C Galaxies	In the Yerkes 1974 system, small, high surface-brightness galaxies which are slightly resolved on medium- and large-scale photographs. [H76]
C Stars	A class of carbon stars ( q.v.), defined by Morgan and Keenan to replace the Harvard R and N spectral classes. [H76]
C Stars	Late type giants with strong bands of carbonated molecules (C2, CN, CH) and no metallic oxide bands. Formerly they were called R or N types, the R types being the hotter and the N types the cooler C stars. [JJ95]
C-S Stars	Group characteristics are: strong bands of CN, outstandingly strong absorption near the Na D lines, usually sufficient structure in the 6400-6500 Å region to suggest ZrO. [H76]
Cabbibo Angle (C)	The measure of the probability that one flavor of quark (u) will change into other flavors (d or s) under the action of the weak force. [CD99]
Cadmium	A transition metal obtained as a by-product during the extraction of zinc. It is used to protect other metals from corrosion, as a neutron absorber in nuclear reactors, in alkali batteries, and in certain pigments. It is highly toxic. Symbol: Cd; m.p. 320.95°C; b.p. 765°C; r.d. 8.65 (20°C); p.n. 48; r.a.m. 112.411. [DC99]
Calabi-Yau Space, Calabi-Yau Shape	A space (shape) into which the extra spatial dimensions required by string theory can be curled up, consistent with the equations of the theory. [G99]
Calcium	A moderately soft, low-melting reactive metal. The electronic configuration is that of argon with an additional pair of 4s electrons. Calcium is widely distributed in the Earth's crust and is the third most abundant element. At ordinary temperatures calcium has the face-centered cubic structure with a transition at 450°C to the close-packed hexagonal structure. Symbol: Ca; m.p. 839°C; b.p. 1484°C; r.d. 1.55; p.n. 20; r.a.m. 40.0878. [DC99]
Calcium Star	Old name for an F star. [H76]
Calendar	A system of reckoning time in which days are enumerated according to their position in cyclic patterns. [S92]
Calibi-Yau Space	These six-dimensional spaces are hypothesized as arising when the ten dimensions of superstring theory are compactified down to four dimensions. They are also related to orbifold spaces. [P88]
California Nebula (IC 1499)	An HII region ionized by Zeta Persei. [H76]
Californium	A silvery radioactive transuranic element of the actinoid series of metals, not found naturally on Earth. Several radioisotopes have been synthesized, including Californium-252, which is used as an intense source of neutrons in certain types of portable detector and in the treatment of cancer. Symbol: Cf; m.p. 900°C; p.n. 98; most stable isotope 251Cf (half-life 900 years). [DC99]
Callisto	(a) Fifth (known) moon out from jupiter, and its second largest. [A84] (b) A Galilean satellite (J IV) of Jupiter, about 5050 km in diameter. Orbital and rotation period 16.7 days (e = 0.0075; i = 0°.3). It has the lowest density (1.7 g cm-3), lowest albedo (0.15), and highest temperature (156 K) of any of the four. [H76]
Cambridge Catalogues	The results of five intensive radio-astronomical surveys (1C, 2C, 3C, 4C and 5C) under the direction of Sir Martin Ryle and Anthony Hewish, during the l950s, 1960s and 1970s, at Cambridge. [A84]
Z Camelopardalis Stars	A class of dwarf novae (q.v.) with standstills in their light curves. Z Cam itself is a semidetached binary (period 7h21m) consisting of a dG1 star and a hot white dwarf or a hot blue subdwarf which is probably degenerate. Mean time between eruptions, 20 days. Peak-to-peak amplitude, about 0.5 mag. [H76]
Canals of Mars	Mistranslation - and consequent misunderstanding - of an Italian description of the pattern or channels (canali) on the Martian surface. Giovanni Schiaparelli in writing his account was careful to say that terrestrial terms were used only for reference, but in the USA Percival Lowell and in France Nicolas Flammarion took his words literally, and elaborated upon them too, with the result that stories of Martian civilizations became popular. The fact remains, however, that there are waterway-like channels on Mars - but not many, and in any case much smaller than any feature that is discernible with terrestrial telescopes. [A84]
Candela	Symbol: cd The SI base unit of luminous intensity; the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has radiant intensity in that direction of 1/683 watt per steradian. Formerly, the unit was defined as the intensity (in the perpendicular direction) of the black-body radiation from a surface of 1/600 000 square meter at the temperature of freezing platinum and at a pressure of 101 325 pascals. [DC99]
Candela	The SI unit of luminous intensity, defined as "the luminous intensity, in the perpendicular direction, of a surface of 1/600,000 square meter of a blackbody at the temperature of freezing platinum under a pressure of 101,325 newtons per square meter." (13th CGPM [1967], Resolution 5.) [H76]
Canis Majoris	see Sirius. [H76]
Canis Majoris Star	see Beta Cephei Star. [H76]
VY Canis Majoris	A peculiar cM3e irregular variable with an extremely strong infrared excess, presumably due to a circumstellar dust shell. It is a class 2b OH emitter, and CO and H2O have been identified in its spectrum. It is a multiple star with at least six components, surrounded by a small reflection nebula, about 1.5 kpc distant, in the galactic plane. It may be a pre-main-sequence star, or it may be a highly evolved object like an extremely young planetary nebula. [H76]
Canis Minoris	see Procyon. [H76]
Canonical	The Canonical Approach to dynamics refers to the scheme in which the basic constituent is a space of states and the evolution of the system is described by a curve in this space parametrized by time. This approach to classical physics is in many respects the basic one to adopt when attempting to include quantum effects. In the case of a field theory, it has the disadvantage that space and time are treated on a different footing, and hence it is not always an easy matter to show that the formalism is compatible with the theory of relativity. [D89]
Canonical Change	A periodic change in one of the components of an orbit (cf. secular change). [H76]
Canopus ( Car)	(a) The brightest star in the constellation Carina and the second brightest star in the night sky. It is spectral type F and shines yellow-white. [C95] (b) A type F0 Ib supergiant, about 55 pc distant, the second brightest star in the southern sky. [H76]
2 CVn Star	see Spectrum Variable. 2 CVn has a period of 5.469 days. Its spectrum shows strong, profuse lines of rare earths, iron-peak elements, and Si. [H76]
AM Canum Venaticorum (HZ 29)	A peculiar blue variable. It may be an accreting, semidetached binary white dwarf system with a period of about 18 minutes (0.012 days). [H76]
