table Name = TermA
1A Spectral type for white stars, such as Sirius, Vega, Altair, Deneb, and Fomalhaut. [C95]
2AAT Anglo-Australian Telescope
3A band One of about a dozen of the strongest Fraunhofer lines seen in the Solar spectrum, the A band at 7600 angstoms is due to telluric lines of molecular oxygen in the Earth's atmosphere.
4A-coefficient Einstein coefficient, where Aji is the coefficient of spontaneous emission from upper level j to lower level i.
5A-number Atomic Mass Number: The total number of protons and neutrons in an atom's nucleus. For example, Oxygen-16 has a mass number of sixteen, because it has eight protons and eight neutrons. [C95]
6A Shell Stars A-type stars in which two different types of line profiles co-exist. [JJ95]
7A Star A star of spectral type A with a surface temperature of about 10,000 K, in whose spectrum the Balmer lines of hydrogen attain their greatest strength. Helium lines can no longer be seen. Some metallic lines are present; in late A stars the H and K lines of ionized calcium appear. A0 stars have a color index of zero. Examples of A stars are Vega and Sirius. [H76]
8ab Variables A sub-class of Bailey type RR Lyrae variables, having asymmetric lioght curves of large amplitude. [H76]
9AB Magnitude System The AB magnitude system is defined such that for any bandpass or filter being considered, the magnitude zero-point corresponds to a flux density of 3631 Jy (1 Jy = 1 Jansky = 10-26 W Hz-1 m-2 = 10-23 erg s-1 Hz-1 cm-2) [BFM03].
10ABT Abbreviation employed in this book to mean After the Beginning of Time, which is here defined as the beginning of the expansion of the Universe. [F88]
11Ae or A Emission Stars A-type stars with emission in one or several Balmer lines. [JJ95]
12Am Stars (a) Peculiar stars whose metallic lines are as strong as those of the F stars but whose hydrogen lines are so strong as to require that they be classed with the A stars. They are generally short-period (<300d) spectroscopic binaries with high atmospheric turbulence and variable spectra, and are slower rotators than normal A stars. [H76] (b) A-type or F-type objects to which no unique spectral type can be assigned. Usually the classifier provides a classification according to the hydrogen, metallic and calcium lines. Also call metallic-line stars. [JJ95]
13Ap Stars Peculiar A-type stars ("magnetic" A stars) that show abnormally strong lines, sometimes of varying intensity, of certain ionized metals. Recent evidence indicates that all Ap stars are slow rotators compared with normal A stars. [H76]
14Abelian Group A mathematical group of transformations with the property that the end result of a series of transformations does not depend on the order in which they are performed. [CD99]
15Aberration (a) Defect in the image formed by a lens, mirror or optical system. Spherical aberration results when different rays of light are brought to more than one focus, producing a blurred image or coma; chromatic aberration when different wavelengths within a ray of light are brought to more than one focus, producing an image distorted by colored fringes. Aberration in lenses can be overcome by the use of an achromatic lens or a combination of lenses made of glasses of different refractive indices. [A84] (b) The apparent angular displacement of the observed position of a celestial object from its geometric position, caused by the finite velocity of light in combination with the motions of the observer and of the observed object. see Aberration, Planetary [S92]
16Aberration A defect in an optical system such that the image is not a true picture of the object. For instance, colored fringes may appear, the image may not be focused, or the shape may show distortion. Techniques of aberration correction exist; these can, however, be complex and costly. Chromatic (color) Aberration is found with a single lens; mirrors do not suffer from chromatic aberration. Because dispersion always accompanies refractive deviation, the `red' image will be farther from the lens than the `blue'. Consequently, the image is surrounded by colored fringes. Chromatic aberration is corrected by forming a compound lens, whose elements have different refractive constants. Spherical Aberration always occurs with rays that are distant from the axis and incident on a spherical mirror or lens. It is the cause of the caustic curve. Spherical aberration is corrected by using parabolic reflecting and refracting surfaces. Astigmatism affects rays neither close nor parallel to the axis. The cone of rays through a lens from an off-axis object does not focus