantum mechanics in which particles are envisioned to travel from one point to another along all possible paths between them. [G99]
Sum Rule	see f-sum Rule. [H76]
Sun	(a) The star that Earth orbits. The Sun is a yellow main-sequence star that is spectral type G2, shines with apparent magnitude -26.74, and has an absolute magnitude of +4.83. The Sun is 4.6 billion years old. It lies 27,000 light-years from the Galactic center, or about 40 percent of the way from the center to the edge of the Galactic disk. [C95] (b) Central body of solar system. Spectral type G2 V. Mass 1.989 × 1033 g; luminosity 3.83 × 1033 ergs s-1 of which 2 × 1024 ergs s-1 fall on Earth. Radius 695,990 km. Mean density 1.409 g cm-3. Density at surface 3 × 10-7 g cm-3. Rotational period at equator 24d6h; at poles, about 35 days. Mean rotation speed 1.9 km s-1. Vesc 618 km s-1; surface gravity 27,398 cm s-2. Surface temperature about 5785 K. Inclination of rotational axis to pole of ecliptic about 7°15'. Central density (Bahcall 1973) 155 g cm-3; central temperature about 14-15 × 106 K (both this density and temperature would be lower in a solar model producing a counting rate of less than 1 SNU). Energy generating mass about 0.35 M. Galactic orbital period about 220 million years (e 0); Vorb about 250-300 km s-1. Motion with respect to nearby stars 20 km s-1 toward R.A. 18h4m, declination + 30° (in Hercules). It is about 10 kpc from the galactic center and about 10-15 pc above the galactic plane. Mv = +4.85; Mbol = +4.67. It takes about 1-10 million years for photons to diffuse from the Sun's interior to its surface. About 3% of the energy radiated is in the form of neutrinos. Every second about 655 million tons of H are being converted into 650 million tons of He. A grazing light ray is deflected 1".7 by the Sun. Magnetic fields about 1-2 gauss over most of its surface; as high as 10-1000 gauss in active regions. If the total angular momentum of the solar system were concentrated in the Sun, its equatorial rotation speed would be about 100 km s-1. [H76]
Sunrise/Sunset	The times at which the apparent upper limb of the Sun is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50', based on adopted values of 34' for horizontal refraction and 16' for the Sun's semidiameter. [S92]
Sunspot	(a) Comparatively dark spot on the Sun's photosphere, commonly one of a (not always obvious) group of two. The center of a vast electrostatic field and a magnetic field of a single polarity (up to 4,000 gauss), a sunspot represents a comparatively cool depression (at a temperature of approximately 4,500 °C). Sunspots occur in cycles of about 11 Earth-years in period although their individual duration - a matter of Earth-days only - is affected by the differential rotation of the Sun; they tend to form at high latitudes and drift towards the solar equator. They are also sources of strong ultra-shortwave radio emissions. [A84] (b) A temporary disturbed area in the solar photosphere that appears dark because it is cooler than the surrounding areas. Sunspots usually occur in pairs of opposite polarity about 30° N and S of the equator, and move in unison across the face of the Sun as it rotates. The leading (or preceding) spot is called the p-spot; the following, the f-spot. Some sunspots have magnetic fields as high as 1000 gauss (highest observed was 5000 gauss [Steshenko 1967]). Typical diameter, 109 cm. [H76]
Sunspot Number	(a) (Also called the Wolf Number or Relative Number.) A quantity (devised by R. Wolf of Zurich in 1852) which gives the number of sunspots, and the number of groups of sunspots, at a given time. R = k (10g + f) where k is a constant depending on observing conditions, individual spots visible on the Sun at a given time. [H76] (b) A number devised by R. Wolf of Zurich in 1852 to describe sunspot activity. The number is sometimes called the Wolf Number. It is defined by the relationship R = k(10g + f) where R is the sunspot number, k is a constant depending on the instrument used, g is the number of disturbed regions and f is the total number of sunspots. [JM92]
Sunspot Radiation	Intense, variable, circularly polarized radio waves in a noise storm. [H76]
Sunyaev-Zel'dovich Effect
Sunyaev-Zel'dovich Process	Compton scattering between the photons of the cosmic microwave background radiation and electrons in galaxy clusters. [C97]
Super-High Frequency	SFH A radio frequency in the range between 30 GHz and 3 GHz (wavelength 1-10 cm). [DC99]
Supercluster	A cluster of clusters of galaxies. Superclusters are typically about one hundred million (108) light-years in diameter and contain tens of thousands of galaxies. [F88]
Superconducting Super Collider	SSC A proposed accelerator of great size and high energy. [F88]
Superconductivity	(a) A phenomenon occurring in some metals at very low temperatures, in which the resistance drops to zero and the metal shows many other anomalous properties. [D89] (b) The phenomenon in which some substances, when cooled to a sufficiently low temperature, lose all resistance to the flow of electricity. [LB90]
