Iapetus	The ninth satellite of Saturn, about 850±100 km in radius; period 79d7h55m, e = 0.028, inclination to Saturn's orbital plane 14°.7. It has the most extreme variation in albedo of any satellite in the solar system (0.04 for the leading side, 0.28 for the trailing side). Discovered by Cassini in 1671. [H76]
IAU	International Astronomical Union. [LLM96]
IBC	Impurity Band Conduction. [LLM96]
IC 4182	A nearby galaxy in which a type Ia supernova exploded in 1937. [C95]
Icarus	Asteroid No. 1566, 1.1 km in diameter, discovered by Baade in 1948. It has the smallest orbit and highest eccentricity (a = 1.07 AU, e = 0.827, i = 23°, P = 408d) of any known minor planet. It is the only asteroid known to come closer to the Sun than Mercury (perihelion distance 0.19 AU). Rotation period 2h16m. [H76]
Ideal Gas	(a) A nondegenerate gas in which the individual molecules are assumed to occupy mathematical points and to have zero volume, and in which the mutual attraction of neighboring molecules is zero. (also called Perfect Gas) [H76] (b) The pressure of a gas is directly proportional to the product of its temperature and density (p = CT). The higher the temperature and the more rarefied a gas, the more closely it obeys the ideal gas laws, so the gases in most stars closely approximate ideal gases. For a degenerate gas, the pressure depends only on the density and is independent of the temperature. [H76]
IDL	Interactive Data Language. [LLM96]
IF	Intermediate Frequency. The beat frequency between the signal and the local oscillator in a radio detection system. [McL97]
Illumination	Symbol: E A measure of the visible-radiation energy reaching a surface in unit time. Once called `intensity of illumination', it is measured in lux (lx). One lux is an illumination of one lumen per square meter. [DC99]
ILR	Intermediate Line Region
Image Dissector Scanner	A specialized television camera used as a light detector (instead of a photographic plate) in the 19705. [LB90]
Image Intensifier	An electronic device for increasing the brightness of a faint optical image. The image is first formed on a thin metallic surface called a photocathode from which electrons are then ejected. The stream of electrons is accelerated and focussed onto a phosphorescent screen which glows brightly as a result of the impact. [McL97]
Image Spectrometers	Refers to a class of instruments which preserve the image field while also determining the spectrum. Integral Field Unit (IFU). Usually implies some kind of image slicing either with facets or fiber optics. [McL97]
Image Tube	An electronic camera in which electrons, emitted from a photocathode surface exposed to light, are focused electronically onto a phosphor or photographic plate. (also called image intensifier) [H76]
Immersion	The disappearance of a celestial body due to eclipse or occultation. [H76]
Impact Parameter	A measure of the distance by which a collision misses being head-on. In astronomy, usually the distance, at closest approach, between the centers of two particles in a collision if there were no attractive force acting between them. [H76]
Imperfect Scattering	See nonconservative scattering. [H76]
Implicate Order	A term coined by the physicist David Bohm to describe the sort of enfolded order that is characteristic of quantum theory. It is to be contrasted with the explicate orders of Newtonian physics. Bohm believes that this implicate order has a universal importance and will be useful in understanding the nature of consciousness. [P88]
Impurity and Conduction	IBC A form of infrared array detector which replaces the photoconductor and provides higher performance. [McL97]
Inclination	(a) In astronomy, the angle between one plane and another. The (equatorial) inclination of a planet is the angle between the plane of its equator and that of its orbit. The inclination of the orbit of a planet in the Solar System other than Earth is the angle between the plane of that orbit and the ecliptic. [A84] (b) The angle between two planes or their poles; usually the angle between an orbital plane and a reference plane; one of the standard orbital elements (see elements, orbital) that specifies the orientation of an orbit. [S92]
Indefiniteness	the suspension of an eventuality between truth and falsity, or of a physical variable among its possible definite values, which occurs, according to quantum mechanics, in certain states of a system. This suspension is not a matter of ignorance on the part of the observer but is rather an objective fact. [D89]
Indeterminacy Principle	Quantum precept indicating that the position and trajectory of a particle cannot both be known with perfect exactitude. Indeterminacy thus indicates the existence of a basic quantum of knowledge of the particle world. And, since information about one quantity can be extracted at the expense of another, it demonstrates that the answer events result to some extent from the questions we choose to ask about them. [F88]
Index of Refraction (n)	The ratio of the speed of light in a vacuum to that in a given medium. [H76]
Indium	A soft silvery metallic element. It is found in minute quantities, primarily in zinc ores, and is used in alloys, in several electronic devices, and in electroplating. Symbol: In; m.p. 155.17°C; b.p. 2080°C; r.d. 7.31 (25°C); p.n. 49; r.a.m. 114.818. [DC99]
Induced Emission	See stimulated emission. [H76]
Induction	System of reasoning in which the conclusion, though implied by the premises and consistent with them, does not necessarily follow from them. [F88]
Inelastic Collision	A reaction involving a change in the kinetic energy of the system, as in ionization, excitation, or capture; or a process which changes the energy level of the system. [H76]
