E component	see L Component. [H76]
e-Folding Time	The time within which the amplitude of an oscillation increases or decreases by a factor e (e = 2.718...). [H76]
E galaxy	In both Hubble's and Morgan's classifications, an elliptical galaxy. [H76]
E layer	The part of Earth's ionosphere (about 150 km) where the temperature gradient reverses and starts to rise. It reflects "short-wave" radio waves. (also called Kennelly-Heaviside Layer) [H76]
E+A Galaxy	One of the most enigmatic types of galaxy, spectrally, are the "E+A" galaxies. They appear to be a combination of an old elliptical galaxy spectrum (E) with that of a significantly younger A-star population (A), thought to be formed in a recent (t < 1 Gyr) episode of star formation. "E+A" galaxies show strong Balmer absorption lines (EW > 5.5 Angstroms) and weak [OII] emission (EW[OII] < 2.5 Angstroms). The standard interpretation is that these are post-starburst galaxies in which star formation ceased within the last gigayear. The mechanisms responsible for the formation of "E+A" galaxies are not fully understood, neither has the environment in which these galaxies have been found been fully defined. These objects are also sometimes referred to as "K+A" galaxies.
E Line	A Fraunhofer line at 5270 Å. It is a blend of Fe I and Ca I. [H76]
e-Process	A hypothetical group of nuclear reactions by which the iron group is assumed to be synthesized. At temperatures > 5 × 109 K and densities > 3 × 106 g cm-3 there are great numbers of collisions between high-energy photons and nuclei. These collisions break up the nuclei, the fragments of which promptly combine with other particles. Thus, there is in effect an equilibrium between formation and breakup. Since the iron group has the largest binding energies, the particles over the long run will tend to be trapped in these nuclei. The e-process (the e stands for equilibrium) is presumed to occur in a supernova explosion. [H76]
Early-Type Emission Stars	see Be Stars. [H76]
Early-Type Spiral	In Hubble's classification, a spiral with a large nuclear bulge and closely coiled arms. [H76]
Early-Type Stars	Hot stars of spectral types O, B, A, and early F. [H76]
Earth	Third planet from the Sun. Mass 5.977 × 1027 g; polar radius 6356.9 km; equatorial radius 6378.17 km; mean density 5.517 g cm-3; effective temperature 287 K. Rotation period 23h56m4s.1. Mean distance from Sun 149,598,500 km (8.3 lt-min): perihelion distance (early January) 147,100,000 km; aphelion distance 152,100,000 km; vorb 29.78 km s-1; orbital period 365d.2564; e = 0.0167, i = 0; obliquity (1973) 23°26'34". Albedo 0.39 (water and land about 0.2; snow and clouds about 0.8). Surface gravity 980 cm s-2; Vesc 11.19 km s-1. Precession 50'.256 per year; relativistic advance of perihelion 4'.6 per century. Atmosphere (by volume) 78% N2; 20.9% O2; 0.9% Ar, 0.03% C02. Particle density of Earth's atmosphere at sea level 1019 per cm3 = 1.3 × 10-3 g cm-3. Atmospheric pressure at sea level 1.013 × 106 dyn cm-2. Magnetic field at surface, about 0.5 gauss; in core, about 100 gauss. Core temperature about 6400 K; core density about 10 g cm-3. Age 4.6 ± 0.1 × 109 years. First forms of life appeared about 3.2 to 3.5 × 109 years ago (Homo sapiens appeared as a species about 105 years ago). [H76]
Earthlight	Light reflected from the Earth's atmosphere onto the dark part of the Moon. [H76]
East Point	The point on the celestial horizon 90° clockwise from the north point. At the equinoxes the Sun rises in the east point. [H76]
EB-CCD	Electron-Bombarded CCD An imaging device containing a thin target material which emits electrons by the photoelectric effect when illuminated and then magnetically focuses these electrons to impact onto a silicon CCD where they generate a large charge. [McL97]
EBS	Electron Bombarded Silicon [LLM96]
Eccentric	An orbit that has a high eccentricity that is, highly elliptical. [C95]
Eccentric Anomaly	In undisturbed elliptic motion, the angle measured at the center of the ellipse from pericenter to the point on the circumscribing auxiliary circle from which a perpendicular to the major axis would intersect the orbiting body. (see Mean Anomaly; True Anomaly.) [S92]
Eccentricity	(a) In astronomy, the extent to which an elliptical orbit departs from a circular one. It is usually expressed as a decimal fraction, regarding a circle as having an eccentricity of 0. [A84] (b) A measure of how round or elliptical an orbit is. A perfect circle has an eccentricity of 0 percent, and an extremely elliptical orbit has an eccentricity of just under 100 percent. The Sun has an orbital eccentricity of 6 percent, which means that at perigalacticon the Sun is 6 percent closer to the Galactic center than its mean distance and at apogalacticon the Sun is 6 percent farther from the Galactic center than its mean distance. [C95] (c) A parameter that specifies the shape of a conic section; one of the standard elements used to describe an elliptic orbit. (see Elements, Orbital.) [S92] (d) The amount by which the orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a is the semimajor axis. [H76]
Eccentrics	In Ptolemaic cosmology, displacement of the center of a rotating celestial sphere from the center of the Universe. [F88]
Echelle	A type of diffraction grating with groove angles of 90°. With the grating at an angle of 45° the grooves resemble a staircase. [McL97]
Echelon	A type of diffraction grating consisting of a number of equal thin glass sheets stacked on a slant. In use the light is reflected from the stepped side of the stack; d in the grating equation is very large, so that very high spectral orders are possible. [DC99]
Eclipse	(a) Occultation of one celestial body by another which passes between it and the observer. The Solar eclipse is caused by the passing of the Moon between the Sun and the Earth in this way; such an eclipse may be complete (total) or incomplete (partial). Eclipsing binary stars also accord with this pattern. Alternatively - and exceptionally - a lunar eclipse is caused by the passage of the Earth between the Sun and the Moon, so that the Earth's shadow falls across the Moon, again either totally or partially, depending upon the position of the observer. [A84] (b) Obscuration of one astronomical object (such as the sun) by another such object (such as the moon). [F88] (c) The total or partial obscuration of the light from a celestial body caused by its passage into the shadow of another body (cf. occultation). [H76] (d) The obscuration of a celestial body caused by its passage through the shadow cast by another body. [S92]
Eclipse, Annular	A Solar eclipse (see Eclipse, Solar) in which the Solar disk is never completely covered but is seen as an annulus or ring at maximum eclipse. An annular eclipse occurs when the apparent disk of the Moon is smaller than that of the Sun. [S92]
Eclipse, Lunar	An eclipse in which the Moon passes through the shadow cast by the Earth. The eclipse may be total (the Moon passing completely through the Earth's umbra), partial (the Moon passing partially through the Earth's umbra at maximum eclipse), or penumbral (the Moon passing only through the Earth's penumbra). [S92]