at a point. Instead, two images in the form of short lines are formed at different distances from the lens. Between the two the image appears blurred. Mirrors forming images of off-axis points show a similar defect. The best method of minimizing astigmatism is to reduce the aperture with stops, thus allowing light only through the center of the lens. Coma is rather similar in cause, effect, and correction to astigmatism. After refraction by a lens, a cone of rays from an off-axis object tends to have a tadpole-shaped section because of coma. Distortion is the result of differences in a lens' magnifying power between different axes. Reduction of aperture is the normal solution to both coma and distortion. [DC99]
17Aberration Annual The component of stellar aberration (see Aberration, Stellar) resulting from the motion of the Earth about the Sun. [S92]
18Aberration Diurnal The component of stellar aberration resulting from the observer's diurnal motion about the center of the Earth. [S92]
19Aberration E-terms of Terms of annual aberration depending on the eccentricity and longitude of perihelion of the Earth (abbreviation for Ecliptic Aberration). [S92]
20Aberrations effects Effects associated with the performance of optical components which give rise to imperfect optical images. [McL97]
21Aberration Planetary The apparent angular displacement of the observed position of a celestial body produced by the motion of the observer and the actual motion of the observed object. [S92]
22Aberration Secular The component of stellar aberration resulting from the essentially uniform and rectilinear motion of the entire Solar System in space. Secular aberration is usually disregarded. [S92]
23Aberration Spherical Always occurs with rays that are distant from the axis and incident on a spherical mirror or lens. It is the cause of the caustic curve. Spherical aberration is corrected by using parabolic reflecting and refracting surfaces.
24Aberrations Effects associated with the performance of optical components which give rise to imperfect optical images. [McL9
25Ablation Erosion of an object (generally a meteorite) by the friction generated when it passes through the Earth's atmosphere. [H76]
26Absolute Brightness The total luminosity radiated by an object. [Silk90]
27Absolute Magnitude (a) A measure of the intrinsic brightness of a star or galaxy. Absolute magnitude is defined as the apparent magnitude the star or galaxy would have if it were 32.6 light-years (10 parsecs) from Earth. The lower an object's absolute magnitude, the greater its intrinsic brightness. For example, the Sun has an absolute magnitude of +4.83, while Sirius, whose intrinsic brightness is greater, has an absolute magnitude of +1.43. A star that is one absolute magnitude brighter than another (e.g., +4 versus +5) is 2.5 times intrinsically brighter; a star that is 5 absolute magnitudes brighter is 100 times intrinsically brighter; and a star that is 10 absolute magnitudes brighter is 10,000 times intrinsically brighter. [C95] (b) The absolute magnitude (g) of a Solar-System body such as an asteroid is defined as the brightness at zero phase angle when the object is 1 AU from the Sun and 1 AU from the observer. [H76]
28Absolute Space Newtonian space, hypothesized to define a cosmic reference frame independent of its content of matter or energy. The existence of absolute space, enshrined in aether theory, was denied in relativity. [F88]
29Absolute Unit A unit defined in terms of fundamental quantities (such as length, mass, time, and electric charge). [DC99]
30Absolute Zero (a) The zero value of thermodynamic temperature; 0 kelvin or -273.15?C. [DC99] (b) The lowest possible temperature, attained when a system is at its minimum possible energy. The Kelvin temperature scale sets its zero point at absolute zero (-273.15? on the Celsius scale, and -434.07? on the Fahrenheit scale). [HH98] (c) The temperature at which thermal disorder completely disappears, and which is therefore the ultimate limit of `coldness'. On the Kelvin (absolute) temperature scale it is by definition the zero of temperature: on the Celsius scale it lies at about -273 degrees. [D89]
31Absorptance Symbol:???The ratio of the radiant or luminous flux absorbed by a body or material to the incident flux. It was formerly called the absorptivity. [DC99]