Superconductor	A piece of superconducting metal below the transition temperature at which superconductivity sets in. [D89]
Supercooling	(a) The process by which a substance is cooled below the temperature at which a phase transition should occur, such as water that has been cooled to below zero degrees Centigrade but that has not yet formed ice. [LB90] (b) The phenomenon in which a system can be cooled below the normal temperature of a phase transition without the phase transition taking place. The original form of the inflationary Universe theory was based on the assumption that the Universe supercooled below the temperature of the grand unified theory phase transition. (see Phase Transition) [G97]
Superfluid	A liquid which undergoes the phenomenon of superfluidity, below the temperature at which this phenomenon sets in. [D89]
Superfluidity	A phenomenon occurring in liquid helium-4 below about 2.17 degrees, in which the liquid flows through thin capillaries without apparent friction and displays many other anomalous properties. Liquid helium-3 is also thought to be superfluid below about 3 × 10-3 degrees. [D89]
Superforce	The force which is dominant in GUT (Grand Unified Theories) It combines the electroweak force with the strong nuclear force. [C97]
Supergalactic Plane	An apparent plane of symmetry, passing through the Virgo cluster of galaxies, about which many of the brightest galaxies in the sky are concentrated. These galaxies form the Local Supercluster. [Silk90]
Supergiant	An extremely luminous star of large diameter and low density. No supergiants are near enough to establish a trigonometric parallax. [H76]
Supergranulation Cells	Convective cells (about 15,000-30,000 km in diameter) in the solar photosphere, distributed fairly uniformly over the solar disk, that last as long as a day. New sunspots develop in the intersections of adjacent supergranulation cells. Most of the magnetic flux through the photosphere is concentrated in the supergranule boundaries. [H76]
Supergravity	(a) A supersymmetric theory of gravity in which the graviton is accompanied by a spin-3/2 particle called the "gravitino". In supergravity theories, supersymmetry has been promoted to the status of a local gauge symmetry. [CD99] (b) The theory that is obtained when the ideas of supersymmetry are applied to general relativity. It predicts that the spin-2 graviton will be accompanied by at least one spin-3/2 gravitino. [D89] (c) Class of point-particle theories combining general relativity and supersymmetry [G99]
Superior Planets	Planets farther from the Sun than the Earth is (i.e., Mars to Pluto). [A84]
Super-Metal-Rich	Used in reference to stars, or stellar populations, which are richer in metals than the Hyades.
Supermultiplet	A multiplet of multiplets. [H76]
Supernova	a stellar explosion in which a star may be completely disrupted, leaving a compact stellar remnant such as a neutron star or black hole. At maximum light, the supernova can have luminosity about 108 or 109 times that of the Sun. The luminosity decays after the initial outburst, in certain classes of supernova, the decline being exponential with a half-life of about 80 days. In massive stars, the supernova occurs when the star has used up all its available nuclear fuel and it reaches a lower energy state through gravitational collapse to form a more compact star. In white dwarf stars in binary systems, accretion of mass onto the surface of a neutron star can be sufficient to take the star over the upper mass limit for stability as a white dwarf and it collapses to form a neutron star resulting in a supernova explosion. [D89]
Supernova	A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which (as in the Crab Nebula) may be a neutron star. Supernovae are of two main types: Type I (Mv = - 14 to - 17) have a nonhydrogen spectrum, lower mass, and high velocity (about 10.000 km s-1), and may be produced by the thermonuclear detonation of a highly degenerate core. Type I supernovae are found in both spiral and elliptical galaxies. Type II (Mv = - 12 to - 13.5) have a hydrogen spectrum, higher mass, and lower velocity (about 5.000 km s-1), and occur in young, massive stars near the edge of spiral arms. Type II supernovae are more common: Tammann (1974) finds that Type II supernovae occur in our Galaxy at the rate of 0.01 to 0.05 per year. (Type III supernovae are similar to Type II but are probably of much higher mass.) Novae release about 1044 ergs of energy; supernovae, about 1049 to 1051 ergs. [H76]
Supernova Remnant	(a) SNR The expanding shell of gas ejected at a speed of about 10,000 km s-1 by a supernova explosion, observed as an expanding diffuse gaseous nebula, often with a shell-like structure. Supernova remnants are generally powerful radio sources. [Silk90] (b) A gaseous nebula, the expanding shell ejected by a supernova, and deriving its energy (at least in some cases) from the conversion by the remanent neutron star of its rotational energy into a stream of high-energy particles being continually accelerated in the SNR. About 100 SNRs are known in our Galaxy. Supernova remnants are usually powerful radio sources. [H76]