Inertia	Property of a moving body to continue moving at the same speed in the same direction - or of a static body to remain static - unless and until acted upon by some force for change. The inertial mass of a body is reckoned as equal to the body's gravitational mass. [A84]
Inertial Frame	a frame of reference in which force-free bodies move along straight lines; postulates of special relativity are said to be valid in an inertial frame. [D89]
Inertial Frame of Reference	Any "standard of rest" or coordinate frame for which Newton's first law is valid. [H76]
Inertial Mass	The mass of an object as measured by the property of inertia. It is equal to the ratio force/acceleration when the object is accelerated by a constant force. In a uniform gravitational field, it is equal to gravitational mass, since all objects have the same gravitational acceleration at the same place. [DC99]
Infinities	Typical nonsensical answer emerging from calculations that involve general relativity and quantum mechanics in a point-particle framework. [G99]
Inflation	(a) The idea that, when it was a fraction of a second old, the universe expanded dramatically. If inflation is correct, then the mass density of the universe () should be 1.0, if there is no cosmological constant; if there is a cosmological constant and inflation is correct, the sum of and the cosmological constant () should be 1.0. [C95] (b) A theory which postulates that, at 10-35 seconds after the Big Bang, the spacetime continuum underwent an intense period of exponential expansion, in response to the separation of the strong nuclear force from the electroweak force. This idea solves the flatness and horizon problems. [C97] (c) The phenomenon by which the universe is driven into exponential expansion by the repulsive gravitational field created by a false vacuum. The inflation would end with the decay of the false vacuum. Although the inflation would occur in far less than a second, it could account for the "bang" of the big bang theory, it could explain the origin of essentially all the matter in the observed universe, and it can solve the horizon problem and the flatness problem. It could also generate the density perturbations that would later become the seeds for galaxy formation. [G97]
Inflaton	The name given to whatever fields are responsible for driving inflation. [G97]
Inflationary Cosmology	Modification to the earliest moments of the standard big bang cosmology in which universe undergoes a brief burst of enormous expansion. [G99]
Inflationary Universe	(a) A cosmological model in which the Universe underwent an epoch of extraordinarily rapid expansion within the first 10-30 s or so after the big bang; in a typical version, the diameter of the Universe increased by a factor at least 1025 times larger (or perhaps much larger still) than had been previously thought; the model was proposed by A.H. Guth in 1981, but the original formulation contained a crucial flaw which was remedied by the development of the new inflationary universe model in 1982 by A.D. Linde, and by A. Albrecht and P. Steinhardt (see new inflationary universe). [D89] (b) A recent modification of the standard big bang model in which the infant universe went through a brief period of extremely rapid (exponential) expansion, after which it settled back into the more leisurely rate of expansion of the standard model. The period of rapid expansion began and ended when the universe was still much less than a second old, yet it provides a physical explanation for the flatness and horizon puzzles. The inflationary universe model also suggests that the universe is vastly larger than the portion of it that is visible to us. (See exponential expansion.) [LB90]
Information	a measure of the delocalization of the state of the system in the space of all possible events. [D89]
Infrared	(a) That part of the electromagnetic spectrum that lies beyond the red, having wavelengths from about 7500 Å to a few millimeters (about 1011-1014 Hz). Infrared radiation is caused by atomic transitions, or by vibrational (near-IR) and rotational (far-IR) transitions in molecules. (IRe1, IRc1, IRs1: the e sources are extended; the c sources are unresolved; the s indicates an infrared nebula surrounding a visible star.) [H76] (b) The region of the electromagnetic spectrum from a wavelength of about 1 µm (10-6 m) to about 200 µm. The region from 1 to 5 µm is the near infrared: 5-30 is the mid infrared and 30-200 µm is the far infrared. [McL97]
Infrared Astronomy	astronomy carried out at wavelengths between about 1 µm and 300 µm. Ground-based observations are possible in a number of astronomical `windows' in the 1-5 µm, 8-13 µm, 18-22 µm and 30*m wavebands from high, dry observing sites. Observations in the wavelength ranges 5-8 µm, 13-l8µm and 30-300µm can only be carried out from above the Earth's atmosphere because of atmospheric absorption. The wavelength region 100 µm to 1 mm is often referred to as the submillimeter waveband. [D89]
Infrared Photometry	The measurement of light intensities using infrared light instead of optical (visible to the human eye) light. Infrared light has longer wavelengths than optical light. (See photometry.) [LB90]
Infrared Slavery	Inability of quarks to escape the bonds of the strong force that confines them to the company of other quarks. [F88]
Inhomogeneous Early Universe	The idea that during the first few minutes after the big bang, the universe had regions of different density. An inhomogeneous early universe can produce elements different from those of the standard homogeneous early universe. [C95]