32Absorption (a) A process in which a gas is taken up by a liquid or solid, or in which a liquid is taken up by a solid. In absorption, the substance absorbed goes into the bulk of the material. Solids that absorb gases or liquids often have a porous structure. The absorption of gases in solids is sometimes called sorption. Compare adsorption. [DC99] (b) Decrease in the intensity of radiation, representing energy converted into excitation or ionization of electrons in the region through which the radiation travels. As contrasted with monochromatic scattering (in which reemission occurs in all directions at the same frequency), the inverse process of emission refers to radiation that is reemitted in general in all directions and at all frequencies. [H76]
33Absorption Band see Band Spectrum
34Absorption Coefficient Fraction of the incident radiation absorbed at a certain wavelength per unit thickness of the absorber. The absorption coefficient is in general a function of temperature, density, and chemical composition. (or k in cm-1) see Lambert's Law. [H76]
35Absorption Edges Sudden rises superposed on the smooth decrease of the curve of the attenuation coefficient, which cause the curve to have a typical sawtooth aspect. They generally occur at the limit of spectral lines. [H76]
36Absorption Lines Dark lines in a spectrum, produced when light or other electromagnetic radiation coming from a distant source passes through a gas cloud or similar object closer to the observer. Like emission lines, absorption lines betray the chemical composition and velocity of the material that produces them. [F88]
37Absorption of Radiation No medium transmits radiation without some energy loss. This loss of energy is called absorption. The energy is converted to some other form within the medium. see also Lambert's Law. [DC99]
38Absorption Spectrum Dark lines superposed on a continuous spectrum, caused by the absorption of light passing through a gas of lower temperature than the continuum light source. [H76]
39Absorption Trough Range of wavelengths (around 21 cm) at which atomic hydrogen absorbs (or emits) radiation; this is a concept used in the attempt to detect intergalactic matter. [A84]
40Abundance (a) The relative amount of a given element among others; for example, the abundance of oxygen in the Earth's crust is approximately 50% by weight. (b) The amount of a nuclide (stable or radioactive) relative to other nuclides of the same element in a given sample. The natural abundance is the abundance of a nuclide as it occurs naturally. For instance, chlorine has two stable isotopes of masses 35 and 37. The abundance of 35Cl is 75.5% and that of 37Cl is 24.5%. For some elements the abundance of a particular nuclide depends on the source. [DC99] (c) The relative amounts of chemical elements. For example, hydrogen makes up about 75 of the mass of the Universe, so its "cosmic abundance" is 75%. [LB90]
41Abundance Ratio The ratio of the number of atoms of an isotope to the number of atoms of another isotope of the same element in a sample. [DC99]
42Acausal Initial Conditions Initial conditions that could not have been caused by any prior physical process. [LB90]
43Acceleration (a) The SI unit is the meter per second per second (m s-2). 1. When considering motion in one dimension, and in unscientific usage, acceleration means rate of increase of speed. This is a scalar quantity, which can be positive or negative. Negative values mean that the speed is decreasing and may be called deceleration or retardation. 2. In scientific study of motion in two or three dimensions acceleration means rate of change of velocity; a = dv / dt. This is a vector quantity having magnitude (which is always positive) and direction. Whenever speed changes (increasing or decreasing), or direction changes, or both speed and direction change, this is an acceleration. By Newton's second law the net force F acting on a body of mass m gives it an acceleration a where F = ma. [DC99] (b) The rate of increase of velocity with time. [HH98]
44Accelerator (a) A machine for speeding subatomic particles to high velocity, then colliding them with a stationary target or with another beam of particles moving in the opposite direction. (In the latter instance, the machine may be called a collider.) At velocities approaching that of light the mass of the particles increases dramatically, adding greatly to the energy released on impact. The resulting explosion promotes the production of exotic particles, which are analyzed according to their behavior as they fly away through a particle detector. [F88] (b) Accelerators are machines that use electric fields to accelerate electrically charged particles (electrons, protons, and their antiparticles) to higher energies. If accelerators are linear, they need to be very long to achieve the desired energies, so in some, magnets are used to bend the particles around and back to the starting point, giving them a little extra energy each time around. [K98]