Capella ( Aur)	(a) The brightest star in the constellation Auriga and the sixth brightest star in the night sky, Capella lies 42 light-years away and consists of two yellow giants. [C95] (b) A spectroscopic triple (F8-G0 III, G5 III, M5 V) with a period of 104.023 days, about 13 pc distant (1974 parallax 0'.079). It has a high lithium content and a nearly circular orbit. It may be an X-ray source. [H76]
Capture	The absorption of one particle by another. For instance, a positive ion may capture an electron to form a neutral atom. In some nuclear reactions, an atomic nucleus may capture a neutron with emission of one or more gamma-ray photons. [DC99]
Carbon	(a) Element with atomic number six and the basis of all terrestrial life. Carbon is produced during helium burning in red giants and is ejected into the Galaxy when these stars form planetary nebulae. Some carbon also comes from high-mass stars that explode as supernovae. [C95] (b) A nonmetallic element; carbon is a universal constituent of living matter and the principal deposits of carbon compounds are derived from living sources; i.e., carbonates (chalk and limestone) and fossil fuels (coal, oil, and gas). It also occurs in the mineral dolomite. Minute quantities of elemental carbon also occur as the allotropes graphite and diamond. A third allotrope, buckminsterfullerene (C60), also exists. Naturally occurring carbon consists of three isotopes: 12C (98.89%), 13C (1.11%) and 14C (minute traces in the upper atmosphere produced by slow neutron capture by 14N atoms). 14C is used for radiocarbon dating because of its long half-life of 5730 years. Symbol: C; m.p. 3550°C; b.p. 4830°C (sublimes); C60 sublimes at 530°C; r.d. 3.51 (diamond), 2.26 (graphite), 1.65 (C60) (all at 20°C); p.n. 6; r.a.m. 12.011. [DC99]
Carbon Black	A form of amorphous carbon (soot) produced by incomplete combustion of gas (or other organic matter). It is used in experiments as a coating for surfaces that need to be good absorbers of radiation, for example in detectors of thermal radiation, such as the thermopile. It is also used to increase the amount of thermal radiation emitted by a surface. [DC99]
Carbon Burning	The stage when a star fuses carbon into heavier elements, making neon and magnesium. Carbon burning eventually occurs in all stars born with more than eight Solar masses. [C95]
Carbon Cycle	A series of nuclear reactions in which carbon is used as a catalyst to transform hydrogen into helium: 12C(p, )13N(p, ) 14O( +)14N(p, )15O( +)15N(p, )12C. The carbon cycle can take place only if the necessary C and N nuclei are present, and it requires higher temperatures (15-20 million kelvins) and is far more temperature-dependent (E T15) than the proton-proton chain (E T4). The cycle yields 26.7 MeV of energy. (On the average, 1.7 MeV of this energy is carried away because of neutrino losses.) [H76] (also called CN cycle or Bethe-Weizsäcker cycle) (discovered in 1938-39)
Carbon Cycle (carbon-nitrogen cycle)	A series of nuclear reactions postulated to account for energy production in stars. In this series 126C is an intermediary in the process by which hydrogen nuclei fuse to form helium with release of energy. The first step is the fusion of carbon and hydrogen nuclei: 126C + 11H 137N + gamma radiation 137N 136C + positron 136C + 11H 147N + gamma radiation 147N + 11H 158O + gamma radiation 158O 157N + positron 157N + 11H 126C + 42He The net result is: 411H 42He with a release of about 4.4 pJ of energy. (see also Proton-Proton Chain Reaction) [DC99]
Carbon Dating	A method of dating - measuring the age of (usually archaeological) materials that contain matter of living origin. It is based on the fact that 14C, a beta emitter of half-life approximately 5730 years, is being formed continuously in the atmosphere as a result of cosmic-ray action. (radiocarbon dating) The 14C becomes incorporated into living organisms. After death of the organism the amount of radioactive carbon decreases exponentially by radioactive decay. The ratio of 12C to 14C is thus a measure of the time elapsed since the death of the organic material. Uncertainties arise because of uncertainty as to the past rate of production of 14C, the possibility of exchange of carbon with carbon of a different age during the elapsed time, the possibility of contamination of the sample, and the effect of burning of fossil fuels on the composition of atmospheric carbon. The method is most valuable for specimens of up to 20 000 years old, though it has been modified to measure ages up to 70 000 years. For ages of up to about 8000 years the carbon time scale has been calibrated by dendrochronology; i.e. by measuring the 12C:14C ratio in tree rings of known antiquity. [DC99]
Carbon Dioxide	A molecule consisting of one carbon atom and two oxygen atoms (CO2). It is a gas in Earth's atmosphere that helps to keep the planet warm by trapping solar heat. [C95]
Carbon Monoxide	A molecule consisting of one carbon atom and one oxygen atom (CO). It is the most abundant interstellar molecule after molecular hydrogen and is especially useful because it radiates at radio wavelengths, so astronomers can use it to map the distribution of molecular hydrogen. [C95]
Carboxyl Group	Also called the COOH group; functional group consisting of a carbon atom double bonded to an oxygen atom and single bonded to another oxygen with a hydrogen on the other side. Terminal group on carboxylic acids. [SEF01]
Catadioptric Lens	A compound lens in which both a mirror(s) and lenses are used to form an image. [DC99]
Cation	A positively charged ion, formed by removal of electrons from atoms or molecules. In electrolysis, cations are attracted to the negatively charged electrode (the cathode). [DC99]
Cavitation	The formation of small cavities in a liquid, caused by a reduction in fluid pressure. The cavities may collapse, generating a large impulsive pressure which may erode and damage nearby solid surfaces (such as pump impellers, turbine blades, and ships' propellers). The phenomenon is exploited for cutting and drilling metals using ultrasonic waves. [DC99]
CNO Cycle	One way that a star converts hydrogen into helium. During the CNO cycle, carbon, nitrogen, and oxygen catalyze the nuclear reaction, so the total number of carbon, nitrogen, and oxygen nuclei remains the same. However, carbon and oxygen gradually get converted into nitrogen. The CNO cycle powers the hydrogen burning that occurs in main-sequence stars with more than 1.5 Solar masses and in giants and supergiants of all masses. [C95]