Superpartners	(a) Particles whose spins differ by 1/2 unit and that are paired by supersymmetry. [G99] (b) if the theory that describes nature has a symmetry called supersymmetry, then every normal particle (the ones we know) has associated with it a partner that differs only in its spin and its mass. [K2000]
Superposition Principle	A quantum mechanical principle according to which any two states can be combined (actually in infinitely many ways) to form states which have characteristics intermediate between those of the two which are combined. In particular, if an eventuality is true in one of the states and false in the other, then it is indefinite in a superposition of the two states. [D89]
Super Radiance	A process by which energy may be extracted from a rotating black hole. A beam of radiation approaches the black hole and "bounces off" with more energy than it had before, analogously to a marble scattering off a rapidly spinning top. The source of the gained energy is the rotational energy of the black hole or top, which slows down in the process. [LB90]
Supersonic	Describing a speed that is greater than the speed of sound in the medium concerned. See subsonic. [DC99]
Superspace	Supersymmetry can be formulated in several ways. One is to imagine ass ociating another coordinate that has special properties with each of our normal spacetime coordinates, giving a kind of space called superspace. Writing theories in superspace makes them supersymmetric. This way of constructing supersymmetric theories is harder to picture than associating superpartners with each Standard Model particle, but it leads to the same results and sometimes facilitates deriving mathematical properties of the theories.
Superstring Theory	(a) A version of string theory which incorporates the ideas of supersymmetry. [D89] (b) String Theory that incorporates supersymmetry. [G99] (c) A new type of theory in physics that unifies all the forces of nature, including the gravitational force, and that may be capable of explaining all of the fundamental laws and particles of nature. In superstring theories, the basic constituent of matter is a 1-dimensional structure, called a string, rather than a point-particle structure. According to superstring theory, space has more than 3 dimensions. [LB90] (d) A proposal for the ultimate laws of nature, a "theory of everything," stemming primarily from discoveries in the mid 1980's. The fundamental entity in this theory is an ultramicroscopic string-like object, with a length of typically 10-33 centimeters and effectively zero thickness. At present our understanding of string theory is very limited. The simplest predictions of superstring theory concern processes at the Planck energy, and so far very little is known about the consequences of string theory at lower energies. [G97]
Supersymmetric Quantum Field Theory	Quantum field theory incorporating supersymmetry. [G99]
Supersymmetric Standard Model	Generalization of the standard model of particle physics to incorporate supersymmetry. Entails a doubling of the known elementary particle species. [G99]
Supersymmetry	(a) A symmetry relating fermions and bosons. If supersymmetry is a true symmetry of nature, then every "ordinary" particle has a corresponding "superpartner" which differs in spin by half a unit. [CD99] (b) A symmetry principle that relates the properties of particles with a whole number amount of spin (bosons) to those with half a whole (odd) number amount of spin (fermions). [G99] (c) A symmetry that relates the fermions (fractional spin particles) to the bosons (elementary particles with integral spin). [P88] (d) Class of theories that seek to identify symmetrical relationships linking fermions and bosons - i.e., particles of half integer spin, like electrons, protoins, and neutrinos, with those of integral spin, like photons and gluons. If attainable, a fully realized supersymmetry theory would provide a unified account of all four fundamental forces, and might well shed light on the very early evolution of the Universe as well. [F88] (e) A mathematical property of some theories of physics proposing that every particle of integer spin (intrinsic angular momentum) has a partner of half integer spin. For example, the photon, which is the particle of light, has a spin of 1 unit. Its hypothesized super symmetric partner is called the photino, which would have a spin of 1/2 units. [LB90] (e) An invariance principle that aspires to place fermions and bosons on an equal footing. [D89] (f) A hypothetical symmetry that describes nature and says that even though fermions and bosons seem to us to be very different in their properties and their roles, in the theory itself they appear in a symmetric way. If supersymmetry is indeed realized in nature, then every particle has a superpartner. [K2000]