45Accretion (a) Collection of material together, generally to form a single body. [A84] (b) A process by which a star accumulates matter as it moves through a dense cloud of interstellar gas; or, more generally, whereby matter surrounding a star flows toward it (as in close binaries). [H76]
46Accretion Disk A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation. [HH98]
47Accumulation Theory The theory by which planetesimals are assumed to collide with one another and coalesce, eventually sweeping up enough material to form the planets. [Silk90]
48Achernar A subgiant of spectral type B5, about 35 pc distant. (
49Achilles Asteroid No. 588, a Trojan 60? ahead of Jupiter (P = 11.98 yr, a = 5.2 AU, e = 0.15, i = 10?.3). It was the first Trojan to be discovered (in 1906). [H76]
50Achromat An achromatic lens. [DC99]
51Achromatic Color A color that has no hue; i.e. black, white, or gray. [DC99]
52Achromatic Objective A lens of two or more components with different refraction indices (e.g., crown glass and flint glass), used to correct for chromatic aberration. [H76]
53Actinic Radiation Radiation that can cause a chemical reaction; for example, ultraviolet radiation is actinic. [DC99]
54Actinium A soft silvery-white radioactive metallic element that is the first member of the actinoid series. It occurs in minute quantities in uranium ores. It can be produced by neutron bombardment of radium and is used as a source of alpha particles. The metal glows in the dark. Symbol: Ac; m.p. 1050?50?C; b.p. 3200?300?C; r.d. 10.06 (20?C); p.n. 89; most stable isotope 227Ac (half-life 21.77 years). [DC99]
55Action A quantity related to the momentum and position of a body or system of particles. The Principle of Least Action asserts that the integral, or sum of this action, taken over a particular path must be a minimum. This Principle of Least Action can be used instead of Newton's Laws to determine the motion of a system. [P88]
56Action-at-a-Distance A description of a force, such as Newton's law of gravity, in which two separated bodies are said to directly exert forces on each other. In the modern description, the bodies produce a gravitational field, which in turn exerts forces on the two bodies. see Gravitational Field [G97]
57Active Galactic Nucleus AGN -- An unusually bright galactic nucleus whose light is not due to starlight. [HH98]
58Active Galaxy Any galaxy which is emitting large quantities of non-thermal radiation. [C97] (b) Active galactic nuclei are very luminous (1043-1046 ergs s-1). Their energy output is in two forms: nonthermal continuum and thermal emission line. [H76]
59Active Optics Controlling the shape of a telescope mirror at a relatively slow rate. [McL97]
60Active Sun The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum. [H76]
61Activity Symbol: A For a radioactive substance, the average number of atoms disintegrating per unit time. [DC99]
62Acuity, Visual The ability of the eye to see separately two points close to each other. It is a measure of the resolving power of the eye's optical system and depends on the density of cells in the retina. The maximum acuity of the normal human eye is around 0.5 minutes of arc - points separated by this angle at the eye should be seen as separate. see Resolution [DC99]
63Adaptive Optics Compensating for atmospheric distortions in a wavefront by high-speed changes in the shape of a small, thin mirror. [McL97]
64ADC Analog-to-Digital Converter -- An electronic circuit which takes an input voltage in a given range (typically 0-10 volts) and provides a corresponding digital output by setting output lines (bits) high or low. A 16-bit ADC has 16 output lines. [McL97]
65ADF Astrophysical Data Facility, located at the NASA Goddard Space Flight Center (GSFC), is responsible for designing, developing, and operating data systems that support the processing, management, archiving and distribution of NASA mission data. The ADF serves three broadly-defined astrophysics disciplines: high-energy astrophysics, UV/optical astrophysics, and infrared/submillimeter/radio astrophysics. The ADF collaborates with the GSFC Laboratory for High Energy Astrophysics (LHEA) and the Laboratory for Astronomy and Solar Physics (LASP) in managing data for specific missions. The ADF staff also support the astrophysics community's access to multi-mission and multi-spectral data archives in the National Space Science Data Center (NSSDC).