Initial Condition	(a) In physics, the state of a system at the time at which a given interaction begins - e.g., the approach of two electrons that are about to undergo an electromagnetic interaction. (b) In cosmology, a quantity inserted as a given in cosmogonic equations describing the early universe. [F88] (c) Data describing the beginning state of a physical system. [G99]
Initial Mass Function	IMF A quantity that determines the number of stars per unit time evolving from the main sequence. [H76]
Inner Bremsstrahlung	The continuous electromagnetic radiation that accompanies the -decay of nuclei. [H76]
Inner Lagrangian Point (L1)	The Lagrangian point (q.v.) through which mass transfer occurs. [H76]
InSb	Indium Antimonide A compound semiconductor used as an infrared photoelectric detector. [McL97]
Insolation	Amount of radiation received from the Sun per unit area on the Earth's surface per unit time. (The word is a contraction of "incoming solar radiation"). [H76]
Instability Strip	A region in the Hertzsprung gap (q.v.) occupied by pulsating stars in a post-main-sequence stage of stellar evolution. Stars traverse the instability strip relatively quickly at least once on their way to their final evolutionary configuration. [H76]
Integrated Circuit	A small electronic component made of semiconductor silicon on which an entire electronic circuit of numerous microscopic transistor amplifiers, diodes and resistors has been constructed. [McL97]
Integrating Detector	Any imaging device, like a photographic emulsion or CCD, which can build up more signal and contrast by a longer exposure to light or other electromagnetic energy. [McL97]
Integration Time	The interval of time used to collect photons of light on a detector and build-up a strong signal. [McL97]
Intelligence	Defined in SETI as the ability and willingness to transmit electromagnetic signals across interstellar space. [F88]
Intensity	A measure of the rate of energy transfer by radiation. The unit of intensity is the watt per square meter (W m-2). (a) The intensity of visible radiation is related to brightness. In photometry special units are used because it is necessary to consider the sensitivity of the eye to different visible radiations. Thus the intensity of a light source - sometimes called luminous intensity - is measured with the candela. The intensity of illumination of a surface is measured in lux. The intensity of visible radiation itself is measured in lumens. (b) The intensity of sound relates to the sensation of loudness though this depends also on frequency. It is inversely proportional to the square of the distance from the source. The intensity level of a sound is the intensity relative to an agreed standard. See decibel. [DC99] (c) A measure of the amount of light received per unit solid angle per unit time per unit area normally from an element of surface. (sometimes called Specific Intensity) [H76]
Intensity Interferometry	The use of two telescopes linked by computer to study the intensity of light received from a star. Analysis of the combined results has enabled measurement of the diameters of stars as apparently small as 2 × 10-4 seconds of arc. [A84]
Interaction	A mutual effect between two or more systems or bodies, so that the overall result is not simply the sum of the separate effects. There are four separate interactions distinguished in physics: 1. Gravitational Interaction The weakest of the four, about 1040 times weaker than the electromagnetic interaction. It is an interaction between bodies or particles on account of their mass, and operates over long distances. 2. Electromagnetic interaction The interaction between charged bodies or particles (stationary or moving). It falls off with the square of distance and operates over all distances. 3. Strong interaction An interaction between hadrons, about 100 times greater than the electromagnetic interaction. It operates at very short range (up to around 10-15 m) and is the force responsible for holding nucleons together in the atomic nucleus. 4. Weak interaction An interaction about 1010 times weaker than the electromagnetic interaction. It occurs between leptons, and is the interaction in beta decay. So far it has not proved possible to formulate a unified field theory for all four types of interaction, although there has been some success in unifying the electromagnetic and weak interactions. [DC99] (a) Event involving an exchange between two or more particles. Since the fundamental forces are portrayed by quantum theory as involving the exchange of force-carrying particles (the bosons), the forces are more correctly described as interactions. [F88] (b) There are four known kinds of interactions between particles. In the order of decreasing strength they are the strong interactions (effective only to distances of about 3 fermis); electromagnetic interactions, which are the interactions of charged particles with electromagnetic fields; weak interactions, responsible for -decay; and gravitational interactions, the weakest of all. The pion is associated with the strong interactions; the photon, with electromagnetic interactions; the neutrino, with weak interactions; and the graviton, with gravitational interactions. If gravitational = 1, then weak = 1028, electromagnetic = 1039, strong = 1041. [H76] (c) Particle physicists use the word "interaction" to refer to anything that particles can do, which includes decaying, scattering, and annihilating. The known interactions of nature are divided into four classes, which from the weakest to the strongest are gravitation, the weak interactions, electromagnetism, and the strong interactions. [G97]
Interarm Region	The area between a spiral galaxy's spiral arms. These areas look dark, not because they lack stars, but because they contain none of the young, luminous stars that light the arms. [C95]