Eclipse, Solar	An eclipse in which the Earth passes through the shadow cast by the Moon. It may be total (observer in the Moon's umbra), partial (observer in the Moon's penumbra), or annular. (see Eclipse, Annular.) [S92]
Eclipse Year	The interval of time (346.62 days) between two successive passages of the Sun through the same node of the Moon's orbit. [H76]
Eclipsing Binary	(a) A binary star of which, from the viewpoint of Earth, one of the two bodies regularly passes in front of the other. The resulting variation is perceived luminosity of some eclipsing binaries has led to their classification as variable stars. [A84] (b) A double star in which at least one of the two stars passes in front of and/or behind the other so that the system's total light periodically fades. The most famous eclipsing binary is Algol. [C95]
Ecliptic	(a) Apparent linear path through the 12 constellations of the zodiac that the Sun seems to take during one Earth year, also representing therefore the "edge" of the plane of Earth's orbit. Because the equator of the Earth is at an angle of more than 22° to the plane of its orbit, the ecliptic is at the identical angle to the celestial equator, intersecting it at two points: the vernal and autumnal equinoxes. [A84] (b) Plane of the Earth's orbit. (Strictly speaking, the ecliptic is a mathematical fiction corresponding not to the actual plane of the Earth's orbit, but to one with all minor irregularities smoothed out.) [H76] (c) The mean plane of the Earth's orbit around the Sun. [S92]
Eddington Approximation	An approximation used in the study of radiative transfer. It is the assumption that the ratio of the second moment of the radiation field to the mean intensity is everywhere equal to 1/3, the value of this ratio for an isotropic field. [H76]
Eddington-Lemaître Universe	A cosmological model in which the cosmological constant plays a crucial role by allowing an initial phase that is identical to the Einstein static Universe. After an arbitrarily long time, the Universe begins to expand. The difficulty with this model is that the initiation of galaxy formation may actually cause a collapse rather than initiate an expansion of the Universe. [Silk90]
Eddington Limit	In essence, radiation pressure must not exceed gravity. It is the limit beyond which the radiation force on matter in the emitting region is greater than the gravitational forces that hold the star together. LE = 4cGM/Ks, where Ks = Thomson and/or Compton scattering opacity. Eddington limit for a 1 M star, 1038 ergs s-1. [H76]
Eddington's Standard Model	A stellar model in which energy is transported by radiation throughout the whole star and the ratio of the radiation pressure to the gas pressure is assumed to be constant. [H76]
Eddy Currents	Induced currents set up in a conductor by a changing magnetic field. They occur in transformers and other electrical devices. The currents produce a heating effect corresponding to a loss of useful energy (eddy-current loss). Metal cores in electrical machines are usually laminated (built of thin sheets) to reduce such losses; the surface layers between the laminations have high electrical resistance. One useful application of eddy currents is in damping moving-coil instruments - the coil is wound on a piece of soft iron. When the coil moves in the instrument's magnetic field, eddy currents are induced in the iron so as to oppose the motion, causing the coil to settle quickly. As Lenz's law would predict, eddy currents always flow so as to oppose the effect producing them. [DC99]
Edge Effects	Absorption in the spectra of galaxies at the edges of some passbands by lines broadened by velocity dispersion. [H76]
Effective Radius	The distance from the center of a galaxy within which half of the total luminosity is included (cf. Holmberg radius). [H76]
Effective Temperature (Teff)	The temperature that a blackbody would have which emitted the same amount of energy per unit area as the star does: it is a temperature characteristic of the surface region. Teff of the Sun is 5800 K. [H76]
Effective Theory	Each part of the physical world can be described by a sub-theory that applies over a certain distance scale or energy scale. Such sub-theor ies are called effective theories. Explanations in a given effective theory can ignore much of the rest of the world, which has effects on the part of inter est through a few inputs or parameters. Every part of our description of the physical world is an effective theory, except the ultimate theory that is called the Primary Theory. [K2000]
Eigenfunctions	The wave functions corresponding to the eigenvalues. Eigenfunctions represent the stationary states ("standing waves") of a system. [H76]
Eigenstate, Eigenvalue	(a) The eigenvalue of a matrix M is a number which satisfies the equation M = , with 0. In quantum mechanics, the matrix M will correspond to a particular dynamical variable (such as position, energy or momentum) and will correspond to the value obtained by measuring that dynamical variable if the system is in the state described by . is called an eigenstate of the system. [CD99] (b) The specified values of E (quantized energy) for which the Schrödinger wave equation is soluble, subject to the appropriate boundary conditions. [H76]
Eightfold Way	classification scheme for elementary particles established c. 1960. Forerunner of quark model. [D89]
Eikonal Approximation	An approximation in which the oscillation of a wave front is replaced by the direction of the ray which is normal to the oscillation. [H76]
Einstein Coefficient	An emission (or absorption) coefficient. Aji is the coefficient of spontaneous emission; Bji, is the coefficient of stimulated emission, where i is the lower level and j is the upper level. [H76]
Einstein-de Sitter Cosmology	(a) A Friedmann model of the Universe in which the spacetime continuum is not curved. [C97] (b) A homogeneous world model (the simplest relativistic model) of finite density, zero curvature, and nonzero cosmological constant, subject to the field equations of general relativity in an expanding Euclidean space. The radius increases rapidly from zero, and, although it always increases, the rate of increase becomes less as time goes on. [H76] (c) The flat (k = 0), pressureless standard model of the Universe. [HH98] (d) A particular solution to Einstein's cosmological equations in which the Universe is flat. (see Flat Universe) [LB90] (e) The Friedmann-Lemaître model in which space is Euclidean was advocated by Einstein and de Sitter in 1932. [Silk90]
Einstein Effect	Displacement of spectral lines due to the gravitational redshift. [H76]
Einstein Equations	The equations of Einstein's theory of gravity, called general relativity. The Einstein equations quantitatively specify the gravity produced by matter and energy. Since gravity is believed to be the principal force acting over very large distances, the Einstein equations are used in modern theories of cosmology. [LB90]
Einstein Equivalence Principle	Foundation for curvcd space-time, it states that bodies fall with the same acceleration and that physics in freely falling reference frames is independent of the velocity and location of the frames. [D89]