CNO Bi-Cycle	Similar to the CN cycle, except that it also includes a cycle in which the next-to-last step becomes 15N(p, )16O(p, )17F( +)17O(p, )14N. This reaction occurs once in about 2,000 CN cycles. For main-sequence stars greater than a few Solar masses, hydrogen burning by the CNO bi-cycle is the main source of energy. (It produces about 2% of the Solar energy.) [H76]
CNO Tri-Cycle	Similar to the CNO bi-cycle, with the addition of the cycle 17O(p, )18F( +)18O(p, )15N. [H76]
CO	see Carbon Monoxide. [C95]
CO2	see Carbon Dioxide. [C95]
Carbon Detonation Supernova Model	A supernova model involving the explosive ignition of carbon in the high-density (108 - 1010 g cm-3), electron-degenerate carbon-oxygen core of a 6±2 - 7±2 M star by the formation and propagation of a detonation wave. A carbon-detonation supernova seems to leave no dense remnant and converts its C-O core entirely to iron. [H76]
Carbon-Nitrogen Cycle	Use of carbon and nitrogen as intermediates in the nuclear fusion process of the Sun. Cooler stars undergo the proton-proton cycle. [A84]
Carbon Reaction	An important nuclear fusion process that occurs in stars. Carbon-12 both initiates it and, following interactions with nuclei of nitrogen, hydrogen, oxygen, and other elements, reappears at its conclusion. [F88]
Carbon Stars	In the HD system, a rather loose category of peculiar red-giant stars, usually of spectral types R and N, whose spectra show strong bands of C2, CN, or other carbon compounds and unusually high abundances of lithium. Carbon stars resemble S stars in the relative proportion of heavy and light metals, but they contain so much carbon that these bands dominate their spectra (see also C Stars). (C2,0. The number following the comma is an abundance parameter.) [H76]
Carbonaceous Chondrites	Chondrites (stony meteorites) characterized by the presence of carbon compounds. They are the most primitive samples of matter in the Solar System. [H76]
Carina	(a) A constellation in the southern sky and home of the bright star Canopus. (b) A dwarf galaxy that orbits the Milky Way. Discovered in 1977, Carina lies 350,000 light-years from the Galactic center. [C95]
Carina OB 2	A rich association of OB stars near Carinae. [H76]
Carinae	see Canopus. [H76]
Carinae	A peculiar nova-like variable about 2 kpc distant. For 50 years in the middle of the nineteenth century it was the second brightest star in the southern sky, reaching magnitude - 1 in 1843. Presently its visual apparent magnitude is + 7 (although at 20 µ it is still the brightest source in the sky). It may be a "slow supernova" with its slowness due to the large size of the parent star. [H76]
Carrier Boson	A particle that carries one of the fundamental forces between other interacting particles. For example, the carrier boson for the electromagnetic force is the photon. [HH98]
Carter's Theorem	Sequences of axisymmetric metrics external to black holes must be disjoint, i.e., have no members in common. [H76]
Cassegrain Focus	An optical arrangement in which light rays striking the parabolic concave primary mirror of a reflecting telescope are reflected to the hyperbolic convex secondary mirror, and re-reflected through a hole bored in the primary to a focus behind it. [H76]
Cassegrain (focus, telescope)	Refers to a design of reflecting telescopes in which the light collected and focussed by the large concave primary mirror is refocussed by a smaller convex secondary mirror on the same axis as the primary. The refocussed beam passes through a central hole cut into the primary mirror and emerges behind the primary. [McL97]
Cassegrain (reflecting) Telescope	Telescope devised by Cassegrain in which an auxiliary convex mirror reflects the magnified image, upside down, through a hole in the center of the main objective mirror - i.e., through the end of the telescope itself. It was, however, no improvement on the gregorian telescope invented probably slightly earlier. [A84]
Cassini's Division	A gap about 1800 km wide between the outermost rings of Saturn. It was discovered by Cassini in 1675. The period of a particle in Cassini's division is about two-thirds that of Janus, one-half that of Mimas, one-third that of Enceladus, and one-quarter that of Tethys. [H76]
Cas A (3C 461)	A radio source in Cassiopeia, the strongest extrasolar source in the sky, perhaps 3 kpc distant, believed to be the remnant of a Type II supernova whose light reached Earth about 1667. Optically it is a faint nebula. It has an expansion velocity of about 800 km s-1 and a mass of a few Solar masses. It is also an extended source of soft X-rays (3U 2321+58). [H76]
AO Cassiopeiae	A binary in which the larger, less massive, hot primary is highly distorted, and in which rapid mass exchange is occurring. [H76]
B Cassiopeiae	see Tycho's Star. [H76]
WZ Cassiopeiae	A carbon star (the most super-rich carbon star known) with a very high abundance of lithium. Its effective temperature is 2420 K. [H76]
Castor ( Geminorum)	A visual triple system about 14 pc distant. Each component is itself a spectroscopic binary. Component A is A 1 V, with a period of 9.22 days; component B is Am5 with a period of 2.93 days. Period of components A and B is about 380 years. Component C (YY Gem), a flare star, is a double-lined eclipsing binary with a period of 0.814 days. Both components are dM1e, and both components exhibit flares. [H76]
Cataclysmic Variable	A type of variable including flare stars and novae (common, recurrent, and dwarf), all of which are believed to be very close binary systems in which hydrogen-rich matter flows from a late-type star onto a hot white-dwarf primary. [H76]
Cataclysmic Variables	A collective name for stars in which the brightness increases suddenly because of an explosive event. The class comprises supernovae, novae, recurrent novae, dwarf novae and flare stars. [JJ95]
Catalog Equinox	The intersection of the hour circle of zero right ascension of a star catalog with the celestial equator. (see Dynamical Equinox; Equator.) [S92]
Catastrophism	Nineteenth-century hypothesis that depicted the many changes evinced by the geological record as having resulted from cataclysms occurring during a relatively brief period of history. Compare uniformitarianism. [F88]
Cathode-Ray Tube (CRT)	Basis of the TV tube and the oscilloscope. Electrons emitted by a heated filament are channeled into a very narrow beam and steered by electric fields between charged plates to impact a phosphorescent screen which emits a flash of light for each collision. [McL97]
Cauchy Dispersion Formula	An approximate empirical formula for the index of refraction n as a function of wavelength: n = A + B / 2 + C / 4 + . . ., where A, B, C, . . . are constants depending on the refracting medium. [H76]
