A	Spectral type for white stars, such as Sirius, Vega, Altair, Deneb, and Fomalhaut. [C95]
AAT	Anglo-Australian Telescope
A 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.
A-coefficient	Einstein coefficient, where Aji is the coefficient of spontaneous emission from upper level j to lower level i.
A-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]
A Shell Stars	A-type stars in which two different types of line profiles co-exist. [JJ95]
A 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]
ab Variables	A sub-class of Bailey type RR Lyrae variables, having asymmetric lioght curves of large amplitude. [H76]
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].
ABT	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]
Ae or A Emission Stars	A-type stars with emission in one or several Balmer lines. [JJ95]
Am 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]
Ap 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]
Abelian 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]
Aberration	(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]
Aberration	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]
Aberration, Annual	The component of stellar aberration (see Aberration, Stellar) resulting from the motion of the Earth about the Sun. [S92]
Aberration, Diurnal	The component of stellar aberration resulting from the observer's diurnal motion about the center of the Earth. [S92]
Aberration, E-terms of	Terms of annual aberration depending on the eccentricity and longitude of perihelion of the Earth (abbreviation for Ecliptic Aberration). [S92]
Aberration, 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]
Aberration, 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]
Aberration, 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.
Aberration, Stellar	(a) Difference in a star's apparent position in the sky from the apparent position it would have if the Earth were stationary. Such displacement caused by the Earth's sidereal motion results in an optical positioning difference of up to about 20.5 seconds of arc, much greater than any displacement observed by parallax. [A84] (b) Angular displacement in the apparent direction of a star, due to Earth's orbital motion (vE) and the finite speed of light (c). The displacement = arc tan (vE/c) 20'.49. Thus to an Earth-based observer a star describes an ellipse on the celestial sphere with a semimajor axis of 20'.49. (The eccentricity of the ellipse is zero - i.e., a circle - for a star on the ecliptic pole; for a star on the ecliptic plane the ellipse degenerates into a straight line.) [H76] (c) The apparent angular displacement of the observed position of a celestial body resulting from the motion of the observer. Stellar aberration is divided into diurnal, annual, and secular components. [S92]
Aberrations	Effects associated with the performance of optical components which give rise to imperfect optical images. [McL97]
Ablation	Erosion of an object (generally a meteorite) by the friction generated when it passes through the Earth's atmosphere. [H76]
Absolute Brightness	The total luminosity radiated by an object. [Silk90]
Absolute Luminosity	see Luminosity
Absolute 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]
Absolute 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]
Absolute Temperature	(a) Temperature measured on the Kelvin scale: 0 Kelvin = -273.15° Celsius. Absolute temperature is directly related to (kinetic) energy via the equation E = kBT, where kB is Boltzmann's constant. So, a temperature of 0 K corresponds to zero energy, and room temperature, 300 K = 27°C, corresponds to an energy of 0.025 eV. [CD99] (b) Symbol: T A temperature expressed on the thermodynamic (ideal gas) scale, measured from absolute zero. If is the temperature on a Celsius scale calibrated against the International Practical Temperature Scale, then: T = + 273.15 [DC99]
Absolute Unit	A unit defined in terms of fundamental quantities (such as length, mass, time, and electric charge). [DC99]
Absolute 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]
Absorptance	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]
Absorption	(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]
Absorption Band	see Band Spectrum
Absorption 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]
Absorption 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]
Absorption 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]
Absorption 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]
Absorption 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]
Absorption 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]
Absorptivity	Symbol: The ratio of the radiant or luminous flux absorbed by a body or material to the incident flux. It is now called the Absorptance. [DC99]
Abundance	(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]
Abundance 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]
Acausal Initial Conditions	Initial conditions that could not have been caused by any prior physical process. [LB90]
Acceleration	(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]
Accelerator	(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]
Accretion	(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]
Accretion 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]
Accumulation 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]
Achernar	A subgiant of spectral type B5, about 35 pc distant. ( Eridani) [H76]
Achilles	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]
Achromat	An achromatic lens. [DC99]
Achromatic Color	A color that has no hue; i.e. black, white, or gray. [DC99]
Achromatic Lens	(a) Lens (or combination of lenses) that brings different wavelenghts within a ray of light to a single focus, thus overcoming chromatic aberration. It was first successfully made by Joseph von Fraunhofer. [A84] (b) A compound lens whose elements differ in refractive constant in order to minimize chromatic aberration. Simple achromatic doublets are formed by combining two lenses of different glass. The condition for achromatism is: 1 P1 + 2 P2 = 0 where 1 and 2 are the dispersive powers of the glasses of the lenses, and P1 and P2 are the powers of the lenses. Achromatic lenses are corrected for chromatic aberration at two different wavelengths. see also Apochromatic Lens [DC99]
Achromatic 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]
Actinic Radiation	Radiation that can cause a chemical reaction; for example, ultraviolet radiation is actinic. [DC99]
Actinium	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]
Action	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]
Action-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]
Active Galactic Nucleus	AGN -- An unusually bright galactic nucleus whose light is not due to starlight. [HH98]
Active 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]
Active Optics	Controlling the shape of a telescope mirror at a relatively slow rate. [McL97]
Active Sun	The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum. [H76]
Activity	Symbol: A For a radioactive substance, the average number of atoms disintegrating per unit time. [DC99]
Acuity, 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]
Adaptive Optics	Compensating for atmospheric distortions in a wavefront by high-speed changes in the shape of a small, thin mirror. [McL97]
ADC	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]
ADF	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).