Einstein's General Theory of Relativity	The theory of gravity in which the gravitational force is described mathematically by a curvature in space or space-time. [D89]
Einstein Static Universe	A cosmological model in which a static (neither expanding nor collapsing) Universe is maintained by introducing a cosmological repulsion force (in the form of the cosmological constant) to counterbalance the gravitational force. [Silk90]
Einstein Universe	A world model of a static Universe with a positive cosmological constant, whose radius of curvature is constant and independent of time. [H76]
Einsteinium	A radioactive transuranic element of the actinoid series, not found naturally on Earth. It can be produced in milligram quantities by bombarding 239Pu with neutrons to give 253Es (half-life 20.47 days). Several other short-lived isotopes have been synthesized. Symbol: Es; m.p. 860 ± 30°C; p.n. 99; most stable isotope 254Es (half-life 276 days). [DC99]
Ekman Layer	Upper boundary layer within which the amplitude changes exponentially. [H76]
Elastic Collision	A collision between two particles which conserves the total kinetic energy and momentum of the system. For atomic collisions it is one involving energy less than the excitation potential of the atom. [H76]
Elastic Scattering	Particle reactions in which the same particles emerge from the reaction as entered it (e.g. - p -> - p). In inelastic scattering, where different and/or new particles emerge, energy is used to create new particles. [CD99]
Electrodes	Also called gates or phases. Small electrically conducting plates connected to a voltage source (battery or power supply) and arranged in strip patterns to define the picture elements or pixels of a CCD. The plates create an electric field within the semiconductor which therefore forms a storage site for photo-generated charges. Also used as a generic term for any conductor with an applied voltage. [McL97]
Electrodynamics	Study of the behavior of electromagnetic force in motion. [F88]
Electromagnetic Field	Force field of the electromagnetic force, consisting of electric and magnetic lines of force at each point in space. [G99]
Electromagnetic Force	(a) Fundamental force of nature that acts on all electrically charged particles. Classical electromagnetics is based on Maxwell's and Faraday's equations, quantum electromagnetics on the theory of quantum electrodynamics (QED). [F88] (b) The force between charged particles, which accounts for electricity and magnetism. One of the four fundamental forces of nature, it is carried by photons and is responsible for all observed macroscopic forces, except for gravity. [HH98] (c) One of the four fundamental forces of nature. Electricity and magnetism arise from the electromagnetic force. The other three fundamental forces are the gravitational force, the weak nuclear force, and the strong nuclear force. (see Gravitational Force; Nuclear Forces) [LB90]
Electromagnetic Gauge Symmetry	Gauge symmetry underlying quantum electrodynamics. [G99]
Electromagnetic Unit	EMU A system of electrical units based on the electromagnetic properties of an electric current. [H76]
Electromagnetic Radiation	(a) "Waves" of electrical and magnetic "disturbance", radiated as visible light, radio waves, or any other manifestation of the electromagnetic spectrum. The distance between successive crests of each wave is known as the wavelength, and varies considerably between electromagnetic forms. The velocity of such radiation in a vacuum is the speed of light. The units of electromagnetic radiation are quanta or photons ("packets" of energy). [A84] (b) Visible light, radio waves, infrared radiation, ultraviolet radiation, x-rays, and gamma rays. In a vacuum, all electromagnetic radiation travels at the speed of light. The shorter its wavelength, the more energetic it is. [C95]
Electromagnetic Spectrum	(a) Complete range of electromagnetic radiation, from very short-wavelength (high-frequency) gamma-rays, through X-rays and ultraviolet light to the small range of visible light, and further to infrared radiation, microwave, and the comparatively long-wavelength low-frequency radio waves. [A84] (b) A wavelike disturbance in an electromagnetic field; all such waves travel at the speed of light. Visible light, X rays, microwaves, and infrared radiation are examples. [G99]
Electromagnetic Wave	A pattern of electric and magnetic fields that moves through space. Depending on the wavelength, an electromagnetic wave can be a radio wave, a microwave, an infrared wave, a wave of visible light, an ultraviolet wave, a beam of X rays, or a beam of gamma rays. (see Photon) [G97]
Electromagnetism	(a) One of the four fundamental forces of nature, governing the electric and magnetic interaction between particles. [C97] (b) The phenomena associated with electrical and magnetic forces. Electrical and magnetic forces are intimately related, since a changing electric field produces a magnetic field, and vice versa. Electromagnetic waves are an example of electromagnetism. [G97]
Electron	(a) Negatively charged fundamental particle (also called a beta particle) found in the atoms of all elements, where it "orbits" (at different energy levels and with different directions of spin) round the central nucleus. The combined charge of the orbiting electrons is balanced (in a neutral atom) by the charge of an equal number of positively charged protons in the atomic nucleus. An electron is also the fundamental unit of electricity. [A84] (b) A small, negatively charged particle that appears in every neutral atom, surrounding the positively charged nucleus like bees around honey. [C95] (c) A negatively charged spin-1/2 particle, which interacts via the electromagnetic, weak and gravitational forces. It has a mass of 0.511 MeV / c2, some 1800 times lighter than the proton. [CD99] (d) A lepton with an electric charge of -1. An electron is also a fermion because it has a spin of one half. [C97] (e) A stable, negatively charged elementary particle - the lightest massive particle known. The classical electron radius is 2.82 × 10-13 cm; me = 9.1 × 10-28 g = 5.48597 × 10-4 amu. The electron family (see Lepton) includes the electron e-, the positron e+, the electron neutrino e, and the electron antineutrino ebar. Rest-mass energy of electron 8.186 x 10-7 ergs. Electron charge = 1.60219 × 10-19 coulombs. [H76] (f) Light elementary particles with a negative electrical charge. Electrons are found in shells surrounding the nuclei of atoms; their interactions with the electrons of neighboring atoms create the chemical bonds that link atoms together as molecules. [F88]
Electron Affinity	(a) The work needed to remove an electron from a negative ion and move it to infinity. 2. A measure of the electronegativity of an atom (i.e. its tendency to acquire electrons). An element's position in the electromotive series gives an indication of its electron affinity. [DC99] (b) The energy released when an electron is added to an atom. [SEF01]
