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A radiofrequency band at a wavelength of 11.1 cm.
An orbital electron whose l quantum number is zero.
A nuclear cross-section factor measured in keV-barns.
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.
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.
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.)
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.
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.
South Atlantic Anomaly
Smithsonian Astronomical Observatory
Stars which appear to be intermediate in type between S stars and carbon stars (C/O ratio near unity).
Subdwarf B-type stars with very broad and shallow Balmer lines; fewer lines of the Balmer series are visible than for normal dwarfs.
Subdwarf O stars showing few very broad and shallow Balmer lines and a very strong He II 4686 line.
Secondary Electron Conduction
Star Formation Rate
Star Formation History
see International System of Units.
Superconductor Insulator Normal
Solid-state Imaging Spectrometer (ASCA X-ray satellite).
Superconductor Insulator Superconductor
Silicon Intensified Target
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.
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.
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.)
The very center of the Milky Way, Sagittarius A* is a strong source of radio waves and probably a massive black hole.
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.
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.
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.
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.
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.
(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.
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).
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.
|Satellite Lines||Of an OH source: The lines arising from transitions at 1612 and 1730 MHz.|
major planet out from the Sun. The most spectacular of the Solar System,
it is circled by a series of concentric 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  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.
A double-ring planetary about 700 pc distant (NGC 7009).
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.
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.)
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.)
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.
The mean distance of a group of stars from the Galactic plane. In general,
old stars have larger scale heights than young ones.
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.
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.
A measure of the size of a physical system or region of space.
(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.
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
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.
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.
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.
Photographic plates obtained with a Schmidt telescope, which is a type of telescope with a particularly large field of view.
(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.
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.
A metal to semiconductor interface without any insulation layer produces an energy barrier in the semiconductor which can be used like a diode.
(a) Equation governing the evolution of probability
waves in quantum mechanics.
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.
|Schwarzschild Black Hole|
A nonrotating, spherically symmetric black hole derived from Karl Schwarzchild's 1916 exact solution to Einstein's vacuum field equations.
|Schwarzschild Filling Factor|
Ratio of the actual density to the limiting value for a system.
(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.
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.
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.
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
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).
A device used with a photomultiplier tube to detect or count charged particles or gamma rays.
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.
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.
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.
An extremely young association of OB stars in Scorpius about 2 kpc distant.
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)
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.
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.
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.
A dwarf elliptical galaxy (Mv = - 11.28 mag, mass about 3 × 106 M), about 85 kpc distant, in the Local Group. Discovered in 1938.
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)
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.
|Second Law of Thermodynamics|
A physical law formulated in the nineteenth century and stating that
any isolated system becomes more disordered in time.
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.
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.
|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.
|Secondary Cosmic Rays|
Atomic fragments - mainly muons - produced by collisions between primary cosmic rays and the molecules in Earth's atmosphere.
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.
In astronomy, gradual, taking aeons to accomplish.
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).
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).
Instability caused by the dissipation of energy.
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).
The condition in which the equilibrium configuration of a system is stable over long periods of time.
Nuclei from which other nuclei are synthesized.
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.
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.
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.
A rule whereby changes in quantum numbers can take only certain allowed values: e.g., l = ± 1 or 0 for dipole transitions.
The reddening of starlight in passing through fine particles of interstellar dust.
The supersymmetric partner of the electron.
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.
With reference to, or pertaining to, the center of the Moon.
Reduction in relative intensity in the central portion of spectral lines resulting from selective absorption by a cooler shell surrounding the hot source.
|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.
spontaneous emergence of order, arising when certain parameters built in a system reach critical values.
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.
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.
The angle at the observer subtended by the equatorial radius of the Sun, Moon, or a planet.
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.
Half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit.
Half the length of the minor axis of an ellipse; a standard element used to describe an elliptical orbit.
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.
One of two opposite directions describable by the motion of a point, line, or surface.
That part of an electronic system responsible for the accurate phasing of time-critical events such as CCD clocking and readout.
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.
A sometimes-eclipsing binary composed of a Wolf-Rayet star and a B0 star with a period of 29.6 days.
A G0 V star almost identical to the Sun in its energy distribution.
The Search for Extraterrestrial Intelligence, by using radiotelescopes to listen for signals transmitted by intelligent alien beings.
A dwarf companion to the Milky Way. Discovered by computer in 1990, Sextans lies about 300 kpc from the Galactic center.