Causality	pertaining to the time development of a system and the requirement of special relativity whereby energy cannot be propagated at a speed faster than that of light. [D89]
Causation, Causality	The doctrine that every new situation must have resulted from a previous state. Causation underlay the original atomic hypothesis of the Greeks, and was popular in classical physics. It is eroded in quantum mechanics and has, in any case, never been proved essential to the scientific world view. (see Chance; Determinism) [F88]
Causality Puzzle	see Horizon Problem. [LB90]
CBR	Cosmic Background Radiation [HH98]
CCD	Charge-Coupled Device (a) A small photoelectronic imaging device (typically 1.5 cm square) made from a crystal of semiconductor silicon in which numerous (at least 250,000) individual light-sensitive picture elements (pixels) have been constructed. Each tiny pixel (less than 0.03 mm in size) is capable of storing electronic charges created by the absorption of light. The name derives from the method of extracting the locally stored charges from each pixel which is done by transferring or "coupling" charges from one pixel to the next by the controlled collapse and growth of adjacent storage sites or "potential wells". Each "well" is formed inside the silicon crystal by the electric field generated by voltages applied to tiny, semi-transparent metallic electrodes on the CCD surface. [McL97] (b) Highly sensitive photoelectric devices that can electronically record the intensity and point of arrival of tiny amounts of light. CCDs are placed at the receiving end of telescopes, to "take pictures" of very faint astronomical objects; they have almost completely replaced photographic plates. [LB90]
cD Galaxy	In Morgan's classification, a supergiant elliptical galaxy with a large, faint halo; an outstandingly large, luminous D galaxy. cD galaxies occur centrally located in rich clusters of galaxies. [H76]
CDA	Centre de Donnees Astronomiques (Strasbourg, France). [LLM96]
CDM	Cold Dark Matter. [HH98]
CD-ROM	Compact Disk - Read Only Memory A computer data storage technology. The disk resembles an audio compact disk 120 mm (4.75 in) in diameter, with each platter containing digital information accessible by a laser beam reading system. [McL97]
CDS	Correlated Double Sampling A technique used with CCDs to remove an unwanted electrical signal, associated with resetting of the tiny "on-chip" CCD output amplifier, which would otherwise compromise the performance of the detector. It involves making a double measurement of the output voltage before and after a charge transfer and forming a difference to eliminate electrical signals which were the same, i.e., correlated. [McL97]
Celestial	Of the heavens; in the sky; in space. [A84]
Celestial Ephemeris Pole	The reference pole for nutation and polar motion; the axis of figure for the mean surface of a model Earth in which the free motion has zero amplitude. This pole has no nearly diurnal nutation with respect to a space-fixed or Earth-fixed coordinate system. [S92]
Celestial Equator	Projection of the Earth's equator as a line across the sky (so that to an observer actually on the equator, such a line would pass through the zenith). The directional bearing of a star is given in terms of its right ascension round the celestial equator. [A84]
Celestial Longitude	Angular distance along the ecliptic from the vernal equinox eastward. [H76]
Celestial Mechanics	Study of the movements and physical interactions of objects in space; astrophysical mathematics. [A84]
Celestial Meridian	The great circle on the celestial sphere which passes through the celestial poles and the zenith of the observer. [H76]
Celestial Poles	The two points at which the Earth's axis of rotation, if extended, would intersect the celestial sphere. [H76]
Celestial Sphere	An imaginary sphere of arbitrary radius upon which celestial bodies may be considered to be located. As circumstances require, the celestial sphere may be centered at the observer, at the Earth's center or at any other location. [S92]
Centauri (Rigil Kent)	A binary system (G2 V, K5 V) 1.3 pc distant. Period of system about 80 years. Parallax 0'.754, proper motion 3'.68 per year. [H76]
Proxima Centauri	An eleventh magnitude (M bol = 11.66 mag) star, probably associated with the Cen system. It is a flare star of spectral type dM4e with a parallax of 0'.765, which makes it our closest known stellar neighbor. M = 0.1 M ; R = 1.3 x 1010 cm. [H76]
Centauri	A metal-poor halo-population globular cluster of more than 3 × 106 M, according to Poveda. It is the closest known globular cluster (about 5.2 kpc distant) and is barely visible to the naked eye in Earth's southern hemisphere. [H76]
Cen A	Centaurus A Strong radio source. Optically, it is an elliptical galaxy (NGC 5128) with a dark obscuring lane. It is the nearest known violent galaxy. Probably about 4 Mpc distant. It is also an X-ray source (3U 1322-42). [H76]
Centaurus Cluster	A cluster of galaxies about 200 Mpc distant. It is also an extended X-ray source. Its radio counterpart is compact and located inside NGC 4696. (3U 1247-41) [H76]
Centaurus X-3	A pulsating (period 4.8 s) binary X-ray source in the galactic plane, recently found to be a member of an occulting binary system (e < 0.002, period of system 2.087 days; X-ray eclipse lasts 0.488 days). Optical component is Krzeminski's star, a B0 giant or supergiant, about 5-10 kpc distant. The X-ray component is probably a rotating neutron star of about 0.65-0.83 M. Cen X-3 is speeding up at a rate of about 1 part in 103-105 per year and will at this rate fall into its companion in about 1000 years. (3U 1118-60) [H76]
Center of Curvature	Each surface of a simple curved lens or mirror is part of a sphere. The center of curvature c of such a surface is the center of the sphere of which the surface is part. [DC99]
Cen X-2 and Cen X-4	are sporadic X-ray sources. [H76]
Cephei Stars	A small group of short-period (P = 3½ to 6 hr) pulsating variables (O9-B3) lying slightly above the upper main sequence. They have a doubly periodic light curve, and are confined within a narrow band of the H-R diagram which lies near the end of core hydrogen-burning stars of roughly 10-20 M. Beta Cephei itself has at least three components. (also called Beta Canis Majoris Stars) [H76]
VV Cephei Stars	Eclipsing binaries with M supergiant primaries and blue (usually B) supergiant or giant secondaries. They have a rich emission spectrum. Sandage (1974) suggests Mv = - 7.3 for the M2p component of VV Cep. [H76]