Electron Collider	Short for electron-positron collider. One import ant way to study particle interactions and search for new particles is to accelerate an electron and a positron to high energies and then collide them, using a detector to study what emerges. The energy to which they are accelerated is chosen to f it the question of interest. For example, to study CP violation in b-qua rk decays, the energy is chosen to maximize the production of b's in an appropriate way, whereas to produce new heavy particles, the energy i s made as large as possible. All uses of electron colliders require ver y large luminosity (intensity). [K2000]
Electron Conduction	A process in astrophysics occurring in highly ionized stellar interiors where the density is high, whereby the bulk of the energy is transported by "hot" electrons moving in one direction and cooler electrons in the other. In degenerate matter electron conduction, not radiation, is the main mechanism of energy transport. [H76]
Electron Gas	a system of electrons whose mutual interactions are sufficiently weak that they can be regarded as moving independently, subject only to the effects of the exclusion principle. [D89]
Electron-Hole Pairs	When a photon is absorbed in silicon its energy causes an electron in the valence band to be ejected into the conduction band leaving a (positively charged) vacancy or hole in the valence band. [McL97]
Electron-Phonon Scattering	Electron scattering by ions oscillating about equilibrium positions which form a perfect lattice. [H76]
Electron Shells	Zones in which the electrons in atoms reside. Their radius is determined by the quantum principle, their population by the exclusion principle. [F88]
Electron Temperature	The temperature that appears in the Maxwell distribution of velocities for electrons. [H76]
Electronuclear Force	Single fundamental force thought to have functioned in the very early Universe and to have combined the attributes thereafter parceled out to the electromagnetic and the strong and weak nuclear forces. (see Grand Unified Theory) [F88]
Electron Volt (eV)	(a) Unit of energy. Typically 1-10 eV is the amount of energy per atom involved in chemical reactions. 1 eV is the energy gained when an electron is accelerated by a potential of one volt. [D89] (b) Measure of energy, equal to 1.6 x 10-12erg. [F88] (c) A unit employed to indicate the energy of a charged particle in terms of the energy received by the charge on an electron (160.219 x 10-21 coulomb) due to a potential difference of one volt. It is equal to an energy of 160.219 × 10-21 joule. An approximate value (1 in 104) for the energy of electromagnetic radiation expressed in electron volts is given by 1234 / , where is the wavelength in nanometres. In recent years it has become customary to write MeV and GeV for mega (106) and giga (109) electron volts. In the USA 109 electron volts are often written as BeV, the letter B being used in this case as an abbreviation for the American billion (109), but in 1948 the International Union of Pure and Applied Physics disapproved of the use of BeV and expressed a preference for GeV or 109 eV. The electron volt was called the equivalent volt when it was originally introduced in 1912. [JM92] (d) A unit of energy or of mass. The electron weighs about 10-27 grams, which is equivalent to about 500 thousand electron volts of energy. Thus, the electron volt is tiny by ordinary standards. The energy released by dropping a penny (3 grams) to the floor is about 4 × 1017 electron volts. [LB90] (e) The energy released when a single electron passes through an ideal one-volt battery. [G97] (f) The energy acquired by an electron when accelerated through a potential difference of 1 volt. 1 electron volt = 1.6 x 10-12 ergs. [Silk90] (g) A unit of energy used in atomic and nuclear physics; the kinetic energy acquired by one electron in passing through a potential difference of 1 volt in vacuum. 1 eV = 1.60 × 10-12 ergs. An electron with an energy of 1 eV has a velocity of about 580 km s-1. The wavelength associated with 1 eV is 12,398 Å. [H76]
Electrostatic Unit (esu)	A unit of charge defined as the charge which exerts a force of 1 dyne on a charge of equal magnitude at a distance of 1 cm. [H76]
Electroweak Force	(a) The combination of the electromagnetic force and the weak nuclear force which takes place at high energy. [C97] (b) A unification of electromagnetism and the weak nuclear force. [P88] (c)The descriptions of the electromagnetic and weak forces have been unified into a single description, the electroweak force. The electromagnetic and weak forces appear to be different because the W and Z bosons that mediate the weak force are massive, whereas the photon that mediates the electromagnetic force has no mass. Consequently, it is easier to emit photons than W and Z bosons. The electroweak unified theoretical description treats all the bosons on an equal footing and explains why they appea r to be different, because of their masses. [K2000]
Electroweak Interactions	The unified description of the weak interactions and electromagnetism, developed between 1967 and 1970 by Sheldon Glashow, Steven Weinberg, and Abdus Salam. [G97]
Electroweak Theory	(a) Theory demonstrating links between the electromagnetic and the weak nuclear forces. Indicates that in the high energies that characterized the very early Universe, electromagnetism and the weak force functioned as a single, electroweak force. Also known as the Weinberg-Salam theory. [F88] (b) The theory that unifies the electromagnetic force and the weak nuclear force into a single force. This theory was developed in the 1960s by Sheldon Glashow, Steven Weinberg, and Abdus Salam and has been subsequently confirmed in the laboratory. One of the mathematical properties of this theory is called the electroweak symmetry. [LB90]
Element	Different elements are distinguished by the number of protons in their nuclei. All hydrogen atoms have one proton; all helium atoms have two protons; all oxygen atoms have eight protons. [C95]
Elements, Besselian	Quantities tabulated for the calculation of accurate predictions of an eclipse or occultation for any point on or above the surface of the Earth. [S92]
Elements, Orbital	(a) Parameters that specify the position and motion of a body in orbit. (see Osculating Elements; Mean Elements) [S92] (b) Seven quantities that must be established from observations in order to define the size, shape, and orientation of an orbit in space. The six elements needed to determine the orbit of a Solar-System body are the semimajor axis a (in AU), the eccentricity e, the inclination i of the object's orbital plane to the ecliptic, the longitude of the ascending node, the argument of the perihelion, and the epoch T (usually the time of perihelion passage). To determine the orbit of a binary star system in which the mass is not known, a seventh element, the period, must be established. [H76]
Elevation	The angle in degrees above the horizon toward the zenith or overhead point. Sometimes loosely called the "altitude" of a star, but not to be confused with height above sea level. Elevation angle is 90° minus the zenith distance (or zenith angle). [McL97]
Eleven-Dimensional Supergravity	Promising higher-dimensional supergravity theory developed in the 1970s, subsequently ignored, and more recently shown to be an important part of string theory. [G99]