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.
(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
A compact group of galaxies surrounding NGC 6027. It has both spiral and irregular members.
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.
A catalogue of all galaxies brighter than seventeenth magnitude (a measure of brightness). There are about a million galaxies in the Shane-Wirtanen catalogue.
A catalogue of galaxies brighter than thirteenth magnitude, completed in 1932. There are about 1200 galaxies in this catalogue.
A stress applied to a body in the plane of one of its faces.
The boundary layer of charged particles between a plasma and its surrounding material.
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.
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.
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)
The cosmological distance scale which uses a Hubble constant of approximately 100 km/s/Mpc.
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).
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.
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.
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.
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).
|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.
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.
(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.
An iron (or iron and nickel) meteorite. Siderites comprise about 6 percent of known falls. (lit.: "star stone")
A stony iron meteorite. Siderolites comprise less than 2 percent of known falls. (lit.: "sky stone")
The SI unit of electrical conduction (reciprocal ohm).
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.
The wavelength interval within which a feature (e.g., the 21-cm line) is measured (cf. comparison band).
The ratio of the amount of intelligible meaning in a signal to the amount of background noise.
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.
with atomic number fourteen and the sixth most common metal in the Universe.
It is produced by high-mass stars that explode.
The end of the line for a high-mass star, silicon burning creates iron and other elements of similar mass and presages a supernova.
Element with atomic number 47. It is produced by both the r-process
and the s-process, but more by the former.
In science, simulations of physical systems with a computer. (see N-body Simulations.)
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.
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)
In general, a region where energy is given up, in contrast to a source, where energy is released.
|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. (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.
Superconductor-Insulator-Superconductor Junction. Can be used as the mixer in a radio receiver system.
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.
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. See night-sky light.
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.
The supersymmetric partner of any of the leptons.
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.
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.
|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.
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).
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.
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)
A spatial region in which the fabric of space is flat or gently curved, with no pinches, ruptures, or creases of any kind.
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)
A putative satellite galaxy of the Milky Way, reported in 1975. Its reality is in dispute.
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.
(a) Supernova Remnant
Solar Neutrino Unit
Former name of the southern constellation Scutum.
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
Stratospheric Observatory for Far-Infrared Astronomy.
Of the Sun.
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).
Mean radiation received from the Sun at the top level of Earth's atmosphere:
1.95 cal cm-2 min-1.
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.
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.
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.
The amount of mass in the Sun, and the unit in which stellar and galactic masses are expressed.
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.
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.
|Solar Neutrino Unit (SNU)|
1 SNU = 10-36 solar-neutrino captures per second per target atom.
The parallax of the Sun, now measured as 8.794".
|Solar Phase Angle ()||Angular distance at the planet between the Earth and the Sun.|
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.
Is differential, the equatorial rotation taking less time than the polar by up to 9.4 Earth-days.
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.
Velocity of the Sun (19.4 km sin the direction lII = 51°, bII = 23°) with respect to the local standard of rest.
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
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.
Usually implies crystalline semiconductor materials used in the electronics industry.
A finite-amplitude hydrodynamic disturbance which is propagated through a fluid without any change of shape. MHD solitons are also known.
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).
A spiral galaxy in the constellation Virgo. It was the first galaxy whose rotation was detected. (M104, NGC 4594)
A rocket or balloon carrying instruments to probe conditions in the upper atmosphere.
Anything which is emitting electromagnetic radiation.
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.
|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.
|South Galactic Pole|
A point in the constellation Sculptor toward which our line of sight is perpendicular to and below the Galactic disk.
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.
|Space Charge Wave|
An electrostatic wave brought about by oscillations of the charges.
A trajectory along which U · U > 0.
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).
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.
four-dimensional framework in which events take place.
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.
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
The process in which an incoming beam of particles or energy collides with a substance, reacts with it, and knocks off pieces of it.
Space Physics Analysis Network
A means of detecting high energy particles by the trail of ionizations left as they pass through a chamber containing many charge plates.
The spectra of ions often produced by a spark discharge (cf. arc spectra).
Hypothetical particles which are predicted by some Grand Unified Theories.
Einstein's theory of time and space, formulated in 1905, which shows
how measurements of length and time differ for observers in relative
Ratio of the mass of a given volume of a substance to that of an equal volume of water.
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.
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.