66Adhesion A force of attraction between atoms or molecules of different substances. For example, adhesion between water molecules and glass creates a meniscus. [DC99]
67Adiabatic Change A change taking place in a system that has perfect thermal insulation, so that heat cannot enter or leave the system and energy can only be transferred by work. In practice, a close approximation to an adiabatic change can be achieved by the process being too rapid for significant heat transfer, or by the large scale of the system (e.g. a large volume of air in the atmosphere). In an adiabatic expansion of a gas, mechanical work is done by the gas as its volume increases and the gas temperature falls. For an ideal gas undergoing a reversible adiabatic change it can be shown that
68Adiabatic Index The ratio of the fractional change in pressure to the fractional change in density as an element of fluid expands (or contracts) without exchange of heat with its surroundings. [H76]
69Adiabatic Fluctuations Fluctuations in both the matter and radiation density, as though a volume of the Universe were slightly squeezed but allowing no radiation to escape. Prior to the Decoupling Era, adiabatic fluctuations behaved like waves, on scales smaller than the horizon size. After decoupling, gravitational instability sets in on scales above about 1013 M , smaller adiabatic fluctuations having been damped at earlier eras. [Silk90]
70Adsorption A process in which a layer of atoms or molecules of one substance forms on the surface of a solid or liquid. All solid surfaces take up layers of gas from the surrounding atmosphere. The adsorbed layer may be held by chemical bonds (chemisorption) or by weaker van der Waals' forces (physisorption). Compare absorption. [DC99]
71ADU Analog-to-Digital Units see DN [McL97]
72Advance of the Perihelion The slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due to gravitational interactions with other planets and/or other effects (such as those due to general relativity). [H76]
73Advection The transfer of matter such as water vapor or heat through the atmosphere as a result of horizontal movement of an air mass. [DC99]
74Aeon (or Eon) In astronomical terms, 1,000 million years. [A84]
75Aerolite A stony meteorite, composed primarily of silicates. About 93 percent of all known falls are aerolites. They include the carbonaceous chondrites, other chondrites, and achondrites. (lit. "air stone.") [H76]
76After-Image An image seen after the eye's retina has been exposed for a time to an intense or stationary light source. It may be negative or positive, or appear in complementary colors. [DC99]
77Aether (1) In Aristotelian physics, the fifth element, of which the stars and planets are made. (2) In Classical physics, an invisible medium that was thought to suffuse all space. [F88]
78Age of the Universe The time elapsed since the singularity predicted by the Big Bang theory, estimated to be around 13 billion years. [BFM02]
79AIPS Astronomical Image Processing System -- National Radio Astronomy Observatory
80Airglow Light in the nighttime sky caused by the collision of atoms and molecules (primarily oxygen, OH, and Ne) in Earth's geocorona with charged particles and X-rays from the Sun or outer space. The airglow varies with time of night, latitude, and season. It is a minimum at zenith and maximum about 10? above the horizon. (also called nightglow)[H76]
81Airy Diffraction Disk The central spot in the diffraction pattern of the image of a star at the focus of a telescope. Named for Sir George Airy (1801-1892), seventh Astronomer Royal. [McL97]
82Albedo (a) The ratio of the amount of light reflected from a surface to the amount of incident light. [DC99] (b) Ratio of the total flux reflected in all directions to the total incident flux. see Bond Albedo; Geometric Albedo [H76]
83Alchemy Art of bringing parts of the Universe to the perfect state toward which they were thought to aspire - e.g., gold for metals, immortality for human beings. [F88]
84Alcyone (Tau) -- The brightest star in the Pleiades (spectral type B5). [H76]
85Aldeberan (a) The brightest star in the constellation Taurus, Aldebaran is an orange K-type giant that lies 60 light-years away. [C95] (b) A K5 III subgiant (a foreground star in the Hyades) about 21 parsecs distant. It has a faint M2 V companion. It is now known to be slowly and irregularly variable. [H76]
86Alfven numberA dimensionless number characterizing steady fluid flow past an obstacle in a uniform magnetic field parallel to the direction of flow. It has a partial analogy to the Mach number. The Alfven number is given by vl(?)1/2 B-1/2 where v is the velocity of flow, l is length of obstacle,
87Alfv?n-Klein Cosmology A cosmological model in which the early Universe is depicted as a giant collapsing spherical cloud of matter and antimatter. When a critical density is reached, the matter and antimatter begin to annihilate, the resulting release of radiation and energy causing the Universe to expand. There are many difficulties with this model of the expanding Universe, which is largely discredited on observational grounds. [Silk90]