Adhesion	A force of attraction between atoms or molecules of different substances. For example, adhesion between water molecules and glass creates a meniscus. [DC99]
Adiabatic 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 pV = K1 T p1 - = K2 and T V - 1 = K3 where K1, K2, and K3 are constants and is the ratio of the principal specific heat capacities. Compare isothermal change. [DC99]
Adiabatic Demagnetization	A method of producing temperatures close to absolute zero. A sample of a paramagnetic salt is cooled in liquid helium in a strong magnetizing field. The sample is then thermally isolated by pumping away the helium, and the magnetic field is removed. The sample demagnetizes itself at the expense of its internal energy so that the temperature falls. Temperatures of the order of a millikelvin can be obtained. [DC99]
Adiabatic 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]
Adiabatic 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]
Adsorption	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]
ADU	Analog-to-Digital Units see DN [McL97]
Advance 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]
Advection	The transfer of matter such as water vapor or heat through the atmosphere as a result of horizontal movement of an air mass. [DC99]
Aeon (or Eon)	In astronomical terms, 1,000 million years. [A84]
Aerolite	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]
After-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]
Aether	(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]
Age of the Universe	The time elapsed since the singularity predicted by the Big Bang theory, estimated to be around 13 billion years. [BFM02]
AIPS	Astronomical Image Processing System -- National Radio Astronomy Observatory
Airglow	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]
Airy 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]
Albedo	(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]
Alchemy	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]
Alcyone	( Tau) -- The brightest star in the Pleiades (spectral type B5). [H76]
Aldebaran	( Tau) -- (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]
-- Theory	Explanation of the big-bang theory in terms of nuclear physics, proposed by Ralph Alpher, Hans Bethe and George Gamow in 1948; it was later slightly corrected by Chushiro Hayashi. [A84]
Alfvén Number	A 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( µ)1/2 B-1/2 where v is the velocity of flow, l is length of obstacle, is density, µ is permeability and B is magnetic flux density. It is named after H. 0. G. Alfven (1908- ), the Swedish astrophysicist and Nobel Prize winner, who introduced the term magnetohydrodynamics. [JM92]
Alfvén-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]
Alfvén Speed	The speed at which hydromagnetic waves are propagated along a magnetic field: (VA) = B / (4 )1/2. [H76]
Alfvén 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]
Algol	( 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]
Aliasing	In a discrete Fourier transform, the overlapping of replicas of the basic transform, usually due to undersampling. [H76]
Allotropy	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]
Almagest	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]
-Particle	The nucleus of a 4He atom, consisting of two protons and two neutrons. Mass of -Particle 4.00260 amu. [H76]
-Particle Nuclei	Nuclei formed by the -process. see Even-Even Nuclei [H76]
-Process	A hypothetical process of nucleosynthesis (now considered obsolete terminology), which consisted of redistributing -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]
Alpha 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]
Alpha 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 The 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]
Alpha Particles	Particles first discovered in radioactive decay, and later identified as helium nuclei (two protons and two neutrons bound together). [CD99]
Altair	( Aql) (a) A bright (mv = 0.78) A7 V star about 4.8 pc distant. [H76] (b) The brightest star in the constellation Aquila, Altair is a white A-type main-sequence star that lies 16 light-years away. [C95]
Altitude-Azimuth	(a) Comprising a means of measuring or precisely locating in coordinates the position of objects at any altitude or azimuth. The term is now used mainly to describe a type of mounting for a telescope. (also Alt-Az)[A84] (b) A form of mounting similar to that of a radar which allows the telescope tube to be moved horizontally (by rotation in azimuth or compass direction) and vertically (by rotation in altitude or elevation). To follow a star the telescope must be adjusted simultaneously in both axes. [McL97]
Altitude	(a) Angular distance above the horizon. [A84] (b) The angular distance of a celestial body above or below the horizon, measured along the great circle passing through the body and the zenith. Altitude is 90 deg. minus zenith distance. [S92]
Aluminum	A soft moderately reactive metal. Aluminum has the electronic structure of neon plus three additional outer electrons. There are numerous minerals of aluminum; it is the most common metallic element in the Earth's crust (8.1% by weight) and the third in order of abundance. Symbol: Al; m.p. 660.37°C; b.p. 2470°C; r.d. 2.698 (20°C); p.n. 13; r.a.m. 26.981539. [DC99]
am	Abbreviation for the Latin Ante Meridiem (before noon). [LLM96]
Amagat	A unit of molar volume at 0° C and a pressure of 1.0 atmosphere. This unit varies slightly from one gas to another, but in general it corresponds to 2.24 × 104 cm3. Also, a unit of density equal to 0.0446 gram mole per liter at 1 atm pressure. [H76]
Amalthea	Jupiter V, the innermost satellite of Jupiter. Diameter about 140 km; i = 0°.4, e = 0.0028, period 0.498 days. Discovered by Barnard in 1892. (also called Barnard's satellite)[H76]
Americium	A highly toxic radioactive silvery element of the actinoid series of metals. A transuranic element, it is not found naturally on Earth but is synthesized from plutonium. 241Am has been used in gamma-ray radiography. Symbol: Am; m.p. 1172°C; b.p. 2607°C; r.d. 13.67 (20°C); p.n. 95; most stable isotope 243Am (half-life 7.37 × 103 years). [DC99]
Amorphous	Denoting a solid that has no crystalline structure; i.e. there is no long-range ordering of atoms. Many substances that appear to be amorphous are in fact composed of many tiny crystals. Soot and glass are examples of truly amorphous materials. [DC99]
Ampere	The SI unit of electric current. "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per meter of length" (CIPM [1946], Resolution 2, approved by the 9th CGPM 1948). A current of 1 A is equivalent to the passage along the filament of a light bulb of about 6 × 1018 electronic charges per second. [H76]
Amplifier	A device that increases an electrical signal applied to it as an input. If the input is an alternating voltage, the output voltage has a similar waveform with an increased amplitude. The ratio of the output signal to the input signal (called the gain), will usually vary with the signal frequency. Amplifiers are usually designed to give a particular current, voltage, or power gain over the required frequency range. Some circuits containing a number of amplifying stages can cope with frequencies from 0 hertz (steady direct current) to radiofrequencies. In modern solid-state electronics, all of the amplifier circuit components, including many individual amplifying stages, are manufactured in a single integrated circuit. [DC99]
Amplitude	(a) The maximum value of a varying quantity from its mean or base value. In the case of a simple harmonic motion - a wave or vibration - it is half the maximum peak-to-peak value. [DC99] (b) The maximum height of a wave peak or the maximum depth of a wave trough. [G99] (c) Maximum displacement from the equilibrium position. [H76] (d) Total range of light variation when used in the context of variable stars.
Amplitude Modulation	(AM) A type of modulation in which the amplitude of a carrier wave is modulated by an imposed signal, usually at audio frequency. In this way communication of a signal is made between two distant points using a radio transmission as carrier. When the carrier wave is received the audio component is extracted by the process of demodulation, and the original sound may be reproduced. [DC99]
Analyzer	A device for determining the plane of polarization of plane-polarized radiation. Maximum intensity is transmitted if the plane is parallel with the analyzer's direction of polarization; the intensity is a minimum (theoretically zero) if the two are perpendicular. For visible radiation, analyzers are usually Polaroid sheets or Nicol prisms. [DC99]
Anamorphic Magnification	The difference in magnification along the spectrum and perpendicular to the spectrum in a spectrograph. [McL97]
Anastigmastic Lens	A lens designed so as to minimize its astigmatic aberration. Anastigmatic lenses have different curvatures in different directions; the surface of an anastigmatic lens is part of a toroid. [DC99]
Andromeda	A constellation near Perseus and Pegasus.