Atoms can exist in a number of discreet energy levels. They emit or ab sorb 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.
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.
|Spectral Energy Distribution|
SED: The distribution of a star's light among various wavelengths.
The power of the frequency to which the intensity at that frequency is proportional. It is positive for thermal radiation, negative for nonthermal radiation.
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.
The ratio of electromagnetic wavelengths from different cosmic epochs. This gives the expansion factor of the Universe.
(a) All spectral lines of a given atom arising from
transitions with a common lower energy level.
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.
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.
Device with which spectra of the various regions of the Sun are obtained and photographed.
A spectroscope fitted with a device such as a photoelectric cell for measuring the spectra observed with it.
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).
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.
Study of spectra; in astronomy, the investigation of the composition
of celestial bodies using information derived from spectral lines.
1. The production and analysis of
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.
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)
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)
|Speed of Light|
c = 299,792 km sec-1 (186,180 miles sec-1).
see Velocity-of-Light Radius.
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.
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.
Initial stage in the collapse of a star, followed by gravitational collapse and finally singularity.
A three-dimensional space whose geometry resembles that of the surface of a sphere and is said to have positive curvature.
( Vir): The brightest star in the
constellation Virgo, Spica consists of two blue B-type stars about 220
light-years from Earth.
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.
(a) A quantum property
of all particles which denotes the intrinsic angular momentum of the
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)
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.
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)
A discontinuous increase in the pulse frequency of a pulsar.
|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.
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.
A spiral galaxy - not really a nebula at all (although many do appear nebulous).
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.
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.
|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.
|Spörer's Law of Zones|
The equatorward drift of average sunspot latitudes.
Radiation emitted by an isolated body.
|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.
Satellite Probatoire d'Observation de Ia Terre
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.
First artificial Earth satellite, launched by the Soviet Union on 4 October 1957. (lit.: companion)
The supersymmetric partner of any of the quarks.
Single Side Band
Solid State Photomultiplier
Particles that do not decay into others. (dee also Decay)
A measure of how hard it is to displace an object or
system from equilibrium.
|Standard Big Bang Model|
The Friedmann - Lemaītre 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.
An object - usually a star or a galaxy of known intrinsic brightness. Measuring the apparent brightness of a standard candle yields its distance.
|Standard Deviation ()|
The root mean square deviation from the arithmetic mean.
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.
|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.
|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.
|Standard Model of Particle Physics|
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.
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.
A pattern of oscillations in space in which the regions of maximum displacement and of zero displacement (the nodes) remain fixed in position.
An interval in the cycle of a variable star during which the brightness temporarily stops changing.
Standard Temperature and Pressure
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.
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.
Determination of the number of stars in a region of the sky as a function of apparent magnitude and sometimes color.
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.
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.
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.
Any galaxy in which an anomalously large rate of star formation is taking place.
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.
Energy (seen as light) produced by a star through nuclear fusion.
A software environment and suite of programs for astronomical data analysis developed in the UK and supported by the Rutherford-Appleton Labs.
The unit of charge in the cgs electrostatic system. 1 stat-coulomb = 3.3 × 10-10 coulombs.
the mathematical space whose points represent the states of a physical system.
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)
A Universe whose radius of curvature is constant and independent of time, as in the Einstein Universe.
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)
|stationary nonequilibrium state|
time-independent state of a system subjected to fixed constraints.
(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.
A standing wave; the pattern formed when two waves of the same amplitude and frequency move simultaneously through a medium in opposite directions.
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.
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.
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.
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.
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.
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.
|Steady State Theory|
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.
(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.
The flux of radiation from a blackbody is proportional to the fourth power of its absolute temperature: L = 4R2T4.
Of a star, of the stars.
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.
How a star changes with time.
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.
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.
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.
|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.
The fraction of all atoms (e.g., hydrogen) incident on an interstellar dust grain that become adsorbed.
1 stilb = 1 candela per square centimeter.
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.
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.
1 St = 1 cm2 s-1.
(a) A way of characterizing the polarization state
of light which is closely related to actual measurements.
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.
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)
A flavor of quark. (see Flavor)
(a) A path in phase space that is not closed. Strange
attractors are characteristic of chaotic behavior. (see Attractor; Phase
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.
(a) A property of hadrons which may have a zero or
non-zero value, depending on their rate of decay.
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.
