Tyler's Astronomy Glossary (O-Z)
Virgo A
A strong radio source. Optically, it is an elliptical galaxy (M87) with a luminous blue jet about 1500 pc long. It is also an X-ray source (3C 274, Virgo X-1, 2U 1228+12). [H76]
YY Ori stars
A subgroup of T Tau stars with inverse P Cyg type profiles in the CaII and H line emissions. [JJ95]
Zwicky Compact Galaxy
A type of very high-surface-brightness galaxy first brought to general attention by Fritz Zwicky. [BFM02]
Rydberg
A unit of energy (R = 3c / me4) equal to 13.5978 eV (the ionization potential of hydrogen).
Torr
A unit of pressure equal to 1/760 of an atmosphere, or about 1 mm Hg. [H76]
Trapped Surface
A surface (e.g., of a black hole) from which light cannot escape to infinity. [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]
Perturbation Method
A system of successive approximations to the solution of a problem, by starting with a closely similar problem whose solution is known, applying small departures from equilibrium, and then calculating their consequences. [H76]
Week
An arbitrary period of days, usually seven days; approximately equal to the number of days counted between the four phases of the Moon. (see Lunar Phases) [S92]
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]
Predictability
the ability to predict the future behavior of a dynamical system on the basis of the present knowledge available on this system. [D89]
State Space
the mathematical space whose points represent the states of a physical system. [D89]
Open String
A type of string with two free ends. [G99]
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]
Wormhole
"(a) A bridge to another universe created by a black hole. [LB90]
Point Source
"(a) A source whose angular extent cannot be measured (< 0"".05). [H76]
Stability
"A measure of how hard it is to displace an object or system from equilibrium.
Zinc
"A bluish-white transition metal, applied as a coating (galvanizing) to protect steel from corrosion.
Tellurium
"A brittle silvery metalloid element belonging to group 16 of the periodic table. It is found native and in combination with metals. Tellurium is used mainly as an additive to improve the qualities of stainless steel and various metals.
Xenon
"A colorless odorless monatomic element of the rare-gas group. It occurs in trace amounts in air. Xenon is used in thermionic tubes and strobe lighting.
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.
Quantum Tunneling
"A quantum leap through a barrier. [F88]
Vanadium
"A silvery transition element used in alloy steels.
Polaroid (Trade name)
"A synthetic doubly refracting substance, that strongly absorbs polarized light in one plane, while easily passing polarized light in another plane at right angles. Unpolarized light passed through a sheet of Polaroid is plane-polarized. Spectacle lenses made of this material normally absorb light vibrating horizontally - as produced by reflection from horizontal surfaces. They thus reduce reflected glare.
Osmium
"A transition metal that is found associated with platinum. Osmium is the most dense of all metals. The metal is used in catalysts and in alloys for pen nibs, pivots, and electrical contacts.
van der Waals Equation
"An equation of state for real gases. For one mole of gas the equation is
WR or Wolf-Rayet stars
"Hot stars characterized by wide emission lines of highly ionized elements, standing out distinctly from the continuous spectrum. There exist three varieties:
Trapezium
( 1 Ori) Four very young stars (O6-B3) (there are six stars in the system) in the center of the Orion Nebula. They form the vertices of a trapezoid. [H76]
Thuban
( Dra) A fourth-mag A0 star. It was the "Pole Star" at the time the Egyptians built the Pyramids. [H76]
White Light
"Visible radiation that gives a sensation of whiteness. The effect is very subjective and depends very much upon conditions and contrast. Light from a lamp with filament temperature about 2000K may be regarded as white, but in contrast to a filament at 3000K it appears yellow. Sunlight normally appears white, but a small beam of sunlight entering a room lit mainly by filament lamps can appear blue-green. Yet the Sun is sometimes classified as a yellow star, in contrast to some stars with radiating surfaces at higher temperature.
WIMP
"Weakly Interacting Massive Particle
Wolf Number (R)
(Also called the Relative Number) A quantity 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, g is the number of sunspot groups, and f is the number of individual spots visible on the Sun at a given time. [H76]
Quantum Yield
(In photochemistry, the number of molecules decomposed per photon absorbed. [H76]
Orion Molecular Cloud 1
(OMC-1) -- Centered approximately 1' north-west of the Trapezium, it contains the Becklin-Neugebauer and Kleinmann-Low infrared sources. [H76]
Degree Fahrenheit °F
-459.67 32.018
(b) A dimensionless number used in heat transfer given by the expression l3g
/ k where l denotes length, g is the acceleration due to gravity, is the cubic expansion coefficient, is the density, is the dynamic viscosity coefficient, k is thermal conductivity and is the temperature difference. The number is equal to the product of the Grashof and Prandtl numbers. [JM92]"
Stilb (sb)
1 stilb = 1 candela per square centimeter. [H76]
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]
Tarantula Nebula
30 Doradus Nebula. [H76]
Orbiting Collision
A "collision" in which an ion and an atom approach each other very closely and spend a long time (several orbits of the atomic electrons) in close proximity. [H76]
Pleione
A B8pe star (28 Tau), one of the brightest stars in the Pleiades, which developed an envelope or shell first observed in 1938. The shell increased in strength and attained its maximum intensity in 1945; thereafter it weakened and was scarcely visible by 1954. In 1972 it developed another shell. It is rotating so fast that it is unstable. [H76]
Three-Phase CCD
A CCD construction in which three overlapping metal electrodes are used to define a pixel and effect the transfer of charge, in either direction along a column, by the charge-coupling method. If only two electrodes are used then the device is two-phase. [McL97]
Weak G-band Stars
A G-type giant (G5 to K5) with a very weak or absent G band of CH and weak CN bands. These stars are C-deficient. [JJ95]
Tau Ceti
A G-type main-sequence star that lies in the constellation Cetus, 11.4 light-years away. It is a single star like the Sun. [C95]
Triangulum Galaxy
A Sc II-III spiral galaxy, a satellite of the Andromeda Galaxy, about 700 kpc distant. Mv = -18.9 mag (M33, NGC 598) [H76]
Omega Centauri
A bright globular cluster in the constellation Centaurus. [C95]
AI Velorum Stars
A class of dwarf Cepheids. They are all RR Lyrae stars with periods shorter than 0.25 days. [H76]
Transputer
A compact computer chip with a special design for linking to other transputers to make the program run faster. [McL97]
Printed Circuit
A compact double-sided circuit board with no wires but instead fine tracks, etched on a copped-clad board, perform the same function. [McL97]
Seyfert's Sextet
A compact group of galaxies surrounding NGC 6027. It has both spiral and irregular members. [H76]
Radiation Damping
A decrease in the amplitude of an oscillation due to the emission of energy by radiation. [H76]
W3
A dense cloud of gas about 3 kpc distant in the Perseus arm. [H76]
z pinch
A diffuse toroidal pinch in which the magnetic field runs around the plasma column. [H76]
Ring Nebula
A famous planetary nebula (M57, NGC 6720) in the constellation Lyra. [C95]
Soliton
A finite-amplitude hydrodynamic disturbance which is propagated through a fluid without any change of shape. MHD solitons are also known. [H76]
Vela Supernova Remnant
A gaseous nebula in the middle of the Gum Nebula, the remnant of a Type II supernova whose light reached Earth about 10,000-30,000 years ago. It consists of bright filaments that form a D-shaped ring in H and a rough circle in the ultraviolet. It includes the Vela X, Y, and Z radio complexes and is a strong X-ray source. [H76]
Stephan's Quintet
A highly disturbed cluster of five peculiar galaxies (NGC 7317, NGC 7318A, NGC 7318B, NGC 7319, NGC 7320) in Pegasus which seem to exhibit gaseous connecting bridges. Four have large redshifts (of the order of 5700-6700 km s-1), but the fifth member (NGC 7320) has a much smaller redshift (800 km s-1). Discovered in 1877 by M. E. Stephan. [H76]
Perturbation Expansion
A method of successive approximations that is used to obtain predictions from a theory such as quantum electrodynamics, which cannot be solved exactly. [G97]
Supermultiplet
A multiplet of multiplets. [H76]
3 Process
A nuclear reaction (3 4He 12C + + 7 MeV) by which helium is transformed into carbon. The process is dominant in red giants. At a temperature of about 2 × 108 K and a density of 105 g cm-3, after core hydrogen is exhausted, three -particles can fuse to form an excited nucleus of 12C, which occasionally decays into a stable 12C nucleus. The overall process can be looked upon as an equilibrium between three 4He nuclei and the excited 12C*, with occasional irreversible leakage out of the equilibrium into the ground state of 12C. Further capture of -particles by 12C nuclei produces 16O and 20Ne. (also called the triple- process) [H76]
Vector
A quantity which has both magnitude and direction, such as the spin of a magnetic atom in the Heisenberg model. [D89]
Q-Branch
A set of lines in the spectra of molecules corresponding to changes in vibrational energy with none in rotational energy. [H76]
R-Branch
A set of lines in the spectra of molecules corresponding to unit decreases in rotational energy. [H76]
U line
A sodium line at 3302 Å. [H76]
Runge-Kutta Method
A step-by-step method of numerical integration. [H76]
Phase Switching
A technique used in radio astronomy to suppress background noise so that the receiver records only point sources. [H76]
Refractor
A telescope in which the light is focused by a lens at the viewing side of the telescope. By contrast, a reflecting telescope is one in which light is focused by a mirror. [LB90]
Trillion
A thousand billion (1012) in American usage. [F88]
Spherical Space
A three-dimensional space whose geometry resembles that of the surface of a sphere and is said to have positive curvature. [Silk90]
t-Time
A time scale in which the relative motion of two observers is nonzero but unaccelerated (see -Time). [H76]
Open Inflationary Universe Theory
A version of the inflationary universe theory, proposed by Bharat Ratra and Jim Peebles in 1995, which produces an open Universe. The usual inflationary prediction of a flat universe is avoided by proposing an energy diagram for the inflation-driving field with a hill of just the right length, so inflation ends before the universe is driven to flatness. [G97]
van Biesbroeck's Star
A very faint (Mv = 18.6; Mbol = 13.12), nearby (parallax 0".168, about 8 pc distant) dM5e star of very low mass (0.07 M). Temperature about 2250 K. (Gliese 752b, BD+4°4048B) [H76]
pc
Abbreviation for Parsec. [H76]
Photovoltaic Effect
Absorption of a photon leads to the production of a voltage across a junction. [McL97]
Photoconductivity
Absorption of light increases the number of charge carriers. [McL97]
Tauri
Also know as Aldebaran. [H76]
Rosette Nebula
An HII region (M16, NGC 2237-2244) in Monoceros, more than 1 kpc distant. It has a fairly high degree of symmetry about a centrally located cluster of about six hot, young O-type stars. [H76]
Ramsauer Effect
An anomalously large mean free path for low-energy electrons. [H76]
Vesta
An asteroid 500 km in diameter (P = 1325 days, a = 2.361 AU, e = 0.09, i = 7°.1). It is the brightest of all minor planets, at times approaching naked-eye visibility (mag 5.5). Rotation period 5h20m31s.665. (Its spectrum can also be interpreted to mean a rotation period of 10h40m58s.84.) Albedo 0.24. Discovered by Olbers in 1807. [H76]
Plane-Parallel Atmosphere
An atmosphere stratified in parallel planes normal to the direction of gravity. [H76]
Thermalization
An atomic or molecular transition is thermalized when the Boltzmann factor for the two levels of the transition takes on the value it would have in thermodynamic equilibrium. [H76]
Richardson-Lucy Method
An image reconstruction algorithm. [McL97]
Pitch Angle
Angle specifying the direction of electron velocity; or the angle between a tangent to a spiral arm and the perpendicular to the direction of the galactic center. [H76]
Position Angle
Angular distance (in degrees, measured from north through east) between the primary and secondary components of a binary system. [H76]
Solar Phase Angle ()
Angular distance at the planet between the Earth and the Sun. [H76]
Zenith Distance
Angular distance on the celestial sphere measured along the great circle from the zenith to the celestial object. Zenith distance is 90° minus altitude. [S92]
Right Ascension
Angular distance on the celestial sphere measured eastward along the celestial equator from the equinox to the hour circle passing through the celestial object. Right ascension is usually given in combination with declination. [S92]
Photoelectric Devices
Any detector which uses the photoelectric effect to convert photons to electrons. [McL97]
Sub-Atomic Particle
Any particle that is contained in an atom, or any particle that can be created in collisions of such particles, is loosely called subatomic, whether it is composite like a proton or elementary like a quark or electron. [K2000]
Reflecting Telescope
Telescope that uses mirrors to magnify and focus an image onto an eyepiece. (reflector) [A84]
Quark Jet
Because quarks must end up in hadrons, quarks that are produced in collisions actually appear in detectors as a narrow jet of hadrons, mostly pions. see also Gluon Jet [K2000]
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]
Orionis
Betelgeuse
Thermal Radiation
Blackbody radiation; radiation caused by the high temperature of the radiating objects, as opposed to nonthermal radiation, which is caused by energetic (not necessarily hot) electrons. [H76]
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]
RS CVn Stars
Close binaries, which show H and K emissions. [JJ95]
Rich Clusters
Clusters with a particularly large number of galaxies. [LB90]
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]
White Noise
Completely random and uncorrelated noise, with equal power at all frequencies. [Silk90]
Sunyaev-Zel'dovich Process
Compton scattering between the photons of the cosmic microwave background radiation and electrons in galaxy clusters. [C97]
Ray Tracing
Computer simulation of light ray paths through an optical system. [McL97]
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]
One-Loop Process
Contribution to a calculation in perturbation theory in which one virtual pair of strings (or particles in a point-particle theory) is involved. [G99]
Pulse Counting
Counting each individual photon as it comes off. [H76]
Radiocarbon Dating
Determination of the age of a substance containing radioactive carbon by means of its radioactive half-life. [F88]
Radiometric Dating
Determination of the age of objects - e.g., earth and moon rocks - by means of the half-life of the unstable elements they contain. [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]
Threshold Energy
Difference between the energy at the first excited level and that of the ground state. [H76]
Oef
Early O stars that show double emission lines in He II 4686. [H76]
Proto-Planets
Early stage in the formation of planets according to the theory by which planetary systems evolve through the condensation of gas clouds surrounding a young star. The theory is not, however, generally accepted. [A84]
Photometric Binaries
Eclipsing variables like Per whose orbital plane lies so nearly in the line of sight that eclipses, as seen from the Earth, can occur and can be detected from their light curves. [H76]
Starlight
Energy (seen as light) produced by a star through nuclear fusion. [A84]
Thermal Energy
Energy associated with the motions of the molecules, atoms, or ions in a substance. [H76]
Royal Society
English organization founded in the seventeenth century and dedicated to the advancement of science. [F88]
Sum-Over-Paths
Formulation of quantum mechanics in which particles are envisioned to travel from one point to another along all possible paths between them. [G99]
Vidicon
General name for the class of vacuum tube imaging devices which employ a scanning electron beam to read out the image. [McL97]
Supra-Thermal
High-energy. [H76]
Subgiant CH Stars
Hot Ba stars (spectral type < G5). [JJ95]
R CrB Stars
Hydrogen-deficient C-type stars. [JJ95]
Plurality of Worlds
Hypothesis that the Universe contains inhabited planets other than Earth. [F88]
Vacuum Genesis
Hypothesis that the universe began as nothingness, from which matter and energy arose by a process analogous to the appearance of virtual particles from a vacuum. [F88]
Spherical Collapse
Initial stage in the collapse of a star, followed by gravitational collapse and finally singularity. [A84]
Secular Instability
Instability caused by the dissipation of energy. [H76]
Transit Circle
Large instrument for the accurate observation and measurement of a transit. [A84]
Zeroth Law of Thermodynamics
Law of thermal equilibrium. [H76]
Rocard Scattering
Linearly anisotropic scattering. [H76]
Sense
One of two opposite directions describable by the motion of a point, line, or surface. [H76]
Thermal Noise
Low-frequency electromagnetic radiation associated with thermal fluctuations, which is emitted by all bodies whose temperature is above 0 K. Also known as Johnson noise. [H76]
Reversing Layer
Lower chromosphere of the Sun, a comparatively cool region in which radiation at certain wavelengths is absorbed from the continuous spectrum emitted from the Sun's photosphere. [A84]
T Tauri Stars
Luminous variable stars with low effective temperatures and strong emission lines, associated with interstellar gas clouds and found in very young clusters. They are believed to be still in the process of gravitational contraction from their protostellar phase and have not yet arrived at the main sequence and begun to burn hydrogen. (see also Nebular Variable Stars) [Silk90]
Observational Cosmology
The application of observational data to the study of the Universe as a whole. [F88]
Raster
The area of an oscilloscope upon which the image is produced. [H76]
Packing Fraction
Mass defect per nuclear particle. The term has been largely superseded by the related quantity, binding energy per nuclear particle. [H76]
Photoelectric Filtering
Means of measuring the astronomical colour index of a star, involving colour filters on photoelectric cells to define the color index between two set wavelengths. The filters correspond to the UBV photometry system. [A84]
ppm
Parts per Million [LLM96]
Wave Noise
Noise in the current of a detector, caused by fluctuations in the electromagnetic radiation falling on the detector (cf. shot noise). [H76]
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]
Sub-Atomic
Of a scale smaller than that of an atom. [F88]
Rotation
Of a single body in space: spinning on an axis. Of a planetary system, rotation is generally planar in relation to the parent star. Galactic rotation, however, is usually differential. [A84]
Stellar
Of a star, of the stars. [A84]
OVLA
Optical Very Large Array. [LLM96]
OVV
Optically Violent Variables.
OAO
Orbiting Astronomical Observatory [LLM96]
Supra-Thermal Proton Bremsstrahlung
Ordinary electron-proton bremsstrahlung viewed from the rest frame of the electron rather than the proton; in other words, the electron is at rest and the heavy particle (proton) is moving. [H76]
Oberon
Outermost satellite of Uranus, discovered by Herschel in 1787. P = 13.46 days (rotational and orbital); R 500 km. [H76]
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]
Radial Pulsation
Periodic expansion and contraction of a star that may be merely an optical effect of recession. [A84]
Polytropic Index (n)
The polytropic index may have any value from zero (uniform density throughout) to 5 (entire mass concentrated at the center). A polytropic index of 1.5 corresponds to a fully degenerate, nonrelativistic electron gas; it also describes a perfect-gas star in convective equilibrium. (See also Polytrope) [H76]
PM
PhotoMultiplier [LLM96]
PC
Photoconductor [LLM96]
pm
Post Meridiem [LLM96]
P-Wave
Primary Wave: A longitudinal seismic acoustic wave that moves by compression. The p-waves travel faster than s-waves and can penetrate the core of the Earth. [H76]
Voigt Profile
Profile of a spectral line allowing for the effects of Doppler broadening combined with a Lorentz (damping) profile. [H76]
PSR
Pulsar [LLM96]
X-Ray Pulsars
Pulsars that radiate in the X-ray region of the spectrum. Best verified examples are Her X-1 and Cen X-3. They are thought to be rotating, strongly magnetic neutron stars of about 1 M in a grazing orbit around a more massive star from which they are accreting matter. [H76]
PHA
Pulse Height Analyzer
PSD
Pulse-Shape Discrimination [LLM96]
QCD
Quantum Chromodynamics
QEH
Quantum Efficiency Hysteresis An increase in QE after exposure to light. [McL97]
QSO
Quasi-Stellar Object (Quasar)
Royal Greenwich Observatory
RGO: Primary national 0bservatory in Great Britain, first sited at Greenwich in 1675, but in 1958 moved to Herstmonceux, Sussex. From the first, Directorship of the Observatory has entailed appointment as Astronomer Royal. In the 1980s the Observatory will lose its primary national status with the completion of the Northern Hemisphere Observatory in Las Palmas, the Canary Islands. [A84]
Residual Intensity
Ratio of correlated flux in the line to correlated flux in the continuum. [H76]
Schwarzschild Filling Factor
Ratio of the actual density to the limiting value for a system. [H76]
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]
Proper Mass
Rest mass. [H76]
Orionis
Rigel
Spectral Energy Distribution
SED: The distribution of a star's light among various wavelengths. [H76]
Super-High Frequency
SFH A radio frequency in the range between 30 GHz and 3 GHz (wavelength 1-10 cm). [DC99]
SPOT
Satellite Probatoire d'Observation de Ia Terre [LLM96]
Regge-Wheeler Equations
Schrödinger-type equations for small, odd- parity perturbations on the Schwarzschild metric. [H76]
SEC
Secondary Electron Conduction [LLM96]
Radiotelescopes
Sensitive radio antennae employed to detect the radio energy emitted by nebulae, galaxies, pulsars, etc. [F88]
SNU
Solar Neutrino Unit [LLM96]
Viking Space Probes
Series of 2 US spaceprobes that successfully effected landings on Mars and relayed data back to Earth. [A84]
SAA
South Atlantic Anomaly
SPAN
Space Physics Analysis Network [LLM96]
Virginis
Spica. [H76]
STP
Standard Temperature and Pressure [LLM96]
SFH
Star Formation History
Radio Astronomy
Study of the universe at the radio wavelengths of electromagnetic energy. [F88]
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]
SIN
Superconductor Insulator Normal [LLM96]
Zetta-
Symbol: Z A prefix denoting 1021. For example, 1 zettameter (Zm) = 1021 meter (m). [DC]
Zepto-
Symbol: z A prefix denoting 10-21. For example, 1 zeptometer (zm) = 10-21 meter (m). [DC99]
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]
THz
TeraHertz. [LLM96]
Sequencer
That part of an electronic system responsible for the accurate phasing of time-critical events such as CCD clocking and readout. [McL97]
q0
The cosmological deceleration parameter.
Visibility Function
The Fourier transformation of a distant radio source, normalized to its value at small antenna spacings. [H76]
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]
Solar Mass
The amount of mass in the Sun, and the unit in which stellar and galactic masses are expressed. [C95]
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]
Photoemission
The emission of photoelectrons by the photoelectric effect or by photoionization. [DC99]
Radiation
The energy carried by waves or particles. [G99]
Virial Mass
The mass of a cluster of stars or galaxies in statistical equilibrium derived by using the virial theorem that the mean square velocity of all the stars or galaxies in a cluster is proportional to the mass of the cluster divided by its radius. [H76]
Uniform Vibration
The overall motion of a string in which it moves without changes in shape. [G99]
Umbra
The portion of a shadow cone in which none of the light from an extended light source (ignoring refraction) can be observed. [S92]
Radiative Transfer
The process by which radiation travels through a medium. [LB90]
Relaxation
The process of gravitational interaction (in the case of a cluster of stars or galaxies) whereby a random distribution of motions is eventually established. The system is said to relax to a state of thermal equilibrium. [Silk90]
Rotational Invariance
The property of being unchanged by a rotation. A sphere is rotationally invariant, but a rectangle is not. As far as we know the fundamental laws of physics are rotationally invariant. [G97]
Polarization (polarimeter)
The property of transverse electromagnetic waves that describes the plane of vibration of the wave and its behavior as the wave progresses. Linearly polarized light implies that all the waves vibrate in the same plane. Circular polarization occurs when the plane of vibration rotates as the wave progresses. A polarimeter is used to measure these properties. [McL97]
Pogson's Ratio/Scale
The ratio between two successive stellar magnitudes, introduced by N. Pogson in 1856. [H76]
Standard Deviation ()
The root mean square deviation from the arithmetic mean. [H76]
Optics
The science of light. [F88]
Top
The sixth flavor of quark. [CD99]
Regression of the Nodes
The slow (19°.35 per year, 360° in 18.6 years), westward motion of the nodes of the Moon's orbit due to perturbations of the Earth and Sun. [H76]
Stat-Coulomb
The unit of charge in the cgs electrostatic system. 1 stat-coulomb = 3.3 × 10-10 coulombs. [H76]
X Process
The unknown nucleosynthetic process that Burbidge, Burbidge, Fowler and Hoyle said had formed the light nuclei Deuterium, Lithium, Beryllium, and Boron. [C95]
Velocity Field
The velocities of a group of objects with different velocities at different positions of space. [LB90]
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]
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]
Relativistic Beaming
Theory devised by Francis Smith regarding the generation of polarized radiation in neutron stars (pulsars). [A84]
Projectile
To study particles and their interactions, it is necessary to probe them with projectiles. The projectiles are other particles (electrons, photons, neutrinos, and protons) because these are small enough and can be given enough energy. [K2000]
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]
TCP
Transmission Control Protocol. [LLM96]
TDRSS
Transmission Data and Relay Satellite System. [LLM96]
Radio Interferometer
Type of radio telescope that relies on the use of two or more aerials at a distance from each other to provide a combination of signals from one source which can be analyzed by computer. Such an analysis results in a resolution that is considerably better than that of a parabolic dish aerial by itself because of the greater effective diameter. [A84]
Ultra-High Frequency
UHF A radio frequency in the range between 3 GHz and 0.3 GHz (wavelength 10 cm-1 m). [DC99]
UCD
Ultra-Compact Dwarf galaxy. [BFM03]
Oersted
Unit of magnetic field strength. 1 Oersted corresponds to 1000 / 4 amperes per meter. [H76]
Rad
Unit of radiation, equal to 100 ergs of ionizing energy absorbed per gram of absorber. [H76]
VBLR
Very Broad Line Region
Transverse Waves
Waves vibrating at right angles to the direction of propagation - e.g., electromagnetic waves. [H76]
Stress
When a system of opposing forces acts on a body the material is subject to some form of stress. This is expressed in terms of one of the forces divided by the area on which it acts. Bulk stress is a change of pressure applied to a fluid, or applied by a fluid to a solid. Tensile stress is a stress that stretches a body. Shear stress causes a deformation without any change of volume. (see also Strain) [DC99]
Vibration Number
Whole number describing the energy in the uniform vibrational motion of a string; the energy in its overall motion as opposed to that associated with changes in its shape. [G99]
XRT
X-Ray Telescope.
XBL
X-Ray selected BL Lac Object
XMM
X-ray Multi-mirror Mission. [LLM96]
Thermodynamic Potential
a function of the state of a system which takes its extreme value on the asymptotically stable state reached by the system in the course of time. [D89]
Potts Model
a generalisation of the Ising model in which the two states for each lattice site of the ising model are replaced by n equivalent states. [D89]
Turbulence
a hydrodynamic flow characterized by an irregular space and time dependence. Chaotic dynamics and fractal geometry constitute the natural models capturing the essence of ae turbulence. [D89]
Subspace
a subset of a vector space which is closed under the operations of vector addition and scalar multiplication. [D89]
Open System
a system communicating with the environment by the exchange of energy and matter.[D89]
PPN Parameters
denotes Parametrized Post-Newtonian Parameters. dimensionless parameters that describe the first relativistic corrections beyond Newtonian gravity in the Solar System; their values depend on the theory of gravity adopted. [D89]
Rydberg atom
extremely highly excited atoms are called Rydberg atoms (Rydberg was an early systematizer of atomic spectra). These atoms are huge (the size of viruses for the largest) and fragile but interact very strongly with radiation. Quantum opticians use them to test basic ideas about atom-photon interactions.[D89]
Weber Bar
gravitational-wave detector pioneered by Joseph Weber, consisting of a solid aluminum cylinder, suspended and isolated from surrounding vibrations. The cylinder can weigh several tons. [D89]
Weinberg Angle (w)
parameter in the electroweak theory. Relates properties, such as mass, of the W and Z bosons and their interactions. [D89]
rms
root mean square. [LLM96]
U(1)
the symmetry group associated with electromagnetic gauge invariance. [D89]
stationary nonequilibrium state
time-independent state of a system subjected to fixed constraints. [D89]
SUSY
A common abbreviation for Supersymmetry [K2000]
Strange
A flavor of quark. (see Flavor) [G97]
Yellow Supergiant
A supergiant star with a spectral type of G. [C95]
Titania
Fourth satellite of Uranus, discovered by Herschel in 1787, R 850 km; P = 8d17h. [H76]
PDA
Photodiode Array [McL97]
PI
Pulse Invariant (also Principal Investigator).