Andromeda Galaxy (M31)	(a) Major spiral galaxy, 2.2 million light-years from Earth. Gravitationally bound to the Milky Way galaxy with which it shares membership in the Local Group, it is currently approaching us, rather than receding as is the case for most galaxies. [F88] (b) A spiral galaxy (Sb in Hubble's classification; kS5 in Morgan's classification) in the Local Group, about 650-700 kpc distant (MV = -21), visible to the naked eye as a fuzzy patch in the constellation of Andromeda. Total mass about 3.1 x 1011 M ; i = 77°, heliocentric velocity - 180 km s-1. Its nucleus exhibits noncircular gas motions. It is similar to but slightly larger than our Galaxy. [H76] (c) The largest galaxy in the Local Group. also known as the Great Spiral and M31. It is about one and a half times the size of our own galaxy, and contains at least 300 globular clusters. Two smaller, elliptical galaxies (M32 and NGC 205) lie close to it. [A84]
Andromeda I, II, III	Three dwarf spheroidal galaxies, in the Andromeda subgroup of the Local Group, discovered by van den Bergh in 1972. They are among the intrinsically faintest members of the Local Group. [H76]
S Andromedae	A supernova seen in 1885 in the Andromeda Galaxy. [H76]
Angstrom	(a) A unit that measures the wavelength of light and equals 0.00000001 of a centimeter. Blue light has a wavelength of about 4400 angstroms, yellow light 5500 angstroms, and red light 6500 angstroms. [C95] (b) A unit of length equal to 10-10 meters; symbol Å. Named for Swedish physicist Anders Jonas Angstrom (1814-1874). [McL97] (c) A unit of length equal to 10-10 m. It is used in atomic and molecular measurements and for the wavelength of electromagnetic radiation in the visible, near infra-red and near ultraviolet regions of the spectrum. The definition of the unit is based on the red emission line of the cadmium spectrum which has an internationally agreed wavelength of 6438.4696 Å in dry air at standard atmospheric pressure at a temperature of 15°C and containing 0.03% carbon dioxide by volume. The unit was introduced by the International Union for Solar Research in 1907[19]. It was named after A. J. Ångström (1814-1874), the Scandinavian scientist who used units of 10-10 m to describe wavelengths in his classical map of the Solar spectrum made in 1868. The ångström was not confirmed as a unit of length by the International Congress of Weights and Measures until 1927. For over half a century the ångström was equal to 1.0000002 × 10-10 m but when the metre was defined in terms of the wavelength of krypton in 1960[20] the ångström became equal to 10-10 m exactly. The ångström is sometimes called a tenth metre. X-ray wavelengths are often given ångström stars (Å*). This unit was devised by J. A. Bearden[21] in 1965 and is based on the wavelength of the K1 line of tungsten which he took to be 0.2090100 Å*, where Å* = 1.00001481 Å = 1.00001481 × 10-10 m. [JM92]
Angular Acceleration	Symbol: The rotational acceleration of an object about an axis: = d / d t or = d2 / d t2 Here is angular velocity; is angular displacement. Angular acceleration is directly analogous to linear acceleration, a. [DC99]
Angular Displacement	Symbol: The rotational displacement of an object about an axis. If the object (or a point on it) moves from point P1 to point P2 in a plane perpendicular to the axis, is the angle P1OP2, where O is the point at which the perpendicular plane meets the axis. [DC99]
Angular Dispersion	The rate of change of angle (due to refraction or diffraction) with wavelength of the emergent beam in a spectrograph. [McL97]
Angular Frequency	(Pulsatance) Symbol: The number of complete rotations per unit time. A simple harmonic motion of frequency f can be represented by a point moving in a circular path at constant speed. The foot of a perpendicular from the point to a diameter of the circle moves backward and forward along the diameter with simple harmonic motion. The angular frequency of this motion is 2f, where f is the frequency. The unit is the hertz. [DC99]
Angular Momentum	(a) The angular momentum of a system about a specified origin is the sum over all the particles in the system (or an integral over the different elements of the system if it is continuous) of the vector products of the radius vector joining each particle to the origin and the momentum of the particle. For a closed system it is conserved by virtue of the isotropy of space. [H76] (b) The product of mass and angular velocity for an object in rotation; similar to linear momentum. In quantum mechanics, angular momentum is quantized, i.e., is measured in indivisible units equivalent to Planck's constant divided by 2 pi. This corresponds classically to only certain frequencies of rotation being allowed.[F88] (c) A property of rotary motion analogous to the familiar concept of momentum in linear motion. [D89]
Angular Size	The angle subtended by an object on the sky. For example, the angular size of the moon is 30 arcminutes. [HH98]
Anharmonic Oscillator	A system whose vibration, while still periodic, cannot be described in terms of simple harmonic motions (i.e. sinusoidal motions). In such cases, the period of oscillation is not independent of the amplitude. [DC99]
Anisotropic Models	see Mixmaster Model
Anisotropic Superfluid	A system of fermions in which Cooper pairs form in a state of finite relative orbital motion and possibly finite total spin. [D89]
Anisotropy	(a) A medium is anisotropic if a certain physical quantity differs in value in. different directions. Most crystals are anisotropic electrically; important polarization properties result from differences in transmission of electromagnetic radiation in different directions. [DC99] (b) The condition in which the Universe appears different in different directions. [LB90] (c) The characteristic of being dependent upon direction. (Light coming with equal intensity from all directions is isotropic; a spotlight's beam is anisotropic.) The cosmic background radiation is generally isotropic - i.e., its intensity is the same in all parts of the sky - but small anisotropies have been detected which are thought to reflect the earth's proper motion relative to the framework of the Universe as a whole. [F88]
Annihilation	A reaction between a particle and its antiparticle; for example, between an electron and a positron. The energy released is equal to the sum of the rest energies of the particles and their kinetic energies. In order that momentum be conserved two photons are formed, moving away in opposite directions. This radiation (annihilation radiation) is in the gamma-ray region of the electromagnetic spectrum. The quantum energy is about 0.51 MeV. Annihilation also can occur between a nucleon and its antiparticle. In this case mesons are produced. see Pair Annihilation [DC99]
Annual Variation	The direction and strength of the Earth's magnetic field at any point changes with time. This must be allowed for by navigators. One such change is a variation with a period of a year, but there are others. The amplitude of the annual variation is greatest during maximum sun-spot activity. [DC99]
Annular Eclipse	An eclipse of the Sun in which the Moon is too far from Earth to block out the Sun completely, so that a ring of sunlight appears around the Moon. [H76]
Anomalistic Month	The interval (27.555 days) between two successive perigee passages of the Moon.[H76]
Anomalistic Year	The interval (365.2596 ephemeris days) between two successive perihelion passages of Earth. [H76]
Anomalous Dispersion	The refractive index of a transparent medium normally increases as the wavelength is reduced. There is then a range of wavelengths (usually in the ultraviolet) in which the radiation is absorbed fairly strongly. Such little radiation as is transmitted in this region shows anomalous dispersion, that is the refractive index decreases as the wavelength is reduced. see Dispersion [DC99]
Anomalous Expansion	An increase in volume resulting from a decreased temperature. Most liquids increase in volume as their temperature rises. The density of the liquid falls with increased temperature. Water, however, shows anomalous behavior. Between 0 and 4°C the density increases with increasing temperature. [DC99]
Anomalous Zeeman Effect	Splitting of spectral lines into several components, in contrast to the normal Zeeman effect which results in only two distinct components. The anomalous Zeeman effect is due to the fact that the electrons in the magnetic field have opposite directions of spin. [H76]
Anomaly	An angular value used to describe the position of one member of a binary system with respect to the other. The true anomaly of a star is the angular distance (as measured from the central body and in the direction of the star's motion) between periastron and the observed position of the star. The mean anomaly is the angular distance (measured in the same manner) between periastron and a fictitious body in the direction of the star, which is moving in a circular orbit with a period equal to that of the star. [H76]