88Alfv?n Speed The speed at which hydromagnetic waves are propagated along a magnetic field: (VA) = B / (4 )1/2. [H76]
89Alfv?n Waves Waves moving perpendicularly through a magnetic field. They are caused by the oscillation of magnetic lines of force by the motions of the fluid element around its equilibrium position, which in turn is caused by the interactions between density fluctuations and magnetic variations. [H76]
90Algol Per (a) The most famous eclipsing binary, Algol was probably the first variable star discovered. It lies in the constellation Perseus and consists of two stars that orbit each other every 2.87 days. When one star passes in front of the other, the light of the system dims. [C95] (b) An eclipsing system of at least three components (B8 V, K0, Am), about 25 pc distant. Period of components A and B is about 68.8 hours; period of components A, B, and C is about 1.9 years. Long term observations also indicate a massive, unseen fourth component with a period of about 190 years. Algol is also an erratic radio source of about 0.5 AU diameter. [H76]
91Aliasing In a discrete Fourier transform, the overlapping of replicas of the basic transform, usually due to undersampling. [H76]
92Allotropy The existence of a solid substance in different physical forms. Tin, for example, has metallic and non-metallic crystalline forms. Carbon has two crystalline allotropes: diamond and graphite. [DC99]
93Almagest Arabic title for Ptolemy of Alexandria's Syntaxis, the writings in which he combined his own astronomical researches with those of others. Although much of the work is inaccurate even in premise, until Nicolaus Copernicus published his results fourteen centuries later the Almagest remained the standard reference source in Europe. [A84]
94Alph -Particle The nucleus of a 4He atom, consisting of two protons and two neutrons. Mass of -Particle 4.00260 amu. [H76]
95Alpha -Process A hypothetical process of nucleosynthesis (now considered obsolete terminology), which consisted of redistributing alpha-particles in the region from 20Ne to 56Fe (and perhaps slightly higher). The -process has been replaced by explosive and nonexplosive C, O, and Si burning occurring in rapidly evolving or even explosive stages of stellar evolution which at higher temperatures and densities becomes the e-process. [H76]
96Alpha Centauri (a) Bright binary star in which both components contribute to a magnitude of -0.27: it is also the nearest of the bright stars (at a distance of 4.3 light years). [A84] (b) The nearest star system to the Sun and the third brightest star in the night sky. Unfortunately, Alpha Centauri is so far south that it is visible only from latitudes below 25 degrees north. The system consists of three stars: Alpha Centauri A, the brightest, which is a yellow G-type main-sequence star like the Sun; Alpha Centauri B, the second brightest, which is an orange dwarf; and Alpha Centauri C, by far the faintest, which is a red dwarf. Alpha Centauri A and B both lie 4.35 light-years from Earth and orbit each other every 80 years; Alpha Centauri C lies far from its mates and 4.25 light-years from Earth. Because it is closer to Earth than are A and B, Alpha Centauri C is usually called Proxima Centauri. [C95]
97Alpha Decay (a) A type of radioactive decay in which the unstable nucleus emits a helium nucleus. The resulting nuclide has a mass number decreased by 4 and a proton number decreased by 2. An example is: 88226 Ra 86222 Rn + 24 He
98Alpha decayThe particles emitted in alpha decay are alpha particles. Streams of alpha particles are alpha rays or alpha radiation. They penetrate a few centimeters of air at STP or a metal foil of mass/area a few milligram/cm2. see also Beta Decay [DC99] (b) The disintegration of an atomic nucleus, in which the final products are an alpha particle and a nucleus with two fewer protons and two fewer neutrons than the original. [G97] (c) Spontaneous emission by a heavier element (such as uranium) of positively charged helium nuclei - alpha particles - comprising 2 protons and 2 neutrons. The result of this radioactive decay is that the original element is very gradually converted into another element, with a decreased atomic number and mass. Alpha particle emission may be simultaneous with beta particle decay. [A84]
99Alpha Particles Particles first discovered in radioactive decay, and later identified as helium nuclei (two protons and two neutrons bound together). [CD99]

first table = 9   AB Magnitude System    The AB magnitude system is defined such that for any bandpass or filter being considered, the magnitude zero-point corresponds to a flux density of 3631 Jy (1 Jy = 1 Jansky = 10-26 W Hz-1 m-2 = 10-23 erg s-1 Hz-1 cm-2) [BFM03].