Particle-to-Antiparticle Ratio
Same as matter-to-antimatter ratio. [LB90]
SSB
Single Side Band [LLM96]
Turbidity
The appearance of a star as a disk in a long-exposure photograph, due to the scattering of adjacent continuum light by emulsion grains. [H76]
Sulfur
"(a) Element with atomic number sixteen and the eighth most common metal in the Universe. It was produced by oxygen burning in high-mass stars that exploded. [C95]
Quadrature
"(a) Elongation of a planet when it makes a 90° angle with the Sun as seen from Earth.
Schrödinger Equation
"(a) Equation governing the evolution of probability waves in quantum mechanics. [G99]
String
"(a) Fundamental one-dimensional object that is the essential ingredient in string theory. [G99]
Photon
"(a) Smallest packet of the electromagnetic force field; messenger particle of the electromagnetic force; smallest bundle of light. [G99]
Proton Decay
"(a) Spontaneous disintegration of the proton, predicted by grand unified theory but never observed experimentally. [F88]
Principle of Equivalence
"(a) States that inertial mass is indistinguishable from gravitational mass. [c97]
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]
Ruthenium
"A transition metal that occurs naturally with platinum. It forms alloys with platinum that are used in electrical contacts. Ruthenium is also used in jewelry alloyed with palladium.
Phase
"(a) A number (usually expressed as an angle between 0° and 360°) which characterizes a wave, where the phase of a wave corresponds to the position in its cycle relative to some arbitrary reference point.
Platinum
"(a) Element with atomic number 78. It is produced almost entirely by the r-process in supernovae. [C95]
Tungsten
"A transition metal, formerly called wolfram. It is used as the filaments in electric lamps and in various alloys.
Rigel
"(a) A blue supergiant some 900 light-years away in the constellation Orion. Rigel is the brightest star in Orion and the seventh brightest star in the night sky. [C95]
Planetary Nebula
"(a) A bubble of gas surrounding a hot, dying star. The star is so hot that it makes the planetary nebula glow, which allows astronomers to see it. The star was once the core of a red giant, which ejected its outer atmosphere and created the planetary. A planetary nebula has nothing to do with a planet, but through a small telescope, it looks like a planet's disk, hence the misleading name. [C95]
Ritchey-Chrétien
"(a) A class of reflecting telescope with a hyperbolic primary and secondary. [McL97]
Quantum Efficiency
"QE
Quantum Electrodynamics
"QED
Quantum Efficiency Hysteresis
"QEH
Solar Neutrino Unit (SNU)
1 SNU = 10-36 solar-neutrino captures per second per target atom. [H76]
Stokes
1 St = 1 cm2 s-1. [H76]
Paul Trap
A radio-frequency quadrupole ion trap in which charged particles can be suspended by radiofrequency electric fields for times limited primarily by collisions with the background gas. [H76]
S Band
A radiofrequency band at a wavelength of 11.1 cm. [H76]
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]
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]
uvby System
A four-color, intermediate-bandwidth, photometric systemdevised by Strömgren consisting of measures in the ultraviolet, violet, blue, and yellow regions of the spectrum. [BFM02]
Weak-Field Condition (gravitational)
<< c2, where is the Newtonian gravitational potential. [H76]
Serpentis
A G0 V star almost identical to the Sun in its energy distribution. [H76]
Stellar Population
A Galaxy-wide group of stars of all types that have similar ages, locations, kinematics, and metallicities. As astronomers presently know the Milky Way, they recognize four stellar populations: the thin disk; the thick disk; the stellar halo; and the bulge. [C95]
Zerilli's Equation
A Schrödinger-type equation for even-parity perturbations on the Schwarzschild metric. [H76]
Static Universe
A Universe whose radius of curvature is constant and independent of time, as in the Einstein Universe. [H76]
Pressure Broadening
A broadening of spectral lines, particularly in white dwarfs, caused by the pressure of the stellar atmosphere, which in turn is caused by the surface gravity of the star. [H76]
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]
X-Ray Sources
A class of celestial objects whose dominant mechanism of energy dissipation is through X-ray emission. Galactic X-ray sources appear optically as starlike objects, peculiar in their ultraviolet intensity, variability (on time scales ranging from milliseconds to weeks), and spectral features. All known compact X-ray sources are members of close binary systems; a current popular model is mass accretion onto a compact object from a massive companion. (Four X-ray sources - all variable - are known to be associated with globular clusters.) The 21 known extended X-ray sources associated with clusters of galaxies seem to be clouds of hot gas trapped in the cluster's gravitational field. [H76]
Persei stars
A class of eclipsing binaries (see Algol) with periods of from 2 to 5 days, the depth of whose secondary minimum is almost negligible. [H76]
Trumpler Stars
A class of extremely luminous (and formerly considered extremely massive) stars. [H76]
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]
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]
Vela X
A compact radio source about 400-500 pc distant associated with the Vela supernova remnant. It has a nonthermal radio spectrum and is about 20 percent polarized. It is associated with the Gum Nebula, the Vela pulsar, and the X-ray source 2U 0832-45, although the pulsar and the X-ray source are displaced about 0°.7 from the center of the Vela X radio emission. Vela Y and Vela Z are outlying components, also nonthermal, but too weak to exhibit polarization. [H76]
Plasma
A completely ionized gas; the so-called fourth state of matter (besides solid, liquid, and gas) in which the temperature is too high for atoms as such to exist and which consists of free electrons and free atomic nuclei. [H76]
Overshoot
A condition that obtains when the momentum of a particle carries it past its equilibrium point. [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]
Quantum Solid
A degenerate gas in which the densities are so great that the nuclei are fixed with respect to each other so that they resemble a crystalline lattice. [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]
Retarder
A device for introducing a phase delay, such as half-wave or quarter-wave, between two orthogonally polarized components of an electromagnetic wave. [McL97]
Spin-Up
A discontinuous increase in the pulse frequency of a pulsar. [H76]
Pinch Machine
A fusion device containing a plasma heated by a shock wave generated within the plasma as it is constricted by the rapidly increasing magnetic field. [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]
Taylor Instability
A hydrodynamic instability which occurs whenever there is a density inversion. This configuration is said to be Taylor unstable (or Rayleigh-Taylor unstable) against perturbations that would cause mixing of layers of unequal densities. [H76]
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]
W Ursae Majoris Stars
A large class of double-lined eclipsing binaries with very short periods (a few hours) whose spectra indicate mass transfer. They are distinguished by the fact that their primary and secondary minima are equal. They are all F or G binaries on or near the main sequence. They may be the progenitors of dwarf novae. [H76]
Riemannian Geometry
A large class of non-Euclidean geometries. The mathematics of general relativity uses Riemannian geometry. (See general relativity.) [LB90]
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]
Ozone Layer
A layer in the lower part of Earth's stratosphere (about 20-60 km above sea level) where the greatest concentration of ozone (03) appears. This is the layer responsible for the absorption of ultraviolet radiation. [H76]
Photodiode
A light-sensitive device made from the junction of two differently doped species of a semiconductor such as silicon. Also known as a pn junction. An internal electric field is generated at the junction of p and n type material. Photons absorbed in the junction create electron-hole pairs which are separated by the field and create a current. [McL97]
Preamplifier
A low-noise amplifier designed to be located very close to the source of weak electronic signals, but capable of delivering amplified signals down tens of meters of cables. [McL97]
Red Dwarf
A main-sequence star with spectral type M. Red dwarfs are much fainter, cooler, and smaller than the Sun but are the most common type of star in the Galaxy, accounting for 70 percent of all stars. The nearest red dwarf, Proxima Centauri, lies just 4.25 light-years away, but neither it nor any other is visible to the naked eye. [C95]
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]
Protogalactic Gas Cloud
A massive gas cloud that collapsed to form a galaxy. Such clouds were produced as a result of the continued growth of density fluctuations after the Decoupling Era. [Silk90]
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]
47 Tucanae
A metal-rich globular cluster about 5.1 kpc distant. It has roughly one-quarter the solar metal abundance. It has a high galactic latitude and low reddening. [H76]
Wesselink Analysis
A method of determining the radius of a variable star. (also known as the Baade-Wesselink Method [H76]
Uranium-Lead Dating
A method of radioactive dating used for estimating the age of certain rocks. It is based on the decay of 238U to 206Pb (half-life 4.5 × 109 years) or 235U to 207Pb (half-life 0.7 × 109 years). The technique is useful for time periods from 107 years ago back to the age of the Earth (about 4.6 × 109 years). [DC99]
Thermal Diffusion
A method of separating gas molecules of different masses by maintaining one part of the gas at a lower temperature than the other (i.e. producing a temperature gradient along a column of gas) - the more massive molecules tend to stay at the low-temperature end. It can be used for the separation of isotopes. [DC99]
Thermal Bremsstrahlung
A mode of X-ray production by very energetic electrons accelerated in the field of a positive ion. [H76]
Thin-Screen Model
A model in which Gaussian angular scattering is concentrated near one point along the path. [H76]
Pancake Model
A model of galaxy formation in which the first structures to condense out of the smooth background of primordial gas were very large in size. These large masses then collapsed into thin sheets (pancakes) and fragmented into many smaller pieces the size of galaxies. A competing theory, sometimes called the hierarchical clustering model, proposes that the first structures to form were the size of galaxies. As galaxies clustered together, due to gravity, larger and larger structures were formed. (See Hierarchical Clustering Model) [LB90]
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.
Schwarzschild Black Hole
A nonrotating, spherically symmetric black hole derived from Karl Schwarzchild's 1916 exact solution to Einstein's vacuum field equations. [H76]
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]
Resonant Reaction
A nuclear reaction that has an energetically favorable probability of occurring (see resonance capture). [H76]
Optical Pair
A pair of stars that appear close together on the sky as a result of perspective only, and that have no physical relation. [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]
Tevatron
A particle accelerator capable of attaining an energy of 1 TeV. [F88]
Owl Nebula
A planetary nebula (M97, NGC 3587) in the constellation of Ursa Major, approximately 600 pc distant. [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]
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]
String Mode
A possible configuration (vibrational pattern, winding configuration) that a string can assume. [G99]
Tera
A prefix meaning 1012. [H76]
Pressure Gradient
A pressure difference between two adjacent regions of fluid results in a force being exerted from the high pressure region toward the low pressure region. In a star, the hot, dense interior and the cooler, more tenuous surface layers supply an outward pressure gradient, which balances the inward attractive force of gravity and stabilizes the star. [Silk90]
Weak-Equivalence Principle
A principle derived from the equality of the inertial and gravitational mass, which states that if we observe two bodies experiencing equal accelerations, we cannot, by observing the motion, tell whether they are being subjected to a uniform acceleration by some external mechanism or whether they happen to be in a uniform gravitational field. [H76]
Wollaston Prism
A prism used to obtain plane-polarized light. [H76]
Pumping (optical)
A process of raising matter from lower to higher energy levels. In order for a maser to work continuously, there must be some mechanism that replenishes the energy depleted by the emission and that provides population inversion. Such a mechanism is known as a pump. [H76]
Restoration
A process used by radio astronomers to eliminate the smoothing effect observed in radio maps that is caused by the finite width of the telescope beam. [H76]
Wave
A propagating pattern of disturbance. One example is a sound wave, in which a pattern of alternating high and low pressures propagates through air. Another example is an electromagnetic wave, in which a pattern of electric and magnetic fields propagates through empty space. [G97]
Rotation Curve
A quantitative description of how fast each part of a galaxy is rotating about the center. A rotation velocity is the velocity of a rotating galaxy at a certain distance from the center of the galaxy. A "flat" rotation curve is one in which the rotational velocity of the galaxy remains constant with distance away from the center of the galaxy. This indicates that the mass of the galaxy increases linearly with distance from its center. (See Keplerian rotation curve.) [LB90]
Potential
A quantitative measure of how much energy is associated with each possible arrangement of a physical system. An arrangement with a relatively high value of the potential is one with a relatively large amount of energy. For example, the gravitational potential of a pendulum at the top of its swing is large; the potential is small when the pendulum is at the bottom of its swing. Since systems in nature generally evolve toward arrangements of lower energy, as in the tendency of upended books to fall over, the final resting point of a system is in a configuration at the minimum value of the potential. The minimum of the potential for empty space corresponds to the vacuum. [LB90]
Pairing Energy ()
A quantity which expresses the fact that nuclei with odd numbers of neutrons and protons have less energy and are less stable than those with even numbers of neutrons and protons. [H76]
Superposition Principle
A quantum mechanical principle according to which any two states can be combined (actually in infinitely many ways) to form states which have characteristics intermediate between those of the two which are combined. In particular, if an eventuality is true in one of the states and false in the other, then it is indefinite in a superposition of the two states. [D89]
X-Band
A radio band at a wavelength of 3.7 cm (8085 MHz). [H76]
Percolation
A random system defined mathematically on an infinite space, such as the random formation of bubbles in the early Universe, is said the percolate if the objects merge to form an infinite cluster. In the original inflationary theory the bubbles that ended the inflation were found not to percolate, implying that they could never be homogeneous enough to describe our Universe. The failure of the original inflationary theory became known as the graceful exit problem. (See Decay of the False Vacuum) [G97]
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]
pep Reaction
A reaction occurring in the proton-proton chain. The first step, instead of p + p -> d + e+ + ve, is p + e- + p -> d + ve. This latter reaction occurs only once in 400 p-p reactions but produces far more energetic neutrinos (1.44 MeV as against 0.42 MeV). [H76]
URCA process
A series of nuclear reactions, primarily among the iron group of elements, accompanied by a high rate of neutrino formation and postulated as a cause of stellar collapse. Neutrinos carry away energy quickly and invisibly, so this process was named for the Urca Casino in Rio de Janeiro, which carried away money the same way. [H76]
Power Series
A series of the form a0 + a1v + a2v2 + . . . + anvn = n=1anvn. [H76]
Vector Space
A set of elements (called Vectors) for which a binary operation of vector addition is defined, such that u1 + u2 is a vector if u1 and u2 are vectors: and a binary operation scalar multiplication is defined, such that cu is a vector if u is a vector and c is a scalar (a real number or a complex number, according to specification of the kind of vector space); and a standard collection of conditions governing these two operations is satisfied. [D89]
P-Branch
A set of lines in the spectra of molecules corresponding to unit increases in rotational energy. [H76]
Vacancy
A site on a lattice on which there is no atom present. [D89]
Yerkes System
A spectral classification system for stars; see MKK system. [H76]
Pickering Series
A spectral series of He II lines found in very hot O-type stars. It is associated with the fourth energy level - Pi at 10124 Å; Pi 6560 Å. The series limit is at 3644 Å. [H76]
Pfund Series
A spectral series of hydrogen lines in the far-infrared, representing transitions between the fifth energy level and higher levels. [H76]
Spectrometer
A spectroscope fitted with a device such as a photoelectric cell for measuring the spectra observed with it. [H76]
Spiral Nebula
A spiral galaxy - not really a nebula at all (although many do appear nebulous). [A84]
Sombrero Galaxy
A spiral galaxy in the constellation Virgo. It was the first galaxy whose rotation was detected. (M104, NGC 4594) [C95]
Stationary Wave
A standing wave; the pattern formed when two waves of the same amplitude and frequency move simultaneously through a medium in opposite directions. [H76]
Subgiant
A star whose position on the H-R diagram is intermediate between that of main-sequence stars and normal giants of the same spectral type. [H76]
Tangential Velocity
A star's velocity across an observer's line of sight. To calculate a star's tangential velocity, one must know the star's distance and proper motion. [C95]
VV Cep Stars
A subgroup of composite spectrum stars. One observes a spectrum of a K or M supergiant, showing emissions of hydrogen and [FeII] plus the spectrum of the secondary, which is generally of type B. [JJ95]
Young Thin Disk
A subpopulation in the thin disk whose stars range in age from 0 to 1 billion years old. The stars of the young thin disk have a scale height of 350 light-years and have very circular orbits around the Galaxy. [C95]
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]
White Supergiant
A supergiant star with a spectral type of A. White supergiants are rare; the nearest is Deneb, which lies 1500 light-years away. (rarely used term) [C95]
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]
Red Supergiant
A supergiant with spectral type M. Red supergiants are the largest stars in the universe: if put in place of the Sun, some would touch Saturn. The two brightest red supergiants in Earth's sky are Betelgeuse and Antares. [C95]
Puppis A
A supernova remnant 104-105 years old, about 1-2 kpc distant. It is an extended nonthermal radio source, and also a source of soft X-rays (2U 0821-42). [H76]
Reticle
A system of cross-hairs in the eyepiece of a telescope. [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]
Vignetting
A systematic error in the measurement of stellar magnitudes when the object being measured is far off axis. [H76]
Rankine Scale
A temperature scale with the same division as the Fahrenheit scale and the zero point at 0° absolute. 0° R = - 470° F. [H76]
Twin-Exhaust Model
A theoretical model for radio galaxies in which a compact source in the galactic nucleus is assumed to emit twin beams of rapidly moving plasma that traverse hundreds of thousands of light-years, eventually splattering to a halt in the ambient intergalactic gas, where the resulting dissipation energizes the radio lobes. [Silk90]
Variable-Mass Theory
A theory of Hoyle and Narlikar in which the masses of fundamental particles are assumed to vary with time in a manner that precisely accounts for the Hubble redshift law. [Silk90]
Zanstra's theory
A theory of emission lines in planetary nebulae which supposes that the emission lines in hydrogen (and helium) arise from a process of ionization (by the ultraviolet radiation of the central star) and recombination, and that the forbidden lines arise from the collisional excitations of the metastable state. [H76]
Toy Theory
A theory which is known to be too simple to describe reality, but which is nonetheless useful for theorists to study because it incorporates some important features of reality. For example, most of what is known about magnetic monopoles in grand unified theories was discovered first in a toy theory that includes only three Higgs fields, while the simplest realistic grand unified theory includes twenty-four of them. [G97]
-Time
A time scale in which there is no relative motion between two observers (cf. t-time). [H76]
Steradian (sr, 0)
A unit of solid (three-dimensional) angular measure. One steradian is equal to the angle subtended at the centre of a sphere by an area of surface equal to the square of the radius. The name for the unit seems to have come into use about 1880 and was comparatively common by the turn of the century. The surface of a sphere subtends an angle of 4 steradians at its centre. [JM92]
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]
Tempon
A unit of time equal to the length of time it takes light to cross the classical radius of an electron (about 10-23) seconds). [H76]
Transparent
Able to pass radiation without significant deviation or absorption. Note that a substance transparent to one radiation may be opaque to another. The divide between transparency and translucency is not well defined. Thus some people call a filter transparent (as it does not distort radiation): others would call it translucent (as it absorbs some of the radiation). (see also Translucent) [DC99]
Translucent
Able to pass radiation, but with much deviation and/or absorption. Compare transparent. [DC99]
Planck Tension
About 1039 tons. The tension on a typical string in string theory. [G99]
Planck Mass
About ten billion billion times the mass of a proton; about one-hundredth of a thousandth of a gram; about the mass of a small grain of dust. The typical mass equivalent of a vibrating string in string theory. [G99]
Primordial Quarks
All baryons and mesons are believed to be composed of quarks, which are elementary particles of fractional charge. In the high-density, hot-temperature phase of the very early Universe, prolific numbers of quarks would have been present in equilibrium with the other elementary particles. As the Universe expanded and cooled, some of these quarks may have been frozen out. To what extent independent free quarks could survive is an unresolved issue of elementary particle physics. [Silk90]
Valence
Also valence band or valence electrons; the electrons in the outermost orbit. [McL97]
Toro
An Earth-crossing asteroid (No. 1685) discovered by Wirtanen in 1948 and rediscovered in 1964, of irregular shape (about 5 × 3 km), whose closest approach takes it within 0.13 AU of Earth. Perihelion distance 0.77 AU; aphelion distance 1.96 AU; e = 0.44. Orbital period 584.2 days (8/5 that of Earth); rotation period 10h11m; a = 1.37 AU. Radar observations indicate a rocky surface, thinly covered with dust. High albedo ( 0.15). [H76]
Virgo X-1
An X-ray source identical to Virgo A. It is also one of the most powerful extragalactic sources of radiation at infrared wavelengths. (2U 1228+12) [H76]
Trifid Nebula
An emission nebula in Sagittarius, ~ 1 kpc distant. (M20, NGC 6514)[H76]
Poisson's Equation
An equation (2 = 4G) which relates the gravitational (or electromagnetic) potential to the mass density (or charge density). [H76]
Vis Viva Equation
An equation governing the conservation of angular momentum. [H76]
Standstill
An interval in the cycle of a variable star during which the brightness temporarily stops changing. [H76]
Ursa Minor Dwarf Galaxy
An intrinsically faint (Mv - 9) dwarf elliptical galaxy about 70 pc distant, in the Local Group. [H76]
Thermion
An ion, either positive or negative, which has been emitted by a heated body. Negative thermions are electrons. [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]
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]
Starburst Galaxy
Any galaxy in which an anomalously large rate of star formation is taking place. [C97]
Relativistic
Approaching the velocity of light. Particles moving at these speeds demonstrate effects predicted by the special theory of relativity - increased mass, slowing of time, etc. - that must be taken into account by combining relativity with quantum theory if accurate predictions are to be made. [F88]
Patroclus
Asteroid 617, a Trojan 60° behind Jupiter. P = 11.82 yr, a = 5.19 AU, e = 0.14, i = 22°.1. [H76]
Planetesimals
Asteroid-sized solid bodies that are hypothesized to form when the protosolar nebula collapsed into a disk and fragmented. Most of the planetesimals subsequently accumulated into planets. [Silk90]
Ultraviolet Astronomy
Astronomy carried out in the waveband 300nm to about 10nm. At these wavelengths, the atmosphere is opaque to radiation and hence these astronomies have to be conducted from above the Earth's atmosphere. The wavelength band 300 to 120nm can be successfully explored using telescopes which form a natural extension of optical techniques. At shorter wavelengths, new approaches have to be taken because the mirror materials become nonreflecting. In addition at short ultraviolet wavelengths, < 91.2 nm, the inrerstellar medium is likely to become opaque because of Lyman-continuum absorption. [D89]
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]
Secondary Cosmic Rays
Atomic fragments - mainly muons - produced by collisions between primary cosmic rays and the molecules in Earth's atmosphere. [H76]
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]
Photon Decoupling
The release of photons from constant collisions with massive particles as the Universe expanded and its matter density diminished. (See Decoupling) [F88]
Program Stars
The stars being observed or measured, as contrasted with the comparison stars. [H76]
Terminal Velocity
The steady final velocity reached by a body in a fluid when the resultant force on it is zero.[DC99]
Surface Gravity
g: Also called acceleration due to gravity. The rate at which a small object in free fall near the surface of a body is accelerated by the gravitational force of the body, g = GM / R2. Surface gravity of Earth is equal to 980 cm s-2 32 feet s-2. [H76]
radio map of the sky
Celestial chart depicting sources and intensities of radio emission. [A84]
Rossby Waves
Cyclonic convection waves in a rotating fluid. Such waves occur in the atmosphere, in the oceans, and in the fluid core of the Earth. [H76]
Plasma Clouds
Clouds of electrically charged particles embedded in the Solar wind. [H76]
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]
Three-Body Problem
Eighteenth- and nineteenth- century problem in celestial mechanics to analyze the gravitational effects of three celestial bodies in finding a stable orbital configuration. [A84]
Ultraviolet Radiation
Electromagnetic radiation "beyond the violet" with wavelengths in the approximate range 100-4000 Å. [H76]
Ultraviolet Light
Electromagnetic radiation of a wavelength slightly shorter than that of visible light. [F88]
Polychromatic Radiation
Electromagnetic radiation that has a mixture of different wavelengths. Compare monochromatic radiation. [DC99]
Radio
Electromagnetic radiation with the lowest energy and longest wavelength. Unlike visible light, radio waves penetrate dust and can be detected from throughout the Galaxy. [C95]
Photoelectrons
Electrons ejected from a solid, liquid, or gas by the photoelectric effect or by photoionization. [DC99]
Transuranic Elements
Elements that have a proton number greater than 92 (i.e. greater than that of uranium). The transuranic elements are all unstable (radioactive) and are produced by nuclear reactions induced either by bombarding heavy elements with high-energy nuclei of light elements or by the capture of slow neutrons, followed by beta decay. In nuclear explosions, transuranic elements may be produced by numerous successive captures of neutrons by uranium nuclei. [DC99]
Vlasov-Maxwell Equations
Equations that describe the propagation of radiation in hot, collisionless plasmas. [H76]
Unstable Equilibrium
Equilibrium such that if the system is disturbed a little, there is a tendency for it to move farther from its original position rather than to return. (see Equilibrium; Stability) [DC99]
T Tauri Stars
Eruptive variable subgiant stars associated with interstellar matter and believed to be still in the process of gravitational contraction on their way to the main sequence. They are found only in nebulae or very young clusters. They have low-temperature (G-M) spectra with strong emission lines and broad absorption lines. Their absolute magnitudes are brighter than those of main-sequence stars of similar spectral types. They have a high lithium abundance. T Tau itself is dG5e. (sometimes called RW Aurigae stars) [H76]
ROSAT
ROentgen SATellite, was an X-ray observatory developed through a cooperative program between the Germany, the United States, and the United Kingdom. The satellite was designed and operated by Germany, and was launched by the United States on June 1, 1990. It was turned off on February 12, 1999.