Supersynthesis	A radio interferometer system in which two synthesis aerials are used; one is static and utilizes the rotation of the Earth to provide a field of scan, the other is mobile. [A84]
Superunified	While grand unified theories attempt to describe three of the four known interactions of nature - the weak, strong, and electromagnetic interactions - in a unified way, the fourth interaction, gravity, is omitted. Theories which attempt to include gravity as well, such as superstrings, are called superunified. [G97]
Superunified Theory	Hypothetical theory that presumably would show how all four fundamental forces of nature functioned as a single force in the extremely early Universe. The best current candidates for such a potential achievement are thought to be supersymmetry and string theory. [F88]
Supra-Thermal	High-energy. [H76]
Supra-Thermal Proton Bremsstrahlung	Ordinary electron-proton bremsstrahlung viewed from the rest frame of the electron rather than the proton; in other words, the electron is at rest and the heavy particle (proton) is moving. [H76]
Surface Brightness	The measure of the amount of light that an object, especially a galaxy, emits per area of the sky. Even a luminous galaxy can be hard to see if it has a low surface brightness. [C95]
Surface Channel	A semiconductor device construction in which the electron charges are held or moved near the surface of the silicon crystal. [McL97]
Surface Gravity	g: Also called acceleration due to gravity. The rate at which a small object in free fall near the surface of a body is accelerated by the gravitational force of the body, g = GM / R2. Surface gravity of Earth is equal to 980 cm s-2 32 feet s-2. [H76]
Surface Tension	Symbol: or The attraction between molecules (cohesion) in the plane of the surface of a liquid, which thus acts a bit like an elastic skin containing the liquid. Surface tension explains why water can drip slowly from a tap and why mercury gathers into globules on a flat surface. Molecules that are surrounded by others are, on average, repelled equally in all directions, since the liquid is under pressure. At the surface, however, the intermolecular spacings are larger in the plane of the surface, and the molecules attract each other. Consequently, the surface layer is in tension. is defined as the force per unit length acting normally in the plane of the surface on either side of a line in the surface. It is measured in units of newton meter-1. Sometimes called free surface energy, it may also be defined as the work done in increasing the surface area by a square meter. Both are defined at given temperature, as surface tension falls with temperature rise. [DC99]
Susceptibility	Symbol: X The ratio, for a given substance, of the magnetization of a sample to the magnetic field strength applied. In SI it equals (µr - 1), where µr is the relative permeability. The value of X determines whether a substance shows Paramagnetism, Diamagnetism, or Ferromagnetism. A diamagnetic material has a negative susceptibility while paramagnetic and ferromagnetic materials have small and large positive susceptibilities respectively. [DC99]
SUSY	A common abbreviation for Supersymmetry [K2000]
Swan Bands	Spectral bands of the carbon radical C2 first investigated in 1856 by W. Swan. They are a characteristic of carbon stars. Swan bands pass through a minimum between spectral types R4 and R6 and increase again toward N6. [H76]
Swan Nebula	see Omega Nebula. [H76]
Symbiotic Stars	(a) Objects exhibiting a spectrum corresponding to a low-temperature star (generally a giant) plus emission lines corresponding to a hot plasma.[JJ95] (b) A term originally used by P. Merrill to describe stars of two essentially dissimilar kinds which seem to occur together and which seem to "need" each other. In practice, it has come to signify a peculiar group of objects (usually spectral type Me) that display a combination of low-temperature absorption spectra and high-temperature emission lines. These objects undergo semiperiodic nova-like outbursts and display the spectral changes of a slow nova superposed on the features of a late-type star. Their spectra are midway between those of planetary nebulae and true stellar objects. A symbiotic star is now usually taken to be a small, hot, blue star surrounded by an extensive variable envelope. As of 1973 about 30 were known. [C97]