Cepheid	(a) A yellow supergiant that pulsates, alternately brightening and dimming. Cepheids allow astronomers to measure distances, because the longer a Cepheid's period of variation, the greater the Cepheid's mean intrinsic brightness. To determine a Cepheid's distance, all an astronomer has to do is measure the Cepheid's period; comparing the star's mean intrinsic brightness with the star's mean apparent brightness then yields the distance. Cepheids are so bright that we can see them in other galaxies, allowing us to establish distances to entire galaxies beyond the Milky Way. [C95] (b) One of a group of very luminous supergiant pulsating stars named for the prototype Cep. Type I (or classical Cepheids) are extreme Population I with characteristic periods of 5-10 days. They are about 4 times more luminous (< Mv > = -0.5 to -6) than those of type II, probably because of their higher metal content (although mass may also be a factor). Type II (or W Virginis stars) are Population II (< Mv > = 0 to -3) with characteristic periods of 10-30 days, and are primarily found in globular clusters. The luminosities of all Cepheids are proportional to their periods, but a different P-L relation applies to each type. No Cepheid is near enough for an accurate trigonometric parallax (Polaris is the nearest). Cepheids are useful distance indicators to about 3 Mpc. [H76]
Cepheid Variable	(a) A type of variable star whose period of variation is tightly related to its intrinsic luminosity. [HH98] (b) A class of stars named after Delta Cephei which vary in brightness over a regular period of time (typically a few days). The period of change is directly related to the true, average brightness or luminosity of the star. Once the period is known the true brightness can be calculated and the distance estimated by observing the "apparent" brightness of the object as seen from Earth. [McL97]
Cerenkov Radiation	(a) A bluish light that is emitted when charged particles travel through a transparent medium at a speed that exceeds the speed of light in the medium. [DC99] (b) Visible (and more energetic) radiation caused by an electromagnetic shock wave arising from charged particles moving with velocities greater than the speed of light in the medium. (It is the electromagnetic analog to an acoustical shock wave, or sonic boom.) The particles will continue to lose energy by radiation until their velocity is less than the speed of light in the medium. [H76]
Ceres	Largest of the known asteroids, and the first to be discovered (by Piazzi in 1801). R 510 km, mean distance from Sun 2.7673 AU, e = 0.079, i = 10°.6. Rotation period 0.38 days, sidereal period 1,682 days, synodic period 466.6 days. Photographic albedo 0.06. Mean orbital speed 17.9 km s-1, Mass 1.17 × 1024 g. Spectrum suggests carbonaceous chondrite. [H76]
Cerium	A ductile malleable gray element of the lanthanoid series of metals. It occurs in association with other lanthanoids in many minerals. It is used in several alloys (especially for lighter flints), as a catalyst, and in compound form in carbon-arc searchlights, etc., and in the glass industry. Symbol: Ce; m.p. 799°C; b.p. 3426°C; r.d. 6.7 (hexagonal structure, 25°C); p.n. 58; r.a.m. 140.15.[DC99]
CERN	(a) The European Laboratory for Particle Physics (formerly the Conseil Europeen pour la Recherche Nucleaire ), located near Geneva in Switzerland. Here, the resources of the European member nations are pooled to construct the large particle accelerators needed for high-energy experiments. The major facilities at CERN include the Super Proton Synchrotron (SPS) and the Large Electron-Positron (LEP) collider. [CD99]
Cesium	A soft golden highly reactive low-melting element. Cesium is used in photocells, as a catalyst, and in the cesium atomic clock. The radioactive isotopes 134Cs (half life 2.065 years) and 137Cs (half life 30.3 years) are produced in nuclear reactors and potentially dangerous atmospheric pollutants. (var. Caesium) Symbol: Cs; m.p. 28.4°C; b.p. 678.4°C; r.d. 1.873 (20°C); p.n. 55; r.a.m. 132.91. [DC99]
Cesium Clock	An apparatus used to produce the steady frequency used in defining the second. It depends on the fact that, in a magnetic field, cesium-133 atoms can have two different energy levels between which transitions occur by absorption of radio-frequency radiation of a frequency of 9 192 631 770 hertz. In a cesium clock, the number of atoms in the higher state is detected, and the signal used to stabilize the oscillator producing the radiation. [DC99]
o Ceti	see Mira. [H76]
Ceti	A G8 Vp star about 3.6 pc distant. [H76]
UV Ceti Stars	Late-type dwarfs (dKe-dMe) with spectra showing hydrogen emission lines. UV Cet itself is a faint M6e V flare star (component B of Luyten 726-8) of very low mass (0.15 M), 2.8 pc distant. Like other flare stars, it is a member of a binary system in which both components are of nearly equal brightness (Mv = 15.3 and 15.8). Period of the system is about 26.5 years (angular separation 1".0, e = 0.615). Radio flares have also been observed. [H76]
Cetus Arc	A gaseous nebula, probably about 100 pc distant, centered on or near Peg. It may be a supernova remnant. [H76]
CFHT	Canada France Hawaii Telescope. [LLM96]
CH Stars	G-type giants (G5 to K5) in which the molecular bands of CH are very strong. [JJ95]
Chain Reaction	The progressive disintegration of fissile material (e.g. 235U) by bombardment with neutrons, which in turn results in the production of more neutrons. These may, under suitable conditions, produce further fissions. Fission of 235U yields a varying number of neutrons depending on the energy of the incident neutrons. Neutrons may escape from the mass of uranium or be absorbed by nonfissile nuclei and so be ineffective in promoting the chain reaction. If one fission produces neutrons which, in turn, cause more than one fission, there is a branching chain reaction, which may be explosive. On the other hand, if insufficient neutrons are captured the chain reaction will die out. To sustain a controlled chain reaction in natural uranium, it is necessary to slow down the neutrons in a moderator. Slow neutrons are more likely to cause fission in 235U than to be captured by the more abundant nonfissile isotope 238U. A chain reaction can only be sustained in uranium without using a moderator if the proportion of 235U is considerably increased (enrichment). The artificial fissile nuclides 239Pu, 241Pu, and 233U can also sustain chain reactions. [DC99]
Chandler Period	The period of the variation of the celestial poles (about 416-433 days, with a peak at 428 days). Pole wandering (by as much as 15 meters from its mean position) causes minute variations in the meridian. [H76]
Chandra	NASA's premier x-ray observatory was named the Chandra X-ray Observatory in honor of the late Indian-American Nobel laureate, Subrahmanyan Chandrasekhar.