Ellipse	A plane curve in which the sum of the distances of each point along its periphery from two points - its "foci" - are equal. [F88]
Elliptical Galaxy	(a) A galaxy that looks round or elliptical. One example is M87, in the constellation Virgo. [C95]
	(b) A galaxy without spiral arms and with an ellipsoidal shape. Ellipticals have little interstellar matter and no blue giants - the only giants are red, and they give ellipticals a slightly redder color than spirals. The most massive galaxies known (about 1013 M) as well as some of the least massive known, are ellipticals. No giant elliptical is near enough for any individual stars to be resolved. Ellipticals apparently produce only Type I supernovae. [H76] (c) A galaxy whose structure is smooth and amorphous, without spiral arms and ellipsoidal in shape. Ellipticals are redder than spirals of similar mass. Giant ellipticals contain over 1013 M in stars, whereas dwarf ellipticals have masses as low as 107 M. [Silk90] (d) Of the shape of a planet or galaxy, the amount of distortion by which it departs from a perfect sphere. The overall ellipticity of the Earth is given as 1/299. One class of galaxy is defined in terms of ellipticity, subdivided E0 to E7, according to degree. [A84]
Ellipticity	A quantitative measure of the shape of a galaxy. A completely spherical galaxy has zero ellipticity. A galaxy shaped like a cigar has a very high ellipticity. [LB90]
Elongation, Greatest	The instants when the geocentric angular distances of Mercury and Venus are at a maximum from the Sun. [S92]
Elongation, Planetary	(a) The geocentric angle between a planet and the Sun, measured in the plane of the planet, Earth and Sun. Planetary elongations are measured from 0° to 180°, east or west of the Sun. [S92] (b) The angle planet-Earth-Sun. Eastern elongations appear east of the Sun in the evening; western elongations, west of the Sun in the morning. An elongation of 0° is called conjunction; one of 180° is called opposition: and one of 90° is called quadrature. [H76]
Elongation, Satellite	The geocentric angle between a satellite and its primary, measured in the plane of the satellite, planet and Earth. Satellite elongations are measured from 0° east or west of the planet. [S92]
ELS	Eggen, Lynden-Bell and Sandage An influential paper published in 1962 by Olin Eggen, Donald Lynden-Bell, and Allan Sandage, who argued that the Galaxy formed from a single huge cloud of gas that rapidly collapsed. [C95]
Emersion	The reappearance of a celestial body after eclipse or occultation. [H76]
Emission	The process of transition of an electron from an outer orbit to an inner orbit around the nucleus results in a characteristic amount of energy being radiated (as line emission) that corresponds to the lost energy of the electron. [Silk90]
Emission Coefficient	Radiant flux emitted per unit volume per unit solid angle. [H76]
Emission Lines	Bright lines produced in a spectrum by a luminous source, such as a star or a bright nebula. Compare absorption lines. [F88]
Emission Measure (EM)	The product of the square of the electron density times the linear size of the emitting region (in parsecs). [H76]
Emission Nebula	An HII region whose spectrum consists of emission lines. [H76]
Emission Spectrum	A spectrum consisting of emission lines produced in the laboratory by a glowing gas under low pressure. [H76]
Emissivity	A measure of the efficiency of a source to radiate like a perfect black body; 0% is perfectly black and 0% is perfectly reflecting. [McL97]
Empiricism	An emphasis on sense data as a source of knowledge, in opposition to the rationalist belief that reasoning is superior to experience. [F88]
Enceladus	Third satellite of Saturn, about 500 km in diameter. Orbital period 1.37 days. Discovered by Herschel in 1789. [H76]
Encke's Comet	The comet with the shortest known period (3.30 years) (a = 2.21 AU, e = 0.847, i = 12°.4). It has been observed at every apparition since its discovery in 1819. Its period is gradually decreasing. Named after J. F. Encke, who computed its orbit. (It was discovered by Pons.) [H76]
Encke's Division	(a) Gap within Saturn's Ring A. [A84] (b) A region of decreased brightness in the outermost ring of Saturn. [H76]
Encounter	see Gravitational Encounter. [H76]
Endoergic Process	A process in which some of the energy of the incoming particle is transferred to the nucleus. [H76]
Endothermic Process	An adjective applied to a reaction in which a net input of energy is required for the reaction to occur. [H76]
Energy	(1) The capacity to do work. (2) Manifestation of a particular variety of force. [F88]
Energy Band	a continuous range of energies in a solid in which there are possible states for the electrons. Energy bands are separated from one another by energy gaps. [D89]
Energy Curve	A plot of the intensity of the continuous spectrum versus the wavelength. [H76]
Energy Density	The amount of energy in the form of radiation per unit volume, expressed in ergs cm-3. The energy density of blackbody radiation at temperature T is aT4, where the radiation constant a = 7.56 × 10-15 erg cm-3 (K)-4. [Silk90]
Energy Distribution	The amount of energy radiated at each range of wavelengths. [H76]
Energy Gap	A range of energies in a solid for which there are no quantum states of the electrons. [D89]
Energy Level	(a) Any of the several discrete states of energy in which an atom or ion can exist. For example, an orbital electron can exist only in those energy levels that correspond to an integral number of deBroglie wavelengths in a Bohr atom. [H76] (b) The allowed energy states for electrons orbiting the nucleus of an atom. [McL97]
Energy Spectrum	(In cosmic-ray studies, a plot of number of particles versus energy. [H76]
Ensemble Average	An average over an ensemble of all possible systems. [H76]
Ensemble (of universes)	A hypothetical group of many universes of varying properties. Some physicists attempt to estimate how "probable" are the properties of our Universe by imagining it as a sample from an ensemble of universes. [LB90]
Entanglement	the impossibility of expressing certain quantum mechanical states of a system with two or more parts as the conjunction of definite quantum states of the separate parts. [D89]
Enthalphy (H)	The heat content of a body. H = U + pV, where U is the internal energy, p is the pressure, and V is the volume. [H76]
Entrance Pupil	The real object or image which defines the limit of valid light paths through an optical system. [McL97]
Entropy	(a) A thermodynamic property of a macroscopic body which corresponds intuitively to the degree of disorder. [D89] (b) A measure of the amount of unavailable heat in a system; or a measure of the amount of disorder in a system. [H76] (c) A quantitative measure of the degree of disorder of a physical system. Highly disordered systems have a large entropy; highly ordered systems have low entropy. One of the laws of physics, the second law of thermodynamics, says that the entropy of any isolated physical system can only increase in time. [LB90] (d) A thermodynamic measure of the degree of order in a system. Also used as a measure of information content. [McL97] (e) A measure of the amount of disorder in a system. [Silk90]