Intercloud Medium	The sparsely populated (about 0.5 atoms per cm3) regions of space between the interstellar concentrations of gas and dust. [H76]
Intercombination Lines	Spectral lines emitted in transitions between two levels with different values of S. [H76]
interference	When similar waves with a regular phase relationship pass through the same region they are said to interfere. The resultant displacement at any point is the sum of the individual displacements, taking into account their directions and phases. The waves emerge from the overlapping region unaffected. The combination of such coherent waves is to be contrasted with the combination of incoherent waves, for example light from two lamps falling on a surface, where the resultant intensity is just the sum of the separate intensities. There are two simple cases. If the waves travel in the same or very nearly the same direction, interference fringes are produced. If waves of comparable intensity travel in opposite directions stationary waves are produced. In all other cases, a complex system of interference fringes and stationary waves is caused, which does not admit simple analysis or description. Interference fringes can be observed with all types of waves. They can be understood most easily by considering waves from just two sources but the ideas can then be generalized to more complicated systems. The two sources must give waves of the same frequency and with a regular phase relationship. This is easily achieved using sources of sound, ripples or radiowaves but in optics it is normally necessary to derive the two sources from one original source by division of wavefront or division of amplitude. The first method is used in Young's double slit, Fresnel's biprism, and Lloyd's mirror. The second is used in Newton's rings, the air wedge, and thin films. Interference fringes are normally observed at a distance from the sources that is very large compared with their separation. Thus the waves travel in almost exactly the same direction and, if the sources are equal, they have equal intensity. There are then maxima of intensity (light fringes) where the path difference is zero or an integral number of wavelengths, and minima (dark fringes) where the path difference is an odd number of half wavelengths. See also Coherence. [DC99] (b) Alternate reinforcement and cancellation of two or more beams of electromagnetic radiation from the same source. In constructive interference the two component beams are in phase, and light results. In destructive interference the components are out of phase, and darkness results. [H76] (c) Waves interfere with each other to produce a new wave motion which depends on whether the interfering waves are in step (in which case a large wave motion is produced) or out of step (in which case a small wave motion is produced). Optical interference was discovered by Thomas Young in 1801. [D89]
Interference Filter	(a) A filter used to shut out all light except the desired wavelengths. [H76] (b) A method of constructing an optical filter to select a particular wavelength band for transmission and reject wavelengths outside this band. Similar to a Fabry-Perot etalon. The construction relies on constructive and destructive interference effects in a multilayer stack of quarter-wave reflective layers and half-wave spacer layers. [McL97]
Interference Pattern	Wave pattern that emerges from the overlap and the intermingling of waves emitted from different locations. [G99]
Interferometer	(a) A device for observing the interference of waves of light or similar emanations caused by a shift in the phase or wavelength of some of the waves. [F88] (b) Stellar Interferometer: A device for measuring small angles by using the principle of interference (q.v.). In radio astronomy, it consists of two or more separate antennas at some distance from one another and each receiving radiation from the same source, joined to the same receiver. The advantage of interferometers is that they reject background noise; their disadvantage is that they are sensitive only to radio waves from sources of small angular size. [H76]
Interferometry	(a) Technique for studying sources of electromagnetic radiation (light or radio waves) through interference patterns caused when two waves are combined. [A84] (b) A measurement technique that relies on interference between coherent waves that results in regions of enhanced signal (constructive interference) when the waves are in phase and regions of no signal (destructive interference) when the waves are exactly out of phase. For light, the effect is usually to produce a series of light and dark bands called fringes. A record of the fringe pattern is called an interferogram. [McL97]
Intergalactic Gas	Matter that is present in the region between the galaxies. It has been detected in considerable amounts in great clusters of galaxies, where the intergalactic gas is so hot that it emits copious amounts of x-radiation. In several groups of galaxies, including the Local Group, the evidence for the presence of intergalactic gas is controversial; clouds of atomic hydrogen may be present. [Silk90]
Intergalactic Matter	Hypothetical material within a cluster of galaxies, whose gravitational effect is to maintain the equilibrium of the cluster. Theoretically comprising 10-30 times the mass of the galaxies themselves (in order to have the observed effect), it has yet to be detected in any form - although the most likely form is as hydrogen. [A84]
Intergalactic Medium	Hypothetical matter (in the form of gas) in the regions between galaxies. It has not yet been detected (but see Magellanic Stream), but Oort (1970) has argued that as much as a factor of 16 or more matter may be present in uncondensed form. [H76]