Chandrasekhar Limit	(a) A limiting mass for white dwarfs. If the mass exceeds this critical mass (1.44 Solar masses, for the expected mean molecular weight of 2), the load of the overlying layers will be so great that degeneracy pressure will be unable to support it, and no configuration will be stable. [H76] (b) The upper limit to the masses of stars which are held up by degeneracy pressure. The mass is more or less the same for white dwarfs and neutron stars, being about the mass of the Sun. In the case of white dwarf's, the degeneracy pressure is due to the electrons and in the case of neutron stars to neutron degeneracy pressure. [D89] (c) The maximum mass, approximately 1.4 M, above which an object cannot support itself by electron degeneracy pressure; hence, the maximum mass of a white dwarf. [HH98]
Chandrasekhar-Schönberg Limit	(a) Mass above which the helium core of a star begins to contract (eventually to collapse altogether). The limit is now reckoned as 10 to 15 per cent of the star's total mass. [A84] (b) The mass limit for an isothermal core. In order to maintain its luminosity by hydrogen burning just outside the isothermal core, the star must keep a high temperature and a high pressure at the surface of the core. When the helium core exceeds about 12% of the star's total mass, the star can no longer adjust by small changes, but must drastically increase in radius and move rapidly from the main sequence. [H76]
Chance	Characteristic of a regime in which predictions cannot be made exactly, but only in terms of probabilities. In classical physics, chance was thought to pertain only where ignorance limited our understanding of an underlying mechanism of strict causation. But in the Copenhagen interpretation of quantum mechanics, chance is portrayed as inherent to all observations of nature. [F88]
Channel Stops	Narrow, heavily doped strips in a silicon CCD which act like walls to prevent sideways movement of charge in a pixel. [McL97]
Chaos	originally used by the Greeks to describe the limitless void, it is now used to describe unpredictable and apparently random structures. The study of chaos using topology and computers has become a major part of modern mathematics, revealing universal and fundamental laws of a remarkably simple kind. [D89]
Chaos Theory	The theory of systems that exhibit apparently random unpredictable behavior. The theory originated in studies of the Earth's atmosphere and the weather. In such a system there are a number of variables involved and the equations describing them are nonlinear. As a result, the state of the system as it changes with time is extremely sensitive to the original conditions. A small difference in starting conditions may be magnified and produce a large variation in possible future states of the system. As a result, the system appears to behave in an unpredictable way and may exhibit seemingly random fluctuations (chaotic behavior). The study of such non-linear systems has been applied in a number of fields, including studies of fluid dynamics and turbulence, random electrical oscillations, and certain types of chemical reaction. (see also Attractor, Butterfly Effect) [DC99]
Chaotic Dynamics	time-dependent aperiodic regime in which individual histories corresponding to initially close states tend subsequently to diverge exponentially. [D89]
Chaotic Inflation	(a) A model in which many distinct universes form from different regions of a "mother" universe, with some inflating and others perhaps not. [HH98] (b) A variation of the inflationary universe model in which random quantum fluctuations are continually forming new universes. (see Inflationary Universe Model; New Inflation; Old Inflation; Quantum Fluctuations; Quantum Mechanics.) [LB90]
Chaotic Inflationary Universe Theory	A version of the inflationary universe theory, proposed by Andrei Linde in 1983, for which the energy density diagram for the fields driving inflation can be as simple as a bowl, with a unique minimum at the center. If the initial randomly chosen value of the fields corresponds to a point high up the hill on the side of the bowl, then sufficient inflation can occur as the fields roll towards the state of minimum energy density. [G97]
Chapman's Equation	An equation expressing the velocity of a gas in terms of certain molecular constants. [H76]
Chapman-Jouguet Detonation	A detonation in which the velocity of the shock front with respect to the material behind it is equal to the corresponding sound velocity. [H76]
Characteristic Value	see Eigenvalue. [H76]
Charf	A permanent blemish on an image-tube phosphor. [H76]
Charge	The fundamental property of a particle that causes it to be affected by the electromagnetic force. [HH98]
Charge Bleeding	The overflow of charge up and down a column in a CCD when the pixel or storage well becomes saturated with photoelectrons. [McL97]
Charge Conjugation	The technical term for mathematical operations which interchange particles and antiparticles. [H76]
Charge Multiplet	A group of particles (such as the two nucleons or the three pions) which differ in electrical charge but which are nearly identical in mass and other respects (such as lifetime and angular momentum) and which seem to experience identical nuclear forces. [H76]
Charge Number	see Atomic Number. [H76]
Charles's Law	The pressure of an ideal gas at constant volume varies directly as the absolute temperature. [H76]
Charm	(a) The fourth flavor (i.e. type) of quark, the discovery of which in 1974 contributed both to the acceptance of the reality of quarks and to our understanding of their dynamics. The charmed quark exhibits a property called "charm" which is conserved in strong interactions. [CD99] (b) The fourth flavor of quarks. Predicted by theory, charmed quarks were discovered in 1974. [F88] (c) The property that distinguishes one of the types of quarks. At present, there are six types of quarks known, one of which is the "charmed" quark. [LB90]
Charmonium	A bound state consisting of a charmed quark and a charmed antiquark. A major impetus for the quark theory was the discovery in 1974 of the J/ particle, a particle whose properties closely matched the predictions for charmonium. [G97]
Chemical Clock	an asymptotically stable regime of a chemical system in which the concentrations of the reagents are periodic functions of time. Both the period and the amplitude are determined solely by the system's intrinsic parameters. [D89]
Chemical Differentiation	The separation of different elements, often heavier elements from lighter elements, as a consequence of different chemical reactions. [Silk90]
Chemical Elements	Ninety-two different stable or long-lived nuclei can be formed from neutrons and protons bound together. Each forms atoms by binding as many electrons to the nucleus as it has protons (so the nucleus is electrically neutral), giving ninety-two different atoms. These atoms are the smallest recognizable units of the ninety-two chemical elements. [K2000]
Chemical Enrichment	The process in which a star manufactures chemical elements, such as carbon and oxygen, in the nuclear reactions in its interior and then ejects these elements into space. The gas in the surrounding space is said to be chemically enriched. [LB90]
Cherenkov Detector	Apparatus through which it is possible to observe the existence and velocity of high-speed particles important in experimental nuclear physics and in the study of cosmic radiation. It was originally built to investigate the Cherenkov radiation effect, in which charged particles travel through a medium at a speed greater than that of light in that medium. [A84]