Ansae	(a) The "handles", or extremities, of Saturn's rings as viewed from Earth (b) The extremities of a lenticular galaxy. [H76]
Antalgol Star	An old name for an RR Lyrae star. [H76]
Antapex	The direction in the sky (in Columba) away from which the Sun seems to be moving (at a speed of 19.4 km s-1) relative to general field stars in the Galaxy. [H76]
Antares	( Sco) (a) A red supergiant star in the constellation Scorpius. Antares is the brightest star in Scorpius and lies about 500 light-years from Earth, on the inner edge of the Orion spiral arm. [C95] (b) A red M1 Ib supergiant, about 125 parsecs distant. It has a B3 V companion, which is a radio source. [H76]
Antenna	The part of a radio telescope responsible for detecting an electromagnetic wave. (or Aerial) [McL97]
Antenna Gain	A measure of the directivity of a radio telescope. It is the ratio of the amount of power received in the direction the dish is pointing to the smaller amount of power from other directions in the sidelobes. [H76]
Antenna Temperature	A term used to describe the strength of a signal received from a radio source. It is the convolution of the true brightness distribution and the effective area of the antenna. [H76]
Antennae	A famous pair of interacting galaxies in the constellation Corvus. Each galaxy's tidal force has drawn out a long tail of stars from the other. The Antennae are also known as NGC 4038 and NGC 4039. [C95]
Anthropic Principle	(a) The doctrine that the value of certain fundamental constants of nature can be explained by demonstrating that, were they otherwise, the Universe could not support life and therefore would contain nobody capable of worrying about why they are as they are. Were the strong nuclear force slightly different in strength, for instance, the stars could not shine and life as we know it would be impossible. [F88] (b) The weak form of the anthropic principle states that life can exist only during a brief period of the history of our Universe. The strong form of the principle states that out of all possible values for the fundamental constants of nature and the initial conditions of the Universe, only a small fraction could allow life to form at all, at anytime. see Boundary Conditions; Fundamental Constants of Nature [LB90]
Anthropocentrism	The belief that humans are central to the Universe. [HH98]
Anthropomorphism	The projection of human attributes onto nonhuman entities such as animals, the planets, or the Universe as a whole. [HH98]
Anti-Baryon	The antiparticle of a baryon. [G97]
Anticenter	The direction of the sky (in Auriga) opposite to that toward the center of the Milky Way. [H76]
Anti-Coincidence Counter	A particle counter in which the circuit has been designed so as not to register the passage of an ionizing particle through more than one counting tube. [H76]
Anti-Ferromagnet	a solid in which the spins of neighboring atoms are oppositely aligned. The lattice is composed of two equivalent sublattices, and on each sublattice the spins are magnetized, as in a ferromagnet, but the directions of the magnetisations are opposed so that there is no net magnetisation. [D89]
Anti-Ferromagnetism	A kind of magnetism found in many solids at low temperatures. The molecular magnets form two arrays, aligned antiparallel. At the lowest temperatures there are equal numbers with equal magnetic moments in opposite directions, giving zero resultant magnetization. As the temperature is raised, the susceptibility increases up to the Néel temperature above which the substance is paramagnetic. [DC99]
Anti-Matter	(a) For every variety of particle there exists an antiparticle with opposite properties such as sign of electrical charge. When a particle and its antiparticle meet they can mutually annihilate and produce energy. Thus, antiquark, antiproton, etc. [D89] (b) Atomic particles that have the same mass as, but opposite charge and orbital direction to, an ordinary particle. Thus, instead of negatively charged electrons, atoms of antimatter have positrons. A quantity of antimatter coming into contact with matter would "cancel out" - annihilate, with total conversion of mass to energy - an exact proportion of matter corresponding to the original quantity of antimatter, provided that the elements in the matter also corresponded with the "elements" in the antimatter, i.e., that the atoms were equivalent but opposite. [A84] (c) Matter made of particles with identical mass and spin as those of ordinary matter, but with opposite charge. Antimatter has been produced experimentally, but little of it is found in nature. Why this should be so is one of the questions that must be answered by any adequate theory of the early Universe. [F88]
Antimony	A metalloid element existing in three allotropic forms; the most stable is a brittle silvery metal. It is used in alloys - small amounts of antimony can harden other metals. It is also used in semiconductor devices. Symbol: Sb; m.p. 630.74°C; b.p. 1635°C; r.d. 6.691; p.n. 51; r.a.m. 112.74. [DC99]
Anti-Neutrino	The antiparticle of a neutrino. [Silk90]
Anti-Neutron	The antiparticle of a neutron. A neutron and antineutron both have the same mass and zero electric charge, but can be differentiated by their interactions: a neutron and an antineutron can annihilate into gamma rays, while two neutrons cannot. [G97]
Anti-Particle	(a) An elementary particle of opposite charge but otherwise identical to its partner. Most of the observable Universe consists of particles and matter, as opposed to antiparticles and antimatter. [Silk90] (b) For every known type of particle, there exists an antiparticle with exactly the same mass, but with the opposite electric charge. When a particle and its antiparticle come together, they can always annihilate to form gamma rays. The antiparticle of an electrically neutral particle is sometimes the same as the original particle (e.g., photons) and sometimes it is distinct (e.g., neutrons). [G97] (c) Particles predicted by combining the theories of special relativity and quantum mechanics. For each particle, there must exist an antiparticle with the opposite charge, magnetic moment and other internal quantum numbers (e.g., lepton number, baryon number, strangeness, charm, etc.), but with the same mass, spin and lifetime. Note that certain neutral particles (such as the photon and 0) are their own antiparticles. [CD99] (d) A particle of the same mass and spin, but opposite charge (and other properties) to its corresponding particle. For example, a proton and antiproton both have mass 1836 times that of an electron and spin ½ unit, but the charge on the proton is + 1 unit, while that on the antiproton is -1 unit. For unstable particles, such as an isolated neutron, the particle and antiparticle have the same half-life. For uncharged particles the antiparticle is indicated by a bar above the symbol, such as for the antineutron. For charged particles the distinction is indicated by the sign, for example, e+ is the positron, the antiparticle of an electron. Antiparticles of fermions are subject to a conservation law according to which new particles can only be created in particle-antiparticle pairs, while particles can be destroyed only by annihilation with their antiparticles. This rule does not apply to bosons. see also Fermion [DC99] (e) Every particle has an associated antiparticle, another particle with the same mass but all charges opposite. If a particle has no charges, such as the photon, it is its own antiparticle. Often the antiparticle is denoted by writing a bar over the particle's name; hence, for example, the electron antiparticle (also called the positron) is denoted .
Anti-Proton	The antiparticle of a proton, identical in mass and spin but of opposite (negative) charge. [Silk90]
Anti-Quark	The antiparticle of the quark. [G97]
Anti-Reflection Coating	Also AR coating. A layer of material of lower refractive index of just the right thickness (1/4 wave) is deposited on the optical surface to be coated. More complex coatings are possible which cover a large wavelength range. [McL97]
Apastron	The point in the orbit of one component of a binary system where it is farthest from the other. [H76]
Aperture	(a) The effective diameter of the primary mirror or lens of a telescope. [H76] (b) A measure of the effective diameter (d) of a mirror or lens compared with its focal distance (f): aperture = d/f Thus a 50-mm camera lens may be used with an aperture diameter of 12.5 mm. Then, aperture = 12.5/50. This is usually described with the f-number. In this case the aperture diameter is f/4, often written as f4. The transmitted light intensity depends on aperture diameter, so that I is proportional to d2. However, large apertures lead to large aberrations although diffraction effects are more serious at small apertures. In many optical instruments, iris diaphragms vary the aperture to obtain the optimum results. [DC99]