Symmetry	(a) A property of a physical system that does not change when the system is transformed in some manner. For instance, a sphere is rotationally symmetrical since its appearance does not change if it is rotated. [G99] (b) State of a system such that it has a significant quantity that remains invariant after a transformation. More generally, an apt or pleasing proportion based upon such a state. [F88] (c) The property of being unchanged after some transformation. A square, for example, has a 4-sided rotational symmetry. It appears the same after it is rotated by 90 degrees. [LB90] (d) If a theory or process does not change when certain operations are performed on it, then we say that it possesses a symmetry with respect to those operations. For example a circle remains unchanged under rotation or reflection. It therefore has rotational and reflection symmetry. [D89] (e) Abstract mathematical relationships that relate elementary particles together and allow them to be grouped into families. A particular symmetry transformation has the effect of, in a theoretical way, transforming one elementary particle into another. Important symmetries include: U(1): the symmetry of the electromagnetic field. SU(2): the symmetry of the weak nuclear interaction. SU(2) × U(1): the symmetry of the unified electroweak interaction. SU(3): the symmetry corresponding to quark theory and the strong nuclear interaction. SU(5): One of the suggested symmetries of the grand unified theory in which the gluon and electroweak forces are united. It includes the group SU(3) × SU(2) × U(1). [P88]
Symmetry Breaking	(a) A reduction in the amount of symmetry a system appears to have, usually associated with a phase transition. [G99] (b) A set of mathematical transformations that represent a symmetry. [LB90] (c) The process by which an intrinsic symmetry of a system is disrupted. For example, a compass, in the absence of any outside magnetic field, has rotational symmetry and is equally likely to point in any direction. The magnetic field of the earth breaks the symmetry and causes the compass to point in a particular direction, toward the earth's north magnetic pole. In some cosmological models, the infant Universe was much more symmetric than it is today. As the Universe aged and cooled, some of these symmetries were permanently broken. [LB90] (d) The spontaneous emergence of a state of lower symmetry than the symmetry of the hitherto prevailing state. [D89]
Symmetry Principle	A principle that requires a physical system to have a symmetry. For example, the notion that empty space should be devoid of any preferred directions and should thus be unchanged by rotations is a symmetry principle. A collection of compasses in empty space would be expected to point in all directions, reflecting this hypothesized rotational symmetry. [LB90]
Synchronous Rotation	Rotation whose period is equal to the orbital period. [H76]
Synchrotron	A modern form of particle accelerator. [D89]
Synchrotron Radiation	(a) The radiation emitted by charged relativistic particles spiraling in magnetic fields. The acceleration of the moving charges causes the particles to emit radiation. Radio galaxies and supernova remnants are intense sources of synchrotron radiation. Characteristics of synchrotron radiation are its high degree of polarization and nonthermal spectrum. (sometimes called magnetic bremsstrahlung) [Silk90] (b) The radiation emitted by ultrarelativistic charged particles that are circulating in strong magnetic fields. The acceleration of the particles causes them to emit radiation. A characteristic of such radiation is that it is polarized, and the wavelength region in which the emission occurs depends on the energy of the electron - e.g., 1 MeV electrons would radiate mostly in the radio region, but GeV electrons would radiate mostly in the optical region. [H76]
Synodic Month	The period of time (29.53 days) between two successive identical phases of the Moon, e.g., new Moon to new Moon or full Moon to full Moon (see Lunation). [H76]
Synodic Period	(a) Time between one opposition and the next, of any superior planet or asteroid. [A84] (b) The period of revolution of one body about another with respect to the Earth. (synodic period)-1 = ± (sidereal period)-1  (Earth's period)-1. [H76] (c) For planets, the mean interval of time between successive conjunctions of a pair of planets, as observed from the Sun; for satellites, the mean interval between successive conjunctions of a satellite with the Sun, as observed from the satellite's primary. [S92]
Synodic Time	Pertaining to successive conjunctions; successive returns of a planet to the same aspect as determined by Earth. [S92]
Synthesis Aerial	A radio interferometer system utilizing a number of small aerials to achieve the effect of an impossibly large single one. [A84]
System Flow	The evolution of the spectrum of configurations, or the associated effective coupling constants, under the action of repeated coarse-graining. [D89]
System I, II & III Longitude	In the case of Jupiter, because of its differential rotation, two different rotation states are used to keep track of the cloud markings: 9h50m30s for the equator (System I) and 9h55m41s for the high latitudes (System II). Since many of the apparently localized sources of radio noise on Jupiter near a wavelength of 15 m have a shorter period than System II for optical nonequatorial features, the IAU has officially adopted a System III (9h55m29s) for radio astronomy. [H76]
System Noise	The noise in a radio telescope; composed of the receiver noise and the sky noise. [H76]