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 medi