Aperture Efficiency	The ratio of the effective aperture of the antenna, A, to its geometric aperture, Ag = d2 / 4. The beam and aperture efficiencies are related by A = B 2 / AgM, where M is the solid angle of the main beam. [H76]
Aperture Function	In radio astronomy, a distribution of direction assignments applying to a uniform background. [H76]
Aperture Photometry	Usually refers to magnitude measurements made from digital images by deriving the flux that would have been recorded within a circular aperture large enough to enclose the star's seeing disk. [McL97]
Aperture Ratio	The ratio of the aperture of a telescope to the focal length. [H76]
Aperture Synthesis	The method of combining the signals received by several smaller telescopes distributed over a very large area or baseline to provide the angular resolution of a much large telescope. Used extensively in radio astronomy, e.g. the VLA. [McL97]
Apex	see Solar Apex
Aphelion	The point in a planetary orbit that is at the greatest distance from the Sun. [S92]
Aplanatic Lens	A lens designed so as to minimize both its astigmatic and coma aberration. [DC99]
Aplanatic System	A system of three lenses which, taken together, correct for spherical aberration, chromatic aberration, and coma. [H76]
Apocenter	The point in the orbit of one component of a binary system which is farthest from the center of mass of the system. [H76]
Apochromatic Lens	A lens designed to correct for chromatic aberration at three different wavelengths. Apochromatic lenses are constructed of three or more kinds of glass. They thus have better correction than achromatic lenses, which correct at two different wavelengths (usually in the red and blue regions of the spectrum). see Achromatic Lens [DC99]
Apodization	A mathematical process performed on the data received from an interferometer before carrying out the calculations of the Fourier transformation to obtain the spectrum, in order to modify the instrumental response function. [H76]
Apogee	The point at which a body in orbit around the Earth reaches its farthest distance from the Earth. [S92]
Apollo Asteroid	One of a small group of asteroids whose orbits intersect that of Earth. They are named for the prototype, Apollo (P = 622d, a = 1.486 AU, e = 0.57, i = 6°.4). [H76]
Apollo Space Program	Successful US lunar exploration program in which the Apollo spacecraft 1 to 6 were unmanned; 7 to 10 were manned but did not land; and 11, 12 and 14 to 17 landed and returned safely. (Apollo 13 was an aborted mission.) The first men to land on the Moon were Neil Armstrong and Edwin Aldrin, from Apollo 11, on 20 July 1969. The final Apollo flight (17) lasted from 7 to 19 December 1972, and left a considerable quantity of exploratory devices on the lunar surface. [A84]
Apogalacticon	The point in a star's orbit farthest from the Galactic center. [C95]
Apparent Depth	Because radiation travels at different speeds in different media, the apparent depth or thickness of a transparent sample is not the same as its real depth or thickness. The effect is very obvious when one looks down into a glass of water or a clear pool. It is associated with the fact that a long object partly submerged in water seems bent at the water surface. The refractive constant of the substance can be measured on this basis: refractive index = real depth / apparent depth The relation is used in a number of methods for finding the refractive constant of a transparent medium. It applies to all wave radiations, not just to visible radiation. [DC99]
Apparent Magnitude	(a) A measure of how bright a star looks in the sky. The brighter the star, the smaller the apparent magnitude. A star that is one magnitude brighter than another (e.g., +1 versus +2) looks 2.5 times brighter. The brightest star of all, of course, is the Sun, whose apparent magnitude is -26.74, followed by Sirius, whose apparent magnitude is -1.46, Canopus (-0.72), Alpha Centauri (-0.27), Arcturus (-0.04), and Vega (+0.03). Stars of the Big Dipper are fainter, most of them around magnitude +2. On a clear, dark night, the unaided eye can see stars as faint as apparent magnitude +6, and the largest telescopes penetrate to apparent magnitude +30. [C95] (b) Measure of the observed brightness of a celestial object as seen from the Earth. It is a function of the star's intrinsic brightness, its distance from the observer, and the amount of absorption by interstellar matter between the star and the observer. The mv, of Sun, -26.5 mag. A sixth-magnitude star is just barely visible to the naked eye. [H76]
Apparent Place	The position on a celestial sphere, centered at the Earth, determined by removing from the directly observed position of a celestial body the effects that depend on the topocentric location of the observer; i.e., refraction, diurnal aberration. see Aberration, Diurnal), and geocentric (diurnal) parallax. Thus the position at which the object would actually be seen from the center of the Earth, displaced by planetary aberration (except the diurnal part - see Aberration, Planetary; Aberration, Diurnal) and referred to the true equator and equinox. [S92]
Apparent Solar Day	Interval between two successive culminations of the Sun - i.e., the period from apparent noon to apparent noon. The apparent Solar day is longest in late December. [H76]
Apparent Solar Time	The measure of time based on the diurnal motion of the true Sun. The rate of diurnal motion undergoes seasonal variation because of the obliquity of the ecliptic and because of the eccentricity of the Earth's orbit. Additional small variations result from irregularities in the rotation of the Earth on its axis. [S92]
Apparition	The period during which a celestial body is visible. [H76]
Appleton Layer	(F-layer) The upper of the two main layers in the ionosphere, at a height above about 150 km. It reflects radio waves. see Ionosphere [DC99]
Appulse	A penumbral eclipse of the Moon. [H76]
Apsidal Motion	Rotation of the line of apsides in the plane of the orbit; (in a binary) precession of the line of apsides due to mutual tidal distortion. [H76]
Apsides, Line of	The major axis of an elliptical orbit. [H76]
Arcminute	One sixtieth of a degree of angular measure. The Moon is 31 arcminutes across. [C95]
Archiving	Making a permanent record which can be accessed later at any time. [McL97]
Arcsecond	One sixtieth of an arcminute, or 1/3600 of a degree. Jupiter is 40 arcseconds across. [C95]
Arc Spectra	The spectra of neutral atoms produced in a laboratory arc (cf. spark spectra). [H76]
Arcturus	( Boo) (a) A beautiful orange star that is the brightest in the constellation Bootes and the fourth brightest in the night sky. It lies 34 light-years away and is a member of the thick-disk population. Historically, Arcturus is famous because it was one of the first stars to have its proper motion measured. [C95] (b) An old subgiant disk star (K2 IIIp, mv = 0.06) about 11 pc distant. [H76]
Areas, Law of	see Kepler's Second Law
Argand Diagram	a diagram in which the length and phase-angle of a complex quantity is displayed. {D89]
Argelander Method	A method of classifying stars according to image size. If the sequence stars are labeled a, b, etc., in order of image size and if the image size of a variable appears to be, say, 0.7 of the way from sequence star a to sequence star b, its brightness is listed as a7b. (also called the step method )[H76]
Argon	An inert colorless odorless monatomic element of the rare-gas group. It forms 0.93% by volume of air. Argon is used to provide an inert atmosphere in electric and fluorescent lights, in welding, and in extracting titanium and silicon. The element forms no known compounds. Symbol: Ar; m.p. -189.37°C; b.p. -185.86°C; d. 1.784 kg m-3 (0°C); p.n. 18; r.a.m. 39.95. [DC99]
Argument of the Perihelion ()	Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point. [H76]
Ariel	Second satellite of Uranus about 1600 km in diameter, discovered by Lassell in 1851. Period 2.52 days. [H76]
Aristotelian Physics	Physics as promulgated by Aristotle; includes the hypothesis that our world is comprised of four elements, and that the Universe beyond the moon is made of a fifth element and so is fundamentally different from the mundane realm. [F88]
Arm Population	Young stars typical of those found in spiral arms (Population I stars). [H76]
Armillary Sphere	Ancient Greek, Arabic and medieval alt-azimuth device, comprising a calibrated ring fixed in the meridian plane, within which a second concentric ring, also calibrated, was mobile around a vertical axis. [A84]
Aromatic, Aromaticity	In organic chemistry, carbon which has bonds that are between single and double bonds (e.g., the molecule has a bond with a delocalized electron.) [SEF01]
Array	In radio astronomy, an arrangement of antenna elements designed to produce a particular antenna pattern. [H76]
Arrival Time	see Dispersion