Interline Transfer	A CCD construction consisting of vertical strips which are alternately opaque and light sensitive. The opaque strips conceal charge transfer registers. [McL97]
Intermediate Vector Boson	(a) IVB Generic name for W and Z bosons - the carriers of the weak force. [D89] (b) A hypothetical elementary particle that acts as intermediary for the weak interaction, carrying its effect from one particle to another as the photon does for electromagnetic interactions and as various mesons do for the strong interactions. (sometimes called W Boson) [H76]
Internal Symmetry	The properties of different elementary particles can be related to each other by mathematical transformations that look very much like the more familiar symmetry properties of our own physical space. Physicists have therefore hypothesized an abstract internal space in which these internal symmetries are defined. With the help of these internal symmetries, the elementary particles can be gathered into families. The relationship between space-time and these internal symmetries remains to be fully explained. [P88]
International Atomic Time	TAI The continuous scale resulting from analyses by the Bureau International des Poids et Mesures of atomic time standards in many countries. The fundamental unit of TAI is the SI second (see second, Système International), and the epoch is 1958 January 1. [S92]
International System of Units	SI Units A practical system of units of measurement adopted in 1969 by the 11th International General Conference of Weights and Measures (CGPM). The seven base units are the meter, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. [H76]
Internet	A global spider-web-like network of computers and computer systems with no central hub or single point of control. [McL97]
Interpulse	The weaker component of a pulsar pulse when its period is roughly half that of the main pulse. [H76]
Interstellar Cloud	A collection of gas and dust that lies between the stars. [C95]
Interstellar Dust	(a) Dust particles in the space between the stars. These are responsible for the dark patches of obscuration seen on astronomical photographs. The particles are composed of common heavy elements such as carbon and silicon but there is no agreement about the exact composition of the dust grains. Typically, the particles have size about 1 µm but there must be a wide range of particle sizes present to explain the interstellar extinction curve. The dust plays a key role in giant molecular clouds in protecting the fragile molecules from intense interstellar ionising and dissociating radiation. The energy absorbed by the grains is emitted in the far-infrared waveband, and this form of dust emission is one of the most important energy loss mechanisms for regions of star formation. [D89] (b) Small grains (primarily silicates) in the interstellar gas. Their dimension is roughly that of optical wavelengths - i.e., about 4000-7000 Å - so they absorb photons of visible light and reemit in the far-infrared. The particles are polarized, needle-shaped grains. Interstellar dust affects the entire spectrum, and leads to dimming and reddening of starlight. Temperature of interstellar dust 5-20 K. [H76] (b) Small grains or particles in the interstellar medium. [McL97]
Interstellar Extinction	The reddening of starlight passing through interstellar dust, caused by the fact that dust scatters blue light more than red. [H76]
Interstellar Gas	Sparse, cool gas (mainly hydrogen) in interstellar space. Dust absorbs and scatters radiation; gas does not interact directly with radiation but is coupled to the dust by collisions. Interstellar gas affects only light of certain wavelengths. Temperature 10-100 K. [H76]
Interstellar Grains	Small needle-shaped particles in the interstellar gas with dimensions from 10-6 to 10-5 cm. They are primarily composed of silicates and strongly absorb, scatter, and polarize visible light at wavelengths comparable to their size, reemitting the light in the far-infrared region of the spectrum. The amount of visual extinction is wavelength-dependent and leads to a dimming and reddening of starlight. [Silk90]
Interstellar Hydrogen	The presence of hydrogen gas between the stars of a galaxy, thus "filling out" the shape of the galaxy in a way that can be detected by spectral analysis and radio monitoring. [A84]
Interstellar Lines	Sharp, distinct absorption lines superposed on stellar spectra, produced by the interstellar gas located between the source and the observer. Strongest are the D lines, followed by the H and K lines, and the K I doublet at 7699 and 7644 Å.
Interstellar Matter	Interstellar gas (99%) and dust (1%). The observed density of the interstellar medium is about 1-5 atoms per cm3. The two other components of the interstellar medium are magnetic fields and cosmic-ray electrons. [H76]
Interstellar Medium	The medium of gas and dust that fills the space between the stars. [LB90]
Interstellar Molecules	Molecules in interstellar space. As of late 1974, at least 33 molecular species had been identified with reasonable certainty: methylidyne CH, ionized methylidyne CH+, the cyanogen radical CN, the hydroxyl radical OH, ammonia NH3, water vapor H2O, formaldehyde H2CO, carbon monoxide CO, hydrogen cyanide HCN, hydrogen isocyanide HNC, molecular hydrogen H2, X-ogen, cyanoacetylene HC3N, methyl alcohol CH3OH, formic acid HCOOH, carbon monosulfide CS, carbonyl sulfide 0CS, formamide NH2CHO, silicon monoxide SiO, methyl cyanide CH3CN, isocyanic acid HNCO, methyl formate HCOOCH3, methyl acetylene CH3C2H, acetaldehyde CH3CHO, thioformaldehyde H2CS, hydrogen sulfide H2S, methanimine H2CNH, ethynyl, sulfur monoxide SO dimethyl ether (CH3)2O, methyl amine CH3NH2, silicon monosulfide SiS, and ethyl alcohol C2H5OH. [H76]