Virial Theorem
For a bound gravitational system the long-term average of the kinetic energy is one-half of the potential energy. [H76]
Relativistic Zone
For a pulsar, the region in which M[grams] / R[cm] is not negligible compared with unity. [H76]
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]
Vector Boson
Force-carrying particles of nature. Three vector bosons are responsible for the weak nuclear force. By admitting the photon on an equal footing it is possible to create a unified electroweak theory. As a result of symmetry-breaking processes, however, this photon remains massless while the three other vector bosons pick up mass. [P88]
Sobieski
Former name of the southern constellation Scutum. [H76]
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]
Planck Units
Fundamental units of length, time, mass, energy, etc. involving Planck's quantum constant, , Newton's gravitational constant, G, and the speed of light, c. As they incorporate both the quantum and gravitational constants, the Planck units play a key role in theories of quantum gravity. [CD99]
Particles
Fundamental units of matter and energy. All may be classed as fermions, which have half-integral spin and obey the exclusion principle, and bosons, which have integral spin and do not obey the exclusion principle. The term particle is metaphoric, in that all subatomic particles also evince aspects of wave-like behavior. [F88]
Weak Gauge Symmetry
Gauge symmetry underlying the weak force. [G99]
Supersymmetric Standard Model
Generalization of the standard model of particle physics to incorporate supersymmetry. Entails a doubling of the known elementary particle species. [G99]
Plaskett's Star
HD 47129: A very massive O-type giant with known anomalies in its spectrum. It is a spectroscopic binary in which mass exchange is occurring. Its spectrum can be interpreted to mean that each component has a mass of 75 M. [H76]
Simulations
In science, simulations of physical systems with a computer. (see N-body Simulations.) [LB90]
Point-Like
If matter is probed with projectiles that are large and have energies that are less than what is needed to change the energy levels of an atom, then atoms will seem to be point-like objects. If the energy is increased, eventually the projectile will penetrate the atom but will encounter the nucleus, which will seem to be point-like. With higher energy, the nucleus will appear to be made of point-like protons and neutrons. With still higher energies, the protons and neutrons will be seen to be made of point-like quarks and gluons. As the energies of projectiles were increased still more, quarks and leptons might have been seen to be made of something still smaller, but that has not happened. Rather, they behave as point-like up to the highest energies they have been probed with - energies well beyond those for which we would have expected to find more constituents if history were to repeat itself once more. Further, the structure of the Standard Model theory suggests that quarks and leptons are the fundamental, point-like constituents of matter. [K2000]
Vortex
In a planar spin model a vortex is a pattern of spins in which the spin direction rotates by 360 degrees along any path which surrounds the centre of the vortex. The name is taken from hydrodynamics, where it denotes the kind of circular flow pattern that can be observed in water flowing out of a bathtub. The vortices in superfluid helium films are of this sort. [D89]
Radiative Equilibrium
In a star, represents an even process by which energy (heat) is transferred from the core to the outer surface without affecting the overall stability of the star. [A84]
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]
Sunspot Radiation
Intense, variable, circularly polarized radio waves in a noise storm. [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]
T Tauri Stars
Late type irregular variables associated with bright or dark nebulosity. The spectrum exhibits emission in both CaII and H lines. [JJ95]
Weak-Line Stars
Late type objects in which the lines of all metals are weakened when compared with normal stars of the same temperature. Also called metal-weak stars. [JJ95]
Wolf Diagram
Logarithmic plot of N (number of stars or galaxies counted at successive magnitude limits) versus apparent magnitude. [H76]
Triangulation
Measurement of the distance of a planet or nearby star by sighting its apparent position against background stars from two or more separate locations. (see Parallax) [F88]
Twistor Theory
Model of the Universe proposed by Roger Penrose, based on the application of complex numbers (involving (-1)1/2) used in calculations in the microscopic world of atoms and quantum theory to the macroscopic ordinary world of physical laws and relativity. The result is an eight-dimensional concept of reality that although complicated is possibly a more logical understanding of the constitution of the Universe. [A84]
Quantum Geometry
Modification of Riemannian geometry required to describe accurately the physics of space on ultramicroscopic scales, where quantum effects become important. [G99]
Para-Hydrogen
Molecular hydrogen in which the two protons of the diatomic molecule have opposite directions of spin. It is a lower energy state than ortho-hydrogen. [H76]
Ortho-Hydrogen
Molecular hydrogen in which the two protons of the diatomic molecule have the same direction of spin. It is a higher energy state than the para form. Terrestrial H2 is 75% ortho-hydrogen, 25% parahydrogen. [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]
Radio Telescope
Non-optical telescope (of various types) which, instead of focusing light received from a distant object, focuses radio signals onto a receiver-amplifier. [A84]
z
Notation for redshift. Objects of higher z are further away. A redshift of z near 0 corresponds to objects nearby; a redshift of z = 1 corresponds to objects at a distance of about 10 billion light years. [LB90]
Seed Nuclei
Nuclei from which other nuclei are synthesized. [H76]
Type I String Theory
One of the five superstring theories; involves both open and closed strings. [G99]
Type IIB String Theory
One of the five superstring theories; involves closed strings with left-right asymmetric vibrational patterns. [G99]
Type IIA String Theory
One of the five superstring theories; involves closed strings with left-right symmetric vibrational patterns. [G99]
Weak Nuclear Force
One of the four fundamental forces of nature. Controls the interaction of neutrinos. [c97]
Weak (Nuclear) Force
One of the four fundamental forces, best known for mediating radioactive decay. [G99]
Relaxation Time
Period required for the reestablishment of thermal equilibrium; in particular (in the astronomical context) the period required for the reestablishment of a random distribution of motion in a cluster of stars. [H76]
RV Tau Variables
Periodic variables with periods 60-100 days, and of spectral types G and K. [JJ95]
Panstellar
Pertaining to more than one star. [F88]
POSS
Palomar Observatory Sky Survey [BFM02]
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]
Terrestrial
Pertaining to the Earth.
Schmidt Plates
Photographic plates obtained with a Schmidt telescope, which is a type of telescope with a particularly large field of view. [LB90]
Promethium
Pm: An unstable rare earth. The longest-lived isotope, 145Pm, has a half-life of only 18 years. [H76]
PSF
Point Spread Function -- The size and shape of the actual image of a point source as a result of the combined effects of atmosphere, optics, guiding. [McL97]
PSD
Power Spectral Density [LLM96]
RADAR
Radio Detection and Ranging. [LLM96]
Taurus A
Radio source designation for Crab Nebula.
Whistlers
Radio waves generated by a flash of lightning, which travel along Earth's magnetic field out beyond the ionosphere and back to Earth. They arrive back with a descending pitch because the high-frequency end of the wave train arrives first (see Dispersion). [H76]
RF Power
Radio-Frequencypower. Electromagnetic fields alternating at the frequencies of radio waves (up to 1010 Hz), which can be used to accelerate charged particles in accelerators. [CD99]
Oblateness
Ratio of the difference between the equatorial and polar radii to the equatorial radius. Oblateness usually is an indication of how fast the body is rotating. [H76]
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]
SC Stars
Stars which appear to be intermediate in type between S stars and carbon stars (C/O ratio near unity). [H76]
Polarization of Light
Reduction of light, considered to travel in three-dimensional transverse waves (vibrating in all directions perpendicular to the direction in which it is traveling), to two dimensions. To achieve this a filter is used. The results may vary from a beam of light in which the waves vibrate in one plane only (plane-polarized light) to one in which the plane rotates but the amplitude is constant (circular polarization). Because light is also polarized by reflection, investigation of polarized light reflected from, for example, the lunar surface enables that surface to be analyzed. [A84]
R Zones
Regions in the solar corona in which short-lived radiofrequency variations are observed. [H76]
Tycho's Star
Remnant of a Type I supernova (B Cas), 3-5 kpc distant, which Tycho observed and described in 1572. At its peak it was as bright as Venus and was visible in the daytime, reaching a magnitude of about -4. It is an X-ray source (2U 0022+63). (3C 10) [H76]
RTI
Rise Time Invariance.
Radio Stars
Stars with detectable emission at radio wavelengths. They include pulsars, flare stars, some infrared stars, and some X-ray stars. [H76]
Peculiar Stars
Stars with spectra that cannot be conveniently fitted into any of the standard spectral classifications. They are denoted by a p after their spectral type. [H76]
Synchronous Rotation
Rotation whose period is equal to the orbital period. [H76]
Superconducting Super Collider
SSC A proposed accelerator of great size and high energy. [F88]
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]
Unification
Scientists have sought for centuries to unify the descript ions of apparently different phenomena by showing that they were due to the same underlying natural laws and that complex levels of matter were made of simpler levels. This unification process is a subject of very active research about the forces of nature today. The possible unification of the strong, electromagnetic, and weak forces is called a grand unification. There is a continuing effort to unify these forces with gravity. String theories seem to do that successfully. [K2000]
Perfect Gas
See Ideal Gas. [H76]
Russell-Saunders Coupling
See LS coupling. [H76]
Past Light Cone
See Light Cone. [H76]
Profile
See Line Profile. [H76]
Primordial Black Holes
Small black holes hypothesized to have formed during the first 10-43 seconds of the Universe, when quantum effects were very large. [LB90]
Weak Gauge Boson
Smallest bundle of the weak force field; messenger particle of the weak force; called W or Z boson. [G99]
SAO
Smithsonian Astronomical Observatory [LLM96]
Refraction, Law of
Snell's law. [H76]
SSPM
Solid State Photomultiplier [LLM96]
SIS
Solid-state Imaging Spectrometer (ASCA X-ray satellite).
Phillips Bands
Spectral bands of the C2 molecule in the red and near-infrared (0-0 band at 1.207 µ). [H76]
Swan Bands
Spectral bands of the carbon radical C2 first investigated in 1856 by W. Swan. They are a characteristic of carbon stars. Swan bands pass through a minimum between spectral types R4 and R6 and increase again toward N6. [H76]
Werner Lines
Spectral lines of molecular hydrogen in the ultraviolet, in the same general region as the Lyman lines. [H76]
SOFIA
Stratospheric Observatory for Far-Infrared Astronomy. [LLM96]
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]
SIS
Superconductor Insulator Superconductor [LLM96]
SIS Junction
Superconductor-Insulator-Superconductor Junction. Can be used as the mixer in a radio receiver system. [McL97]
Superspace
Supersymmetry can be formulated in several ways. One is to imagine ass ociating another coordinate that has special properties with each of our normal spacetime coordinates, giving a kind of space called superspace. Writing theories in superspace makes them supersymmetric. This way of constructing supersymmetric theories is harder to picture than associating superpartners with each Standard Model particle, but it leads to the same results and sometimes facilitates deriving mathematical properties of the theories.
SU(5)
Symmetrical Unitary of Order 5: The simplest type of grand unified theory, proposed in the 1970s. (see Grand Unified Theories) [LB90]
Terrestrial Dynamical Time
TDT The independent argument for apparent geocentric ephemerides. At 1977 January 1d00h00m00s TAI, the value of TDT was exactly 1977 January 1d.0003725. The unit of TDT is 86400 SI seconds at mean sea level. For practical purposes TDT = TAI + 32s.184. (see Barycentric Dynamical Time; Dynamical Time; International Atomic Time.) [S92]
TDT
Terrestrial Dynamical Time. The independent argument for apparent geocentric ephemerides. At 1977 January 1d00h00m00s TAI, the value of TDT was exactly 1977 January 1d.0003725. The unit of TDT is 86400 SI seconds at mean sea level. For practical purposes TDT = TAI + 32s.184. (see Barycentric Dynamical Time; Dynamical Time; International Atomic Time.) [S92]
Orion Spur
That part of the local spiral arm in which the Sun is embedded. (The Sun is on an inner edge of the Orion spur.) [H76]
True Equator and Equinox
The Celestial Coordinate System determined by the instantaneous positions of the celestial equator and ecliptic. The motion of this system is due to the progressive effect of precession and the short-term, periodic variations of nutation. (see Mean Equator and Equinox.) [S92]
Ursa Minor
The Little Bear (or Little Dipper), a constellation in the northern sky that contains Polaris, the North Star. [C95]
Tropical Year
The interval of time between two successive vernal equinoxes. It is equal to 365.242 mean solar days. [H76]
SETI
The Search for Extraterrestrial Intelligence, by using radiotelescopes to listen for signals transmitted by intelligent alien beings. [F88]
Resolution
The ability of an optical system, including detector, to separate two adjacent objects - this is called "spatial resolution" - or two adjacent wavelengths in a spectrometer - this is called "spectral resolution". [McL97]
r-Process
The creation of elements heavier than zinc through the rapid bombardment of other elements by neutrons. The r process occurs in supernovae. Examples of reprocess elements are gold, iodine, and europium. [C95]
Wavefront Error
The departure of the true wavefront in an electromagnetic wave propagating through an optical system from the ideal spherical wave at that point. [McL97]
Tesla
The derived SI unit of magnetic flux density. 1 Tesla = 1 Wb m-2 = 104gauss. [H76]
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]
Winding Energy
The energy embodied by a string wound around a circular dimension of space. [G99]
Thermal Convection
The energy transfer in a fluid by a mechanism of bulk hydrodynamic movement. [D89]
Post Hoc Fallacy
The erroneous assumption that, because B follows A, B therefore was caused by A. More strictly, the fallacy of calculating, in retrospect, the odds of B's having occurred by adding up a long sequence of such putative causes. [F88]
Primary Mirror
The first mirror encountered by incident light in a telescope system. [McL97]
Valence Band
The highest completely filled energy band of a solid. In an insulator or semiconductor empty states in the valence band can carry an electric current as positively charged `holes'. [D89]
Primeval Fireball
The hot, dense, early stage of the Universe (predicted by the Big Bang theory) when the Universe was predominantly filled with highly energetic radiation, which subsequently expanded and cooled and is now observed as the cosmic microwave background radiation. [Silk90]
Tired Light
The hypothesis that light may be degraded in energy, thereby increasing in wavelength and becoming redshifted, during its passage through intergalactic space. This would provide an alternative to the Big Bang model in accounting for the redshifts of distant galaxies. However, there is no evidence for any such tired-light effect. [Silk90]
Triton
The inner satellite of Neptune, discovered by Lassell in 1846. It is larger than the Moon (R 2900 km), with an almost circular retrograde orbit of 5 days 21 hours. [H76]
Resonance Line
The longest-wavelength line arising from the ground state. [H76]
Propagator
The mathematical expression used to describe the propagation in space-time of virtual particles. [CD99]
Radiation Length
The mean distance traveled by a relativistic particle in a given medium before its energy is reduced by a factor e by its interaction with matter. [H76]
Statistical Parallax
The mean parallax for a group of stars which are all at approximately the same distance, determined from their radial velocities and from the tau components of their proper motion. [H76]
Photometry
The measurement of the intensity of light from an astronomical object. [LB90]
Primary Theory
The name used in this book for the theory sought by many particle physicists that includes not only the Standard Theory but also the theory of gravity, explains why the primary theory itself takes the form it does, explains what quarks and other particles are, explains what space and time are, and more. (See Theory of Everything) [K2000]
Triton ()
The nucleus of the tritium atom. [H76]
Old Inflation
The original (1981) Inflationary Universe model. see Inflationary Universe Model; New Inflation [LB90]
Phoebe
The outermost satellite of Saturn, discovered by Pickering in 1898. Period 550 days retrograde: radius about 100 km. [H76]
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]
Quantum Defect
The principal quantum number responsible for a spectral series, minus the Rydberg denominator for any actual spectral term of the series. (also called Rydberg correction) [H76]
Velocity-of-Light Radius
The radius of a rotating neutron star at which the rotational velocity of the plasma approaches the velocity of light. (also called velocity-of-light cylinder) [H76]
Statistical Distribution
The range of variation of some quantity in a population, obtained by sampling many members of the population. For example, the statistical distribution of the height of American males could be obtained by sampling 10,000 randomly chosen males and counting the number of them within each range of heights. In cosmology, the distance between pairs of galaxies, averaged over a large number of galaxies, would constitute a statistical distribution. [LB90]
Reaction Rate
The rate at which a chemical or nuclear reaction proceeds. Particles interacting via the strong nuclear force react together in roughly 10-23 seconds, while particles interacting via the weak nuclear force, such as the disintegration of a neutron, might take seconds. [LB90]
Photon-to-Baryon Ratio
The ratio of the number of photons to the number of baryons in any typical, large volume of space. (See Baryons; Photon) [LB90]
Transmittance
The ratio of the transmitted energy that a substance allows through to the energy incident upon it. (Transmission Coefficient) [DC99]
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]
Selective Absorption
The reddening of starlight in passing through fine particles of interstellar dust. [H76]
Plasmapause
The region in Earth's ionosphere (at about 4-7 Earth radii) where the particle density (100 particles per cm3 just below the plasmapause) drops off very rapidly. It marks the transition from high to low density. [H76]
Stratosphere
The region of Earth's atmosphere immediately above the troposphere. It starts at a height of about 15 km and goes to a height of about 50 km. The temperature increases from about 240 K to about 270 K. [H76]
Reconnection
The rejoining of magnetic lines of force severed by the annihilation of the field across the neutral region. [H76]
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.
Slepton
The supersymmetric partner of any of the leptons.
Squark
The supersymmetric partner of any of the quarks. [K2000]
WINO
The supersymmetric partner of the W boson [K2000]
Zino
The supersymmetric partner of the Z boson.
Selectron
The supersymmetric partner of the electron.
Photino
The supersymmetric partner of the photon. [K2000]
von Zeipel's Theorem
The surface brightness of a rotating star or a component of a binary at any point on its surface is proportional to the local value of gravity. [H76]
World Surface/World Sheet
The surface traced in space-time by an extended object such as a string. [P88]
Preflash
The technique of illuminating the CCD with a low light level flash before beginning a long exposure in order to "fill up" any charge traps. [McL97]
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]
Sunrise/Sunset
The times at which the apparent upper limb of the Sun is on the astronomical horizon; i.e., when the true zenith distance, referred to the center of the Earth, of the central point of the disk is 90°50', based on adopted values of 34' for horizontal refraction and 16' for the Sun's semidiameter. [S92]
Universe
The total celestial cosmos. According to Gott et al. the universe seems to be on a large scale isotropic, homogeneous, matter-dominated, and with negligible pressure. The total proper mass content of about 1023 M (Sandage derives 1056 g from his determination of the deceleration parameter q0) and radius of about 2 × 1028 cm are the order of magnitude that most cosmologists would accept if the universe is bounded. Total mass contributed by luminous matter, about 3 × 1053 g (see Mass Discrepancy). Age about 18 × 109 yr for a Hubble constant H0 = 55 km s-1 Mpc-1. [H76]
Poincaré's Theorem
The total kinetic energy of all the stars in a cluster is equal to half the negative gravitational potential energy of the cluster. [H76]
Spectral Bandwidth
The total wavelength or frequency range over which photons can be detected with reasonable efficiency.
Tincle
The track of a charged particle in a meteorite. (track in the cleavage)[H76]
Radiation Pressure
The transfer of momentum by electromagnetic radiation incident on a surface: prad = (4/3)T4 / c. [H76]
Twistor
The twistor is a sort of generalization of a spinor, being a massless object having both linear and angular momentum. It can be defined in terms of a pair of spinors. Twistors are the coordinates of twistor space, but they also have a geometrical interpretation in space-time. Twistors with zero helicity correspond to null lines while more general twistors must be pictured as congruences of null lines. [P88]
Reset Noise
The unwanted and uncertain electrical signal transmitted to the output pin of a CCD during the process of recharging, via the reset transistor, the output storage capacitor to its preset value in readiness for the next pixel charge. [McL97]
Wien's Law
The wavelength at which a blackbody emits the greatest amount of radiation is inversely proportional to its absolute temperature. [H76]
Signal Band
The wavelength interval within which a feature (e.g., the 21-cm line) is measured (cf. comparison band). [H76]
Phase Conjugation
This novel form of nonlinear mixing of optical waves generates an output wave which retraces precisely the path taken by the input wave. The phase conjugation reverses the wave front variation in the incident field and can compensate exactly aberrations and distortions in the input. The technique is a kind of holography in real time. [D89]
True Vacuum
This phrase has the same meaning as vacuum, with the word "true" being used only to emphasize the distinction with the false vacuum. [G97]
TDG
Tidal Dwarf Galaxy. A self-gravitating entity of dwarf-galaxy mass, built from tidal material expelled during interactions. Examples include AM0547-244, AM1054-325 and AM1353-272 (see Weilbacher, et al., Astron. Astrophys., 358, 819, 2000)
TDI
Time Delay and Integration or drift-scan. Methods for averaging the response of a CCD along columns by reading out at the same rate as a mechanical motion is shifting the optical image along the CCD. [McL97]
Topologically Distinct
Two shapes that cannot be deformed into one another without tearing their structure in some manner. [G99]
VLT
Very Large Telescope [LLM96]
Superunified
While grand unified theories attempt to describe three of the four known interactions of nature - the weak, strong, and electromagnetic interactions - in a unified way, the fourth interaction, gravity, is omitted. Theories which attempt to include gravity as well, such as superstrings, are called superunified. [G97]
Topocentric
With reference to, or pertaining to, a point on the surface of the Earth, usually with reference to a coordinate system. [S92]
Selenocentric
With reference to, or pertaining to, the center of the Moon. [S92]
[WR] stars
Wolf-Rayet stars that are the central objects of a planetary nebulae. [JJ95]
XBONG
X-Ray Bright Optically Normal galaxy. X-ray satellites have now detected a sizeable number of X-ray sources spectroscopically identified with "normal" galaxies otherwise having no any obvious signs of nuclear activity in their optical spectra. The large X-ray-to-optical flux ratio exceeds the average value for early-type galaxies of similar optical lumisosity, by more than an order of magnitude. The hardness of the X-ray spectrum suggests that highly obscured AGN activity is taking place in their nuclei. However, the lack of optical emission lines could also be explained if the nuclear light is being overwhelmed by either the stellar continuum or a non-thermal component, or if the lines are not efficiently produced.
Population I
Younger stars, generally formed towards the edge of a galaxy, of the dusty material in the spiral arms, including the heavy elements. The brightest of this Population are hot, white stars. [A84]
Strömgren Spheres
Zones of ionized hydrogen gas surrounding hot stars embedded in interstellar gas clouds; they are called additionally H II zones. [A84]
Quantum Hall Effect
in a two-dimensional electron system at sufficiently low temperature and in sufficiently high magnetic field the ratio of the current to the voltage applied in a direction perpendicular to the current is very accurately a multiple (integer or fraction with small odd denominator) of e2/, where e is the electron charge and is Planck's constant. [D89]
Sublattice Magnetization
in an antiferromagnet the magnetic atoms can be divided into two equivalent classes, each magnetized in opposite directions. The total magnetization of one of these classes is the sublattice magnetization. [D89]
Optical Chaos
in many nonlinear optical systems the output response varies in an unpredictable and uncontrollable fashion despite being governed by deterministic laws. Such optical chaos shows a rich and unexpected variety of phenomena which are only recently being discovered.[D89]
SU(N)
mathematical structure known as a `group' that describes operations on N objects. Examples include SU(2) applied to the two quarks or two leptons in a generation and SU(3) applied to the three colors of quark. The three colors and two flavors have recently been combined to yield a set of live entities that can be described by a grand unified theory exploiting SU(5). [D89]
Tau Lepton
negatively charged lepton in the third generation of particles. A heavier analogue of the electron and muon. [D89]
Swan Nebula
see Omega Nebula. [H76]
Stellar Parallax
see Parallax [C95]
Vertex
see Radiant [H76]
Pressure Scale Height
see Scale Height. [H76]
Population
see Stellar Population. [C95]
Transverse Velocity
see Tangential Velocity [H76]
1 Orionis
see Trapezium
UV Stars
see Ultraviolet Stars.
Speed-of-Light Circle
see Velocity-of-Light Radius. [H76]
Sum Rule
see f-sum Rule. [H76]
Triple- process
see 3 process. [H76]
Spherical Aberration
see Aberration. [A84]
Sco
see Antares. [H76]
Planetoids
see Asteroids [A84]
Visual Binary Star
see Binary System.
Solar Burst
see Burst [H76]
Space Curvature
see Curvature. [LB90]
Veil Nebula
see Cygnus Loop. [H76]
Ultrashort-Period Cepheids
see Delta Scuti Stars. [H76]
Time Delay
see Dispersion. [H76]
World Point
see Event. [H76]
Stellar Halo
see Halo [C95]
Velocity-Distance Relation
see Hubble's Law. [H76]
Specific Intensity
see Intensity [H76]
W Boson
see Intermediate Vector Boson.[H76]
SI Units
see International System of Units. [H76]
Universal Attraction
see Law of Universal Attraction. [A84]
Pulse Window
see Mean Profile. [H76]
Planar Spin Model
similar to the Heisenberg model, except that the spin of the atom is restricted to lie in a plane instead of being free to point in any direction in space. [D89]
radio astronomy
the astronomy associated with radio observations of celestial objects. The waveband extends from low radio frequencies (10 MHz, : 30 m) to centimeter and millimeter wavelengths. At the low frequency end of the range, the limit is imposed by the Earth's ionosphere and at the upper end by water vapor absorption in the atmosphere. Within this waveband, many sophisticated radio telescope systems have been constructed, either using single dishes or combining them in arrays using the principles of aperture synthesis and interferometry to obtain high angular resolution. [D89]
renormalisation group
the way in which coupling constants enter into field theory often involves certain simple scaling relations that are described by a group (in the mathematical sense), In statistical mechanics, the renormalization group method systematically implements some form of coarse-graining operation to expose the character of the large-scale phenomena, in physical systems where many scales are important. [D89]
Rabi frequency
this is the frequency at which atomic population is coherently transferred from one state to another by a resonant radiation field; it is named after its discoverer I. Rabi. It plays a central role in atom-field interactions, showing up in emitted light spectra and in time dependence. [D89]
Quantum Electronics
this is the name used for those parts of quantum optics which have practical device applications.[D89]
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]
Tides
"(a) A differential gravitational force. In the Galaxy, a tide results because the Milky Way's gravity pulls more strongly on the side of an object facing the Galactic center than on the object's other side, so the object may get torn apart. [C95]
Reynolds Number
"(a) A dimensionless number (R = Lv/, where L is a typical dimension of the system, v is a measure of the velocities that prevail, and is the kinematic viscosity) that governs the conditions for the occurrence of turbulence in fluids. [H76
Pulsar
"(a) A fast-spinning neutron star that emits radiation toward Earth every-time it rotates. [C95]
Space-Time Continuum
"(a) A four-dimensional framework in which events take place. [C97]
Protogalaxy
"(a) A galaxy in the process of formation. None are observed nearby, indicating that all or most galaxies formed long ago. [F88]
Strong Equivalence Principle
"(a) A generalization of the Einstein equivalence principle, stating that all bodies, including those with self-gravitational binding, fall with the same acceleration, and that physics in freely falling reference frames, including local gravitational physics, is independent of the velocity and location of the frame. [D89]
Parton
"(a) A generic term used to describe any particle which may be present inside nucleons. It includes quarks, antiquarks and gluons. [CD99]
Tachyon
"(a) A hypothetical subatomic particle that can travel faster than the speed of light. [DC99]
Orion Nebula
"(a) A large cloud of gas and dust giving birth to young stars in the constellation Orion and visible to the naked eye. It is an HII region 1500 light-years away. [C95]
Streamline
"(a) A line following the direction of the fluid in laminar or streamline flow. Where the speed increases, as it does in a narrower section of a pipe, the streamlines are closer together. (see also Laminar Flow) [DC99]
Perturbation Theory
"(a) A mathematical approximation in which a small disturbance added to an exactly soluble system is analysed by a series expansion in powers of the small disturbance.[D89]
Radian
"(a) A measure of angular distance; 2 radians equals 360 degrees. [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]
Opacity
"(a) A measure of the resistance of a medium to the transmission of visible light or other forms of radiation. A material that is opaque is said to have a high opacity; one that is transparent has low opacity. [LB90]
Subdwarf (sd)
"(a) A metal-poor main-sequence star. On the H-R diagram, subdwarfs lie slightly below the metal-rich Main Sequence, because they are fainter than metal-rich main-sequence stars of the same color. [C95]
Rayleigh Number
"(a) A non-dimensional parameter involving the coefficients of thermal conductivity and kinematic viscosity which determines when a fluid, under specified geometrical conditions, will become convectively unstable. [H76]
Z particle
"(a) A particle that is identical to the photon in all respects except mass. It conveys part of the weak force between hadrons and leptons, and its existence was a distinctive prediction of the Glashow-Salam-Weinberg theory of the amalgamated weak-electromagnetic force. [D89]
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]
Radioactivity
"(a) Emission of particles by unstable elements as they decay. [F88]
Quark
"(a) Fundamental particle of which protons, neutrons and electrons are now thought perhaps to be made up. There are possibly three or four types of quark. It is even possible that quarks themselves may be made up of still more fundamental particles. [A84]
Quanta
"(a) Fundamental units of energy. [F88]
Resonance Particles
"(a) Hadronic particles which exist for only a very brief time (10-23 seconds) before decaying into hadrons. (also called resonances) [CD99]
Oort Cloud
"(a) Home of most Solar System comets. [F88]
Stellar Evolution
"(a) How a star changes with time. [C95]
Quantum Genesis
"(a) Hypothesis that the origin of the Universe may be understood in terms of a quantum chance. [F88]
Retrograde
"(a) In a backwards direction; in astronomy this means in a direction corresponding to east-to-west. [A84]
Unified Theories
"(a) In particle physics, any theory exposing relationships between seemingly disparate classes of particles. More generally, a theory that gathers a wide range of fundamentally different phenomena under a single precept, as in Maxwell's discovery that light and magnetism are aspects of a single, electromagnetic force. [F88]
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]
World Line
"(a) In space-time, the history of a particle is represented by a world-line. The position of the particle in space at any particular time t is found by slicing space-time at time t and seeing where the slice cuts the world-line. [D89]
Raman Effect
"(a) In spectroscopy, the change in the wavelength of light scattered by molecules. [A84]
Red Giant
"(a) Large, highly luminous but relatively cool star that has reached a late stage in its ""life"". It is running out of nuclear ""fuel"" and has accordingly expanded greatly and become less dense. Many also become variable stars of long periodicity. Its next evolutionary stage is to become a white dwarf, in developing into which the star has to cross the main sequence on the Hertzsprung-Russell diagram. [A84]
Penumbra
"(a) Less than full shadow (umbra). [A84]
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]
UBV Photometric System
"(a) Measurement of the astronomical color index of a star, utilizing the ultraviolet, blue and yellow visual images over two pre-set wavelengths obtained by photoelectric filtering. Other standardized filter wavebands are also used. [A84]
Opposition
"(a) Occurs when the Earth comes directly between that planet and the Sun; it can thus only happen in relation to the superior planets and the asteroids. [A84]
Strong Nuclear Force
"(a) One of the four fundamental forces of nature. It governs the interaction between particles in atomic nuclei. [C97]
Weak Interaction
"(a) One of the fundamental forces of nature. Its most famous manifestation is in -decay; it is also involved in some radioactive decays of nuclei, and neutrino interactions. [D89]
Resonance
"(a) One of the natural states of oscillation of a physical system. [G99]
Voyager
"(a) Pair of unmanned American spacecraft launched in 1977 on missions to Jupiter, Saturn, and beyond. [F88]
Ultraviolet
"(a) Part of the electromagnetic spectrum immediately above visible light (but below Gamma-rays and X-rays); it therefore comprises a range of radiation of shorter wavelength and higher frequency than those of visible light. (UV) [A84]
Superpartners
"(a) Particles whose spins differ by 1/2 unit and that are paired by supersymmetry. [G99]
RR Lyr
"(a) Periodic variables with periods less than one day, and of spectral types A to early F. [JJ95]
Photoelectric Effect
"(a) Phenomenon in which electrons are ejected from a metallic surface when light is shone upon it. [G99]
X-Rays
"(a) Photons of wavelengths between about 0.1 Å and 100 Å - more energetic than ultraviolet, but less energetic than -rays. [H76]
Wave-Particle Duality
"(a) Quantum realization that particles of matter and energy also exhibit many of the characteristics of waves. [F88]
Stimulated Emission
"(a) Radiation emitted by a body, such as an atom, when it is bombarded by radiation. The stimulated radiation has the same wavelength and direction as the bombarding radiation. [LB90]
Spontaneous Emission
"(a) Radiation emitted by an isolated body. [LB90]
Technetium
"(a) Radioactive element with atomic number 43. It was seen in red giants in 1952; because it is unstable, its presence indicated that the stars themselves had made it. [C95]
Prandtl Number
"(a) Ratio of the product of the viscosity coefficient and the specific heat at constant pressure to the thermal conductivity. [H76]
Supernova Remnant
"(a) SNR The expanding shell of gas ejected at a speed of about 10,000 km s-1 by a supernova explosion, observed as an expanding diffuse gaseous nebula, often with a shell-like structure. Supernova remnants are generally powerful radio sources. [Silk90]
Titan
"(a) Seventh (known) moon out from Saturn, and its largest. It is possibly also the largest satellite in the Solar System (although Neptune's Triton may be proved to be larger). It is 20% larger than the planet Mercury and is known to have an atmosphere. [A84]
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]
Pauli Exclusion Principle
"(a) States that particles with half integer spins cannot occupy the same quantum states. This manifests itself as the reason why solid objects cannot exist in the same physical space. [c97]
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]
Strong Force
"(a) Strongest of the four fundamental forces, responsible for keeping quarks locked inside protons and neutrons and for keeping protons and neutrons crammed inside of atomic nuclei. [G99]
Spectroscopy
"(a) Study of spectra; in astronomy, the investigation of the composition of celestial bodies using information derived from spectral lines. [A84]
SNR
"(a) Supernova Remnant [LLM96]
Wave Number
"(a) Symbol: The reciprocal of the wavelength of a wave. It is the number of wave cycles in unit distance, and is often used in spectroscopy. The unit is the meter-1 (m-1). The circular wave number (symbol: k) is given by:
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]
Polaris
"(a) The North Star, a second-magnitude star in the constellation Ursa Minor. The star is a yellow-white F-type supergiant that lies 330 light-years away.