Arrow of Time	The direction, apparently inviolable, of the "flow" of time that distinguishes the past from the future.The direction, apparently inviolable, of the "flow" of time that distinguishes the past from the future. [HH98]
Arsenic	A toxic metalloid element existing in several allotropic forms; the most stable is a brittle gray metal. It is used in semiconductor devices, alloys, and gun shot. Symbol: As; m.p. 817°C (gray) at 3 MPa pressure; sublimes at 616°C (gray); r.d. 5.78 (gray at 20°C); p.n. 33; r.a.m. 74.92159. [DC99]
ASCA	Advanced Satellite for Cosmology and Astrophysics
Ascending Node	In the orbit of a Solar-System body, the point where the body crosses the ecliptic from south to north: for a star, out of the plane of the sky toward the observer. [H76]
Ashen Light	A faint glow from the unlit side of Venus when it is in the crescent phase. Its cause is unknown; it may be the Venusian analog to terrestrial airglow. [H76]
Asinh Magnitude	Magnitudes expressed as the inverse hyperbolic sine (or ``asinh''), sometimes referred to informally as luptitudes. The transformation from linear flux measurements to asinh magnitudes is designed to be virtually identical to the standard astronomical magnitude at high signal-to-noise ratio, but switches over to linear behavior at low S/N thereby accommodating even negative values of flux, where the logarithm, as used in the Pogson magnitude, fails. [sinh-1(x) = asinh(x)*ln(x+SQRT(x2+1))]
ASIC	Application Specific Integrated Circuit
Aspect	The apparent position of any of the planets or the Moon relative to the Sun, as seen from Earth. [S92]
Aspect Ratio	Ratio of the major axis (e.g., of a rocket) to the minor axis. (Of a fusion device) ratio of the plasma diameter to the major diameter of the torus. [H76]
Aspheric	An optical surface with departures in shape from a perfect sphere in order to cancel optical imperfections or aberrations. [McL97]
Association	A sparsely populated grouping (mass range 102-103 M) of very young, massive stars lying along a spiral arm of the Milky Way, whose spectral types or motions in the sky indicate a common origin. The star density is insufficient for gravitation to hold the group together against shear by differential galactic rotation, but the stars have not yet had time to disperse completely. OB associations are composed of stars of spectral types O-B2; T associations have many young T Tauri stars. The internationally approved designation for associations is the name of the constellation followed by an arabic numeral - e.g., Perseus OB2. [H76]
Astatine	A radioactive element belonging to the halogen group. It occurs in minute quantities in uranium ores. Many short-lived radioisotopes are known, all alpha-particle emitters. Symbol: At; m.p. 302°C (est.); b.p. 337°C (est.); p.n. 85; most stable isotope 210At (half-life 8.1 hours). [DC99]
Asteroid	(a) A small rocky body that orbits a star. In the Solar System, most asteroids lie between the orbits of Mars and Jupiter. The largest asteroid is Ceres, about 900 kilometers in diameter. [C95] (b) A small planet-like body of the Solar System, <e> ~ 0.15, <i> ~ 9°.7. More than 1800 have been catalogued, and probably millions of smaller ones exist, but their total mass would probably be less than 3 percent that of the Moon. Their densities are poorly known (about 2.6 g cm-3), but they suggest a composition similar to carbonaceous chondrite. The bright asteroids are presumably original condensations and those fainter than about 14-15 mag are collision fragments. Asteroids and short-period comets have some orbital similarities. Also called minor planet [H76] (c) Also called planetoids or minor planets, the asteroids are tiny planets most of which orbit the Sun between Mars and Jupiter. The largest - and the first discovered - is Ceres, with a diameter of 1,003 km. It is estimated that there may altogether be no fewer than 40,000. A few have very elliptical orbits and cross the orbits of several other (major) planets. One or two even have their own satellites (moons). [A84]
Asteroid Belt	A region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU), where the great majority of the asteroids are found. None of the belt asteroids have retrograde motion. [H76]
Astigmatism	(a) A common eye defect in which the observer cannot focus clearly on objects at any distance. The cause is usually a non-spherical cornea. Visual astigmatism may be corrected with a lens with a suitable degree of cylindrical curvature. [DC99] (b) An optical aberration in which off-axis points tend to become elongated ellipses. [McL97] (c) 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.
Astration	The processing of matter through stars. [H76]
Astrolabe	(a) Ancient Arabic and medieval alt-azimuth device comprising two or more flat, metal, calibrated discs, attached so both or all could rotate independently. For early navigators and astronomers it acted as star-chart, compass, clock and calendar. [A84] (b) Sighting instrument employed since antiquity to determine the elevation above the horizon of celestial objects. Eventually replaced by the sextant. [F88]
Astrology	(a) The belief that human affairs and people's personalities and characters are influenced by (or encoded in) the positions of the planets. [F88] (b) Divination using the positions of the planets, the Sun and the Moon as seen against the stars in the constellations of the zodiac - a "science" almost as old as homo sapiens. Although at one stage in history astrology and astronomy were almost synonymous- the latter has advanced so far during the last three centuries that the two now bear little relation to each other. [A84]
Astrometric Binaries	see Binary System
Astrometric Ephemeris	An ephemeris of a Solar-System body in which the tabulated positions are essentially comparable to catalog mean places of stars at a standard epoch. An astrometric position is obtained by adding to the geometric position, computed from gravitational theory, the correction for light-time. Prior to 1984, the E-terms of annual aberration (see Aberration, Annual; Aberration, E-terms of) were also added to the geometric position. [S92]
Astrometry	The branch of astronomy that deals with measuring the positions of celestial objects, especially stars. Astrometrists measure parallaxes and proper motions, which allow astronomers to determine the distances and velocities of the stars. [C95]
Astronomical Color Index	Difference in a star's brightness when measured on two selected wavelengths, in order to determine the star's temperature. Cooler stars emit more light at longer wavelengths (and so appear redder than hot stars). Modern methods involve photoelectric filtering and the UBV system. [A84]
Astronomical Coordinates	The longitude and latitude of a point on the Earth relative to the geoid. These coordinates are influenced by local gravity anomalies. see Zenith; Longitude, Terrestrial; Latitude, Terrestrial) [S92]
Astronomical Twilight	The period from sunset to the time that the Sun is 18° below the horizon; or the corresponding period before sunrise. [H76]
Astronomical Unit	(a) Mean distance between the Earth and the Sun: 149,598,500km. [A84] (b) The AU is the preferred unit for distances within the Solar System. Mercury, the innermost planet, lies on average 0.39 AU from the Sun; Pluto, normally the farthest planet, lies on average 39.5 AU from the Sun. [C95] (c) The mean distance between the Earth and the Sun. The astronomical unit is defined as the length of the radius of the unperturbed circular orbit of a body of negligible mass moving around the Sun with a sidereal angular velocity of 0.017202098950 radian per day of 86,400 ephemeris seconds. AU = 1.496 × 1013 cm 500 lt-sec. [H76] (d) The mean distance from the earth to the sun, equal to 92.81 million miles or 499.012 light-seconds. [F88] (e) The radius of a circular orbit in which a body of negligible mass, and free of perturbations, would revolve around the Sun in 2 / k days, where k is the Gaussian gravitational constant. This is slightly less than the semi-major axis of the Earth's orbit. [S92]
Astronomy	The science that studies the natural world beyond the earth. [F88]
Astrophysics	(a) The science that studies the physics and chemistry of extraterrestrial objects. The alliance of physics and astronomy, which began with the advent of spectroscopy, made it possible to investigate what celestial objects are and not just where they are. [F88] (b) All applications of the laws of physics, chemistry and the other physical sciences to the understanding of astronomical phenomena. [D89]
Asymmetric Drift	The negative of the mean V velocity of a stellar population. In general, the older the stellar population, the more negative the V velocity and therefore the greater the asymmetric drift. The young thin disk has an asymmetric drift of 0 kilometers per second, whereas the halo has an asymmetric drift of 200 kilometers per second. [C95]
Asymmetry	A violation of symmetry. [F88]
Asymptotic Branch (AGB) Stars	Globular cluster stars, which are found in that part of the HR diagram that connects the top pf the giant tip with the horizontal branch. [JJ95]