Interstellar Reddening	The reddening of starlight passing through interstellar dust, caused by the fact that dust scatters blue light more than red. [H76]
Interstellar Space	Space between the stars of a galaxy. It is generally not, however, a void vacuum, and is the subject of considerable spectral research. [A84]
Interstellar Square Law	Decreasing as one over distance squared (1/r2), where r is the distance from the source. Light and gravity both have this property. [McL97]
Interval	(a) The quantity in Minkowski space-time which replaces length in ordinary space. [D89] (b) The "distance" between two events in four-dimensional spacetime. [H76]
Intrinsic Brightness	The amount of light an object actually emits, as opposed to how bright the object looks from Earth. An apparently bright star can be intrinsically bright and far away or intrinsically faint and nearby. [C95]
Intrinsic Luminosity	The energy per second emitted by an astronomical object, analogous to the wattage of a light bulb. [LB90]
Invariable Plane	The plane through the center of mass of the solar system perpendicular to the angular momentum vector of the solar system. [S92]
Invariant	(a) Any physical property which does not change under the transformation from one frame of reference to another. [C97] (b) An adjective referring to a quantity whose numerical value is the same in all coordinate systems. [H76]
Invariant Plane	The plane defined by the total angular momentum of the solar system. It is within about 1°.5 of the ecliptic. [H76]
Inverse -decay	The relatively rare process p + vbar -> n + e+. Free-electron capture (e + p -> n + v) is sometimes called inverse -decay in astrophysics. [H76]
Inverse Bremsstrahlung	Absorption (free-free absorption) of a photon by an electron in the field of a nucleus.[H76]
Inverse Compton Effect	The collision between a photon and an energetic (cosmic-ray) electron, in which some of the energy of the electron is transferred to the photon. [H76]
Inverse Maser	A mechanism that absorbs radiation and cools a gas so that the number of molecules in an upper level falls below that expected in a condition of thermal equilibrium. The effect is inverse to that found in a maser, where there is an overpopulated upper level. [H76]
Inverse P Cygni Profile	A profile in which the emission is on the violet side of the absorption. It is usually interpreted to mean infall of matter. [H76]
Inverse Plasmon Scattering	Scattering of electrostatic plasma waves by a flux of relativistic electrons. [H76]
Inverse Square Law	A force law that applies to the gravitational and electromagnetic forces in which the magnitude of the force decreases in proportion to the inverse of the square of the distance. [Silk90]
Inversion	The term used with CCDs to indicate that the applied voltage has not only driven away the majority carriers but has actually attracted the minority carriers of the opposite sign. [McL97]
Inversion Layer	a very thin layer of electrons trapped on an interface between a semiconductor and an insulator, or between two different semiconductors. [D89]
Invisible Astronomy	The study of celestial objects by observing their radiation at wavelengths other than those of visible light. [F88]
Io	Jupiter I The innermost Galilean satellite of Jupiter, similar in size and density to the Moon (R 1850 km from Pioneer 10, period 1.77 days; e = 0.01, i = 0.03). Jupiter's decametric radiation has been linked at least partially to Io. Mean density (from Pioneer 10) 3.48 g (the highest of any of the Galilean satellites). Pioneer 10 also detected the presence of an ionosphere, and Na D emission. Albedo 0.91 (?), the highest in the solar system. [H76]
Iodine	A dark-violet volatile solid element belonging to the halogens. Symbol: I; m.p. 113.5°C; b.p. 184°C; r.d. 4.93 (20°C); p.n. 53; r.a.m. 126.90447. [DC99]
Ion	(a) A charged particle consisting of an atom, or group of atoms, that has either lost or gained electrons. Sodium chloride (salt), for example, is made up of positive sodium ions - atoms that have each lost an electron - and negative chlorine ions - atoms that have each gained an electron. See ionization. [DC99] (b) Atom or group of atoms which has a net charge (i.e., there are not enough electrons to properly balance out the charge of the nuclei.) [SEF01]
Ionization	(a) Loss or gain by an atom of one or more electrons, by which process the atom becomes an ion and instead of being neutral, has a charge: positive if it has lost an electron, negative if it has gained one. High temperature is particularly conducive to ionization. [A84] (b) The process of producing ions. There are several ways in which ions may be formed from atoms or molecules. In certain chemical reactions ionization occurs by transfer of electrons; for example, sodium atoms and chlorine atoms react to form sodium chloride, which consists of sodium ions (Na+) and chloride ions (Cl-). Certain molecules can ionize in solution; acids, for example, form hydrogen ions as in the reaction H2SO4 2 H+ + SO42- Ions can also be produced by ionizing radiation; i.e. by the impact of particles or photons with sufficient energy to break up molecules or detach electrons from atoms: A A+ + e-. Negative ions can be formed by capture of electrons by atoms or molecules: A + e- A-. [DC99]
Ionization Potential	(a) The energy required to remove an electron from an atom or molecule to form a positive ion. [DC99] (b) The minimum energy required to remove an electron from an atom. It always takes a higher energy to remove a second electron from a singly ionized atom, a still higher energy to remove a third, etc. The ionization potential for hydrogen is 13.596 eV, which corresponds to a wavelength of 912 Å. [H76]