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]
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]
Ring Galaxy
A galaxy with a ring-like appearance. The ring contains luminous blue stars, but relatively little luminous matter is present in the central regions. It is believed that such a system was an ordinary galaxy that recently suffered a head-on collision with another galaxy. [Silk90]
Optical Window
A gap in Earth's atmospheric absorption spectrum through which visible light can pass down to the surface. The optical window includes the spectral region between the O3 cut-off at 2950 Å and the A band of O2 at 7600 Å. [H76]
Ptolemaic Model of the Universe
A geocentric model in which the Earth remained stationary as the other planets the Sun, the Moon and the stars orbited it on their spheres. It was eventually replaced by the Copernican model. [A84]
Widmanstätten Pattern
A geometric pattern found in some iron meteorites, consisting of groups of parallel lamellae crossing each other at various angles. [H76]
White Giant
A giant star of spectral type A. Some RR Lyrae stars are white giants. (rarely used term) [C95]
Yellow Giant
A giant star with a spectral type of G. The nearest and brightest yellow giants are the two composing the double star Capella. [C95]
Supernova
A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which (as in the Crab Nebula) may be a neutron star. Supernovae are of two main types: Type I (Mv = - 14 to - 17) have a nonhydrogen spectrum, lower mass, and high velocity (about 10.000 km s-1), and may be produced by the thermonuclear detonation of a highly degenerate core. Type I supernovae are found in both spiral and elliptical galaxies. Type II (Mv = - 12 to - 13.5) have a hydrogen spectrum, higher mass, and lower velocity (about 5.000 km s-1), and occur in young, massive stars near the edge of spiral arms. Type II supernovae are more common: Tammann (1974) finds that Type II supernovae occur in our Galaxy at the rate of 0.01 to 0.05 per year. (Type III supernovae are similar to Type II but are probably of much higher mass.) Novae release about 1044 ergs of energy; supernovae, about 1049 to 1051 ergs. [H76]
Thought Experiment
An experiment that cannot be or is not carried out in practice, but can, given sufficient imagination and rigor, be reasoned through by thought and intuition alone. [F88]
Pup
An extremely bright O4f star (the brightest Of star known) embedded in the Gum Nebula. It has an envelope which is rapidly accelerating outward. [H76]
Supergiant
An extremely luminous star of large diameter and low density. No supergiants are near enough to establish a trigonometric parallax. [H76]
Scorpius OB1
An extremely young association of OB stars in Scorpius about 2 kpc distant. [H76]
Parametric Representation
An indirect means of expressing the solution to a differential equation in terms of an arbitrary parameter. As the parameter is allowed to vary, the parametric expression takes on the various values that the actual solution would have. [Silk90]
NML Tau
An infrared source (an M-type Mira variable with a period of 465 days) discovered by Neugebauer, Martz, and Leighton in 1965. ( IK Tau) [H76]
Quadrant
An instrument, based on a quarter of a circle, employed to measure the altitude above the horizon of astronomical bodies. Eventually replaced by the sextant. [F88]
Order
An integer (m) associated with a given interference fringe or diffraction pattern. In interference a bright fringe occurs for a path difference m; a dark fringe is produced if the path difference is (m + 1/2). A bright fringe is first order if it arises through a path difference of one wavelength (m = 1). Similarly, second order corresponds to m = 2, etc. [DC99]
Siderite
An iron (or iron and nickel) meteorite. Siderites comprise about 6 percent of known falls. (lit.: "star stone") [H76]
Virgo Cluster
An irregular cluster of about 2500 galaxies (z = 0.004), including the giant elliptical M87 (the galaxy of greatest known mass). [H76]
Zone of Avoidance
An irregular zone near the plane of the Milky Way where the absorption due to interstellar dust is so great that no external galaxies can be seen through it. [H76]
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]
Tully-Fisher Relation
An observed relation between the intrinsic luminosity of a spiral galaxy and the rotational speed of its stars. More luminous galaxies have stars that are moving faster. [LB90]
Polarization Modulator
An optical device sensitive to the plane of vibration of electromagnetic waves, i.e. to their polarization. It is used to convert the polarization of light into a measurable brightness change. [McL97]
Theodolite
An optical instrument (using prisms and lenses) for measuring horizontal and vertical angles with great accuracy. It is an essential tool in surveying. [DC99]
p-Electron
An orbital electron whose l quantum number is 1. [H76]
s-Electron
An orbital electron whose l quantum number is zero. [H76]
X-ogen
An unidentified molecular transition at 3.36 mm (89.19 GHz) discovered in 1970. [H76]
Vacuum Fluctuation
An unpredictable event occurring in the vacuum as a result of the inherently probabilistic nature of quantum theory. Particles can materialize in conjunction with their antiparticles, and fields can undergo fluctuations in their values. (see Vacuum) [G97]
Source
Anything which is emitting electromagnetic radiation. [C97]
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]
Vertical
Apparent direction of gravity at the point of observation (normal to the plane of a free level surface.) [S92]
Transient X-Ray Sources
As of early 1974, four had been detected: Cen X-2, Cen X-1, 2U 1543-47, and Cep X-4. They resemble slow novae. [H76]
Optical Fibres
Glass and transparent plastics can be made into very thin wires or fibers. Typical dimensions are 10-50 µm. If a ray enters one end of a fiber at the appropriate angle, it will undergo total internal reflection and travel down the fiber without much loss through the sides. [McL97]
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]
Ultra-Relativistic
Having velocities very nearly equal to the velocity of light (E >> mc2). [H76]
P Cyg Stars
High-luminosity, early-type stars, in which all lines have a P Cyg type profile (an emission component on the red side of the absorption line). [JJ95]
Transition Radiation
Radiation emitted (in the X-ray region) when energetic charged particles pass through an interface between two media of different dielectric properties. [H76]
RAM
Random access memory. A silicon micro-chip capable of temporary storage of information in the form of binary digits, either 0 or 1, and enabling rapid access to any part of its storage area. [McL97]
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]
Reproducing Universes
The process in some inflationary universe models whereby the universe is constantly spawning new universes, causally disconnected from each other and from the parent universe. (See chaotic inflation.) [LB90]
Phonon
The quantum associated with lattice vibrations in a solid. Phonons are sound quanta. [H76]
QE Pinned
The quantum efficiency of certain CCDs can be driven to their maximum by UV flooding and pinned there by immediate cooling. [McL97]
Thermal Background
The radiation emitted by the telescope and the atmosphere at infrared wavelengths due to the heat (temperature) of the source. [McL97]
Spectral Ratio
The ratio of electromagnetic wavelengths from different cosmic epochs. This gives the expansion factor of the Universe. [C97]
Signal-To-Noise Ratio
The ratio of the amount of intelligible meaning in a signal to the amount of background noise. [H76]
Velocity Space
The subspace of phase space whose coordinates are the velocities in each of the three directions of ordinary space. [H76]
Photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon for a range of applications ranging from detection to laser energy production to communications and information processing. [McL97]
Thermal Equilibrium, Law of
The temperature of a body in equilibrium is the same at all points (also called zeroth law of thermodynamics). [H76]
Radiation Temperature
The temperature that a blackbody of similar dimensions would have that radiated the same intensity at the same frequency. [H76]
Particle
The term particle is used somewhat loosely and includes not only the elementary quarks and leptons and bosons, but also the composite hadrons. It also includes any (currently hypothetical) new particles that might be discovered, such as the supersymmetric partners of the quarks and leptons and bosons. [K2000]
Rayleigh Scattering
Selective scattering (i.e., preferential scattering of shorter wavelengths) of light by very small particles suspended in the Earth's atmosphere, or by molecules of the air itself. The scattering is inversely proportional to the fourth power of the wavelength (see Mie scattering). [H76]
Ranger Spaceprobes
Series of 9 US spaceprobes only the final 3 of which were successful. All were meant to photograph the surface of the Moon before crashing onto it. Some good results were obtained. [A84]
Pioneer Spaceprobes
Series of US spaceprobes the first 9 of which concentrated predominantly on Solar exploration and research. From then on, Pioneer probes have been sent to the outer planets of the Solar System. [A84]
Orbital Elements
Seven quantities needed to establish the orbit of a celestial body (see elements of an orbit). [H76]
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]
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]
Palomar
The mountain in California upon which sits the largest telescope in the United States, 200 inches (5 meters) in diameter. The telescope itself is sometimes referred to as the Mt. Palomar Telescope. [LB90]
Vulcan
The name of a hypothetical planet at one time thought to exist between the Sun and Mercury. [H76]
Pericenter
The point in the orbit of one component of a binary system which is closest to the center of mass of the system. [H76]
Turn-Off Point
The point on the H-R diagram at which stars turn off from the Main Sequence. The brighter the turn-off point, the younger the cluster age.
Zero-Age Main Sequence
The position on the H-R diagram for stars which have attained hydrostatic equilibrium and have started hydrogen burning in their cores, but which have not yet had time to produce an observable change in their chemical composition (ZAMS). [H76]
Phobos
The potato-shaped inner satellite of Mars (about 18 × 22 km), discovered by A. Hall in 1877. Orbital and rotation period 7h39m14s, e = 0.021, i = 1°.1. Visual geometric albedo 0.06. Infrared observations suggest that its surface is covered with dust. Phobos lies just outside the Martian Roche limit. [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]
Vibrational Pattern
The precise number of peaks and troughs as well as their amplitude as a string oscillates. [G99]
Zero-Point Pressure
The pressure contributed by degenerate electrons, which do not come to rest even at absolute zero. [H76]
Vapor Pressure
The pressure exerted at a particular temperature by a vapor. When a liquid or solid evaporates, molecules are continuously escaping from the surface at a rate that increases rapidly with temperature; they exert a vapor pressure. Those striking the surface tend to re-enter the liquid or solid, so that eventually a state of dynamic equilibrium is reached in which the number of molecules returning to the surface per second is the same as the number leaving it. The vapor now exerts its equilibrium pressure for that temperature, the saturated vapor pressure (SVP). The value of this depends only on the temperature, and is independent of the volume (unless all the vapor condenses or all the liquid evaporates). [DC99]
Unitarity
The principle of conservation of probability. An example would be that if a particle can decay by several modes, the sum of the fractions taking each particular mode should add up to one. [H76]
Transition Probability
The probability that a system in one energy state will undergo a transition into another. Associated with any given pair of energy levels are three transition probabilities: the spontaneous-emission probability, the absorption probability, and the stimulated-emission probability. [H76]
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]
Reddening
The process by which light from an astronomical object grows red as the light travels through interstellar dust. Dust scatters blue light more than red, thus leaving predominantly red light transmitted. [LB90]
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]
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]
Orbital Velocity
Velocity required by a body to achieve a circular orbit around its primary: Vorb = (GM / r)1/2. [H76]
VLA
Very Large Array A network of 27 radio telescopes in New Mexico, USA. [McL97]
VLTI
Very Large Telescope Interferometer [LLM96]
VLBI
Very Long Baseline Interferometry [LLM96]
Ultraviolet Stars
Very hot prewhite-dwarf stars; usually the hot central stars of planetary nebulae which are contracting toward the white-dwarf state. [H76]
PG 1159 Stars
Very hot stars with strong O VI and C IV lines, which are X-ray emitters. Probably these stars are the central stars of planetary nebulae that have dissipated their envelopes. Also called pre-degenerates. [JJ95]
WLRG
Weak-Lined Radio Galaxy
Pulse Counter
When an atom is ionized by collision with a charged particle, the electrons it loses can be collected by applying a voltage. The process of collection gives an electrical pulse that is proportional to the number of free electrons, which in turn is proportional to the energy of the colliding particle. [H76]
Quadrupole
When referred to a system containing charges, a quadrupole is equivalent to the presence of two equal dipoles parallel to each other, but with their corresponding charges reversed; or more generally, that component of the charge distribution which has axial or triaxial symmetry. Similarly, when referred to mass distributions, it arises from unequal components of the moment-of-inertia tensor along three principal directions. [H76]
Planet
"(1) Planet A celestial body that (a) is in orbit around the Sun, (b) having sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (near round) shape, and (c) has cleared the neighborhood around its orbit. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Sunspot Number
"(a) (Also called the Wolf Number or Relative Number.) A quantity (devised by R. Wolf of Zurich in 1852) which gives the number of sunspots, and the number of groups of sunspots, at a given time. R = k (10g + f) where k is a constant depending on observing conditions, individual spots visible on the Sun at a given time. [H76]
Proton-Proton Chain
"(a) (p-p chain) A series of thermonuclear reactions in which hydrogen nuclei are transformed into helium nuclei. The temperature and density required are about 107 K and 100 g cm-3. It is the main source of energy in the Sun, where 1038 of these reactions occur every second. All parts of this reaction have been observed in the laboratory, except for the first step 1H(p, +v)2D, which occurs only a few times in 1012 collisions of protons. But the first two reactions provide about one-third of the Sun's total energy release. The p-p chain divides into three main branches: PP I: 1H(p, +v)2D (p, )3He(3He, 2p)4He + 4 × 10-5 ergs of energy. PP II: 1H(p, +v)2D(p, )3He (4He, ) 7Be(+v)7Li(p,)4He. PP III: 1H(p, +v)2D(p,)3He(4He,)7Be(p,)8B (+v)8Be -> 2 4He. (PP III occurs once in 1000 times:) Although the neutrinos from the PP II and PP III chains are detectable, they have not been observed. [H76]
Stefan-Boltzmann Constant
"(a) : The constant of proportionality relating the luminosity of a star to its absolute temperature: = 5.67 × 10-5 ergs cm-2 (deg-K)-4 s-1. [H76]
Zodiac
"(a) A band about 8° wide on the celestial sphere, centered on the ecliptic. [H76]
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]
Strange Attractor
"(a) A path in phase space that is not closed. Strange attractors are characteristic of chaotic behavior. (see Attractor; Phase Space) [DC99]
Superconductivity
"(a) A phenomenon occurring in some metals at very low temperatures, in which the resistance drops to zero and the metal shows many other anomalous properties. [D89]
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]
Symmetry
"(a) A property of a physical system that does not change when the system is transformed in some manner. For instance, a sphere is rotationally symmetrical since its appearance does not change if it is rotated. [G99]
Strangeness
"(a) A property of hadrons which may have a zero or non-zero value, depending on their rate of decay. [DC99]
Virtual Particle
"(a) A quantum particle that exists only temporarily, for example while being exchanged between other particles. Because of Heisenberg's uncertainty relation a virtual particle need not satisfy the usual relationship between energy, momentum and mass. [D89]
Spin
"(a) A quantum property of all particles which denotes the intrinsic angular momentum of the particle. [C97]
Theory
"(a) A rationally coherent account of a wider range of phenomena than is customarily accounted for by a hypothesis. [F88]
Symmetry Breaking
"(a) A reduction in the amount of symmetry a system appears to have, usually associated with a phase transition. [G99]
Paschen Series
"(a) A series of lines in the infrared spectrum emitted by excited hydrogen atoms. The lines correspond to the atomic electrons falling into the third lowest energy level and emitting energy as radiation. The wavelength () of the radiation in the Paschen series is given by
Tritium
"(a) A short-lived isotope of hydrogen in which each nucleus contains one proton and two neutrons, instead of only one proton as in normal hydrogen. Hydrogen-3 the heaviest isotope of hydrogen. Deuterium is another isotope of hydrogen. Tritium does not exist naturally on Earth; it has a half-life of 12 years and a mass of 3.016 amu.
Perturbation
"(a) A small disturbance which makes the system deviate from its equilibrium state. It is by considering such perturbations that one determines the stability of a system: it is stable if in time the system returns to its equilibrium state: and it is unstable if some initial perturbation makes the system depart from the equilibrium state indefinitely. [H76]
Vacuum
"(a) A space containing gas below atmospheric pressure. A perfect vacuum contains no matter at all, but for practical purposes soft (low) vacuum is usually defined as down to about 10-2 pascal, and hard (high) vacuum as below this. Ultra-high vacuum is lower than 10-7 pascal. [DC99]
Phase Space
"(a) A space whose coordinates are given by the set of independent variables characterizing the state of a dynamical system. [D89]
Variable Star
"(a) A star whose luminosity changes over periods of time; there are many reasons and many types. Periods vary widely in length and even regularity. Novae and supernovae are classed as variables. The present brightest variable star is Betelgeuse (Alpha Orionis). [A84]
Thermal Equilibrium
"(a) A state in which there is no net flow of heat. If two bodies are in thermal equilibrium, then they have the same temperature. (see also Equilibrium) [DC99]
Proton
"(a) A subatomic particle with positive electric charge. Every atom has at least one proton in its nucleus; the number of protons determines the element. For example, all atoms with one proton are hydrogen, all atoms with two protons are helium, and so on. [C95]
Supergravity
"(a) A supersymmetric theory of gravity in which the graviton is accompanied by a spin-3/2 particle called the ""gravitino"". In supergravity theories, supersymmetry has been promoted to the status of a local gauge symmetry. [CD99]
Supersymmetry
"(a) A symmetry relating fermions and bosons. If supersymmetry is a true symmetry of nature, then every ""ordinary"" particle has a corresponding ""superpartner"" which differs in spin by half a unit. [CD99]
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]
String Theory
"(a) A theory in which the fundamental constituents of matter are not particles but tiny one-dimensional objects, which we can think of as strings. These strings are so minute (only 10-33 cm long) that, even at current experimental energies, they seem to behave just like particles. So, according to string theory, what we call ""elementary particles"" are actually tiny strings. each of which is vibrating in a way characteristic of the particular ""elementary particle"". [CD99]
Quantum Gravity
"(a) A theory of gravity that would properly include quantum mechanics. To date, there is no complete and self-consistent theory of quantum gravity, although successful quantum theories have been found for all the forces of nature except gravity. see Quantum Mechanics [LB90]
Zirconium
"(a) Element with atomic number 40. It arises almost entirely from the s-process, in red giant stars. [C95]
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]
Quantum Theory
"(a) A theory which seeks to explain that the action of forces is a result of the exchange of sub-atomic particles. [c97]
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]
Superstring Theory
"(a) A version of string theory which incorporates the ideas of supersymmetry. [D89]
Upsilon Particle
"(a) A very massive (9 GeV) meson built from a bottom quark and bottom antiquark. Discovered in 1977, it is a member of the most massive family of particles known at present. [D89]
Stokes Parameters
"(a) A way of characterizing the polarization state of light which is closely related to actual measurements. [McL97]
Planck Time
"(a) About 10-43 seconds. Time at which the size of the Universe was roughly the Planck length; more precisely, time it takes light to travel the Planck length. [G99]
Spectral Series
"(a) All spectral lines of a given atom arising from transitions with a common lower energy level. [H76]
Phase Transition
"(a) An abrupt change in the equilibrium state of a system.