Eötvös Experiment	(a) An experiment performed in 1909 by the Hungarian physicist Eötvös to establish that the gravitational acceleration of a body does not depend on its composition - i.e., that inertial mass and gravitational mass are exactly equal. [H76] (b) An experiment carried out by Baron Eötvös in the nineteenth century confirming that different objects experience identical accelerations in a gravitational field. (In the absence of air resistance, a cannon ball and a feather, released from the same height, hit the floor at the same time.) [LB90]
Ep Galaxy	In Morgan's classification, an elliptical galaxy with dust absorption. [H76]
Epact	The age of the Moon; the number of days since New Moon, diminished by one day, on January 1 in the Gregorian ecclesiastical lunar cycle. (see Gregorian Calendar; Lunar Phases) [S92]
Ephemeris	(a) A list or tabulation of astronomical phenomena that change with time. [McL97] (b) A tabulation of the positions of a celestial object in an orderly sequence for a number of dates. [S92] (b) A list of computed positions occupied by a celestial body over successive intervals of time. (plural is Ephemerides)[H76]
Ephemeris Hour Angle	An hour angle referred to the ephemeris meridian. [S92]
Ephemeris Longitude	Longitude (see Longitude, Terrestrial) measured eastward from the ephemeris meridian. [S92]
Ephemeris Meridian	A fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by Terrestrial Dynamical Time (TDT). The ephemeris meridian is 1.002738T east of the Greenwich meridian, where T = TDT-UT1. [S92]
Ephemeris Second	The length of a tropical second (1/31,556,925.97474 of the tropical year) on 1900 January 0.5 ephemeris time. [H76]
Ephemeris Time	(a) Time based on the ephemeris second. Ephemeris time is determined primarily from observations of the Moon against the background of stars, whereas Universal Time is determined from observations of the stars and depends on the Earth's current rate of rotation. [H76] (b) The timescale used prior to 1984 as the independent variable in gravitational theories of the Solar System. In 1984, ET was replaced by dynamical time. [S92]
Ephemeris Transit	The passage of a celestial body or point across the ephemeris meridian. [S92]
Epicycle	(a) Circular orbit of a body round a point that is itself in a circular orbit round a parent body. Such a system was formulated to explain some planetary orbits in the Solar System before they were known to be elliptical. [A84] (b) In Ptolemaic cosmology a circular orbit around a point that itself orbits another point. [F88]
Epicycle Theory	A means of accounting for the apparent motions of the planets in terms of circular motions in a geocentric cosmology. Each planet moves in a circle, the center of which moves in a circle of larger radius, and so on, the largest circles being centered on the earth. [Silk90]
Epitaxial	A thin layer of differently doped semiconductor used in the construction of solid-state devices such as the CCD. [McL97]
Epoch	(a) A point of time selected as a fixed reference. [H76] (b) An arbitrary fixed instant of time or date used as a chronological reference datum for calendars (see Calendar), celestial reference systems, star catalogs, or orbital motions (see Orbit). [S92]
EPR	Abbreviation of A. Einstein, B. Podolsky and N. Rosen, who presented an argument in 1935 that the quantum mechanical description of certain composite physical systems cannot be complete. [D89]
EPROM	Erasable Programmable Read Only Memory A small silicon chip containing thousands of individual locations which can be set to either a low or a high voltage level; a 0 or a 1. The settings can be erased by exposure to ultraviolet light. [McL97]
Epsilon Eridani	A young orange dwarf star in the constellation Eridanus that is visible to the naked eye and lies just 10.7 light-years away from the Sun. [C95]
Epsilon Indi	An old orange dwarf star in the southern constellation Indus that lies 11.2 light-years away from the Sun. [C95]
Equation of Center	In elliptic motion the true anomaly minus the mean anomaly. It is the difference between the actual angular position in the elliptic orbit and the position the body would have if its angular motion were uniform. [S92]
Equation Equinoxes	The Right Ascension of the mean equinox (see Mean Equator; Equinox) referred to the true equator and equinox; apparent sidereal time minus mean sidereal time. (see Apparent Place; Mean Place) [S92]
Equation of State	(a) A relation between the pressure, temperature, and density of a fluid. [H76] (b) An equation that describes a relationship between the energy, pressure, and density of a quantity of mass. The equation of state of a substance depends on the material it is made of and on the forces operating within it. [LB90]
Equation of Time	(a) The difference between Apparent and Mean Solar Time. At Greenwich, apparent Solar noon varies between 11h44m05s and 12h14m19s. Maximum contribution from Earth's orbital eccentricity, ~ 8 min; from Earth's obliquity, ~ 10 min. Apparent and Mean Solar Time agree 4 times a year. [H76] (b) The hour angle of the true Sun minus the hour angle of the fictitious mean sun; alternatively, Apparent Solar Time minus Mean Solar Time. [S92]
Equator	The great circle on the surface of a body formed by the intersection of the surface with the plane passing through the center of the body perpendicular to the axis of rotation. (see Celestial Equator.) [S92]
Equatorial Mount	The classic type of telescope mount with one axis parallel to the Earth's polar axis (i.e. pointing at the celestial pole) and the other at right angles. Once the object is located. only the polar axis need be driven by a motor to counteract the Earth's rotation. [McL97]
Equilibrium	A condition of balance between the forces operating on or within a physical system, so that no accelerated motions exist among the parts of a system. For stable equilibrium, a small disturbance will eventually damp out. If a small disturbance continues to grow, the system is said to be in unstable equilibrium. [H76]
Equilibrium Position	The position of an oscillating body at which no net force acts on it. [H76]
Equinox	(a) One of two points in the sky that represent where the Sun appears to cross the plane of the Earth's equator. From the Earth's viewpoint therefore, the Sun reaches one point at a quarter, the other at three quarters of the way through the sidereal year: the vernal (spring) equinox is thus on or around 21 March, the autumnal on or around 22 September. The actual points in the sky change slightly every year through a process called precession. [A84] (b) Either of the two points on the celestial sphere where the celestial equator intersects the ecliptic. [H76] (c) Either of the two points on the celestial sphere at which the ecliptic intersects the celestial equator; also the time at which the Sun passes through either of these intersection points; i.e., when the apparent longitude (see Apparent Place; Longitude, Celestial) of the Sun is 0° or 180°. (see Catalog Equinox; Dynamical Equinox for precise usage.) [S92]