Ionization Radiation	Radiation such that the individual particle or quantum has sufficient energy to ionize substances. Electrons with kinetic energy just greater than the ionization potential will cause ionization, but other particles (e.g. molecular positive ions) require higher energies. Gamma-rays and x-rays ionize indirectly by means of the electrons they eject from substances by the photoelectric effect or Compton scattering. Short-wavelength ultraviolet quanta may ionize individual molecules by the photoelectric effect, but the ejected electrons have insufficient kinetic energy to cause further ionization unless an electric field is applied. At ordinary intensities electromagnetic radiation with quantum energy below the ionization potential cannot cause ionization. But by focusing laser beams it is possible to ionize matter - even when the individual photons have energy less than a hundredth of the ionization potential. Such radiations are not ordinarily classified as ionizing radiations. Ionizing radiations can be dangerous to health and precautions should be taken with their use. [DC99]
Ionized	(a) Having lost one or more electrons from an atom. [McL97] (b) If the atoms of a gas have lost one or more electrons, they are said to be ionized. The gas in the early universe is believed to have been ionized until about 300,000 years after the big bang. Since ionized gases interact very strongly with photons, the photons of the cosmic background radiation could not move freely until after the first 300,000 years. [G97]
Ionized Hydrogen	A hydrogen atom that has lost its electron. Hydrogen gets ionized by hot O and B stars in H II regions. The most famous H II region is the Orion Nebula. [C95]
Ionosphere	(a) The region of Earth's atmosphere (80-500 km), immediately above the stratosphere. The ionosphere consists of the D layer, the E layer, and the F layers (q.v.). It is strongest at the end of the day. [H76] (b) A region in the Earth's upper atmosphere containing ions and free electrons. It is formed by ionization by ultraviolet radiation from the Sun. It is divided into three layers: D-layer (50-90 km), E-layer (90-150 km), and F-layer (150-1000 km). Radio transmission can occur by reflection from the ionosphere. [DC99]
IPCS	Image Photon Counting System. A form of very low light level detector used in astronomy. By means of an image intensifier the IPCS is capable of counting individual photons of light. [McL97]
IR	Infrared. [LLM96]
IRAF	Image Reduction and Analysis Facility. An extensive suite of programs developed for astronomy applications and supported by the US National Optical Astronomy Observatories. [McL97]
IRAM	Institut de RadioAstronomie Millimetrique. [LLM96]
IRAS	Infrared Astronomy Satellite. [LLM96]
IRAS Galaxy	Any galaxy which was discovered by the Infra-Red Astronomical Satellite (IRAS) to possess an excessive amount of infrared emission. [C97]
IRAS Samples	Astronomical objects detected in infrared radiation by the Infrared Astronomical Satellite launched in 1983. Infrared light has longer wavelengths than visible light. [LB90]
Iridium	A white transition metal that is highly resistant to corrosion. It is used in electrical contacs, in spark plugs, and in jewelry. Symbol: Ir; m.p. 2140° C; b.p. 4130° C; r.d. 22.56 (17° C); p.n. 77; r.a.m. 192.217.
Iris	An arrangement able to vary the amount of light that enters an optical instrument. In the mammalian eye, this is a circular muscle that changes the size of the pupil. In many optical instruments, a similar effect is obtained with a diaphragm. In either case the aperture is varied. [DC99]
Iron	(a) Element with atomic number 26, created mostly by type Ia supernovae, with an additional contribution from type Ib, Ic, and II supernovae. It is the most stable element. [C95] (b) A transition element occurring in many ores. It is used in the manufacture of steel and various other alloys. Symbol: Fe; m.p. 1535°C; b.p. 2750°C; r.d. 7.874 (20°C); p.n. 26; r.a.m. 55.845. [DC99]
Iron Peak	A maximum on the element-abundance curve near atomic mass number 56. [H76]
Irradiance	Symbol: E The rate of energy reaching unit area of a surface; i.e. the radiant flux per unit area. Unlike illumination, irradiance is not restricted in use to visible radiation. The unit is the watt per square meter (W m-2). The solar constant is the irradiance of the Earth by the Sun. The value is 1.35 KW m-2. When applying this figure to a consideration of solar power, allowance must be made for atmospheric absorption and reflection, and for the angle of the Sun from the zenith. [DC99]
Irradiation	(a) An optical effect of contrast that makes bright objects viewed against a dark background appear to be larger than they really are. [S92] (b) Exposure to an intense flux of fast neutrons or to ionizing radiation. The outer layers of a supernova are irradiated by neutrons produced as the core collapses to form a neutron star. The neutrons react with atomic nuclei to form heavier elements. [Silk90]
Irregular Galaxy	A galaxy with amorphous structure and with relatively low mass (108-1010 M). Fewer than 10% of all galaxies are classified as irregular. [H76]
Irreversibility	time-asymmetric evolution of an observable quantity of a physical system. For an isolated system irreversibility implies the monotonic evolution in the future toward the state of thermodynamic equilibrium. [D89]
IRS	Infrared source. [C95]
Island Universe Hypothesis	Assertion that the sun belongs to a galaxy and that the spiral nebulae are other galaxies of stars, which in turn are separated from one another by vast voids of space. Compare nebular hypothesis. [F88]
ISAS	Institute of Space and Astronautical Science.