Weinberg-Salam Model
"(a) An alternative name for the Electroweak Theory. [D89]
Planck Energy
"(a) An energy of 1.22 × 1019 GeV (billion electron volts), at which the strength of the gravitational interactions of fundamental particles becomes comparable to that of the other interactions. It is believed that the quantum effects of gravity become important at approximately this energy. [G97]
Robertson-Walker Metric
"(a) An equation which describes the spacetime continuum in a Universe which adheres to the cosmological principle. [c97]
Parallax
"(a) Angle subtended by the apparent difference in a star's position when viewed from the Earth either simultaneously from opposite sides of the planet, or half such an angle, measured after a gap of six months from opposite sides of the planet's orbit; the nearer the celestial body, the greater the parallax. [A84]
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]
Proper Motion
"(a) Apparent angular rate of motion of a star across the line of sight on the celestial sphere. [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]
van der Waals Forces
"(a) Attractive forces existing between molecules. These forces are the ones giving the pressure correction in the van der Waals' equation. They are much weaker than chemical bonds and act over short range (inversely proportional to the seventh power of distance). They are caused by attraction between dipoles of molecules. For atoms or molecules without permanent molecular dipole moments, the attractive forces result from attractions between nucleus-electron dipoles (called dispersion forces). [DC99]
White Dwarf
"(a) Compact star with mass less than about 1.4 solar masses, typical radius of 1000 km; supported against gravity by quantum-mechanical degeneracy pressure of electrons. [D89]
Sunspot
"(a) Comparatively dark spot on the Sun's photosphere, commonly one of a (not always obvious) group of two. The center of a vast electrostatic field and a magnetic field of a single polarity (up to 4,000 gauss), a sunspot represents a comparatively cool depression (at a temperature of approximately 4,500 °C). Sunspots occur in cycles of about 11 Earth-years in period although their individual duration - a matter of Earth-days only - is affected by the differential rotation of the Sun; they tend to form at high latitudes and drift towards the solar equator. They are also sources of strong ultra-shortwave radio emissions. [A84]
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]
Refraction, Refractive Index
"(a) Deflection (or ""bending"") of light - or any ray as it passes from one medium into another of greater or lesser density, representing a change in overall speed of the ray. Refracting telescopes rely on the refraction of light through lenses. The refractive index of a medium (e.g., glass) is a measure of the medium's ""bending"" power. [A84]
X-Ray Astronomy
"(a) Detection of stellar and interstellar X-ray emission. Because X-rays are almost entirely filtered out by the Earth's upper atmosphere, the use of balloon- and rocket-borne equipment is essential. [A84]
Runaway Stars
"(a) Early type stars (O and early B) outside the Galactic plane, which reached large distances (from the Galactic plane) because of their high velocities. [JJ95]
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]
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]
Out-Gassing
"(a) The absorbed gases released from the interior walls and components of a vacuum chamber which has already been ""roughed-out"". [McL97]
Phase Angle
"(a) The angle measured at the center of an illuminated body between the light source and the observer. [S92]
Positron
"(a) The antiparticle of the electron, discovered by Anderson in 1934. It has the same mass and spin as the electron, but opposite charge and magnetic moment. (also called Antielectron) [CD99]
Perfect Cosmological Principle
"(a) The assumption adopted by the steady-state theory, that all observers, everywhere in space and at all times, would view the same large-scale picture of the Universe. [H76]
Topology
"(a) The branch of mathematics that treats the `large-scale' structure of curved spaces. Topological properties of a geometrical space are those properties that are unchanged by continuous distortion of the space. An example is that in two-dimensional space a simple closed curve divides the space into two regions, one inside the curve and the other outside. [D89]
Particle Physics
"(a) The branch of science that deals with the smallest known structures of matter and energy. As their experimental investigation usually involves the application of considerable energy, particle physics overlaps with high-energy physics. [F88]
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]
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]
Procyon
"(a) The brightest star in the constellation Canis Minor and one of the nearest stars, lying just 11.4 light-years from Earth. Procyon is the eighth brightest star in the night sky. It consists of two stars: Procyon A, a bright yellow-white F-type star that has just started to evolve off the main sequence; and Procyon B, a dim white dwarf. [C95]
Recombination
"(a) The capture of an electron by a positive ion. It is the inverse process to ionization. [H76]
Primordial Nucleosynthesis
"(a) The creation of elements that occurred just minutes after the Big Bang. According to standard theory, primordial nucleosynthesis gave the Universe only five nuclei, all lightweight: hydrogen-1, hydrogen-2 (or deuterium), helium-3, helium-4, and lithium-7. [C95]
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]
Planck Length
"(a) The dimension at which space is predicted to become ""foamlike"" and at which Einstein's theory is supposed to break down. [(G / c3)1/2 = 1.6 × 10-33 cm] [H76]
Parsec
"(a) The distance at which one astronomical unit subtends an angle of one second of arc; equivalently, the distance to an object having an annual parallax of one second of arc. (abbreviation for parallax second) [S92]
Wavelength
"(a) The distance between adjacent peaks in a wave-train is the wavelength. [D89]
Partition Function
"(a) The effective statistical weight of an atom or ion under existing conditions of excitation or ionization. [H76]
Zero-Point Energy
"(a) The energy of the lowest state of a quantum system. Amount of vibrational energy allowed by quantum mechanics to be associated with atomic particles at 0 K, whereas classical mechanics requires this to be zero. Also, the energy of an electron in its ground state. [H76]
Planck Constant
"(a) The fundamental constant of nature = 1.05 × 10-34 kg m s-1, which characterizes quantum physics. Its simplest occurrence is in Heisenberg's uncertainty principle. For macroscopic systems, is usually negligibly small. [D89]
Photoionization
"(a) The ionization of atoms or molecules by electromagnetic radiation. Photons absorbed by an atom may have sufficient photon energy to free an electron from its attraction by the nucleus. The process is M + h M+ + e-. As in the photoelectric effect, the radiation must have a certain minimum threshold frequency. The energy of the photoelectrons ejected is given by W = h - I, where I is the ionization potential of the atom or molecule.[DC99]
Universal Time (UT)
"(a) The local mean time of the prime meridian. It is the same as Greenwich mean time, counted from 0 hours beginning at Greenwich mean midnight. UT0 is uncorrected; UT1 is corrected for the Chandler wobble; UT2 is corrected both for the Chandler wobble and for seasonal changes in Earth's rotation rate. [H76]
Wave Function
"(a) The mathematical object in quantum theory which determines probabilities of different results of experiments. It is a complex quantity, so it has an amplitude (whose square gives the probability) and a phase-angle. The phase-angle has no direct physical interpretation, but is important in interference effects, where two wave-functions are added together. [D89]
Renormalization
"(a) The mathematical process which ensures that the basic quantities in quantum field theory (e.g. in QED: the photon, electron and electric charge) are well-defined and not infinite. [CD99]
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]
Solar Parallax
"(a) The parallax of the Sun, now measured as 8.794"". [A84]
Period
"(a) The period of a wave is the time interval between the receipt of two successive peaks (often called crests) of the propagating disturbance. [G97]
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]
Zenith
"(a) The point in the sky directly overhead. [McL97]
Radiant
"(a) The point in the sky from which a meteor shower appears to emanate. [A84]
Parity
"(a) The principle of space-inversion invariance; i.e., no experiment can differentiate between the behavior of a system and that of its mirror image. Parity is conserved in strong interactions, but not in weak ones. [H76]
Uncertainty Principle
"(a) The principle that the fundamental uncertainty in a variable times that in its canonical conjugate is of the order of Planck's constant: xp = h. Thus the uncertainty in the measurement of the position of an electron varies inversely as the uncertainty in the measurement of its momentum. A corollary is that it is impossible to measure an atomic or nuclear process without at the same time disturbing or altering the process. [H76]
Supercooling
"(a) The process by which a substance is cooled below the temperature at which a phase transition should occur, such as water that has been cooled to below zero degrees Centigrade but that has not yet formed ice. [LB90]
Pair production
"(a) The production of an electron and its antiparticle (a positron) from a photon (according to the equation E = mc2) The process can occur in the field of an atomic nucleus. Since the mass of an electron or positron is equivalent to 0.511 MeV, the minimum energy of a photon that can promote pair production is 1.022 MeV. Any surplus energy becomes kinetic energy of the products. [DC99]
Synchrotron Radiation
"(a) The radiation emitted by charged relativistic particles spiraling in magnetic fields. The acceleration of the moving charges causes the particles to emit radiation. Radio galaxies and supernova remnants are intense sources of synchrotron radiation. Characteristics of synchrotron radiation are its high degree of polarization and nonthermal spectrum. (sometimes called magnetic bremsstrahlung) [Silk90]
Resolving Power
"(a) The ratio of the mean wavelength of two lines to the minimum resolvable angle. The resolving power of the human eye is about 1 minute of arc (its integration time is about 1/15 second). [H76]
Strehl (Intensity) Ratio
"(a) The ratio of the peak intensity in the point spread function of an optical system to that of the equivalent diffraction-limited system. [McL97]
Photosphere
"(a) The region of a star which gives rise to the continuum radiation emitted by the star. The visible surface of the Sun (temperature about 6000K), just below the chromosphere and just above the convective zone. The photosphere ends (and the chromosphere begins) at about the place where the density of negative hydrogen ions has dropped to too low a value to result in appreciable opacity. The spectrum of the photosphere consists of absorption lines (unlike that of the chromosphere, which consists of emission lines). [H76]
Quantum Field Theory
"(a) The relativistically invariant version of quantum mechanics. [H76]
Strong Interaction
"(a) The short-range nuclear force which is assumed to be responsible for binding the nucleus together. Strong interactions are so called because they occur in the extremely short time of about 10-23 seconds. Strong interactions can occur only when the particles involved are less than 3 fermis apart. [H76]
Precession
"(a) The slow (once per twenty-six thousand years) gyration of the Earth's axis. [F88]
Zeeman effect
"(a) The splitting of atomic spectral lines into two or more components in a transverse magnetic field. [DC99]
Quantum Fluctuations
"(a) The spontaneous fluctuation of energy in a volume of space. A consequence of the Heisenberg uncertainty principle. [c97]
Vernal Equinox
"(a) The spring equinox, on or around 21 March. [A84]
Sun
"(a) The star that Earth orbits. The Sun is a yellow main-sequence star that is spectral type G2, shines with apparent magnitude -26.74, and has an absolute magnitude of +4.83. The Sun is 4.6 billion years old. It lies 27,000 light-years from the Galactic center, or about 40 percent of the way from the center to the edge of the Galactic disk. [C95]
Thermodynamic Equilibrium
"(a) The state reached ultimately by an isolated system.[D89]
Quantum Cosmology
"(a) The study of the Planck era. [c97]
Thermodynamics
"(a) The study of the behavior of heat (and, by implication, other forms of energy) in changing systems. [F88]
Relativity
"(a) The theory of how motion and gravity affect the properties of time and space. The special theory of relativity establishes, among other things, the nonabsolute nature of time. The amount of time elapsed between two events will not be the same for two observers or clocks in relative motion to each other. The general theory of relativity describes how gravity affects the geometry of space and the rate at which time passes. (See general relativity; special relativity.) [LB90]
Quantum Mechanics
"(a) The theory that explains the dual wave-like and particle-like behavior of matter and the probabilistic character of nature. According to quantum mechanics, it is impossible to have complete and certain information about the state of a physical system, just as a wave cannot be localized to a single point in space but spreads out over many points. This uncertainty is an intrinsic aspect of the system or particle, not a reflection of our inaccuracy of measurement. Consequently, physical systems must be described in terms of probabilities. For example, in a large collection of uranium atoms, it is possible to accurately predict what fraction of the atoms will radioactively disintegrate over the next hour, but it is impossible to predict which atoms will do so. As another example, an electron with a well-known speed cannot be localized to a small region of space but behaves as if it occupied many different places at the same time. Any physical system, such as an atom, may be viewed as existing as a combination of its possible states, each of which has a certain probability. Quantum theory has been extremely successful at explaining the behavior of nature at the subatomic level, although many of its results violate our common-sense intuition. see Copenhagen Interpretation of Quantum Mechanics; Many-Worlds Interpretation of Quantum Mechanics; Uncertainty Principle; Wave Function [LB90]
Ylem
"(a) The word used by Gamow and his collaborators for the primordial material of the Big Bang. In most of his work Gamow assumed that the ylem consisted entirely of neutrons. In inflationary cosmology, the role of the ylem is played by the false vacuum. [G97]
Steady State Theory
"(a) Theory that the expanding Universe was never in a state of appreciably higher density - i.e., that there was no ""big bang"" - and that matter is constantly being created out of empty space in order to maintain the cosmic matter density. [F88]
Synodic Period
"(a) Time between one opposition and the next, of any superior planet or asteroid. [A84]
Van Allen (Radiation) Belts
"(a) Two doughnut-shaped belts in the Earth's magnetosphere (inner belt some 3000 km above the surface; outer belt, 18,000-20,000 km above the surface), where many energetic charged particles from the solar wind are trapped in Earth's magnetic field. The energy of the particles is highest in the inner belt. [H76]
Rayleigh
"(a) Unit of flux. 1 rayleigh = 106 photons emitted in all directions per cm2 vertical column per second. It is used in measuring the luminous intensity of the aurora. [H76]
Radial Velocity
"(a) Velocity along the line of sight toward (-) or away from (+) the observer. [H76]
Peculiar Velocity
"(a) Velocity with respect to the Local Standard of Rest. [H76]
W particle
"(a) Very massive charged (+ or -) particle that conveys part of the weak force between leptons and hadrons. [D89]
Speed of Light
"(a) c = 299,792 km sec-1 (186,180 miles sec-1). [A84]
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]
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.
Root Mean Square (rms)
"1. The square root of the average of the squares of a group of numbers or values. For example, the RMS value of {2, 4, 3, 2.5} is
Period-Luminosity Relation
"A correlation between the periods and mean luminosities of Cepheids, discovered by Henrietta Leavitt in 1912. [H76]
Tunnel Diode
"A highly doped p-n junction diode that has a large reverse current, and, in the forward direction, a negative slope resistance over part of the voltage-current characteristic.
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.
Rutherfordium
"A radioactive metal not found naturally on earth. It is the first transactinide metal. Atoms of rutherfordium are produced by bombarding 249Cf with 12C or by bombarding 248Cm with 18O.
Polonium
"A radioactive metallic element belonging to group 16 of the periodic table. It occurs in very minute quantities in uranium ores. Over 30 radioisotopes are known, nearly all alpha-particle emitters. Polonium is a volatile metal and evaporates with time. It is also strongly radioactive; a quantity of polonium quickly reaches a temperature of a few hundred degrees C because of the alpha emission. For this reason it has been used as a lightweight heat supply in space satellites.
Plutonium
"A radioactive silvery element of the actinoid series of metals. It is a transuranic element found on Earth only in minute quantities in uranium ores but readily obtained, as 239Pu, by neutron bombardment of natural uranium. The readily fissionable 239Pu is a major nuclear fuel and nuclear explosive. Plutonium is highly toxic because of its radioactivity; in the body it accumulates in bone.
Phosphorus
"A reactive solid non-metallic element. There are three common allotropes of phosphorus and several other modifications of these, some of which have indefinite structures.
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.
Yttrium
"A silvery metallic element. It is found in almost every lanthanoid mineral, particularly monazite. Yttrium is used in various alloys, in yttrium-aluminum garnets used in the electronics industry and as gemstones, as a catalyst, and in superconductors. A mixture of yttrium and europium oxides is widely used as the red phosphor on television screens.
Tantalum
"A silvery transition element. It is strong, highly resistant to corrosion, and is easily worked. Tantalum is used in turbine blades and cutting tools and in surgical and dental work.
Titanium
"A silvery transition metal. It is used in the aerospace industry as it is strong, resistant to corrosion, and has a low density. It forms compounds with oxidation states +4, +3, and +2, the +4 state being the most stable.
Palladium
"A silvery white ductile transition metal occurring in platinum ores. It is used in electrical relays and as a catalyst in hydrogenation processes. Hydrogen will diffuse through a hot palladium barrier.
Terbium
"A soft ductile malleable silvery rare element of the lanthanoid series of metals. It occurs in association with other lanthanoids. One of its few uses is as a dopant in solid-state devices.
Strontium
"A soft low-melting reactive metal. The electronic configuration is that of krypton with two additional outer 5s electrons.
Thulium
"A soft malleable ductile silvery element of the lanthanoid series of metals. It occurs in association with other lanthanoids.
Thallium
"A soft malleable grayish metallic element belonging to group 13 of the periodic table. Thallium is highly toxic and was used previously as a rodent and insect poison. Various compounds are now used in photocells, infrared detectors, and low-melting glasses.
Ytterbium
"A soft malleable silvery element having two allotropes and belonging to the lanthanoid series of metals. It occurs in association with other lanthanoids. Ytterbium has been used to improve the mechanical properties of steel.
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.
Potassium
"A soft reactive metal. The atom has the argon electronic configuration plus an outer 4s1 electron.
Rubidium
"A soft silvery highly reactive element. Naturally occurring rubidium comprises two isotopes, one of which, 87Rb, is radioactive (half-life 5 × 1010 years). The element is used in vacuum tubes, photocells, and in making special glass.
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:
Thorium
"A toxic radioactive element of the actinoid series that is a soft ductile silvery metal. It has several long-lived radioisotopes found in a variety of minerals including monazite. Thorium is used in magnesium alloys, incandescent gas mantles, and nuclear fuel elements.
Uranium
"A toxic radioactive silvery element of the actinoid series of metals. Its three naturally occurring radioisotopes, 238U (99.283% in abundance), 235U (0.711%), and 234U (0.005%), are found in numerous minerals including the uranium oxides pitchblende, uraninite, and carnotite. The readily fissionable 235U is a major or nuclear fuel and nuclear explosive, while 238U is a source of fissionable 239Pu.
Tin
"A white lustrous metal of low melting point. Its electronic structure has outer s2p2 electrons ([Kr]4d105s25p2). The element is of low abundance in the Earth's crust (0.004%) but is widely distributed. Tin has three crystalline modifications or allotropes, -tin or `gray tin' (diamond structure), -tin or `white tin', and -tin; the latter two are metallic with close packed structures. Tin also has several isotopes. It is used in a large number of alloys including Babbit metal, bell metal, Britannia metal, bronze, gun metal, and pewter as well as several special solders.
Rydberg Formula
"An equation that gives the wave number of a line in a spectrum:
Scales
"Celsius. This mercury-in-glass scale was devised as early as 1710 and was used by Linnaes at Uppsala certainly before 1737. The zero of the scale represents the melting point of ice and the boiling point of water is taken to be 100 degrees. In continental Europe the scale has always been known as the Celsius scale in the mistaken belief that it was invented by Anders Celsius (1701-1744), whereas Celsius proposed a scale which had zero for the boiling point of water and 100 for the melting point of ice. The scale was inverted by J. P. Christen (1683-1755) in 1743. In England the scale was originally called the Centigrade scale but this name was abandoned in favour of Celsius in 1948.
Wien's Displacement Law
"For black body radiation the rate of energy radiation per unit area per unit wavelength range at constant kelvin temperature T1 can be plotted against wavelength. It is found that there is a peak at wavelength 1. For temperature T2 the peak comes at 2, such that
Planck's Blackbody Formula
A formula that determines the distribution of intensity of radiation that prevails under conditions of thermal equilibrium at a temperature T: Bv = (2hv3 / c2)[exp(hv / kT) - 1]-1 where h is Planck's constant and v is the frequency. [H76]
Pion
"Pi Meson -- (a) An unstable nuclear particle of mass intermediate between that of a proton and an electron (+ and -: 273 me; 0: 264 me). The pions are believed to be the particles exchanged by nucleons, resulting in the strong nuclear force; they play a role in the strong interactions analogous to that of the photons in electromagnetic interactions. A charged pion usually decays into a muon and a neutrino; a neutral pion, into two -rays. Pions have spins of 0. (also called -meson) [H76]
Quantum Chromodynamics
"QCD
Quasar
"QSO
Quasi-Stellar Radio Source
"QSRS
Polarization
"Restriction of the vibrations in a transverse wave. Normally in a transverse wave the vibrations can have any direction in the plane perpendicular to the direction of propagation. If the directions of the vibrations are restricted in any way the radiation is said to be polarized. The simplest case is that of plane polarization. In a plane-polarized transverse acoustic wave in a solid, all the vibrations are parallel to each other. In plane-polarized electromagnetic radiation, all the electric oscillations are parallel to each other and at right angles to the magnetic oscillations.
Surface Tension
"Symbol: or The attraction between molecules (cohesion) in the plane of the surface of a liquid, which thus acts a bit like an elastic skin containing the liquid. Surface tension explains why water can drip slowly from a tap and why mercury gathers into globules on a flat surface. Molecules that are surrounded by others are, on average, repelled equally in all directions, since the liquid is under pressure. At the surface, however, the intermolecular spacings are larger in the plane of the surface, and the molecules attract each other. Consequently, the surface layer is in tension.
Planck Distribution
"The distribution with respect to wavelength of the frequency of the intensity of blackbody radiation can be expressed as
Planck's Radiation Law
"The energy radiated per unit area per unit time per unit wavelength range at wavelength from a black body at kelvin temperature T is given by
Redshift
"The shift of spectral lines toward longer wavelengths in the spectrum of a receding source of radiation. [Silk90]
Orion Molecular Cloud 2
(OMC-2) -- An infrared and molecular emission complex about 12' northeast of the Trapezium, centered on a cluster of infrared sources. [H76]
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]
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]
Work Function
(W): The amount of energy needed to release an electron from the attraction of positive ions in a metal. It is different for different metals. [H76]
WKB method
(Wentzel-Kramers-Brillouin) A method for obtaining an approximate solution to Schrödinger's equation. [H76]
Symbiotic Stars
(a) Objects exhibiting a spectrum corresponding to a low-temperature star (generally a giant) plus emission lines corresponding to a hot plasma.[JJ95] (b) A term originally used by P. Merrill to describe stars of two essentially dissimilar kinds which seem to occur together and which seem to "need" each other. In practice, it has come to signify a peculiar group of objects (usually spectral type Me) that display a combination of low-temperature absorption spectra and high-temperature emission lines. These objects undergo semiperiodic nova-like outbursts and display the spectral changes of a slow nova superposed on the features of a late-type star. Their spectra are midway between those of planetary nebulae and true stellar objects. A symbiotic star is now usually taken to be a small, hot, blue star surrounded by an extensive variable envelope. As of 1973 about 30 were known. [C97]
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]
Orthorhombic Crystal
A crystal in which the atoms are arranged in a rectangular solid, for which each of the three principal lengths are different. Such a crystal provides a simple example of spontaneous symmetry breaking, since the rotational invariance of the underlying physical laws is broken by the randomly chosen orientation of the crystal. Inside such a crystal there are three distinct speeds of light, depending on which of the three axes the light is following. [G97]
Theta Pinch
A fusion device in which the magnetic field runs parallel to the plasma column. It is a long cylindrical tube enclosed in a one-turn magnet coil. [H76]
Objective Grating
A coarse grating placed in front of the telescope objective. [H76]
Rosseland Mean Absorption Coefficient
A coefficient of opacity which is a weighted inverse mean of the opacity over all frequencies. It is applied when the optical depth is very large. [H76]
Penrose's Theorem
A collapsing object whose radius is less than its gravitational radius must collapse into a singularity. [H76]
Oxygen-Rich Giants
A collective designation for giants showing metal oxide molecules - thus M, MS and S stars. [JJ95]
Vlaslov Equation
A collisionless Boltzmann equation which describes stars moving in regular orbits in an averaged self-contained gravitational field. [H76]
Taylor Column
A column that occurs over a fixed region in a rotating fluid because of the two-dimensional character of the motion in the absence of viscosity. This phenomenon is used to interpret the Red Spot of Jupiter. [H76]
Precursor Pulse
A component of a Pulsar pulse which occurs slightly before the main pulse. At energies of about 600 keV the precursor pulse becomes stronger than the main pulse. [H76]
Veneziano Theory
A formula that accounted for the experimental results of the dual resonance model. Nambu discovered that a string theory would reproduce the results of the Veneziano approach. [P88]
3-Kpc Arm
A component of the Sagittarius Arm with noncircular gas motions. It is seen in absorption against Sgr A with a velocity of -53 km s-1, implying that at least part of the arm is expanding away from the galactic center. The nearest "edge" is presently at a radius of 4 kpc from the Galactic center. [H76]
Population Inversion
A condition that exists when there are more molecules in an excited state than an equilibrium distribution would allow. It is necessary for masers. [H76]
Wavefront
A continuous surface associated with a wave radiation, in which all the vibrations concerned are in phase. A parallel beam has plane wavefronts; the output of a point source has spherical wavefronts. [DC99]
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]
Plano-Convex Lens
A converging lens with one plane face and one convex face. [DC99]
Viscosity
The internal friction of a fluid or liquid that tends to resist and dissipate its flow. [Silk90]
Redshift-Magnitude Test
A cosmological test involving the plotting of redshifts and apparent magnitudes of distant galaxies. Deviations from the relation expected in Euclidean space can help determine whether the universe is open or closed. The redshift-magnitude test is very sensitive to evolutionary effects (whether galaxies were brighter or dimmer in the past). [Silk90]
Relativistic Quantum Field Theory
Quantum-mechanical theory of fields, such as the electromagnetic field, that incorporates special relativity. [G99]
Radio Galaxy
A galaxy that is extremely luminous at radio wavelengths. A radio galaxy is usually a giant elliptical - the largest galaxy in a cluster - and is a strong emitter of synchrotron radiation. M87 and M82 are examples. [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]
Theory of Everything
A "Theory of Everything" would not only describe how thing s work but also explain why things are the way they are. The name is unfortunate in one way, because it does not tell how to deduce the behavior of complex systems from a knowledge of their components. In this book I have used the name primary theory instead. [K2000]
String Coupling Constant
A (positive) number that governs how likely it is for a given string to split apart into two strings or for two strings to join together into one-the basic processes in string theory. Each string theory has its own string coupling constant, the value of which should be determined by an equation; currently such equations are not understood well enough to yield any useful information. Coupling constants less than 1 imply that perturbative methods are valid. [G99]
Pollux ( Gem)
A K0 III star 11 pc distant. [H76]
RV Tauri Stars
A class of about 100 semiregular variable yellow supergiants of late spectral type (G-K), similar to W Virginis stars but with longer periods. Their spectra often contain emission lines, and their light curves have alternating deep and shallow minima. They have a large infrared flux. RVa stars maintain an approximately constant mean brightness; RVb stars have long-term (on the order of 1000 days) periodicity. [H76]
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]
Reflection Nebula
A cloud of interstellar gas and dust whose spectrum contains absorption lines characteristic of the spectrum of nearby illuminating stars. The emission component of its spectrum is due to gas; the reflection component, to dust (see also diffuse nebula). [H76]
Supercluster
A cluster of clusters of galaxies. Superclusters are typically about one hundred million (108) light-years in diameter and contain tens of thousands of galaxies. [F88]
Telescope
A device for gathering and amplifying light or other energy. Refracting telescopes gather light by means of a lens, reflecting telescopes by means of a mirror. Radiotelescopes gather radio energy typically by using a metallic dish antenna. Telescopes have also been built that can gather X rays, gamma rays, and other forms of energy. [F88]
Radiometer
A device that detects radio waves from space and measures their direction. [LB90]
Wheatstone Bridge
A device that measures the resistance of an electrical circuit to the flow of electricity. [LB90]
Proportional Counter
A device used in X-ray astronomy which counts the number of ions produced when photons come into a volume of gas and ionize the gas. The more energetic the photon, the more ions are produced. [H76]
Parametric Amplifier
A device used in radio astronomy for increasing the strength of a radio signal. (paramp) [H76]
Scintillation Counter
A device used with a photomultiplier tube to detect or count charged particles or gamma rays. [H76]
Particle Accelerator
A device using electric and magnetic fields to accelerate beams of particles-usually electrons, positrons, protons, or antiprotons-to high energies for experimental purposes. Modern accelerators are often very large: the main ring at Fermilab, for example, is 4 miles in circumference. [G97]
Timing Waveform
A diagram showing the time sequence and voltage levels of a stream of pulses required, for instance, to perform charge-coupling in a CCD. [McL97]
Perseus Cluster
A diffuse, irregular cluster of about 500 galaxies (z = 0.0183) (richness class 2) dominated by and centered on the Seyfert galaxy NGC 1275 (Perseus A). Mass required to bind the cluster, greater than 1015 M; mass of cluster, about 2 × 1015 M. [H76]
Time
A dimension distinguishing past, present, and future. In relativity, time is portrayed as a geometrical dimension, analogous to the dimensions of space. [F88]
Sublimation
A direct change of state from solid to vapor without melting. [DC99]
Quantum
A discrete quantity of energy hv associated with a wave of frequency v. It is the smallest amount of energy that can be absorbed or radiated by matter at that frequency. [H76]
Quantum Field
A distribution of energy that is constantly creating and destroying particles, according to the probabilities of quantum mechanics, and transmitting the forces of nature. see Field Theory; Quantum Mechanics [LB90]
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]
Plano-Concave Lens
A diverging lens with one plane face and once concave face. [DC99]
h and Persei
A double stellar association about 2 kpc distant, visible to the naked eye as a patch of light. It contains many young 0 and B stars and also many M supergiants. (also called Perseus OB1) [H76]
Saturn Nebula
A double-ring planetary about 700 pc distant (NGC 7009). [H76]
SX Phoenicis
A dwarf Cepheid (spectral type A) with the shortest known period (1h19m). [H76]
Wolf-Lundmark-Melotte System
A dwarf E5 elliptical galaxy, sometimes considered a member of the Local Group. (also known as WLM) [H76]
Sextans Dwarf
A dwarf companion to the Milky Way. Discovered by computer in 1990, Sextans lies about 300 kpc from the Galactic center. [C95]
Zodiacal Light
A faint glow that extends away from the Sun in the ecliptic plane of the sky, visible to the naked eye in the western sky shortly after sunset or in the eastern sky shortly before sunrise. Its spectrum indicates it to be sunlight scattered by interplanetary dust. (Pioneer 10 has determined that its brightness varies inversely as the square of the distance out to 2.25 AU and then decreases more rapidly.) The zodiacal light contributes about a third of the total light in the sky on a moonless night. [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]
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]
Positronium
A fleeting combination of an electron and a positron to form an analog to a hydrogen atom. When the two particles have their spins parallel the half-life is about 1.5 × 10-7 s; when they are antiparallel the half-life is shortened to 10-10 s. A positronium `atom' decays to form two photons by annihilation. A combination of two electrons and two positrons also appears to exist, and is known as a positronium `molecule', analogous to a hydrogen molecule. [DC99]
Overstability
A form of instability which, when it sets in, sets in as oscillations of increasing amplitude. [H76]
Twisted-Pair
A form of wiring consisting of two strands of single wire twisted together to form a transmission line. One strand carries the signal and the other is grounded to the single "star" ground point of the system. [McL97]
Rydberg Formula
A formula by which the various lines in a given spectral series are obtained: -1 = R[(m + k1)-2 - (n + k2)-2], where m is an integer, n is any integer greater than m, k1 and k2 are empirical corrections which are different for different series, and R is the Rydberg constant, which has a value of 109,678 if is measured in centimeters. [H76]
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]
Reseau
A grid that is photographed by a separate exposure on the same plate with star (or galaxy) images. [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]
Wilson-Bappu Effect
A linear relation between the width of the K2 emission core in the resonance line of Ca II at 3933 Å, detectable for late-type stars, and the absolute magnitudes of the stars. [H76]
Superfluid