Chiral, Chirality	Feature of fundamental particle physics that distinguishes left- from right-handed, showing that the Universe is not fully left-right symmetric. [G99]
Chirality	An expression of the basic handedness of nature. Fundamental theories of the elementary particles and of superstrings must possess chirality. [P88]
2 Test	A least-squares statistical test that measures the probability of randomness in a distribution. [H76]
Chlorine	A green reactive gaseous element belonging to the halogens. It accounts for about 0.055% of the Earth's crust. Symbol: Cl; m.p. -100.38°C; b.p. -33.97°C; d. 3.214 kg m-3 (0°C); p.n. 17; r.a.m. 35.4527. [DC99]
Chondrite	A stony meteorite usually characterized by the presence of chondrules (q.v.). (Type I carbonaceous chondrites contain no chondrules.) [H76]
Chondrules	Small spherical grains varying from microscopic size to the size of a pea, usually composed of iron, aluminum, or magnesium silicates. They occur in abundance in primitive stony meteorites. Chondrules show evidence that they were formed at about the same time as the planets - it has been suggested that they formed in the Solar nebula by impact between high-velocity grains. True chondrules have never yet been observed in terrestrial rocks. [H76]
Chopping	The method of removing very large background signals at infrared wavelengths by alternating quickly from the object to nearby sky and back using a rocking motion of the telescope's secondary mirror. [McL97]
Christoffel Symbols	Mathematical quantities used in the mathematical formalism of general relativity, Einstein's theory of gravity. [LB90]
Chromatic Aberration	(a) Introduction of spurious colors by a lens. This defect flawed the performance of refracting telescopes for centuries, until attenuated by the introduction of corrective elements into a compound lens. [F88] (b) A defect of refracting telescopes whereby light of different colors is focused at different distances behind the objective. Blue light is refracted more than red light and hence comes to a focus inside that of red light. Images are then surrounded by a rainbow of colors. [H76] (c) The change in the image size in an optical system due to the wavelength dependence of refractive index of the material. [McL97]
Chromium	A transition metal; chromium is used in strong alloy steels and stainless steel and for plating articles. It is a hard silvery metal that resists corrosion at normal temperatures. Symbol: Cr; m.p. 1860±20°C; b.p. 2672°C; r.d. 7.19 (20°C); p.n. 24; r.a.m. 51.9961. [DC99]
Chromosphere	(a) The part of the Sun's atmosphere immediately above the surface (the photosphere) and beneath the corona. [A84] (b) The part of the Solar atmosphere between the photosphere and the corona. It consists of two rather well defined zones: the lower chromosphere extends to about 4000 km ( 10-8 to 10-13 g cm-3) and consists of cool (7500 K) neutral hydrogen; the upper chromosphere extends to about 12,000 km ( 10-16 g cm-3) and consists of hot (106 K), ionized hydrogen. It has an emission spectrum (see flash spectrum). [H76]
Chromospheric Network	A large-scale cellular pattern along the boundaries of which lie bright and dark mottles seen in H and other regions. [H76]
Chronometer	A highly accurate timepiece. [F88]
CID	Charge Injection Device. [LLM96]
Circinus X-1 (3U 1516-56)	A highly variable X-ray source. Many of its properties are similar to those of Cygnus X-1. [H76]
Circle	An ellipse possessing but one focus. [F88]
Circular polarization	A type of polarization of electromagnetic radiation in which the plane of polarization rotates uniformly round the axis as the ray progresses. [DC99]
Cislunar	An adjective referring to the region of space between the Earth and the Moon. [H76]
Clapeyron's Equation	A fundamental relation between the temperature at which an inter-phase transition occurs, the change in heat content, and the change in volume. [H76]
Classical Physics	Physics prior to the introduction of the quantum principle. Classical physics incorporates Newtonian mechanics, views energy as a continuum, and is strictly causal. [F88]
Clocking	The process of raising and lowering the voltages between two levels - high and low - on the electrodes or gates of a CCD in order to move charges from one pixel to the next. The voltage levels themselves are often called the "clocks" or "clock levels". [McL97]
Clock Frequency	The rate at which a CCD is clocked or read out. It is the reciprocal of (or one divided by) the pixel time; e.g. 40 microseconds (µs) per pixel corresponds to 25 kilohertz (kHz). [McL97]
Closed Space	A space of finite volume but without any boundary (in the cosmological context). [Silk90]
Closed String	A type of string that is in the shape of a loop. [G99]
Closed System (isolated system)	A set of one or more objects that may interact with each other, but do not interact with the world outside the system. This means that there is no net force from outside or energy transfer. Because of this the system's angular momentum, energy, mass, and linear momentum remain constant. [DC99]
Closed Universe	(a) Any model of the Universe in which the gravity of the matter content can reverse the expansion and cause a collapse. [C97] (b) A standard universe with a spherical three-dimensional spatial geometry. Such a universe is finite in both space and time, and recollapses. [HH98] (c) A homogeneous, isotropic universe is said to be temporally closed if gravity is strong enough to eventually reverse the expansion, causing the Universe to recollapse. It is said to be spatially closed if gravity is strong enough to curve the space back on itself, forming a finite volume with no boundary. Triangles would contain more than 180°, the circumference of a circle would be less than times the diameter, and a traveler intending to travel in a straight line would eventually find herself back at her starting point. If Einstein's cosmological constant is zero, as is frequently assumed, then a universe which is temporally closed is also spatially closed, and vice versa. [G97] (d) Cosmological model in which the Universe eventually stops expanding and begins to collapse, presumably to end in a fireball like that of the big bang. Compare open universe. [F88] (e) A universe that has a finite size. Closed universes expand for a finite time, reach a maximum size, and then collapse. In closed universes, the inward pull of gravity dominates and eventually reverses the outward flying apart of matter; that is, gravitational energy dominates the kinetic energy of expansion. The value of omega is greater than 1 for a Closed Universe. If a universe begins closed, it remains closed; if it begins open, it remains open; if it begins flat, it remains flat. In the Big Bang model of the Universe, the question of whether the Universe is closed, open, or flat is determined by the initial conditions, just as the fate of a rocket launched from earth is determined by its initial upward velocity relative to the strength of earth's gravitational pull. If the initial rate of expansion of the Universe was lower than a critical value, determined by the mass density, the Universe will expand only for a certain period of time and then collapse, just as a rocket launched with a velocity below a critical value, dependent on the strength of earth's gravity, will reach a maximum height and then fall back to earth. This is the behavior of a Closed Universe. If the initial rate of expansion of the Universe was larger than a critical value, the Universe is open and will keep expanding forever. If the initial rate of expansion was precisely the critical value, the Universe is flat and will expand forever, but with a rate of expansion that approaches zero. (see Flat Universe; Omega; Open Universe.) [LB90]