Ising Model	a simplified version of the Heisenberg model in which the atomic spins must be aligned parallel or antiparallel to a given direction. [D89]
ISO	Infrared Space Observatory. [LLM96]
Isobars	Nuclei with the same A number but different Z numbers. Also, lines connecting equal atmospheric pressures. [H76]
Isochoric	Constant volume. [H76]
Isochrones	Time-constant loci. [H76]
Isoelectronic Sequence	A sequence of ions which have the same number of electrons but different atomic numbers. [H76]
Isomer	(a) A molecule having the same number of atoms of each element as another molecule. [SEF01] (b) Nuclei with the same A and Z numbers but in different energy states. [H76]
Isomer Shift	Displacement of an absorption line due to the fact that the absorbing nuclei have a different s-electron density from that of the emitting nuclei. [H76]
Isophotes	Lines connecting points of equal light intensity. [H76]
Isoplanatic Patch	The angular region on the sky over which the wavefront correction applied by an adaptive optics system remains valid. It is relatively small and therefore a nearby reference star is also required. [McL97]
Isospin	(a) A quantum number which arises from regarding different members of a charge multiplet (q.v.) as different states of a single particle. (also called Isotopic Spin ) [H76] (b) The neutron and proton behave as if they have an additional form of spin that transforms one into the other. This spin cannot, however, exist in our physical space and is referred to an abstract space - isospace. [P88]
Isostasy	The plasticity of the surface layer of a planet - e.g., the ability of the surface layer to adjust its level according to the load (such as ice caps) that it has to carry. [H76]
Isothermal Change	A process that takes place at constant temperature. If work is done on a body in such a way as would otherwise increase its temperature (for example by compressing it) it is possible to keep the temperature constant by heat transfer from the body. Conversely a body may expand doing external work, while heat is transferred to it to prevent the temperature from falling. In the case of an ideal gas, for which the internal energy depends only on the temperature, the heat transferred from the substance must be exactly equal to the work done on it. For real substances the work and heat may not balance in an isothermal process since the internal energy may depend also on the volume. Although no real process can be perfectly isothermal there are many phenomena that approximate very closely to it. Perfectly reversible isothermal volume changes of an ideal gas are assumed in thermodynamic analyzes, for example Kelvin based his thermodynamic temperature scale upon the behavior of an ideal gas in a Carnot cycle. Isothermal changes are contrasted with adiabatic changes in which no heat enters or leaves the system so any work done on or by it changes the internal energy and hence the temperature. See adiabatic change. [DC99]
Isothermal Fluctuations	Fluctuations in the matter density, without any associated perturbation of the radiation density. The radiation temperature therefore remains uniform. Prior to the Decoupling Era, isothermal fluctuations were frozen, neither growing nor decaying after decoupling, isothermal fluctuations became gravitationally unstable, if greater than the Jeans mass, about 106 M. [Silk90]
Isotones	(a) Nuclei with the same number of neutrons but with different A and Z numbers. [H76] (b) Two or more nuclides that have the same neutron numbers but different proton numbers. [DC99]
Isotope	(a) An atomic nucleus having the same number of protons as a more commonly found atomic nucleus but a different number of neutrons. For example, the hydrogen nucleus has a single proton; deuterium has one proton and one neutron and would be called an isotope of hydrogen. (see neutron; proton.) [LB90] (b) Two nuclei with the same number of protons but different numbers of neutrons are said to represent the same element, but different isotopes. For example, helium-3, with two protons and one neutron in each nucleus, and helium-4, with two protons and two neutrons, are two different isotopes of helium. For another example, see deuterium. [G97] (c) Two or more species of the same element differing in their mass numbers because of differing numbers of neutrons in their nuclei. The nuclei must have the same number of protons (an element is characterized by its proton number). Isotopes of the same element have very similar chemical properties (the same electron configuration), but differ slightly in their physical properties. An unstable isotope is termed a radioactive isotope or radioisotope. For example, potassium has 3 naturally occurring isotopes of mass numbers 39, 40, and 41 respectively. 3919K has 19 protons and 20 neutrons 4019K has 19 protons and 21 neutrons 4119K has 19 protons and 22 neutrons 4019K is radioactive with a half-life of 1.3 × 109 years. At least eight other isotopes of potassium can be produced by nuclear reactions but all are highly unstable (radioactive). [DC99]
Isotopic Spin	A concept introduced by Heisenberg in 1932 to describe the charge independence of the strong nuclear force. Since the strong force cannot distinguish between a proton and a neutron, Heisenberg proposed that these particles were actually different states of a single particle - the nucleon. He argued that just as the electron comes in two different spin states, so the nucleon comes in two different "isospin" states. So, isospin is a concept analogous to spin which is conserved by the strong interaction. The nucleon is an isospin-1/2 particle, and its third component of isospin determines whether we are talking about a proton (I3 = +1/2) or a neutron (I3 = -1/2). See Isospin [CD99]
Isotropic	(a) Having equal and uniform properties at all points and in all directions. In astronomy the term describes microwave background radiation. [A84] (b) A universe is said to be isotropic, from the point of view of a given observer, if it looks the same in all directions. The isotropy of the real universe is seen most strikingly in the cosmic background radiation, which has the same temperature in all directions to an accuracy of about one part in 100,000. See also homogeneous. [G97]
Isotropy	(a) Quality of being the same in all directions. Compare anisotropy. [F88] (b) In cosmology, the property that the universe appears the same in all directions. The uniformity of the cosmic background radiation, coming from all directions of space, suggests that on the large scale the universe is isotropic about our position. If we then assume that our position is not unique, we conclude that: the universe appears isotropic about all points. This last result requires that the universe be homogeneous. (See cosmic background radiation; homogeneity.) [LB90]
Israel-Robinson Theorem	The only locally stationary empty space-time which is asymptotically flat with a nondegenerate event horizon is the |a| < m Kerr metric, where a is angular momentum per unit mass. [H76]
IUE	International Ultraviolet Explorer. [LLM96]