A liquid which undergoes the phenomenon of superfluidity, below the temperature at which this phenomenon sets in. [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]
Polytrope
A mathematical model of an inhomogeneous, compressible configuration in equilibrium under its own gravitation in which the relation between the pressure and the density satisfies the relation p = K(n+1)/n, where K is a constant and n is the polytropic index. [H76]
World Model
A mathematical model of the Universe. [H76]
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]
Quantum Mechanical Amplitude
A mathematical quantity in quantum mechanics whose absolute square determines the probability of a particular process occurring. [CD99]
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]
Penrose Process
A means of extracting energy from a rotating black hole. If a particle spirals into the ergosphere of a black hole in a direction counter to the rotation of the black hole, and if the particle then breaks up into two fragments inside the ergosphere, one of the fragments can escape with energy greater than the energy of the original particle. [H76]
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]
Temperature
A measure of the average kinetic energy of the particles of a system. [H76]
Throughput
A measure of the efficiency of an optical system. Formally, the product of the solid angle accepted and the effective collecting aperture. (also referred to as étendue.) [McL97]
Principal Quantum Number
A measure of the major axis of an electronic orbital. In the case of hydrogen, the energies of bound levels are specified completely by n. [H76]
Oscillator Strength (f-value)
A measure of the probability that a transition represented by an electronic oscillator will occur. It is independent of the physical conditions under which the atom is radiating. [H76]
Scale Length
A measure of the size of a physical system or region of space. [LB90]
Two-Component Model
A model of the solar wind which has two thermal components - electron and proton gases of differing temperatures. [H76]
Synchrotron
A modern form of particle accelerator. [D89]
Praesepe
A naked-eye open cluster in Cancer, about 160 pc distant. (Beehive Cluster) (M44, NGC 2632) [H76]
PSI
A name by which the J- or psi- (-)meson is known. [D89]
Polysilicon
A non-crystalline form of silicon with a high conductivity like a metal; preferred in CCD manufacture to the use of metals because it keeps the entire process in silicon and is more transparent to visible light. [McL97]
S-Factor
A nuclear cross-section factor measured in keV-barns. [H76]
Pulse Width
The interval of time between two successive pulses. [H76]
Zel'dovich Spectrum
A particular prescription for how much clumping of matter should occur on each length scale. Specifically, the Zel'dovich spectrum proposes that the strength of inhomogeneity, of clumping, should be the same for each length scale at the moment when that length scale is equal to the size of the horizon. (See horizon.) [LB90]
Time-Like Path
A path whose tangent obeys U · U < 0. In relativity, all material particles travel along timelike paths. [H76]
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]
Year
A period of time based on the revolution of the Earth around the Sun. The calendar year (see Gregorian calendar) is an approximation to the tropical year (see year, tropical). The anomalistic year is the mean interval between successive passages of the Earth through perihelion. The sidereal year is the mean period of revolution with respect to the background stars. (See Julian year; year, Besselian.) [S92]
Quark-Hadron Phase Transition
A phase transition in the early Universe when freely roaming quarks combined to form neutrons, protons, and other strongly interacting particles called hadrons. see Neutron; Phase Transition; Quark [LB90]
Tunneling
A phenomenon in quantum mechanics whereby a particle has a nonzero chance of crossing a potential barrier into a region which in classical mechanics would be forbidden to it. Tunneling is a direct consequence of the wave nature of material particles. [H76]
Superfluidity
A phenomenon occurring in liquid helium-4 below about 2.17 degrees, in which the liquid flows through thin capillaries without apparent friction and displays many other anomalous properties. Liquid helium-3 is also thought to be superfluid below about 3 × 10-3 degrees. [D89]
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]
Superconductor
A piece of superconducting metal below the transition temperature at which superconductivity sets in. [D89]
Pico-
A prefix meaning 10-12. [H76]
Ritz Combination Principle
A principle discovered empirically before the advent of quantum mechanics which states that every spectral line of a given atom corresponds to the difference of some pair of energy levels.[H76]
Symmetry Principle
A principle that requires a physical system to have a symmetry. For example, the notion that empty space should be devoid of any preferred directions and should thus be unchanged by rotations is a symmetry principle. A collection of compasses in empty space would be expected to point in all directions, reflecting this hypothesized rotational symmetry. [LB90]
Super Radiance
A process by which energy may be extracted from a rotating black hole. A beam of radiation approaches the black hole and "bounces off" with more energy than it had before, analogously to a marble scattering off a rapidly spinning top. The source of the gained energy is the rotational energy of the black hole or top, which slows down in the process. [LB90]
Vela Pulsar
A pulsar about 400-500 pc distant, probably associated with the Vela supernova remnant. Period 0.0892 seconds. (PSR 0833-45) [H76]
PSR B 1257+12
A pulsar in the constellation Virgo and the site of the first Solar System to be discovered outside our own. The planets were detected in 1991. [C95]
Snickers
A putative satellite galaxy of the Milky Way, reported in 1975. Its reality is in dispute. [BFM2002]
Wave Mechanics
A quantum-mechanical theory introduced by Schrödinger in 1926 which ascribes wave characteristics to the fundamental entities of atomic structure, and formulates the appropriate wave equation (Schrödinger's equation). [H76]
Supersynthesis
A radio interferometer system in which two synthesis aerials are used; one is static and utilizes the rotation of the Earth to provide a field of scan, the other is mobile. [A84]
Synthesis Aerial
A radio interferometer system utilizing a number of small aerials to achieve the effect of an impossibly large single one. [A84]
W49
A radio source (a giant H II region) about 14 kpc distant. It is the most powerful thermal radio source known in our Galaxy. [H76]
W51
A radio source, a supernova remnant. PSR 1919+14 lies within its radio contours. [H76]
W44
A radio source. It is a supernova remnant about 3 kpc distant and less than 0°.5 from the galactic plane. [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]
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]
Unified Field Theory
A single theory to account for the electromagnetic, gravitational, strong, and weak interactions by one set of equations. So far, attempts to find such a theory have been unsuccessful, although there has been some progress in unifying the weak and electromagnetic interactions. [DC99]
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]
Ophiuchi
A reddened O9.5 V star 170 pc distant (a runaway star from the Sco-Cen association) with a high rotational velocity (396 km s-1). It is well known for its strong interstellar absorption lines in the visible part of the spectrum. [H76]
Prominence
A region of cool (104 K), high-density gas embedded in the hot (106 K), low-density Solar corona. Prominences are the flamelike tongues of gas that appear above the limb of the Sun. [H76]
Perseus-Pisces Region
A region of space containing a huge congregation of galaxies called a supercluster. The galaxies in this supercluster appear to be distributed in a long chain. [LB90]
Storage Ring
A ring in which particles are kept in a circular motion, suspended in a magnetic field, until they can be injected into the larger ring of an accelerator. [F88]
Plume
A rising column of gas over a maintained source of heat. [H76]
Sonde
A rocket or balloon carrying instruments to probe conditions in the upper atmosphere. [H76]
Phasor
A rotating vector used to represent a sinusoidally varying quantity (e.g. an alternating current). The projection of the vector on a fixed axis represents the amplitude variation with time. A phase angle between two quantities (e.g. current and voltage) is represented by the angle between their phasors. [DC99]
Rossiter Effect
A rotational distortion of the velocity curves of eclipsing binaries. [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]
Uhuru
A satellite devoted entirely to the study of cosmic X-ray sources. It was launched off the coast of Kenya on 1970 December 12. [H76]
Themis
A satellite of Saturn discovered by Pickering in 1900, but since lost. [H76]
Umbriel
A satellite of Uranus about 400 km in diameter (period 4.1 days). Discovered by Lassell in 1851. [H76]
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]
Top-Down Scenario
A scenario of galaxy formation in which large structures form first and then fragment to become galaxies. [c97]
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]
Paradox
A self-contradictory proposition. Paradoxes are most useful when they seem most likely to be true, for it is then that they best serve to expose flaws in the data or reasoning that led to their appearance. [F88]
Surface Channel
A semiconductor device construction in which the electron charges are held or moved near the surface of the silicon crystal. [McL97]
Zener Diode
A semiconductor diode with high doping levels on each side of the junction. If the junction is reverse-biased, breakdown occurs at a well-defined potential, giving a sharp increase in current. The effect is called Zener breakdown; it occurs because electrons are excited directly from the valence band into the conduction band. Zener diodes are used as voltage regulators. See also diode. [DC99]
Vela Satellites
A sequence of satellites launched to monitor possible violations of the nuclear test ban treaties. The system consists of four satellites in a circular orbit around the Earth with a radius of 120,000 km. The Vela satellites have detected cosmic gamma-ray bursts (q.v.). [H76]
Osculating Elements
A set of parameters (see elements, orbital) that specifies the instantaneous position and velocity of a celestial body in its perturbed orbit. Osculating Elements describe the unperturbed (two-body) orbit that the body would follow if perturbations were to cease instantaneously. [S92]
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]
Vibrational Transition
A slight change in the energy level of a molecule due to its vibration. If the possibility of the rotation of the molecule as a whole is disregarded, then one gets from each electronic level a sequence of vibrational levels corresponding to various degrees of vibration of the nuclei around their equilibrium position. These are distinguished by the vibrational quantum number v. [H76]
Rotational Transition
A slight change in the energy level of a molecule due to the rotation of its constituent atoms about their center of mass. [H76]
Tektite
A small glassy body containing no crystals, probably of meteoritic origin and bearing no antecedent relation to the geological formation in which it is found. [H76]
Ultraviolet Ga Stars
A small group of Bp stars, which in the ultraviolet spectrum exhibit a strong 1414 line of GaII. [JJ95]
P-Strong Stars
A small subgroup of B-type stars in which P lines are very strong. [JJ95]
STARLINK
A software environment and suite of programs for astronomical data analysis developed in the UK and supported by the Rutherford-Appleton Labs. [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]
Radiosonde
A sounding balloon used to transmit information on Earth's upper atmosphere. [H76]
Radio Source
A source of extraterrestrial radio radiation. The strongest known is Cas A, followed by Cyg A and the Crab Nebula (Tau A) (the capital letters following the name of a constellation refer to the radio sources of the constellation, A being the strongest source). Radio sources are divided into two main categories: Class I, those associated with our Galaxy (which is a weak radio source), and Class II, extragalactic sources. Most radio sources are galaxies, supernova remnants, or H II regions. [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]
Whirlpool Galaxy
A spiral galaxy (M51, NGC 5194) of type Sc in Canes Venatici. [H76]
Triple Star
A star system having three stars that revolve around one another. [A84]
V Velocity
A star's velocity in the direction of Galactic rotation, as measured relative to a nearby star that has a circular orbit. If a star revolves faster than such a star, the V velocity is positive; if it revolves more slowly, the V velocity is negative; and if both revolve at the same rate, the V velocity is zero. The Sun has a V velocity of +12 kilometers per second, so it revolves 12 kilometers per second faster than it would if it had a circular orbit. Since a star on a circular orbit revolves around the Galaxy at 220 kilometers per second, a star with a V velocity of 0 is not stationary; rather, it revolves at 220 kilometers per second. The Sun therefore revolves around the Galaxy at 220 + 12 = 232 kilometers per second. [C95]
W Velocity
A star's velocity perpendicular to the Galactic plane. If a star is moving up, its W velocity is positive; if a star is moving down, its W velocity is negative; and if a star does neither, its W velocity is zero. The Sun has a W velocity of +7 kilometers per second, so it is moving up at 7 kilometers per second. In general, the greater a star's W velocity when it crosses the Galactic plane, the farther above and below the plane the star will travel. [C95]
Pressure Ionization
A state found in white dwarfs and other degenerate matter in which the atoms are packed so tightly that the electron orbits encroach on each other to the point where an electron can no longer be regarded as belonging to any particular nucleus and must be considered free. [H76]
Statistical Equilibrium
A state in which the average density of atoms per cubic centimeter in any atomic state does not change with time and in which, statistically, energy is equally divided among all degrees of freedom if classical concepts prevail. [H76]
Transit Telescope
A stationary support structure for a telescope. Motion is allowed along the meridian from the zenith to the horizon, but stars cannot be tracked east/west. Measurements are only possible when the objects "transit" the meridian due to the Earth's rotation. [McL97]
Siderolite
A stony iron meteorite. Siderolites comprise less than 2 percent of known falls. (lit.: "sky stone")[H76]
Shear
A stress applied to a body in the plane of one of its faces. [H76]
Winding Mode
A string configuration that wraps around a circular spatial dimension. [G99]
Perseus A
A strong radio source (z = 0.0183; recession velocity about 5000 km s-1). Optically it is a Seyfert galaxy (NGC 1275) with a huge amount (about 108 M) of ionized gas receding from it. It is also a strong X-ray source (3U 0316+41). (also known as 3C 84, Abell 426) [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]
Pixons
A term used to describe pixel-like elements in an image reconstruction algorithm. Pixons range in size and shape depending on the information content in that part of the image. [McL97]
Red Spot
An elliptical spot about 40,000 × 15,000 km on the southern hemisphere of Jupiter. Its color and intensity vary with time. It has been observed for at least a century, and an examination of earlier records shows that Cassini had sketched it in the seventeenth century. [H76]
Window
A term used to describe the spectral range within which the Earth's atmosphere is transparent to radiation (see Optical Window; Radio Window). Earth's atmosphere is completely opaque to X-rays; ultraviolet radiation is absorbed by electronic transitions in the ozone layer, but it is possible to get above some of it in balloons and rockets. Infrared radiation is absorbed by water vapor, so a high mountain or desert will let some radiation through (the vibrations of molecules cause absorption in the near-infrared, and the rotation of molecules causes absorption in the far-infrared and short radio). Wavelengths beyond the radio window are absorbed by the ionosphere. [H76]
Thomas-Fermi Theory
A theory of the energy of partially ionized matter in the limit of high density (see also Boltzmann-Saha Theory) [H76]
Photocathode
A thin metallic plate housed inside an evacuated tube capable of releasing electrons through the "photoelectric effect" when illuminated by light. These surfaces are best for optical and ultraviolet light. [McL97]
Space-Like Path
A trajectory along which U · U > 0. [H76]
Unitary Transformation
A transformation (U) whose reciprocal is equal to its Hermitian conjugate. [H76]
Post-Galilean Transformation
A transformation which replaces the Lorentz transformation when first-order corrections due to general relativity are included. [H76]
Omicron2 Eridani
A triple star that lies 16 light-years away and has the first white dwarf ever discovered. [C95]
2 Velorum
A triple system (WC8, B1 IV, O9 I) embedded in the Gum Nebula, probably about 400 pc distant. Period 78.5 days. It is the brightest Wolf-Rayet star in the sky (Mv = - 5.6). [H76]
Tokamak
A type of "magnetic bottle" used in experiments on controlled nuclear fusion. (the name is a Russian acronym) [H76]
Virtual Phase
A type of CCD in which only one electrode is physically outside the silicon and is such as to obscure only half of the pixel. A specially doped layer under the transparent part acts as another or virtual electrode. [McL97]
Violent Galaxy
A type of galaxy differentiated only recently. Violent galaxies include QSOs and exploding galaxies like M82. About 1 percent of the galaxies are classified as violent. Violent galaxies release on the average 1058 ergs of energy, compared with a supernova release of 1049 ergs. Nearest violent galaxy is Cen A. [H76]
Rayleigh-Taylor Instability
A type of hydrodynamic instability for static fluids (see Taylor instability) in which the density increases outward [H76]
Thermohaline Convection
A type of hydrodynamic instability. [H76]
Stellarator
A type of plasma machine. It has a twisted-field configuration in the form of a figure 8 to fold the plasma back on itself; therefore, unlike a pinch machine. it has no ends where the plasma can leak out. Stellarators and tokomaks resemble each other in that both are toroidal devices that attain equilibrium and MHD stability through rotational transform and shear; they differ mainly in the way they attain these properties. [H76]
Photomultiplier Tube
A vacuum encapsulated photocathode from which electrons are ejected by the photoelectric effect followed by multiple cathodes from which many additional electrons are emitted in a cascade. When finally collected, the original single electron may have generated a pulse of over one million electrons. [McL97]
Pulsating Nova
A variable star, probably not a true nova, in which the change between more and less luminous stages is extreme. (also called recurrent novae) [A84]
Pleiades
A very young open cluster of several hundred stars (B6 and later) in Taurus, about 125 pc distant. Six members of the cluster (all of spectral type B or Be) are visible to ordinary sight. (M45, NGC 1432) [H76]
Regulus ( Leo)
A visual triple B8 V star about 26 pc distant. [H76]
Transverse Wave
A wave motion in which the motion or change is perpendicular to the direction of energy transfer. Electromagnetic waves are examples of transverse waves. (see Polarization) [DC99]
Torsional Wave
A wave motion in which the vibrations in the medium are rotatory simple harmonic motions around the direction of energy transfer. [DC99]
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]
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]
van Maanen's Star
A white dwarf 4 pc distant; density 4 × 105 g cm-3. [H76]
Perseus OB2
A young association of OB stars about 350 pc distant. (also called Perseus 2) [H76]
Persei
A young open cluster with a high mean rotational velocity. [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]
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]
Vector Meson
Also called the intermediate vector boson. [H76]
W Virginis Star
Also known as a population II Cepheid, a W Virginis star is a bright yellow star that pulsates like a Cepheid but is older and fainter. W Virginis stars appear in globular clusters. [C95]
Yang-Mills Theories
Also known as gauge theories, these theories were invented in 1954 by Chen Ning Yang and Robert Mills. In 1973 David Gross, Frank Wilczek, and David Politzer showed that these theories possess a property called asymptotic freedom, just what was needed for a theory of how quarks bind to form protons and neutrons. The new theory, dubbed quantum chromodynamics or QCD, proposed that the color of the quarks acts as the charge of the Yang-Mills interactions. [G97]
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]
Zeolite
An absorbent material (in the form of small pellets) used in low-temperature cryostats to trap gases released gradually after active pumping has ceased. See outgassing. [McL97]
Pycnonuclear
An adjective used to describe nuclear processes (such as the proton-proton chain) that take place at relatively low temperatures and that are not strongly temperature-dependent. [H76]
Virgo Supercluster
An aggregation of galaxies - roughly ten thousand of them - to which the Virgo Cluster and our own galaxy belong. [F88]
Supergalactic Plane
An apparent plane of symmetry, passing through the Virgo cluster of galaxies, about which many of the brightest galaxies in the sky are concentrated. These galaxies form the Local Supercluster. [Silk90]
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]
Order-of-Magnitude Estimate
An approximate estimate of the magnitude of something, accurate to within a range of 10 times too big to 10 times too small. For example, given that the population of the United States is 250 million, any estimate of the population lying between 25 million and 2,500 million would be an acceptable order-of-magnitude estimate. [LB90]
Rayleigh-Jeans Law
An approximation of Planck's blackbody formula valid at long wavelengths (hv << kT). It is often used in radio astronomy; it gives the brightness temperature of a radio telescope. [H76]
Poisson Distribution
An approximation to the binomial distribution used when the probability of success in a single trial is very small and the number of trials is very large. [H76]
Rayleigh-Jeans Limit
An approximation valid at sufficiently long wavelengths (longward of the peak intensity) to the energy distribution of a blackbody. [Silk90]
Yellow Spot (Macula Lutea)
An area a few millimeters across in the human retina. It has a high concentration of rods, giving high visual acuity and color vision but low sensitivity to dim light. [DC99]
Vela X-1
An eclipsing X-ray source identified with the seventh-magnitude single-lined spectroscopic binary HD 77581 (B0.5 Ib) with a period of 8.96 days. Estimated mass of unseen companion 1.7-15 M, with a probable value of about 2.6 M. (3U 0900-40) [H76]
Poynting-Robertson Effect
An effect of radiation pressure on a small particle orbiting the Sun that causes it to spiral slowly into the Sun. The radiation falls preferentially on the leading edge of the orbiting particle and acts as a drag force. [H76]
Paschen-Back Effect
An effect on spectral lines obtained when the light source is located in a strong magnetic field, so that the magnetic splitting becomes greater than the multiplet splitting. H76]
Thirring Effect
An effect predicted by general relativity, which causes the dragging of the inertial frame outside a rotating mass. As a pulsar, for example, rotates, it drags along the inertial frames, both inside and outside. (see also Lense-Thirring Effect) [H76]
Tsytovitch Effect
An effect wherein the index of refraction of a medium is much less than unity so that the phase velocity of electromagnetic waves is greater than the speed of light in the medium. In this case, a relativistic electron can no longer keep in phase with the waves it generates, and the intensity of synchrotron radiation is very much reduced. (also called the Razin-Tsytovitch Effect.) [H76]
Pockels Cell
An electro-optic crystal used as a reversible waveplate by applying alternately high positive and negative voltage. [H76]
Space Charge Wave
An electrostatic wave brought about by oscillations of the charges. [H76]
TDB
Barycentric Dynamical Time
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]
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]
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]
Resonance Capture
Capture by an atomic nucleus of a particle whose energy is equal to one of the energy levels of the nucleus. Under such circumstances the particle's chances of being captured are greatly increased. [H76]
Radiative Capture
Capture of a free electron by an ion with the subsequent emission of an X-ray (or gamma-ray) photon (also called Radiative Recombination ). [H76]
Supergranulation Cells
Convective cells (about 15,000-30,000 km in diameter) in the solar photosphere, distributed fairly uniformly over the solar disk, that last as long as a day. New sunspots develop in the intersections of adjacent supergranulation cells. Most of the magnetic flux through the photosphere is concentrated in the supergranule boundaries. [H76]
Planetographic Coordinates
Coordinates for cartographic purposes dependent on an equipotential surface as a reference surface. Longitude (see Longitude, Celestial) of a point is measured in the direction opposite to the rotation (positive to the west for direct rotation) from the cartographic position of the prime meridian defined by a clearly observable surface feature. Latitude (see Latitude, Celestial) of a point is the angle between the planetary equator (normal to the z-axis and through the center of mass) and normal to the reference surface at the point. The height of a point is specified as the distance above a point with the same longitude and latitude on the reference surface. [S92]
O(f)
O-type stars in which NIII is present in emission and He II is weakly present in absorption or emission. [JJ95]
Planetocentric Coordinates
Coordinates for general use, where the z-axis is the mean axis of rotation; the x-axis is the intersection of the planetary equator (normal to the z-axis through the center of mass) and an arbitrary prime meridian; and the y-axis completes a right-hand coordinate system. Longitude (see Longitude, Celestial) of a point is measured positive to the prime meridian as defined by rotational elements. Latitude (see Latitude, Celestial) of a point is the angle between the planetary equator and a line to the center of mass. The radius is measured from the center of mass to the surface point. [S92]
Principle of Relativity
Core principle of special relativity declaring that all constant-velocity observers are subject to an identical set of physical laws and that, therefore, every constant-velocity observer is justified in claiming that he or she is at rest. This principle is generalized by the principle of equivalence. [G99]
Relativistic Cosmology
Cosmological applications of Einstein's theory of general relativity. The Big Bang cosmological models were first derived according to the equations of relativistic cosmology, which are needed for many applications of these models. [Silk90]
Oscillating Universe
Cosmological model in which the Universe is "closed" and its expansion is destined to stop, to be succeeded by collapse and "then" (if ordinary temporal terms may be said to apply) by a rebound into a new expansion phase. [F88]
Pixel
Derived from "picture element." The smallest individual element of an array detector. Note that the size of the detector pixel does not necessarily equate with the resolution of the system (undersampling). [McL97]
Regge Trajectory
Derived from S-matrix theory, the Regge Trajectories were theoretical plots that attempted to account for the position of elementary particle resonances. One of the triumphs of early string theory was to describe the general shape of these Regge Trajectories. [P88]
O((f))
O-type stars in which NIII is present in emission and He is strong in absorption. [JJ95]
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]
Supersonic
Describing a speed that is greater than the speed of sound in the medium concerned. See subsonic. [DC99]
Subsonic
Describing a speed that is less than the speed of sound in the medium concerned. See supersonic. [DC99]
Subcritical
Describing an arrangement of fissile material that does not permit a sustained chain reaction because too many neutrons are absorbed without causing fission or otherwise lost. [DC99]
Paraxial
Describing rays incident on a surface close and parallel to the axis. Only paraxial rays pass or appear to pass through the focal point of a spherical reflecting or refracting surface. [DC99]
UBVRI
Designations for parts of the optical waveband, isolated by means of special glass filters which eliminate the unwanted regions, and used for standard astronomical intensity measurements. [McL97]
Polarimeter
Device that measures the polarization of any form of electromagnetic radiation, particularly light. [A84]
Photomultiplier
Device used in photometry for the amplification of light by the release and acceleration of electrons from a sensitive surface. The result is a measurable electric current that is proportional to the intensity of received radiation. [A84]
Spectroheliograph
Device with which spectra of the various regions of the Sun are obtained and photographed. [A84]
Phases
Differences in the appearance of the Moon, in particular, but also of Mercury and Venus, caused by the Earth observer's seeing only a part of the body lit by the Sun. [A84]
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]
TeV
Equal to one teraelectron volt, or 1,000 GeV. [F88]
Topology-Changing Transition
Evolution of spatial fabric that involves rips or tears, thereby changing the topology of space. [G99]
X Particle
Exceedingly massive (hypothetical) particle predicted by grand unified theories to convey a very short-ranged interaction between quarks and leptons. An X particle would be able to change a quark into a lepton or an antiquark. [D89]
Radio Lobes
Extended regions of diffuse radio emission, often dumbbell shaped, that surround a radio galaxy. [Silk90]
Post-Asymptotic Branch Stars
F-type supergiants with strong sulfur lines. [JJ95]
Time Dilation
Feature emerging from special relativity in which the flow of time slows down for an observer in motion. [G99]
Sputnik 1
First artificial Earth satellite, launched by the Soviet Union on 4 October 1957. (lit.: companion) [A84]
Turbulent Flow
Fluid flow in which the speed at any point varies rapidly in magnitude and direction. Fluid flow becomes turbulent when its speed increases beyond a critical speed. This corresponds to a critical value of the Reynolds number that depends on the geometry of the system. For highly turbulent flow, the resistance to motion is proportional to the product of the density of the fluid, the speed squared, and the square of the linear dimensions. Compare with Laminar Flow. (see also Reynolds Number) [DC99]
Tethys
Fourth satellite of Saturn, discovered by Cassini in 1684. Diameter about 1000 km; P = 1.87 days. [H76]
Strong Force Symmetry
Gauge symmetry underlying the strong force, associated with invariance of a physical system under shifts in the color charges of quarks. [G99]
Receiver
General term for a radio detection system. [McL97]
Relativistic Bremsstrahlung
Hypothetical gravitational radiation emitted when two stars fly past each other with high velocity and deflect each other slightly. [H76]
Sparticles
Hypothetical particles which are predicted by some Grand Unified Theories. [C97]
Superunified Theory
Hypothetical theory that presumably would show how all four fundamental forces of nature functioned as a single force in the extremely early Universe. The best current candidates for such a potential achievement are thought to be supersymmetry and string theory. [F88]
Random Walk
If a point experiences successive displacements such that each displacement is in a random direction and of a length also governed by a frequency distribution, then the point is said to experience a random walk. It is a law of statistical behavior closely allied to Brownian motion and the diffusion of molecules. It can be proved that the root mean square displacement experienced in N mean free paths is related to the diffusion coefficient by Drms = N. [H76]
Vogt-Russell Theorem
If the pressure, the opacity, and the energy generation rate are functions of the local values of density, temperature and the chemical composition only, then the structure of a star is uniquely determined by the mass and the chemical composition. (When isothermal cores occur in the interiors of stars, then multiple-valued solutions become possible.) [H76]
Valence Electron
In an atom, an electron in an incompletely filled (usually outer) shell, available for chemical bonding to form a molecule. [DC99]
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]
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]
Radius Vector
In astronomy, an imaginary line connecting the center of an orbiting body with the centre of the body (or point) that it is orbiting. [A84]
Radius
In astronomy, an old instrument for measuring the angular distance between two celestial objects. [A84]
Secular
In astronomy, gradual, taking aeons to accomplish. [A84]
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]
Obliquity
In general the angle between the equatorial and orbital planes of a body or, equivalently, between the rotational and orbital poles. For the Earth the obliquity of the ecliptic is the angle between the planes of the equator and the ecliptic. [S92]
Sink
In general, a region where energy is given up, in contrast to a source, where energy is released. [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]
Velocity Profiles
In radio astronomy, the output response for all filters for a given position of the beam on the source. [H76]
Parasites
In radio jargon, spiral coils or gratings of wire used on dipole antennas of radio telescopes to give greater sensitivity. [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]
Oe
O-type stars with emissions in the Balmer lines. [JJ95]
Virtual Processes
Quantum-mechanical processes which do not conserve energy and momentum over microscopic timescales, in accordance with Heisenberg's uncertainty principle. These processes cannot be observed. [CD99]
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]
R Galaxy
In the Yerkes9 1974 system, a system showing rotational symmetry, without clearly marked spiral or elliptical structure (formerly called D galaxy). [H76]
System I, II & III Longitude
In the case of Jupiter, because of its differential rotation, two different rotation states are used to keep track of the cloud markings: 9h50m30s for the equator (System I) and 9h55m41s for the high latitudes (System II). Since many of the apparently localized sources of radio noise on Jupiter near a wavelength of 15 m have a shorter period than System II for optical nonequatorial features, the IAU has officially adopted a System III (9h55m29s) for radio astronomy. [H76]
Ultraviolet Catastrophe
In the late nineteenth century it was realized that the short-wavelength region of black-body radiation could not be explained by the theories of physics of the time (classical physics). The problem - sometimes called the ultraviolet catastrophe - was resolved by the concept of quantization of energy. (see Planck's Radiation Law) [DC99]
TAI
International Atomic Time [LLM96]
Traps
Irregularities in the silicon crystal lattice which can absorb free charges created in the semiconductor by, for instance, the absorption of light. [McL97]
Solar Rotation
Is differential, the equatorial rotation taking less time than the polar by up to 9.4 Earth-days. [A84]
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]
Supersymmetric Quantum Field Theory
Quantum field theory incorporating supersymmetry. [G99]
Virtual Interaction
Quantum uncertainties in energy make it possible for virtual particles to be constantly created and annihilated during elementary particle interactions. Elementary particles are able to make use of these virtual particles within their interactions. [P88]
Voids
Large regions of space without galaxies. [LB90]
Ultramicroscopic
Length scales shorter than the Planck length (and also time scales shorter than the Planck time). [G99]
Troposphere
Lowest level of Earth's atmosphere, from zero altitude to about 15 km above the surface. This is the region where most weather occurs. Its temperature decreases from about 290 K to 240 K. [H76]
Pinwheel Galaxy
M33, a spiral galaxy that lies 2.6 million light-years away and is the third largest member of the Local Group, after Andromeda and the Milky Way. [C95]
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]
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]
Photoelectric Emulsions
Materials in which the absorption of light leads to a chemical reaction. [McL97]
Riemannian Geometry
Mathematical framework for describing curved shapes of any dimension. Plays a central role in Einstein's description of spacetime in general relativity. [G99]
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]
Recession
Motion (increasing distance) away. [A84]
Prograde Motion
Motion in the same direction as the prevailing direction of motion.