Asymptotic Freedom	(a) A term used to describe the observed decrease in the intrinsic strength of the color force between quarks as they are brought closer together. At asymptotically small separations, the quarks are virtually free. This is in contrast to the electromagnetic force whose intrinsic strength increases as two charged particles approach each other. [CD99] (b) A force of interaction between particles is said to be asymptotically free if it becomes weaker as the energy of the interacting particles increases. Empirically the force between-quarks in a proton or neutron is found to be asymptotically free, a feature that can be explained by assuming that the force is described by a Yang-Mills theory. [G97] (c) In general, there are strong, QCD forces between quarks due to the exchange of gluons. Asymptotic freedom is the principle which says that these forces become weaker for very close encounters between quarks, so that the quarks become `free' of the forces at very short distances. In high-energy collisions, the quarks do sometimes become very close and, since they then behave almost like free particles, it is not too difficult to calculate their motion during the close collision. [D89]
ATB	Acronym for After The Bang; usually used in reference to time elapsed since the big bang. [G99]
Atmosphere	(a) Mantle of gases round a star planet or moon, sometimes even forming the apparent surface of the body. For a body to retain an atmosphere depends on the body's gravity, and the temperature and composition of the gases. The atmosphere of the Earth is, by volume, 78% nitrogen and 21% oxygen (with 1% of other gases); mean atmospheric pressure at the surface is 10,330 kg/m2, and is also referred to as atmosphere. [A84] (b) Unit of pressure. 1 atm = 1.013 bars. [H76] (b) A unit of pressure equal to 14.07 lb in2, 101 325 pascals or 760 torr. [JM92]
Atmosphere (Solar)	The gaseous outer layers of the Sun, including, from the deeper layers outward, the photosphere, the chromosphere, and the corona. The atmosphere constitutes those layers of the Sun that can be observed directly. [H76]
Atmospheric Dispersion Corrector	An optical device usually comprising two thin prisms which can rotate to compensate for the elongation of a star image caused by the wavelength dependence of the refractive index of air. [McL97]
Atmospheric Extinction	Decrease in the intensity of light from a celestial body due to absorption and scattering by the Earth's atmosphere. The extinction increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they are at the zenith. [H76]
Atom	(a) The smallest component of matter that retains its chemical properties. An atom consists of a nucleus and at least one electron. [HH98] (b) The building block of matter. The nucleus of an atom consists of one or more protons and may contain neutrons as well; any electrons surround the nucleus. The number of protons in the atom - the atomic number - determines the element. [C95] (c) The smallest part of an element that can take part in a chemical reaction. Atoms consist of a small dense positively charged nucleus, made up of neutrons and protons, with electrons in a cloud around this nucleus. The chemical reactions of an element are determined by the number of electrons (which is equal to the number of protons in the nucleus). All atoms of a given element have the same number of protons (the proton number). A given element may have two or more isotopes, which differ in the number of neutrons in the nucleus. The electrons surrounding the nucleus are grouped into shells - i.e. main orbits around the nucleus. Within these main orbits there may be sub-shells. These correspond to atomic orbitals. An electron in an atom is specified by four quantum numbers: 1. The principal quantum number (n) can have values 1, 2, etc. The corresponding shells are denoted by letters K, L, M, etc., the K shell (n = 1) being the nearest to the nucleus. The maximum number of electrons in a given shell is 2n2. This quantum number has the largest effect on the energies of the states; high values of n correspond to weakly bound (higher energy) electrons. 2. The orbital quantum number (l), which specifies the angular momentum. For a given value of n, l can have possible values of n-1, n-2,... 2, 1, 0. For instance, the M shell (n = 3) has three sub-shells with different values of l (0, 1, and 2). Sub-shells with angular momentum, 0, 1, 2, and 3 are designated by letters s, p, d, and f. This quantum number has the second largest effect on the energies; higher values of l give moderately higher energy electrons. 3. The magnetic quantum number (m). This can have values -l, -(l - 1) ... 0 ... +(l + 1), + l. It determines the orientation of the electron orbital in a magnetic field. States with the same values of n and l but different values of m have the same energy in the absence of a magnetic field, but differ slightly when a field is applied. 4. The spin quantum number (ms), which specifies the intrinsic angular momentum of the electron. It can have values +½ and -½. Quantum states in which the spin is parallel to the orbital angular momentum are at slightly higher energy than ones in which it is antiparallel. This results, for example, in the fact that the yellow light from a sodium lamp has two very close lines in its spectrum. Each electron in the atom has four quantum numbers and, according to the Pauli exclusion principle, no two electrons can have the same set of quantum numbers. This explains the electronic structure of atoms. [DC99] (c) An atom has a nucleus surrounded by electrons bound together by the electromagnetic force. Ninety-two different atoms occur naturally, making ninety-two different chemical elements, with nuclei having one to ninety-two protons. The atoms are electrically neutral. The diameter of an atom is about 10,000 times larger than the diameter of its nucleus. [K98]
Atomic Hydrogen	Individual hydrogen atoms that do not belong to molecules. In its neutral form (HI), atomic hydrogen consists of a proton and an electron and generates radio waves that are 21 centimeters long. In its ionized form (HII), atomic hydrogen is simply a proton. HII regions look red because a few of the protons capture electrons, which can radiate red light as they settle into position around the protons. [C95]
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]
Atomic Mass Unit	(a) A unit of mass used for atoms and molecules, equal to 1/12 of the mass of an atom of carbon-12. It is equal to 1.660 33 × 10-27 kg. [DC99] (b) In the notation of physics, adopted by international agreement in 1961, one-twelfth the mean mass of an atom of 126 (including the orbital electrons). Because of the mass defect, the amu is slightly less than the mass of a hydrogen atom, so one H atom has 1.007825 amu. 1 amu = 1.66 × 10-24 g. The energy equivalent of 1 amu is 931 MeV. [H76] (c) The masses of atoms and molecules are generally given in atomic mass units. These units are based on a scale in which the mass of the carbon isotope 126C is taken to be 12. This makes one atomic mass unit equivalent to 1.6605402 × 10-27 kg. The unit is sometimes called a dalton. Atomic masses were originally given as atomic weights on a scale where the mass of the hydrogen atom was unity. In 1885 Ostwald suggested if atomic weights were expressed on a scale in which the mass of oxygen was 16, more of the elements would have integral numbers for their atomic weights. The discovery of oxygen isotopes led to the adoption of two atomic weight scales[27]. The one used by chemists was based on the figure 16 representing the average mass of the oxygen atom in its natural state (16O:18O:17O in the abundance 506:1:0.204), whereas that used by the physicists considered the oxygen isotope 16O as the basic unit on their scale. The ratio of atomic mass on the physical scale to atomic mass on the oxygen chemical scale[28] was 1.000272±0.00005. In 1960[29] the International Union of Pure and Applied Physics followed the proposal made a year earlier by the International Union of Pure and Applied Chemistry that all atomic weights should be based on the 126C scale. This enabled more isotopes to have integral mass numbers than would have been possible on the oxygen scale. This reduced the values given on the previous chemical scale by a factor of 1.000043 and changed the values of the Faraday and Avogadro's constant by a similar amount. Values on the physical scale can be converted to the new scale by multiplying by 0.999685. [JM92]