(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)
|Strehl (Intensity) Ratio
(a) The ratio of the peak intensity
in the point spread function of an optical system to that of the equivalent
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)
Fundamental one-dimensional object that is the essential ingredient
in string theory.
|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.
A possible configuration (vibrational pattern, winding configuration) that a string can assume.
(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".
Zones of ionized hydrogen gas surrounding hot stars embedded in interstellar gas clouds; they are called additionally H II zones.
|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.
(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.
|Strong Force Symmetry|
Gauge symmetry underlying the strong force, associated with invariance of a physical system under shifts in the color charges of quarks.
(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.
|Strong Nuclear Force|
of the four fundamental forces of nature. It governs the interaction
between particles in atomic nuclei.
Theory whose string coupling constant is larger than 1.
Situation in which a strongly coupled theory is dual-physically identical-to a different, weakly coupled theory.
A soft low-melting reactive metal. The electronic
configuration is that of krypton with two additional outer 5s
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.
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).
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.
Symmetrical Unitary of Order 5: The simplest type of grand unified theory, proposed in the 1970s. (see Grand Unified Theories)
Of a scale smaller than that of an atom.
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.
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.
(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.
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.
|Subgiant CH Stars|
Hot Ba stars (spectral type < G5).
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.
A direct change of state from solid to vapor without melting.
Stars fainter than those on the main sequence. Subluminous stars are stars whose age divided by their life span is close to unity.
The weaker component of the pulse of a pulsar.
Describing a speed that is less than the speed of sound in the medium concerned. See supersonic.
a subset of a vector space which is closed under the operations of vector addition and scalar multiplication.
(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.
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.
Formulation of quantum mechanics in which particles are envisioned to travel from one point to another along all possible paths between them.
see f-sum Rule.
(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.
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.
(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
(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
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.
Intense, variable, circularly polarized radio waves in a noise storm.
Compton scattering between the photons of the cosmic microwave background radiation and electrons in galaxy clusters.
SFH A radio frequency in the range between 30 GHz and 3 GHz (wavelength 1-10 cm).
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.
|Superconducting Super Collider|
SSC A proposed accelerator of great size and high energy.
A phenomenon occurring in some metals at very low temperatures, in which
the resistance drops to zero and the metal shows many other anomalous
A piece of superconducting metal below the transition temperature at which superconductivity sets in.
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.
A liquid which undergoes the phenomenon of superfluidity, below the temperature at which this phenomenon sets in.
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.
The force which is dominant in GUT (Grand Unified Theories) It combines the electroweak force with the strong nuclear force.
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.
An extremely luminous star of large diameter and low density. No supergiants are near enough to establish a trigonometric parallax.
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.
(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.
Planets farther from the Sun than the Earth is (i.e., Mars to Pluto).
Used in reference to stars, or stellar populations, which are richer in metals than the Hyades.
A multiplet of multiplets.
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.
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.
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.
Particles whose spins differ by 1/2 unit and that are paired by supersymmetry.
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.
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.
Describing a speed that is greater than the speed of sound in the medium concerned. See subsonic.
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.
A version of string theory which incorporates the ideas of supersymmetry.
|Supersymmetric Quantum Field Theory|
Quantum field theory incorporating supersymmetry.
|Supersymmetric Standard Model|
Generalization of the standard model of particle physics to incorporate supersymmetry. Entails a doubling of the known elementary particle species.
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.
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.
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.
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.
|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.
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.
A semiconductor device construction in which the electron charges are held or moved near the surface of the silicon crystal.
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.
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.
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.
A common abbreviation for Supersymmetry
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.
see Omega Nebula.
(a) Objects exhibiting a spectrum corresponding to a low-temperature star (generally a giant) plus emission lines corresponding to a hot plasma. (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.
(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.
A reduction in the amount of symmetry a system appears to have, usually
associated with a phase transition.
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.
Rotation whose period is equal to the orbital period.
A modern form of particle accelerator.
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)
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).
(a) Time between one opposition and the next, of any
superior planet or asteroid.
Pertaining to successive conjunctions; successive returns of a planet to the same aspect as determined by Earth.
A radio interferometer system utilizing a number of small aerials to achieve the effect of an impossibly large single one.
The evolution of the spectrum of configurations, or the associated effective coupling constants, under the action of repeated coarse-graining.
|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.
The noise in a radio telescope; composed of the receiver noise and the sky noise.