Equipartition of Energy	(a) If all stars have the same kinetic energy, equipartition of energy prevails. Because kinetic energy depends on both a star's mass and its velocity, high-mass stars must move more slowly than low-mass stars, if equipartition of energy prevails. [C95] (b) Equal sharing of the total energy among all components of a system. The principle originally enunciated by Boltzmann states that the mean kinetic energy of the molecules of a gas is equally divided among the various degrees of freedom possessed by the molecules. The average molecular energy associated with any degree of freedom is one-half the product of the absolute temperature and Boltzmann's constant. [H76]
Equivalence Principle	(a) The principle that it is impossible to distinguish between gravitational and inertial forces; gravitational mass is precisely equal to inertial mass. [H76] (b) Tested by Galileo in the famous experiments at Pisa, this principle asserts the identity of the inertial mass of an object with its ability to produce, or react to, a gravitational field. In Newtonian physics this has to be inserted `by hand' whereas in general relativity it emerges from the formalism automatically. [D89] (c) The statement that a gravitational force is completely equivalent in all of its physical effects to an overall acceleration in the opposite direction. For example, a person in an elevator in space accelerating upward at 32 feet per second per second would feel the floor pushing upward against her feet in exactly the same way as if the elevator were at rest on earth, where gravity pulls downward with an acceleration of 32 feet per second per second. The "weak equivalence principle," which is not as strong as the equivalence principle, states that all objects, independent of their mass or composition, fall with the same acceleration in the presence of gravity. The Eötvös experiment, and later refinements of this experiment, have proven the weak equivalence principle. [LB90]
Equivalent Width	A measure of the total amount of energy subtracted from the continuous spectrum by an absorption line on a graph of relative intensity versus wavelength. Since the shapes of line profiles vary - e.g., one may be broad and shallow whereas another is narrow and deep - measurement is facilitated by transforming each profile into a rectangle whose base corresponds to zero intensity and whose area is the same as that of the true absorption line. [H76]
Era	A system of chronological notation reckoned from a given date. [S92]
Erbium
ERG	Extremely Red Galaxy
erg	(a) The cgs unit of energy; the work done by a force of 1 dyne acting over a distance of 1 cm. 1 erg = 10-7joules = 1 g cm2 s-2. (sometimes called dyne cm) [H76] (b) Twice the amount of energy necessary to accelerate a one gram mass to a speed of one centimeter per second. [G97] (c) The CGS unit of work, i.e. the work done by a force of one dyne acting over a distance of one centimetre. The unit was proposed by the British Association in 1873 and the name was suggested by Professor Everett from Queen's College, Belfast. It is derived from the Greek verb to work. [JM92]
Ergodic Motion	Motion by one or more particles which fills phase space with uniform density after a sufficiently long time. [H76]
Ergoregion	That part of space in which no physical object can remain at rest with respect to an observer at infinity; the dragging of inertial frames is so extreme that all timelike world lines rotate with the star. Technically, it is the region in which the asymptotically timelike Killing vector becomes spacelike. [H76]
Ergosphere	The region surrounding the event horizon (but inside the stationary limit) of a rotating Kerr black hole (see Ergoregion). [H76]
Eri	see Achernar. [H76]
AS Eridani	An eclipsing binary whose secondary is close to its Roche limit. [H76]
Eri	A fourth-magnitude K2 V star 3.30 pc distant. In 1973 van de Kamp announced that it has a planet-like object in orbit around it at a distance of about 8 AU and with a period of about 25 years. [H76]
ERO	Extremely Red Object
40 Eridani	A nearby triple system, 5 pc distant. Component A is K0 V; component B is a DA white dwarf; component C is M5e V. [H76]
Eros	A small asteroid, No. 433 (axes 35 × 16 × 17 km) whose closest approach to Earth is less than 0.15 AU. Rotation period 5h16m12s.913, orbital period 642 days, a = 1.48 AU, e = 0.223, i = 10°.8; perihelion distance 1.084 AU. Discovered by G. Witt in 1898. [H76]
Eruptive Galaxy	see Violent Galaxy. [H76]
Eruptive Variable	see Cataclysmic Variable. [H76]
ESA	European Space Agency [LLM96]
Escape Velocity	(a) Speed an object must attain in order to free itself from returning to the parent body under the effects of gravity. [A84] (b) The speed at which an object can leave another object behind, without being recalled by its gravitational force. The escape velocity of Earth - which must, for instance, be attained by a spacecraft if it is to reach another planet - is 25,000 miles per hour. [F88] (c) The velocity that a body requires to achieve a parabolic orbit around its primary [Ve = sqrt(2GM/R)]. Escape velocity at Earth's surface is 11.2 km s-1; of Moon, 2.4 km s-1; of Sun, 617.7 km s-1 (cf. orbital velocity). [H76]
ESO	European Southern Observatory [LLM96]
ESRF	European Synchrotron Research Facility [LLM96]
ET	Ephemeris Time [LLM96]
Eta Aquilae	A pulsating star in the constellation Aquila. It was the first Cepheid variable star discovered, in 1784. [C95]
Etalon	Essentially an optical filter that operates by multiple-beam interference of light reflected and transmitted by a pair of parallel flat reflecting plates. [McL97]
Ethernet	A system for linking computers with a single serial cable. [McL97]
Euclidean Geometry	The geometry developed by the Greek Euclid about 300 BC. Euclidean geometry, like all geometries, deduces certain results from a set of starting assumptions. One of the critical assumptions of Euclidean geometry is that given any straight line and a point not on that line, there is exactly one line that can be drawn through that point parallel to the first line. One of the results of Euclidean geometry is that the interior angles of any triangle sum to 180 degrees. Euclidean geometry is the geometry we learn in high school. [LB90]
Euler Number (Eu)	A number used in fluid dynamics defined by p / v2, where p is pressure, density and v velocity. It is named after the German mathematician L. Euler (1707-1783). [JM92]
Europa (J II)	One of the Galilean satellites of Jupiter 3600 km in diameter. Period 3.55 days, e = 0.00, i = 0°.01, mean density 3.07 g cm-3. [H76]
Europium
EUV	Extreme UltraViolet [LLM96]
Evection	The small irregularity in the Moon's orbital motion due to Solar and planetary perturbations. [H76]
Even-Even Nuclei	see 4N Nuclei. [H76]
Even-Odd Nuclei	Nuclei that contain even numbers of protons but odd numbers of neutrons. [H76]
Event	(a) A happenstance in the spacetime continuum referenced by three spatial coordinates and a complementary temporal ordinate. [C97] (b) A "point" in four-dimensional spacetime. [H76]