Ram Pressure
Motion of a blunt body at supersonic velocity through an ambient gaseous medium causes a strong drag or ram pressure to be exerted on the body. In the case of a galaxy moving through the intergalactic gas, the ram pressure is capable of stripping the galaxy of much of its interstellar gas. [Silk90]
Vibrational Energy
Motion of the pair of nuclei in a diatomic molecule along the direction of the internuclear axis (cf. rotational energy). [H76]
Pair Annihilation
Mutual destruction (annihilation) of an electron-positron pair with the formation of gamma rays, or of a proton-antiproton pair with the formation of pions. The charges cancel, and the total mass of the pair is converted into energy (unlike nuclear fusion, in which less than 1% of the mass is converted into energy). [H76]
Photoneutrinos
Neutrino-antineutrino pairs produced by the collision of high-energy photons with electrons: + e+ -> e- + v + vbar. [H76]
Primary Quantum Number
Number used in electron structure notation, (e.g., 1s, 2s, etc.) Corresponds to the orbit in the Bohr atom where an electron would be found. Written "n".[SEF01]
Structure
Objects have structure if they have parts - that is, if they are made of other things. Whether objects have structure can be learned from experiments that probe them with projectiles. Over the past century, each stage of matter that was found as it became possible to search for ever-smaller things turned out to have structure. Quarks and leptons appear not to have structure, so perhaps the search for the basic constituents has finally ended. There are also theoretical arguments that quarks and leptons are the basic constituents. [K2000]
Satellite Lines
Of an OH source: The lines arising from transitions at 1612 and 1730 MHz. [H76]
Solar
Of the Sun. [A84]
Population II
Older stars, generally formed towards the centre of a galaxy, containing few heavier elements. The brightest of this Population are red giants. [A84]
Wolf-Rayet (WR) Star
One of a class of very luminous, very hot (as high as 50,000 K) stars whose spectra have broad emission lines (mainly He I and He II), which are presumed to originate from material ejected from the star at very high (~ 2000 km s-1 ) velocities. Some W-R spectra show emission lines due to carbon (WC stars); others show emission lines due to nitrogen (WN stars). (Hiltner and Schild classification: WN-A, narrow lines; WN-B, broad lines.) [H76]
Transition Metal
One of the metals such as iron, manganese or platinum in the centre of the periodic table. [D89]
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]
Oort's Constants
Parameters A and B that, to first order, characterize the rotation of our Galaxy in the neighborhood of the Sun. IAU Standard values (1985): A = +14.4 [1.2] km s-1 kpc-1; B = -12.0 [2.8] km s-1 pc-1. [BFM2002]
Virtual Pairs
Particles and antiparticles that exist for an extremely short time, often as the intermediate stage of a nuclear transition. According to Dirac's theory, the vacuum can be visualized as consisting of a sea of virtual electron-positron pairs that can only be released or separated when sufficient energy is made available. [Silk90]
Stable Particle
Particles that do not decay into others. (dee also Decay) [K2000]
W and Z Particles
Particles that transmit the unified electromagnetic and weak nuclear forces. These particles were predicted by the Weinberg-Salam theory of the 1960s and later discovered in the 1980s. (see Electroweak Theory; Weinberg-Salam Theory.) [LB90]
Relativistic Particles
Particles whose velocities approach the speed of light. [Silk90]
ppb
Parts per Billion [LLM96]
Synodic Time
Pertaining to successive conjunctions; successive returns of a planet to the same aspect as determined by Earth. [S92]
Quantum Physics
Physics based upon the quantum principle, that energy is emitted not as a continuum but in discrete units. [F88]
Superior Planets
Planets farther from the Sun than the Earth is (i.e., Mars to Pluto). [A84]
PAH
Polycyclic Aromatic Hydrocarbon
Young Disk Cepheids
Population I Cepheids. [H76]
W Virginis Stars
Population II Cepheids. [H76]
Predict
Predict is used in the normal sense that a theory may predict some unanticipated or as-yet-unmeasured result. It is also used in another sense: A theory can be said to predict a result that is already known, because once the theory is written, it gives a unique statement about that result. Sometimes an in-between situation holds, in that the theory predicts a result uniquely in principle, but the prediction depends on our knowing some other quantity or requires very difficult calculations. [K2000]
Sum-Over-Histories
Probabilistic interpretation of a system's past, in which quantum indeterminacy is taken into account and the history is reconstructed in terms of each possible path and its relative likelihood.
Waves
Propagation of energy by means of coherent vibration. [F88]
Ultraviolet Excess
Property of a star that emits more ultraviolet radiation than one would have expected, based on its visual color. In general, the greater the ultraviolet excess, the lower the star's metallicity, because metals in a star's atmosphere absorb ultraviolet radiation. [C95]
Quantum Determinism
Property of quantum mechanics that knowledge of the quantum state of a system at one moment completely determines its quantum state at future and past moments. Knowledge of the quantum state, however, determines only the probability that one or another future will actually ensue. [G99]
QED
Quantum Electrodynamics
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]
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]
Primordial Fireball Radiation
Same as cosmic background radiation. [LB90]
Venus
Second planet from the Sun. Mass 4.872 × 1027 g; radius of solid surface 6056 km; radius of cloud surface 6100 km. Mean density 5.16 g cm-3. Vesc 10.3 km s-1; surface gravity 8 m s2. Surface temperature (from Venera 8) 743 ± 8 K: temperature of cloud tops about 250 K. Mean distance from Sun 0.7233 AU; orbital period 224.7 days (synodic period 583.9 days); e = 0.0068, i = 3°.39. Rotation period 243.09 ± 0.5 days retrograde (Mariner 10 has established that the cloud tops rotate every 4 hours retrograde). Obliquity 3° R. Orbital velocity 35 km s-1. Radar experiments have established that the surface is somewhat smoother than the Moon, but there are mountains and there is extensive cratering. Atmospheric pressure 92-95 atm. Atmosphere (by volume 1972) 90-95% CO2, remainder primarily N2, traces of water vapor, oxygen, HF, HC1. Maximum elongation 48°. Last transit of Sun was in 1882; next one will be 2004. Venus's rotation period is in synchronism with Earth - that is, at inferior conjunction the same side is always toward the Earth. Albedo 0.76. [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]
P-L Relation
See Period-Luminosity relation. [H76]
Rydberg Correction
See Quantum Defect. [H76]
P-Spot
See Sunspot. [H76]
Russell-Vogt Theorem
See Vogt-Russell theorem. [H76]
Relative Number
See Wolf number. [H76]
Z-number
See atomic number. [H76]
Perseus OB1
See h and Persei. [H76]
Overtone
See harmonic overtone. [H76]
Radiative Recombination
See radiative capture. [H76]
Recombination Radiation
See radiative capture. [H76]
Radio Recombination Lines
See spectral lines. Radio recombination lines are the result of electron transitions between high-n (n > 50) levels in an atom or ion. [H76]
Uranus
Seventh planet from the Sun, discovered by Herschel 1781 March 13. Mass 8.78 × 1028 g; radius 25,400 km; oblateness 0.07. Mean density 1.21 g cm-3. Rotation period 10h49m26s retrograde. Mean distance from Sun 19.18 AU. Orbital-period 84.0 years; orbital velocity 6.8 km s-1; e = 0.04; i = 0°.8; obliquity 97°.9. Escape velocity 22 km s-1; surface gravity 0.96 Earth's. Synodic period 369.66 days. Albedo 0.66. Maximum brightness +5.7 mag. Surface temperature about 110 K. Atmosphere H2 and CH4. Five satellites, all of which orbit in its equatorial plane. [H76]
Principia Mathematica
Short form of the title of Isaac Newton's great work, published in 1687; title also of the mathematically philosophical work of Bertrand Russell and Alfred North Whitehead, published in 1910-13 [A84]
SIT
Silicon Intensified Target [LLM96]
Strong-Weak Duality
Situation in which a strongly coupled theory is dual-physically identical-to a different, weakly coupled theory. [G99]
Rhea
Sixth satellite of Saturn, discovered by Cassini in 1672. Diameter about 1500 km; rotation period 4d12h25m. Albedo 0.57. [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]
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]
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]
Telluric Lines
Spectral lines or bands that originate from absorption by gases such as O2, H2O, or CO2 in the Earth's atmosphere. [H76]
OB
Spectral type O or B - that is, hot and blue. [C95]
O
Spectral type for the hottest blue stars, even hotter than B-type stars. O-type stars are rare and short-lived. [C95]
Para-Spectrum
Spectrum of singlet (l = 0). [H76]
Ortho-Spectrum
Spectrum of triplet (l = l). [H76]
SFR
Star Formation Rate
Peculiar A Star
Stars can be classified according to their surface temperatures, which determine, in large part, the spectrum of radiation they emit. A Stars have surface temperatures between about 7,500 and 11,000 degrees centigrade. Peculiar A Stars are A stars whose emitted radiation spectra have many of the characteristics of A stars but are peculiar in certain ways. [LB90]
Sub-Luminous Stars
Stars fainter than those on the main sequence. Subluminous stars are stars whose age divided by their life span is close to unity. [H76]
O Star
Stars of spectral type O are very hot blue stars with surface temperatures of about 35,000 K, whose spectra are dominated by the lines of singly ionized helium (see Pickering series). (Most other lines are from at least doubly ionized elements, though H and He I lines are also present.) O stars are useful because they are found in dust clouds and virtually define the spiral arms. Most O stars are very fast rotators. O stars have lifetimes of only 3 to 6 million years. [H76]
R Star
Stars of spectral type R are stars with spectral characteristics similar to those of K stars except that molecular bands of C2, CN, and CH are present instead of TiO bands. [H76]
Ultraviolet-Bright Stars
Stars that are brighter than the horizontal-branch stars and bluer than the giant-branch stars. [H76]
Old Stars
Stars that, according to contemporary stellar evolution theory, have an age comparable to that of the galaxy to which they belong. This is not an observational definition. [JJ95]
Ultraviolet Excess Screening
Technique devised by Sir Martin Ryle and Allan Sandage to measure the spectral red shift of suspected quasars. It was this process that resulted in the discovery of quasi-stellar objects. [A84]
Refracting Telescope
Telescope that uses lenses to magnify and focus an image onto an eyepiece. (refractor) [A84]
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]
T Associations
Stellar associations containing many T Tauri stars. About 20 are known. [H76]
sdO
Subdwarf O stars showing few very broad and shallow Balmer lines and a very strong He II 4686 line. [JJ95]
Unit
Symbol Absolute Zero Triple Point of Water
Poynting Vector
Symbol: S The vector product of the electric field vector E and the magnetic field vector H (= B0/µ0) in an electromagnetic wave. The Poynting vector gives, in magnitude and direction, the power radiated through unit area at any instant. The unit is the watt per sqare meter. In a simple harmonic wave, the average value is 1/2E0H0, where E0 and H0 are the amplitudes. [DC99]
Susceptibility
Symbol: X The ratio, for a given substance, of the magnetization of a sample to the magnetic field strength applied. In SI it equals (µr - 1), where µr is the relative permeability. The value of X determines whether a substance shows Paramagnetism, Diamagnetism, or Ferromagnetism. A diamagnetic material has a negative susceptibility while paramagnetic and ferromagnetic materials have small and large positive susceptibilities respectively. [DC99]
SU(3)
Symmetrical Unitary of Order 3: A symmetry found in sub-nuclear spectra. It is a concept in group theory, by which Gell-Mann and others, using eight quantum numbers, have been able to combine particles into family groups or supermultiplets, as the lowest-lying eightfold group of the nucleon doublet, the singlet, the triplet, and the doublet. The SU(3) theory applies only to the strongly interacting particles. [H76]
Precession of the Equinoxes
The First Point of Aries (0 hr Right Ascension) moves backwards (westward) along the equator at 50.2 arcseconds per year due to the 26 000-year conical motion of the Earth's rotation axis caused by the gravitational pull of the Sun and Moon on the Earth's equatorial bulge. Correcting for this effect yields the mean equator and mean equinox. Nutation is the wobble of the Earth's axis as it precesses. Correction for this effect gives the true equator and true equinox. Because of precession the tropical year is about 20 minutes shorter than the sidereal year. It takes about 25,800 years for Earth's axis to complete one circuit. [McL97]
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]
Planck Scale
The Planck scale refers to certain values of length, time, and energy or mass. To understand how these values originate, suppose you were trying to explain to an intelligent being in another galaxy how long humans typically lived. You couldn't use hours or years, because those units are defined on earth (for example, by how long it happens to take our planet to go around its sun once), so a being in another galaxy wouldn't know what you meant. However, every physicist in the universe knows the values of Planck's constant (h), the speed of light in vacuum (c), and the universal strength of the gravitational force (G). You could use those values to form ratios that define a universal unit of time called the Planck time and then tell the being from another galaxy our typical lifetime in units of Planck times. Similar units for length and mass or energy can be defined. Max Planck realized this possibility and defined these units at the beginning of the twentieth century. Because the Planck scale units are the only universal ones, we expect the fundamental laws of nature to be simple in form when expressed in those units. [K2000]
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]
Siemens (S)
The SI unit of electrical conduction (reciprocal ohm). [H76]
Watt
The SI unit of power. 1 W = 107 ergs s-1. [H76]
Photosphere of the Sun
The Solar "surface". Granular in appearance, it comprises spicules of gaseous helium at an average temperature of 6,000°C. Each spicule averages 7,000 km in height but lasts for less than 8 minutes. The sunspots are cooler depressions in the photosphere. [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]
Quiet Sun
The Sun when the 11-year cycle of activity is at a minimum. [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]
Semi-Diameter
The angle at the observer subtended by the equatorial radius of the Sun, Moon, or a planet. [S92]
True Anomaly
The angle, measured at the focus nearest the pericenter of an elliptical orbit, between the pericenter and the radius vector from the focus to the orbiting body; one of the standard orbital elements (see Elements; Orbital; Eccentric Anomaly; Mean Anomaly) [S92]
Statistical Mechanics
The area of physics that analyzes the behavior of a system with very many members, such as a gas with many individual molecules. In such a situation, the behavior of the whole system is obtained by averaging over the behavior of individual members. [LB90]
Trojans
The asteroids located at the points of Jupiter's orbit around the Sun that are equidistant from the Sun and Jupiter (see Lagrangian Points). The first Trojan (Achilles) was discovered in 1906. About 15 are now known. [H76]
Polar Axis
The axis of an equatorially mounted telescope that points towards the Celestial Poles and is therefore parallel to the Earth's axis of rotation. [McL97]
Terminator
The boundary between the illuminated and dark areas of the apparent disk of the Moon, a planet or a planetary satellite. [S92]
Sheath
The boundary layer of charged particles between a plasma and its surrounding material. [H76]
Plage
The bright rim of a sunspot, observed in emission in monochromatic light of some spectral line (H[alpha] or Ca II). It is a chromospheric phenomenon associated with and often confused with a facula. (sometimes called flocculus)[H76]
Vega
The brightest star in the constellation Lyra and the fifth brightest star in the night sky. Vega is a white A-type main sequence star 25 light-years away. [C95]
Uniformitarianism
The hypothesis that the extensive changes in the earth, as evinced in the geological record, have resulted, not from massive catastrophes, but from the slow operation of wind, weather, volcanism, and the like over many millions of years. Compare catastrophism. [F88]
Rare Gases
The inert gases He, Ne, Ar, etc.
r-Process
The capture of neutrons on a very rapid time scale (i.e., one in which a nucleus can absorb neutrons in rapid succession, so that regions of great nuclear instability are bridged), a theory advanced to account for the existence of all elements heavier than bismuth (up to A 298) as well as the neutron-rich isotopes heavier than iron. The essential feature of the r-process is the release of great numbers of neutrons in a very short time (less than 100 seconds). The presumed source for such a large flux of neutrons is a supernova, at the boundary between the collapsing neutron star and the ejected material. However, other proposed sources have included such things as supernova shocks and black-hole-neutron-star collisions. The heavier r-process elements are synthesized at a temperature of about 109 K and an assumed neutron density of 1020-1030 per cm3. The r-process is terminated by neutron-induced fission. The existence of 244Pu (half-life 82 million years) in the early solar system shows that at least one r-process event had occurred in the Galaxy just before the formation of the solar system. [H76]
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]
Refraction, Astronomical
The change in direction of travel (bending) of a light ray as it passes obliquely through the atmosphere. As a result of refraction, the observed altitude of a celestial object is greater than its geometric altitude. The amount of refraction depends on the altitude of the object and on atmospheric conditions. [S92]
Strange Particles
The collective name for a group of strongly interacting particles possessing the property of strangeness. According to one theory, the strange particles are regarded as the higher quantum states of the nucleus. [H76]
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]
U Velocity
The component of a star's motion away from the Galactic center. If a star moves away from the Galactic center, the star's U velocity is positive; if a star moves toward the Galactic center, the U velocity is negative; and if the star moves neither toward nor away from the Galactic center, the U velocity is zero. The Sun has a U velocity of -9 kilometers per second, so the Sun is moving toward the Galactic center at 9 kilometers per second. [C95]
Planetary Precession
The component of general precession caused by the gravitational coupling between the center of mass of the Earth and that of the other planets. The effect of planetary precession is to move the equinox eastward by 0".11 / year and to diminish the angle between the ecliptic and the equator by about 0".47 / year. [H76]
Primordial Chaos
The concept that the early Universe might have been highly irregular and inhomogeneous. It could enable us to understand the origin of structure in the Universe and why the Universe is homogeneous and isotropic on the very largest scales. [Silk90]
Secular Stability
The condition in which the equilibrium configuration of a system is stable over long periods of time. [H76]
Umklapp Scattering
The contribution to scattering caused when the exchange of momentum crosses the boundary of a Brillouin zone. [H76]
Redshift-Distance Relation
The correlation between redshift in the spectra of galaxies and their distances. See Hubble law. [F88]
Primary Cosmic Rays
The cosmic rays that arrive at Earth's upper atmosphere from outer space (see also Secondary Cosmic Rays). [H76]
Vortex Theory
The cosmogony of the solar system by Descartes (1644), who argued that the planets and sun accumulated from matter that moved in a system of vortices extending over all scales. [Silk90]
Short Scale
The cosmological distance scale which uses a Hubble constant of approximately 100 km/s/Mpc. [C97]
Weber (Wb)
The derived SI unit of magnetic flux. 1 weber = 108 maxwells. [H76]
Pascal
The derived SI unit of pressure. 1 Pa = 1 N m-2 = 10-5 bars. [H76]
Photon Counting
The detection of individual photons. Photomultiplier tubes are capable of detecting single photons. Photon counting statistics says that if N is the total number of photoelectrons counted then the error in N is ± N. [McL97]
Warp
The deviation from flatness in the outer Galactic disk. Some parts of the outer disk lie above the Galactic plane; others lie below it. [C95]
Tidal Force
The differential gravitational pull exerted on any extended body in the gravitational field of another body. [Silk90]
Quantum Leap
The disappearance of a subatomic particle - e.g., an electron - at one location and its simultaneous reappearance at another. The counter-intuitive weirdness of the concept results in part from the limitations of the particle metaphor in describing a phenomenon that is also in many respects a wave. [F88]
Particle Horizon
The distance a photon of radiation could have traveled since the creation of the particle. [c97]
Radio Source Counts
The integral number of radio sources per unit solid angle whose measured flux density at the operating frequency of a radio telescope exceeds a certain given value; plot of log N (number of sources) versus log S (where S is in flux units). [H76]
21-cm Radiation
The emission line (in the radio range at a frequency of 1420 MHz) of neutral (atomic) hydrogen, caused when an electron "flips" from spinning in a direction parallel to the proton's spin to the opposite direction of lower energy. Spontaneous transitions from one level to another occur only once every 11,000,000 years on the average for a single electron, but there are so many billions of electrons in the Milky Way that this radiation can be detected by radio telescopes. (The analogous deuterium line is at 91.6 cm.) First detected in 1951; 2 years later extragalactic H I was detected. [H76]
Rest-Mass Energy
The energy which a particle has even when it is at rest. According to the famous relation E = mc2 of special relativity, this rest energy is equal to the rest mass of the particle-the mass it has when a rest-times the square of the speed of light. If the mass is in grams and the speed of light in centimeters per second (c = 2.998 × 1010 centimeters per second), then the energy is given in ergs. [G97]
Spörer's Law of Zones
The equatorward drift of average sunspot latitudes. [H76]
Radiation Era
The era from about 10 sec to about 1012 sec after the Big Bang, when the temperature had dropped to 109K and the rate of electron-positron pair annihilation exceeded the rate of their production, leaving radiation the dominant constituent of the universe. At t = 200 sec, nucleosynthesis began rather abruptly and virtually all Deuterium was synthesized to Helium. The radiation era was followed by the matter era. [H76]
Probable Error (p.e.)
The error which will not be exceeded by 50 percent of the cases. The probable error is equal to 0.6745 times the standard error. [H76]
System Flow
The evolution of the spectrum of configurations, or the associated effective coupling constants, under the action of repeated coarse-graining. [D89]
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]
Proxima Centauri
The faintest of the three stars that make up the Alpha Centauri star system. (See Alpha Centauri) [C95]
Wave Zone
The field of a pulsar beyond the velocity-of-light radius. (also called Far Fie ) [H76]
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]
Planck Era
The first 10-43 seconds of the Universe's existence. Physics can currently say very little about this time. Quantum gravity is needed before quantum cosmology can be fully realised.