Atomic Number	(a) The number of protons in an atom's nucleus. This determines the type of element. For example, hydrogen has an atomic number of one, so all hydrogen atoms have one proton; helium has an atomic number of two, so all helium atoms have two protons; and oxygen has an atomic number of eight, so all oxygen atoms have eight protons. [C95] (b) The atomic number was introduced by J. A. R. Newlands[30] (1837-1898) in 1865 to describe the position of an element in the periodic table. The work of Rutherford and Moseley (1913) showed that the atomic number also indicated both the number of electrons in an atom and the number of positive charges in the nucleus[31]. The values of the atomic numbers of the elements so far discovered lie between one for hydrogen and 103 for lawrencium. [JM92]
Atomic Orbital	Representation of the electron cloud surrounding an atom. Named by primary quantum number and shape (i.e., 1s, 2s, 2p). [SEF01]
Atomic Second	see Second; Systeme International [S92]
Atomic Theory	The theory that matter is made up of atoms that combine to form molecules. Each chemical element has a particular type of atom, which may join with like atoms to form molecules of the element, or with atoms of other elements to form molecules of a compound. The atom consists of a dense positively charged nucleus containing protons and neutrons, surrounded by electrons. The number of protons in the nucleus determines the number and distribution of the electrons, which are held by the positive charge of the nucleus. Because the outer electrons form the chemical bonds between atoms, the chemical properties of an element depend on the electronic structure of the atom, and therefore also on the number of protons. The number of neutrons in the nucleus may vary, forming different isotopes of an element. These cannot usually be separated by chemical means. [DC99]
Atomic Time	Time based on the atomic second (see Second). Atomic time was officially adopted 1972 January 1. From 1972 January to 1974 January 1, 3 leap seconds had to be introduced to keep atomic time within 0.7 seconds of Universal Time. [H76]
Atomic Weight	The mean atomic mass of a particular element in atomic mass units. [H76]
Atoms	The fundamental units of a chemical element. An atom consists of a nucleus, which may contain protons and neutrons, and electrons, which occupy shells that surround the nucleus and are centered on it. [F88]
Attenuation	(a) The reduction of intensity of a radiation as it passes through a medium. It includes reductions due to both absorption and scattering. (b) Reduction in current, voltage, or power of an electrical signal passing through a circuit. [DC99] (c) The falling off of the energy density of radiation with distance from the source, or with passage through an absorbing or scattering medium. [H76]
Attenuation Factor	In a rocket-borne or satellite-borne telescope, the ratio of the expected counting rate to the observed counting rate. [H76]
Attitude	Position of a rocket with respect to the horizon or some other fixed reference plane. [H76]
Attractor	(a) A mechanical system may be such that its dynamical evolution causes it to approach a stable end-state. In the phase space representing the system, the representative point tends to a fixed set of points called an attractor. The attractor may be a point, a line, or a fractal. [D89] (b) The point or set of points in phase space to which a changing system moves with time. The idea of an attractor for a system comes from chaos theory. The attractor of a system may be a single point (in which case the system reaches a fixed state that is independent of time). Alternatively, it may be a closed curve, known as a limit cycle. This is the type of behavior found in oscillating systems. In some systems, the attractor is a curve that is not closed and does not repeat itself. This, known as a strange attractor, is characteristic of chaotic systems. see also Chaos Theory; Phase Space [DC99]
Atto-	A prefix meaning 10-18. [H76]
A-type	Having a stellar spectral type of A, that is, hot and white, like Sirius and Vega. [C95]
AU	Astronomical Unit
Auger Effect	(a) The ejection of an electron from an atom or ion without the emission of radiation (x-rays or gamma rays). It results from the de-excitation of an excited electron within the atom. It can be regarded as the internal conversion of the photon that would otherwise have been emitted. [DC99] (b) A radiationless quantum jump that occurs in the X-ray region. When a K-electron is removed from an atom and an L-electron drops into the vacancy in the K-shell, the energy released in the latter transition goes not into radiation, but into the liberation of one of the remaining L-electrons. [H76]
Augmentation	The amount by which the apparent semidiameter of a celestial body, as observed from the surface of the Earth, is greater than the semidiameter that would be observed from the center of the Earth. [S92]
AE Aurigae	An O9.5 V runaway star. [H76]
Aurigae	see Capella
Aurigae	An eclipsing binary with an invisible supergiant companion. The primary is an extremely luminous A8 Ia supergiant of 30 M in a post-main-sequence stage of evolution; the secondary may be a collapsed star or black hole. The period of the system is about 27 years. Probably on the order of 1 kpc distant. It has at least six components. [H76]
RW Aurigae	A dG5e T Tauri star with a strong ultraviolet excess. [H76]
Aurigae Stars	In general, binaries with a K supergiant primary and a main-sequence secondary. [H76]
Aurora	(a) Spectacular array of light in the night sky, caused by charged particles from the Sun hitting the Earth's upper atmosphere. The aurora borealis is seen in the north of the Northern hemisphere; the aurora australis in the south of the Southern. [A84] (b) Light radiated by ions in the Earth's atmosphere, mainly near the geomagnetic poles, stimulated by bombardment by energetic particles ejected from the Sun (see Solar Wind). Aurorae appear about 2 days after a Solar flare and reach their peak about 2 years after sunspot maximum. [H76]
Auto-Catalysis	the ability of certain chemicals to enhance by their presence the rate of their own production in a sequence of chemical reactions. Part of the more general class of feedback processes. [D89]
Auto-Ionization	(a) The spontaneous ionization of excited atoms, ions, or molecules, as in the Auger effect. see Auger Effect; Ionization [DC99] (b) A phenomenon occurring when a discrete double-excitation state of an atom lies in the ground-state continuum. In the autoionization process one of the excited electrons is ejected, leaving the ion in an excited state (see Dielectronic Recombination; see also Auger Effect). (also called Pre-ionization)[H76]
Autumnal Equinox	see Equinoxes [A84]
Avalanche	A process such as that in which a single ionization leads to a large number of ions. The electrons and ions produced ionize more atoms, so that the number of ions multiplies quickly. [DC99]
Average Life	see Mean Life [H76]
Avogadro Constant	(a) Symbol: NA number of particles in one mole of a substance. Its value is 6.002 52 × 1023. [DC99] (b) The Avogadro constant is the number of atoms in a gram-atom (mass in grams numerically equal to the atomic weight) or the number of molecules in a gram-molecule (mass in grams numerically equal to the molecular weight). It is named after Amadeo Avogadro (1776-1856), an Italian who in 1811 introduced the famous hypothesis. The value is 602.213 67(36) × 1021 mol-1 [JM92]
Avogadro's Law	Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. It is often called Avogadro's hypothesis. It is strictly true only for ideal gases. [DC99]
Avogadro's Number	(6.02 × 1023): The number of atoms in 12 grams of 12C; by extension, the number of atoms in a gram-atom (or the number of molecules in a mole) of any substance. [H76]
Avoidance	The fact that galaxies appear to "avoid" the Milky Way, and are most numerous in other parts of the sky. When galaxies were known as spiral nebulae and their nature was not yet understood, avoidance was thought by some researchers to indicate a connection between them and the Milky Way. Now the effect is understood to be due to dark clouds of dust and gas in our galaxy, which obscure our view of the Universe beyond in those quarters of the sky. [F88]
AXAF	Advanced X-ray Astronomical Facility
Axion	A hypothetical spin-0 particle with a very small mass of 10-5-10-3 eV. It was postulated in order to provide a natural solution to the "strong CP problem".
Axis	Theoretical straight line through a celestial body, around which it rotates. [A84]
Axisymmetric Collapse	Collapse of mass in such a way that the mass maintains the symmetry of a cylinder. [LB90]
Azeotropic Mixture	Aseotrope: A mixture of two liquids that boils without any change in composition. The proportions of components in the vapor are the same as in the liquid. Azeotropic mixtures cannot be separated by distillation. [DC99]
Azimuth	(a) Directional bearing around the horizon, measured in degrees from north (0°). [A84] (b) Angular distance from the north point eastward to the intersection of the celestial horizon with the vertical circle passing through the object and the zenith. [H76]
Azimuthal Quantum Number (k)	A measure of the minor axis of an elliptic orbital of an electron according to the Bohr-Sommerfeld theory. [H76]