Event Horizon	(a) The "edge" of a black hole; the interface between four-dimensional space and a singularity. [A84] (b) The surface surrounding a black hole with the property that any light ray emitted inside it cannot escape because of the strength of the gravitational field. [D89] (c) The surface of a black hole: the boundary of a region in space from which no matter can escape and hence from which no signals can be received by an external observer (cf. stationary limit). Bodies inside event horizons disappear not temporarily but for all time. In the Schwarzschild geometry, the event horizon and the stationary limit coincide. [H76] (d) The one-way surface of a black hole; once penetrated, the laws of gravity ensure that there is no turning back, no escaping the powerful gravitational grip of the black hole. [G99]
Eventuality	a contingency concerning a system which is either true or false if it is definite, but which (in view of a fundamental conceptual innovation of quantum mechanics) may he indefinite. A near-synonym for this term is `proposition'. [D89]
Evershed Effect	The radial motion outward (from the central umbra) of the gases in the penumbral regions of sunspots. [H76]
Evolution	(a) In Biology the theory that coniplex and multifarious living things developed from generally simpler and less various organisms. (b) In astronomy, the theory that more complex and varied atoms develop from simpler ones, as through the synthesis of heavy atomic nuclei in stars. [F88]
Exchange Correlation	The correlation of particles and spins which is embodied in a Slater-determinant wave function. [H76]
Exchange Interaction	the spin-dependent part of the interaction between particles with spin. [D89]
Excitation Potential	Amount of energy required to bring an electron from its ground state to a given excited state (measured in electron volts). [H76]
Exclusion Principle	(a) Pauli's exclusion principle says that there could not be more than one electron in each quantum state. [D89] (b) The rule that no two fermion can occupy the same quantum state. [F88] (c) No two electrons in an atom can have the same set of values for the four quantum numbers n, l, mb, ms. The exclusion principle applies only to fermions, not to bosons. [H76]
Existence Theorems	these are the theorems that assert the existence of mathematical objects satisfying a specific set of axioms. In the case of differential equations describing the time evolution of a physical system, they guarantee the existence of solutions to the equations that become unique if enough initial conditions are specified. [D89]
Exoergic Process	A process in which energy is liberated. [H76]
Expanding Arm	A spiral arm of neutral hydrogen lying between 2.5 and 4 kpc beyond the Galactic center and receding from it at about 135 km s-1. [H76]
Expansion of Universe	Constant increase, with time, in the distance separating distant galaxies from one another. Expansion does not take place within individual galaxies or clusters of galaxies, which are bound together gravitationally, but evidences itself on the supercluster level. [F88]
Exploding Galaxy	see Violent Galaxy. [H76]
Explorer	A US series of satellites, many of which remain in orbit round the Earth fulfilling scientific functions. Explorer 1 was in fact the first US orbital satellite (launched on 31 January 1958) and was instrumental in discovering the inner Van Allen belt. [A84]
Explosive Galaxy Formation	A theory of galaxy formation wherein the explosion of a large number of stars creates a giant shock wave that travels outward and compresses the surrounding gas. Galaxies form in the regions of high-density gas. [LB90]
Explosive Nucleosynthesis	(a) The nucleosynthetic processes which are thought to occur in supernovae. These explosive processes are thought to produce the nuclei from neon up to and including the e-process nuclei and possibly the r-process nuclei. Explosive carbon burning occurs for a temperature of about 2 × 109 K and a density of 104-107 g cm-3 and produces nuclei from neon to silicon. Explosive oxygen burning occurs for a temperature of about 4 × 109 K and produces nuclei from silicon to calcium, and the e-process occurs at a temperature greater than 5 × 109 K and produces the iron peak nuclei. [H76] (b) The nucleosynthesis processes that are believed to occur in supernovae. Explosive carbon burning occurs at a temperature of about 2 × 109 K and produces the nuclei from neon to silicon. Explosive oxygen burning occurs near 4 x 109 K and produces nuclei between silicon and calcium in atomic weight. At higher temperatures, still heavier nuclei, up to and beyond iron, are produced. [Silk90]
Explosive Variables	see Cataclysmic Variables. [H76]
Exponential Expansion	(a) Extremely rapid expansion. "Exponential" is a mathematical term that precisely defines the rate of expansion. For example, a balloon that doubles its size every second is expanding exponentially. By contrast, a balloon whose radius is one inch after one second, two inches after two seconds, three inches after three seconds, and so on, is expanding linearly with time, rather than exponentially. According to the inflationary universe model, the early Universe went through a brief period of exponential expansion, during which its size increased enormously. [LB90] (b) An expansion described by a fixed doubling time. The size doubles after one doubling time, quadruples after two doubling times, octuples after three doubling times, etc. [G97]
Extended Dimension	A space (and spacetime) dimension that is large and directly apparent; a dimension with which we are ordinarily familiar, as opposed to a curled-up dimension. [G99]
Extended Inflationary Universe	A version of the inflationary Universe theory proposed in 1989 by Paul Steinhardt and Daile La. Its key new feature was the suggestion that a new field interacts directly with the gravitational field, causing the strength of gravity to change with time. This causes the expansion of the Universe to slow down, allowing the bubbles forming at the end of inflation to catch up with the expansion and smoothly fill the Universe. [G97]
Extended Source	In radio astronomy, formerly a source whose angular extent could be measured, as distinguished from a point source. Now, one which has a large angular extent and is strongest at longer wavelengths (distinguished from a compact source). Most extended sources tend to be polarized. [H76]
Extinction	(a) Attenuation of starlight due to absorption and scattering by Earth's atmosphere, or by interstellar dust. The longer the path through the dust, and the denser the dust, the more the starlight is reddened. The normal relation is AV = 0.8 mag per kpc. The total visual extinction toward the galactic center is on the order of 25 magnitudes. [H76] (b) The combined effects of absorption and scattering of light, usually by interstellar dust, which dims our view of a distant object. [McL97]
Extragalactic Astronomy	The field that deals with objects beyond the Milky Way, especially galaxies and quasars. [C95]
Extragalactic Distance Scale	The set of distances to astronomical objects outside our galaxy. It is difficult to obtain distances to objects further than about 10 million light years with accuracies better than about 25%. [LB90]
Extremal Black Holes	Black holes endowed with the maximal amount of force charge possible for a given total mass. [G99]
Extrinsic	A semiconductor, such as silicon, which has been doped with impurity atoms to provide smaller energy band gaps for detection of lower-energy photons. [McL97]