Roche Lobe
The first equipotential surface for two massive bodies describing circular orbits around one another which forms a figure eight enclosing the two objects. The Roche lobes are the two lenticular volumes enclosing the two bodies. [H76]
Planck Epoch/Planck Time
The first instant following the beginning of the expansion of the Universe, when the cosmic matter density was still so high that gravitational force acted as strongly as the other fundamental forces on the sub-atomic scale. [F88]
Thermodynamics Laws
The first is the law of conservation of energy; the second is the law of entropy. (see also Nernst Theorem) [H76]
Post-Newtonian Effects
The first nontrivial gravitational effects which go beyond the predictions of Newton's theory. [H76]
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]
Platonic Solids
The five regular polyhedrons - the tetrahedron, octahedron, hexahedron, icosahedron, and dodecahedron - esteemed by Plato as embodying aesthetic and rational ideals. [F88]
Stefan's Law
The flux of radiation from a blackbody is proportional to the fourth power of its absolute temperature: L = 4R2T4. [H76]
Prime Focus
The focal point of the large primary reflecting mirror in astronomical telescopes when the light source is extremely distant. This focus actually falls at a point just within the upper structure of the telescope itself and is therefore accessible to CCD cameras and other instruments; it provides a large field of view. [McL97]
Pressure
The force exerted over a surface divided by its area. [H76]
Superforce
The force which is dominant in GUT (Grand Unified Theories) It combines the electroweak force with the strong nuclear force. [C97]
Sticking Coefficient
The fraction of all atoms (e.g., hydrogen) incident on an interstellar dust grain that become adsorbed. [H76]
Strain
The fractional change in dimension produced by a stress applied to a body. Tensile strain applies to the stretching of a body. It is the change in length divided by the original length (l/l). Bulk strain occurs when a body is subjected to a change of pressure. It is the change in volume divided by the original volume. Shear strain occurs when an angular deformation occurs, and is equal to the angular displacement produced. (see also Stress) [DC99]
Passband
The frequency band that is transmitted with maximum efficiency and without intentional loss. [H76]
Radiative Viscosity
The friction produced by the collisions between matter and radiation. [LB90]
Stochastic Cooling
The gathering(i.e., focusing) of clouds of subatomic particles in an accelerator by monitoring their scattering vectors and altering the magnetic environment in an accelerator storage ring to keep them close together. First employed in storing particles of antimatter, which are expensive to manufacture and ought not to be wasted. [F88]
Twilight
The interval of time preceding sunrise and following sunset (see Sunrise; unset) during which the sky is partially illuminated. Civil twilight comprises the interval when the zenith distance, referred to the center of the Earth, of the central point of the Sun's disk is between 90°50' and 96°, nautical twilight comprises the interval from 96° to 102°, astronomical twilight comprises the interval from 102° to 108°. [S92]
Reciprocal
The inverse of a number; for example, the reciprocal of 3 is 1/3, the reciprocal of 1/2 is 2. [G99]
Reciprocal Linear Dispersion
The inverse of the linear dispersion of a spectrometer which is the rate of change of position along the spectrum (in millimeters) with wavelength (in angstroms). The reciprocal gives the number of angstroms per millimeter. [McL97]
Polar Motion
The irregularly varying motion of the Earth's pole of rotation with respect to the Earth's crust. (See Celestial Ephemeris Pole) [S92]
Regolith
The layer of fragmentary debris produced by meteoritic impact on the surface of the Moon or a planet. [H76]
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]
Thomson Scattering
The limit of Compton scattering at low energies. [H76]
Tail
The long streamer (about 107 km long; density about 10-18 atm) behind the comet head. Type I Tails are straight (ionic tails): Type II Tails are curved (dust tails, little or no charge). Dust tails are usually driven by radiation pressure; ionic (gas) tails are driven by the solar wind. Comet tails do not usually appear until the comet is inside the orbit of Mars. [H76]
Rayleigh-Jeans Spectrum
The low-frequency portion of a blackbody spectrum. (see blackbody radiation) [LB90]
Zeropoint
The magnitude corresponding to one data number per second from the array detector for a star of zero color term (like Vega) corrected for absorption in the Earth's atmosphere. [McL97]
O Magnitude
The magnitude derived from observations at 11 microns. [H76]
Visual Magnitude
The magnitude determined with the eye. [H76]
Photographic Magnitude
The magnitude of an object as measured on the traditional photographic emulsions, which are sensitive to a slightly bluer region of the spectrum than is the human eye. (mph)(antiquated term) [H76]
Photovisual Magnitude
The magnitude of an object as measured photographically by filters and emulsions that are sensitive to the same region of the spectrum as the human eye. (antiquated) [H76]
Photoelectric Magnitude
The magnitude of an object as measured with a photoelectric photometer. (mpe) (antiquated term) [H76]
Pencil Beam
The main lobe of an antenna pattern, consisting of a narrow receiving beam of the type obtained with a single parabolic instrument. [H76]
Surface Brightness
The measure of the amount of light that an object, especially a galaxy, emits per area of the sky. Even a luminous galaxy can be hard to see if it has a low surface brightness. [C95]
Redshift Survey
The methodical tabulation of the redshifts of a large number of galaxies in a particular region of the sky. Redshifts directly measure the recessional speeds of galaxies. If Hubble's law is assumed, this speed can be translated to distance. Under such an assumption, a redshift survey provides the third dimension, depth, for the galaxies in a survey. The other two dimensions for each galaxy are provided by its position on the sky. The redshift of a galaxy is obtained by measuring its spectrum of light; in this way it is possible to see how much its colors are shifted. (See spectrum.) [LB90]
Roche Limit
The minimum distance at which a satellite under the influence of its own gravitation and that of a central mass about which it is describing a circular Keplerian orbit can be in equilibrium. For a satellite of negligible mass, zero tensile strength, and the same mean density as its primary, in a circular orbit around its primary, this critical distance is 2.44 times the radius of the primary. (For the Moon, whose density is lower than that of Earth, the Roche limit would be 2.9 Earth radii.) [H76]
Rayleigh Limit
The minimum resolvable angle between the wavelengths of two spectral lines. [H76]
Rods
The more sensitive cells of the retina of the eye. They are of most importance in vision in poor light, but cannot provide color information. Their action is not known in detail. It appears to be based on the photochemical breakdown of a reddish dye called rhodopsin (formerly visual purple). [DC99]
Oxygen
The most abundant metal in the universe, and the third most abundant element overall, after hydrogen and helium. Oxygen has atomic number eight and is produced by massive stars-those born with over eight Solar masses-which eject the element into the Galaxy when they explode. [C95]
Pluto
The most distant known planet from the Sun (39.44 AU), discovered by Clyde Tombaugh in 1930. Orbital period 248.43 years, Vorb 4.7 km s-1. Its orbit has the highest eccentricity (0.249) and highest inclination to the ecliptic (17°.17) of any planet and some astronomers suggest that it may be an escaped satellite of Neptune. Synodic period 366.7 days; albedo less than 0.25: rotation period 6d9h17m49s. In the mid-1970s Pluto crosses Neptune's orbit on its way in, and for the rest of this century Pluto will be closer to the Sun than Neptune. (Pluto and Neptune, however, are never less than 2.6 AU apart.) Perihelion will occur in 1989. Effective temperature about 50-60 K. Its mass and radius have not been determined with any great certainty, but it is probably about 0.1 to 0.2 the mass of the Earth (6 × 1026 g?) and no more than 2900 km in radius. [H76]
P-Process
The name of the hypothetical nucleosynthetic process thought to be responsible for the synthesis of the rare heavy proton-rich nuclei which are bypassed by the r- and s-processes. It is manifestly less efficient (and therefore rarer) than the s- or r-process, since protons must overcome the Coulomb barrier, and may in fact work as a secondary process on the r- and s-process nuclei. It seems to involve primarily (p, ) reactions below cerium (where neutron separation energies are high) and the (, n) reactions above cerium (where neutron separation energies are low). The p-process is assumed to occur in supernova envelopes at a temperature greater than about 109 K and at densities less than about 104 g cm-3. [H76]
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]
System Noise
The noise in a radio telescope; composed of the receiver noise and the sky noise. [H76]
Reciprocity Failure
The non-linear behavior of a photographic emulsion in which an increase in exposure time does not correspond to an increase in sensitivity by the same factor. [McL97]
Particle Distribution Function
The number of particles per unit volume of phase space. [H76]
Plate Scale
The number of seconds of arc on the sky corresponding to 1 mm in the focal plane of the telescope. Originating with photographic plates, but now being replaced by "pixel scale" for digital (generally CCD) detectors. [BFM2002]
Winding Number
The number of times a string is wound around a circular spatial dimension. [G99]
Virgo Infall
The observed gravitational motion of nearby galaxies toward the Virgo cluster of galaxies, about 50 million light years away. The Virgo cluster represents a strong concentration of mass, a strong departure from a uniform distribution of matter, and it therefore causes galaxies in its vicinity to deviate from the Hubble flow. [LB90]
Reduced Proper Motion
The observed proper motion of a star (in seconds of arc per year) reduced to absolute proper motion (in kilometers per second). [H76]
Triple Point
The only point at which the gas, solid, and liquid phases of a substance can coexist in equilibrium. The triple point of water (273.16 K) is used to define the kelvin. Some substances (e.g. carbon dioxide) have pressures at the triple point greater than atmospheric. Such materials can appear as liquids only when under pressure. [DC99]
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]
Tunnel Effect
The passage of a particle through a potential barrier, even though it has not enough energy to pass the barrier on classical grounds. The tunnel effect can be explained by quantum mechanics. (see Tunnel Diode) [DC99]
Transit
The passage of a smaller, nearer astronomical object across the face of a larger object in the background, as in a transit of Venus across the sun. [F88]
Orbit
The path in space followed by a celestial body. [S92]
Osculating Orbit
The path that an orbital body (e.g., a planet) would follow if it were subject only to the inverse-square attraction of the Sun or other central body. In practiced secondary bodies, such as Jupiter, produce perturbations. [H76]
Year (Besselian)
The period of one complete revolution in right ascension of the fictitious mean sun, as defined by Newcomb. The beginning of a Besselian year, traditionally used as as standard epoch, is denoted by the suffix ".0". Since 1984 standard epochs have been defined by the Julian year rather that the Besselian year. For distinction, the beginning of the Besselian year is now identified by the prefix B (e.g., B1950.0). [S92]
Year (Tropical)
The period of one complete revolution of the mean longitude of the sun with respect to the dynamical equinox. The tropical year is longer than the Besselian year (see year, Besselian) by 0s.148 T, where T is centuries from B1900.0. [S92]
Synodic Month
The period of time (29.53 days) between two successive identical phases of the Moon, e.g., new Moon to new Moon or full Moon to full Moon (see Lunation). [H76]
Universality
The phenomenon whereby many microscopically quite different physical systems exhibit critical point behavior in with quantitatively identical features such as critical indices. [D89]
Perigee
The point at which a body in orbit around the Earth most closely approaches the Earth. Perigee is sometimes used with reference to the apparent orbit of the Sun around the Earth. [S92]
Perigalacticon
The point in a star's orbit around the Galaxy when the star lies closest to the Galactic center. The Sun is near perigalacticon now. [C95]
Pericynthion
The point in the orbit of a satellite around the Moon closest to the Moon. [H76]
Periapsis
The point in the orbit of a satellite where it is closest to its primary. [H76]
Perihelion
The point in the orbit of an object orbiting the Sun where it is closest to the Sun's center of mass. Earth's perihelion occurs early in January. [H76]
Periastron
The point in the orbit of one component of a binary system where it is nearest the other component. [H76]
Precession, Constant of
The ratio of the lunisolar precession to the cosine of the obliquity of the ecliptic. It amounts to about 54".94 per annum. [H76]
Quantization
The restriction of various quantities to certain discrete values; or, more generally, to deriving the quantum-mechanical laws of a system from its corresponding classical laws. [H76]
Product
The result of multiplying two numbers. [G99]
Physics
The scientific study of the interactions of matter and energy. [F88]
Radio Scintillation
The scintillation in received radio emission; the equivalent of "twinkling" in visible light from the stars. [A84]
Pallas
The second asteroid to be discovered (by Olbers in 1802). Diameter about 560 km; a = 2.77 AU, e = 0.235, i = 34°.8. Orbital period 1,686 days; rotation period 9-12 hours. Albedo ~ 0.05; mass (1972 est.) 2.6 × 1023 g. Spectrum resembles meteorites of either low-grade carbonaceous chondrite or enstatite achondrite. [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]
Radiative Braking
The slowing down of rotation of a star due to radiation. [H76]
Proto-Solar Nebula
The slowly rotating cloud of gas and dust from which the Solar System formed. [Silk90]
Vector Translation
The small theoretical precession of the axis of an orbiting body due to the gravitational influence of its primary. This effect is predicted by general relativity, but so far it has not been observed. [H76]
Protostellar Core
The smallest opaque clumps into which a collapsing interstellar gas cloud fragments. The characteristic mass of a protostellar core is only 0.01 Solar mass. It grows by accretion as the surrounding matter falls toward it, attaining stellar mass within 105 years after it forms. At this stage, it is a protostar - a large cocoon of contracting matter that is radiating predominantly in the far infrared. [Silk90]
Spark Spectra
The spectra of ions often produced by a spark discharge (cf. arc spectra). [H76]
Velocity
The speed and the direction of an object's motion. [G99]
Perseus Arm
The spiral arm that lies next out from the arm containing the Sun. The most famous members of the Perseus arm are the young star clusters h and Chi Persei. [C95]
Velocity Dispersion
The spread of a velocity distribution - that is, how stars move relative to one another. Technically, the velocity dispersion is the standard deviation of the velocity distribution. Stars with similar velocities have a small velocity dispersion, whereas stars with wildly different velocities have a large velocity dispersion. [C95]
Root Mean Square (rms)
The square root of the mean square value of a set of numbers. [H76]
Oxygen Burning
The stage when a star fuses Oxygen into Silicon and Sulfur. It occurs only in stars born with over eight Solar masses. [C95]
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]
Pole Star
The star - Polaris - that lies near the direction in the sky toward which the North Pole of the Earth points. [F88]
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]
Thick Disk
The stellar population that contains Arcturus and about 4 percent of the other stars near the Sun. It has a scale height of about 3500 light-years and consists of old stars. [C95]
Thin Disk
The stellar population that contains the Sun and most other nearby stars. Most of its stars have a scale height of 1000 light-years and orbit the Galaxy on fairly circular orbits. The stars of the thin disk range in age from 0 to about 10 billion years. The thin disk breaks into two subpopulations, the young thin disk (ranging in age from 0 to 1 billion years) and the old thin disk (ranging in age from 1 to about 10 billion years). The young thin disk has a smaller scale height than the old thin disk, and the former's stars have more circular orbits. [C95]
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]
Razin Effect
The strong suppression of low-frequency (synchrotron) radiation by electrons moving in a cool, collisionless plasma. It is a theoretical calculation of the Tsytovitch effect specifically directed toward radio astronomy. (also called Razin-Tsytovitch Effect ) [H76]
Perseus X-1
The strongest known extragalactic X-ray source, centered on NGC 1275. (3U 0316+41) [H76]
Tidal Theory
The theory of the origin of the solar system involving the near collision of a massive body with the sun. The original version of a tidal theory, due to Buffon (1785), considered passage of a comet, but modern versions of this theory invoke a passing star. The gaseous debris torn from the sun by tidal forces is supposed to have condensed into the planets; however, this theory has been replaced by the nebular theory. [Silk90]
Tertiary (mirror)
The third mirror to be encountered by the light in a telescope system. A tertiary mirror is required on alt-az telescopes to direct light to the stationary Nasmyth foci. [McL97]
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]
Transit-Time Effect
The time required for the radiation to travel from the source to the object which reflects, or absorbs and reemits, it to the observer. [H76]
Rise Time
The time required for the vehicle to achieve its optimum height (in rocket or balloon astronomy). [H76]
White Hole
The time-reversal of a black hole. A white hole is a singularity from which matter emerges unpredictably, but into which matter cannot enter. The initial singularity of the standard big bang theory is an example of a white hole. It can be shown that the creation of a new universe from a false vacuum bubble in the context of classical General Relativity would require a white hole singularity, which means essentially that it cannot be done, even in principle. However, a false vacuum bubble could conceivably grow to become a new universe through a process of quantum tunneling. [G97]
Proper Time
The timelike invariant spacetime interval between the points along the trajectory of a particle. (More prosaically, time measured by an ideal clock at rest with respect to the observer.) [H76]
Torus
The topological name for the shape of a donut. While a donut is a two-dimensional surface in a three-dimensional space, the torus can be generalized to higher numbers of dimensions. [P88]
Statistical Error
The uncertainty resulting from a measurement of purely random events. Such an uncertainty is defined as bracketing a range of values within which the correct value has a 66% chance of lying. For example, a value of (100 ± 10) obtained from a given measurement means that the true value has a 66% chance of lying between 90 and 110, and a 34% chance of being either above or below this range. [H76]
Red Giant Tip
The upper tip of the red-giant branch in the H-R diagram. The red-giant tip represents the "flash" point (e.g., helium flash, carbon flash) where the density and temperature of the core have become high enough that the "ash" in the core is ignited and serves as the fuel for a new series of nuclear reactions. Here a 1 M star ejects its envelope. [H76]
Vacuum Expectation Value
The value of the Higgs field (a constant value different from zero) is called a system's Vacuum Expectation Value. [K2000]
Vacuum Higgs Value
The value that the Higgs field(s) have in the vacuum. (see Higgs Fields) [G97]
Radio Window
The wavelength range between a few millimeters and about 20 meters within which Earth's atmosphere is transparent to radiation. [H76]
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.
Sub-Pulse
The weaker component of the pulse of a pulsar. [H76]
Population I, II, and III
The youngest observed stars are called Population I stars; older observed stars are called Population II; and it is postulated that an even older generation of stars, called Population III, existed still earlier. Population II stars formed mostly from hydrogen and helium. Population I stars, like our sun, formed from hydrogen, helium, and a large range of heavier elements (like carbon and oxygen) believed to have been created in the interiors of earlier Population II and III stars and then blown out into space. [LB90]
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]
Virtual Particle Theory
Theory devised by Stephen Hawking to account for apparent thermal radiation from a black hole (from which not even light can escape). It supposes that space is full of "virtual particles" in a particle and antiparticle relationship, being created out of "nothing" and instantly destroying each other. At an event horizon, however, one particle may be gravitationally drawn into the singularity, and other appear to radiate as heat. [A84]
TOE
Theory of Everything A quantum-mechanical theory that encompasses all forces and all matter. [G99]
Pulsating Universe
Theory that the Universe constantly undergoes a Big Bang, expands, gradually slows and stops, contracts, and gradually accelerates once more to a Big Bang. Alternative theories include an ever-expanding Universe and the Steady-State Universe. (also know as Oscillating Universe) [A84]
Strongly Coupled
Theory whose string coupling constant is larger than 1. [G99]
Weakly Coupled
Theory whose string coupling coyistant is less than 1. [G99]
Photon Detectors
These devices respond directly to individual photons. An absorbed photon then releases one or more bound charge carriers in the detector that may (1) modulate the electric current in the material; (2) move directly to an output amplifier; or (3) lead to a chemical change. Photon detectors are used throughout the x-ray, ultraviolet, visible and infrared spectral regions. (See also Thermal Detectors; Coherent Receivers)
Rossby Number
This gives the ratio of the inertial to the Coriolis forces in a moving fluid. It is named after the Swedish meteorologist C. A. Rossby (1899-1959). [JM92]
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]
Universality Class
This is a way of classifying the behavior of systems near the critical points of continuous phase transitions. Systems in the same universality class have the same behavior in the critical region, when an appropriate matching is made between the physical variables, and have the same critical exponents. [D89]
Populations I and II
Two classes of stars introduced by Baade in 1944. In general, Population I (now sometimes called arm population) are young stars with relatively high abundances of metals, and are usually found in the disk of a galaxy, especially the spiral arms, in dense regions of interstellar gas. Population II (now sometimes called halo population) are older stars with relatively low abundances of metals, and are usually found in the nucleus of a galaxy or in globular clusters. The Sun is a rather old Population I star. [H76]
Phase Difference
Two identical waves of the same wavelength are said to be "in phase" when the peaks and troughs coincide perfectly. If this is not the case then a phase difference is said to occur. For a phase difference of half a wavelength, the peak of one wave coincides with the trough of the other. [McL97]
UHF
Ultra-High Frequency A radio frequency in the range between 3 GHz and 0.3 GHz (wavelength 10 cm-1 m). [DC99]
ULIRG
Ultra-Luminous InfraRed Galaxy
UV
Ultraviolet: wavelengths shorter than about 350 nm. [McL97]
UTC
Universal Time Coordinated [LLM96]
Poseidon
Unofficial name for J VIII, the next outermost satellite of Jupiter. P = 737dR, e = 0.4, i = 147°. Discovered by Melotte in 1908. [H76]
Pan
Unofficial name for Jupiter XI. P = 692d R, e = 0.2; i = 163°. Discovered by Nicholson in 1938. [H76]
Tropopause
Upper boundary of the troposphere (about 15 km), where the temperature gradient goes to zero. [H76]
Super-Metal-Rich
Used in reference to stars, or stellar populations, which are richer in metals than the Hyades.
Solid-State
Usually implies crystalline semiconductor materials used in the electronics industry. [McL97]
Very Large Array
VLA A radio telescope scheduled to be built near Socorro, New Mexico, which will consist of 27 antennas, each 82 feet in diameter, distributed along three 13-mile-long arms of a Y-shaped track. According to the NSF, the array will give radio astronomers as much resolution as the 200-inch gives optical astronomers. [H76]
Very Long Baseline Interferometry
VLBI In radio astronomy, a system of two or more antennas placed several hundred or several thousand miles apart, which are operated together as an interferometer. [H76]
Quantum Space
Vacuum with the potential to produce virtual particles. [F88]
Potentiality
a peculiarly quantum mechanical mode of reality, which is intermediate between full actuality and bare logical possibility. When an eventuality is characterized as a potentiality, it is neither true nor false but indefinite, but it has a definite probability of turning out to be true if the system is subjected to physical conditions which suffice to make it actual. [D89]
relativistic plasma
a plasma consisting of particles which have relativistic energies, i.e. for typical particles, their kinetic energies exceed their rest-mass-energies mc2. Relativistic plasmas are found in such astronomical objects as supernova remnants, radio galaxies, the interstellar medium and the nuclei of galaxies. In many of these cases, the relativistic plasma provides most of the pressure of the medium. The spectrum of the particle energies is generally of power-law rather than Maxwellian form so that many of the particles are ultrarelativistic, i.e. E >> mc2. [D89]
principle of equivalence
a principle which states that all bodies should fall with the same acceleration; also denoted the `weak equivalence principle'.[D89]
Optical Soliton
a soliton is a wave pulse which propagates without changing shape or dispersion. It maintains its shape by a balancing between linear dispersion and nonlinear compression and occurs in many areas of wave physics where nonlinearities are important. Optical solitons are generated in nonlinear media excited by strong laser pulses.[D89]
Stellar Wind
a steady or unsteady outflow of material from the surface of a star. In many classes of star hot coronae are observed and these are believed to be due to heating by waves generated in the upper layers of the star. This results in the outflow of mass in the form of a stellar wind. For a star like the Sun, the mass outflow in the solar wind amounts to only about 10-13 M y-1 but in massive blue supergiant stars the mass loss in the form of stellar winds can amount to as much as 10-4 to 10-5 M y-1. [D89]
Supernova
a stellar explosion in which a star may be completely disrupted, leaving a compact stellar remnant such as a neutron star or black hole. At maximum light, the supernova can have luminosity about 108 or 109 times that of the Sun. The luminosity decays after the initial outburst, in certain classes of supernova, the decline being exponential with a half-life of about 80 days. In massive stars, the supernova occurs when the star has used up all its available nuclear fuel and it reaches a lower energy state through gravitational collapse to form a more compact star. In white dwarf stars in binary systems, accretion of mass onto the surface of a neutron star can be sufficient to take the star over the upper mass limit for stability as a white dwarf and it collapses to form a neutron star resulting in a supernova explosion. [D89]
Quantum Liquid
a system of particles which are both sufficiently mobile and at sufficiently low temperature to display the effects of quantum-mechanical indistinguishability. Examples include the electrons in superconducting metals and the atoms in liquid helium. [D89]
Order Parameter
a variable such as magnetisation used to describe the degree of order in a phase below its transition temperature. In a continuous phase transition the order parameter goes continuously to zero as the critical temperature is approached from below.[D89]
Statistical Weight
g: The probability that the state will appear under a given set of conditions. Usually, the number of ordinarily degenerate substates contained in the state; e.g., the (2l + 1)m states of an atom in the absence of a magnetic field. [H76]
Self-Organisation
spontaneous emergence of order, arising when certain parameters built in a system reach critical values. [D89]
renormalisation
strictly, the rescaling of some parameter in a field theory. In practice, nearly all renormalizations involve an infinite rescaling, so the term has come to be identified with a mathematical procedure for circumventing otherwise nonsensical infinite terms in quantum field theory by absorbing them into observable constants in the theory, such as mass, charge, etc. [D89]
Period Doubling
the motion of a particle under the influence of a force may settle down to a regular orbit with a definite period. If the force acts nonlinearly on the particle, and is increased, then the orbit period (the time taken to return to a previous position) may suddenly double when the motion changes to a more complex pattern. This doubling from a simple motion (called a one-cycle) to the more complex form (a two-cycle) is period doubling. The process may continue until an n-cycle is produced. Period doubling is a major phenomenon in nonlinear systems, especially in lasers where the particle motion is replaced by radiation fields.[D89]
Degree Celsius
°C -273.15 0.01
Degree Kelvin
°K 0 273.16
Degree Rankine
°R 0 491.688
Open Cluster
"(a) A small, loose cluster of stars that typically contains several hundred members. The best examples are the Hyades and the Pleiades, both in the constellation Taurus. Open clusters line the Galactic plane, in contrast with globular clusters, which are members of the Galaxy's halo or thick disk. [C95]
Open Universe
"(a) Any model of the Universe which does not contain enough matter to halt its expansion. [c97]
Observer
"(a) Anything in receipt of electromagnetic radiation. [c97]
Occultation
"(a) Eclipse of a star by another celestial body. [A84]
Objective
"(a) The lens or combination of lenses nearest the object in an optical instrument. The nearest lens to the object in a compound objective is often called the object lens. The large converging mirror in a reflecting telescope can also be described as the objective. [DC99]
Occam's Razor
"(a) The notion that the simplest explanation of a problem is the preferred explanation, unless it is known to be wrong. [LB90]
Olber's Paradox
"(a) The puzzle of why the sky is dark at night. If the Universe extends infinitely in space, as it might, then the accumulated light from an infinite number of distant stars should seemingly cause the sky to be bright at all times, whether our sun is visible or not. This paradox, first posed in the eighteenth century, has been resolved by the big bang theory. In a Universe with a beginning, we can receive light only from that part of the Universe close enough so that light has had time to travel from there to here since the big bang (about 10 billion years ago). Thus, even if space extends infinitely far, only a limited region, and a limited number of stars, are visible to us. And the accumulated light from this limited number of stars is not sufficient to spoil the darkness of the night sky. [LB90]
Orion Arm
"(a) The spiral arm containing the Sun. It lies between the Sagittarius arm and the Perseus arm. [C95]
Order of Magnitude
A factor of ten. Two orders of magnitude indicate a factor of 100, etc. (Not to be confused with astronomical `magnitudes'. [G97]
2 Orionis
A 21.03-day O9.5 Vp spectroscopic binary tentatively identified with 2U 0525-06. [H76]
Orgueil Meteorite
A Type I carbonaceous chondritic meteorite that fell in France in 1864 and that has recently been found to contain amino acids. [H76]
Omega Nebula
A bright HII region in Sagittarius. It is a double radio source. (also called Swan Nebula) (M17, NGC 6618)[H76]
OB Association
A loose gathering of O and B stars that typically stretches over hundreds of light-years and contains a few dozen OB stars. [C95]
Optical Depth
A measure of the integrated opacity along a path through a layer of material, measured by the amount of absorption of a beam of incident light. The intensity ratio I / I0 = e-, where the optical depth = N l (N = column density, = cross section, and l is the path length). Optical depth 1 corresponds to the "visible" surface and occurs when the intensity is reduced by a factor e. [H76]
FU Orionis
A newly formed star, probably a pre-main-sequence star (cF5-G3 Ia) presently near the top of its Hayashi track. In 1936 it suddenly appeared in the middle of a dark cloud, and rose by 6 magnitudes in the photographic band. Its lithium abundance is 80 times that of the Sun. It has developed a reflection nebula. [H76]
Orbifold
A particular space used as a candidate for the compactified space of superstring theory. These six-dimensional orbifolds could be thought of as generalizations of a six-dimensional torus, but containing twenty-seven singular points. [P88]
BM Orionis
A peculiar eclipsing binary (B2-B3) in the Trapezium, with a flat-bottomed light curve suggesting a total eclipse. The spectrum of the secondary has never been seen. [H76]
Orion A
A radio continuum feature (an HII region) centered on the Trapezium, and excited by 1 Ori C. The Orion A molecular cloud, which lies beyond it, is a rich source of molecules CO, OH, HCN, and probably NO, HCO, and H2CO have been observed. [H76]
Orion B
A radio continuum source (NGC 2024).
Orthonormal Tetrad
A set of four mutually orthogonal unit vectors at a point in spacetime, one timelike and three spacelike, which give the directions of the four axes of a locally Minkowskian coordinate system. [H76]
Objective Prism
A small-angle prism placed in front of a telescope objective to transform each star image in a field of stars into an image of its spectrum. [H76]
Open Space
A space of infinite volume and without any boundary (in the cosmological context). [Silk90]
Oblique Rotator
A stellar model in which the rotational and magnetic axes are not coincident. Magnetic stars are generally assumed to be oblique rotators of this kind. [H76]
One-Standard-Deviation Uncertainty
An estimate of the uncertainty of a measurement which is specified so that the probability of the true value of the measured quantity lying within the uncertainty interval is two out of three. There is one chance in three that the true value lies outside the interval. [G97]
YY Orionis
An extremely young star (younger than T Tauri) in the Orion Nebula. YY Orionis stars are very young, late-type, low-mass stars in the gravitationally contracting stage in which the star is still accreting matter from the protostellar cloud. [H76]
OH
An interstellar molecule (the hydroxyl radical) first detected in 1963 at a wavelength of 18 cm. The four transitions that occur near 18 cm are caused by the splitting of the ground level. Galactic OH sources have been divided into three classes according to whether the OH emission is strongest in the main lines, particularly at 1665 MHz (Class 1), whether the emission and absorption are highly anomalous only in the satellite lines (Class 2) (Class 2a, 1720-line emitters; Class 2b, 1612-line emitters), or whether there is only absorption in all four lines (Class 3). [H76]
Of
Peculiar O stars in which emission features at 4634-4641 from N III and 4686 from He II are present. They have a well-developed absorption spectrum, which implies that the excitation mechanism of the emission lines is selective, unlike that of Wolf-Rayet stars. The spectra of Of stars are usually variable, and the intensities of their emission lines vary in an irregular manner. Of stars belong to extreme Population I. All O stars earlier than 05 are Of. [H76]
Optical Path Difference (OPD)
The difference in path length between the actual wavefront in an optical system and the equivalent spherical wavefront. [McL97]
Observable Universe
The extent of the Universe that we can see with the aid of the largest telescopes. Its ultimate boundary is determined by the horizon size. [Silk90]
Oosterhoff Groups
Two groups of globular clusters which differ in the period of transition between Bailey type ab and type c variables, the ratio of type c to type ab stars, in the metallicity of RR Lyrae stars, and in the mean period of the ab variables. On the whole group I clusters have slightly weak metal lines whereas group II clusters have very weak metal lines. [H76]
Oppenheimer-Volkoff Limit
The limiting mass for a neutron star as the density approaches infinity. Beyond this mass all configurations are unstable. [H76]
Old Thin Disk
The older part of the thin-disk population, ranging in age from about 1 to 10 billion years. The Sun and most other nearby stars belong to the old thin disk. The scale height of the old thin disk is about 1000 light-years. [C95]
Omega
The ratio of the average density of mass in the Universe to the critical mass density, the latter being the density of mass needed to eventually halt the outward expansion of the Universe. In an Open Universe, Omega is always less than 1; in a Closed Universe, it is always greater than 1; in a Flat Universe it is always exactly equal to 1. Unless omega is exactly equal to 1, it changes in time, constantly decreasing in an Open Universe and constantly increasing in a Closed Universe. Omega has been measured to be about 0.1, although such measurements are difficult and uncertain. see Critical Mass Density; Closed Universe; Flat Universe; Open Universe [LB90]
Octave
The span over which the frequency doubles; e.g. Middle C is 262 cycles per second; the C one octave above it is 524 cycles per second. The observed electromagnetic spectrum covers a range of 17 decades (about 56 octaves) - from about 106 to about 1023 cycles per second. [H76]
Optical Matching
The use of lenses or other optical devices to match the size of the image of the seeing disk, as it appears in the focal plane of the telescope, to the physical size of the CCD pixels. If the telescope yields 10 arcseconds per millimeter and the seeing is 1 arcsecond then the image is 0.1 mm in size. But a typical CCD pixel is 0.022 mm, five times smaller. [McL97]