Cloud Chamber	A chamber used to show the tracks of ionizing radiation, especially alpha and beta particles. A diffusion cloud chamber has felt strips near the top soaked in water and ethanol. The bottom of the chamber is held at a low temperature, and there is continuous diffusion of vapor down the chamber. At one particular level, water droplets condense only along the tracks of ionizing radiation. The expansion cloud chamber contains moist air, sometimes with ethanol vapor, which is cooled by a sudden adiabatic expansion, causing the air to become supersaturated with water vapor. Water droplets condense out preferentially on ions formed along the tracks. It can be arranged that passage of an ionizing particle through the chamber is detected by a counter and the resulting pulse can be used to operate the pump, so that the expansion takes place just after the ion pairs have been formed. A camera may also be triggered to take a photograph of the tracks just as they become visible. Magnetic fields can be applied and the resulting curvature of the tracks provides information about the charge and energy of the particles. [DC99]
Cloud Chamber	A glass-walled enclosure containing a vapor in which particles can be detected by photographing the tracks of water droplets they leave behind when they pass through the chamber. [F88]
Clouds of Magellan	see Magellanic Clouds. [A84]
Cluster	1. A gathering of hundreds, thousands, or even a million stars. Star clusters come in two varieties: open clusters and globular clusters. 2. A gathering of hundreds or thousands of galaxies. The nearest large galaxy cluster is the Virgo cluster. [C95]
Cluster of Galaxies	An aggregate of galaxies. Bautz and Morgan divide them into three morphological types: type I contains a supergiant cD galaxy; type III contains no members significantly brighter than the general bright population. Coma is type II, Virgo is type III. Rood recognizes three types: A compact group (e.g., Stephan's Quartet) contains a few galaxies separated by a few galaxy diameters. A loose group (e.g., the Local Group; M81) contains on the order of 10 galaxies separated by 10-100 galaxy diameters. A rich cluster (e.g., Virgo; Coma) contains 100 or more galaxies within a volume comparable to that of a loose group. Scale of cluster, about 1 Mpc. 21 known X-ray sources are associated with clusters of galaxies. [H76]
Cluster Variable	see RR Lyrae star. [H76]
Clustering	In cosmology, the observed tendency of galaxies to bunch together, rather than to distribute themselves uniformly and independently of each other. [LB90]
CMD	Color-Magnitude diagram [BFM02]
CMOS	Complementary metal oxide semiconductor. Refers to microelectronic logic circuitry which employs both p-type and n-type MOS transistors in a single circuit (see Doping). Low power consumption is a feature of CMOS circuits. [McL97]
CNES	Centre National d'Etudes Spatiales (Paris, France). [LLM96]
CN-Strong Stars	Late type giants with strong CN bands. Metallic lines are also stronger than in normal giants. [JJ95]
CN-Weak Stars	High-velocity star with both weak metallic lines and weak CN bands. [JJ95]
CNO Stars	Late O-type or early B-type stars (O8 to B4) in whose spectrum the lines of some of the elements C, N and O are weaker or stronger than in the standard stars. [JJ95]
Coalsack	A prominent dark nebula in Crux, near the Southern Cross, readily visible to the naked eye, about 170 pc distant, located on the galactic plane. [H76]
Coarse-Graining	an operation implementing some form of spatial averaging which smoothes out relatively small length-scale configurational structure while preserving the larger length-scale structure. [D89]
Coaxial Cable	A type of electrical wiring. It consists of a copper wire surrounded by an insulator which in turn is surrounded by a braided copper shield which is encased in another plastic insulator. [McL97]
Cobalt	A lustrous silvery-blue hard ferromagnetic transition metal. It is used in alloys for magnets, cutting tools, and electrical heating elements and in catalysts and some paints. Symbol: Co; m.p. 1495°C; b.p. 2870°C; r.d. 8.9 (20°C); p.n. 27; r.a.m. 58.93320. [DC99]
Coded Mask	Coded masks are used for imaging high-enery x-ray and gamma radiation that cannot otherwise be focused by lenses or mirrors. A coded mask telescope is a basically pinhole camera, but one with many pinholes (which can number in the thousands) whose positions and sizes are carefully predetermined. The coded mask is placed above a detector usually together with a mechanical collimator. The holes pass a unique radiation pattern to the (position sensitive) focal plane detector. By analysing the detected pattern of light and shadow a unique solution of the original image of the sky can be recovered. [BFM02]
COBE	COsmic Background Experiment: Satellite used to study the microwave background. [BFM02]
Coherence	The existence of a correlation (statistical or temporal) between the phases of two or more waves. [H76]
Coherent	(a)Two sources of waves are said to be coherent if there is a constant relationship between the phases of the waves emitted by them. For most sources of light or similar electromagnetic radiations (other than radio waves) it is usually necessary to derive the radiations of the two sources from one original source, as in Young's interference experiment. Radiation is said to be coherent if there is a regular phase relationship between widely different parts of a wavefront, or if such regularity is maintained for a considerable time. Radiation from a laser is exceptionally coherent. [DC99] (b) Involving waves at different locations which have a definite (not random) phase relationship to each other and can therefore exhibit interference effects. [McL97]
Coherent Receivers	These devices respond to the electric field strength of the signal, and can preserve phase information about incoming photons. They operate by interference of the electric field of the incident photon with the electric field from a coherent local oscillator. These devices are primarily used in the radio and sub-millimeter regions, and are sometimes useful in the infrared. (See also Thermal Detectors; Photon Detectors)
Coherent Scattering	A scattering process that leaves atoms in the same energy state after the scattered photon departs in a direction different from that of the incident photon. The energy of the scattered photon is the same (in the rest frame of the atom) as that of the incident photon. [H76]
Cohomology	A branch of mathematics concerned with the patching together of spaces. [P88]
Cold Dark Matter (CDM)	(a) Hypothetical subatomic particles that move slowly compared with the speed of light. [C95] (b) Any dark matter candidate which was non-relativistic at the point of decoupling. [C97] (c) A model of structure formation in which an exotic particle whose energy is low at the time it decouples from other matter is responsible for structure formation. [HH98] (d) Particle physics theories that extend the Standard Model often predict the existence of new, stable particles that were present in the early universe and survive today, making up a large fraction of the matter of the universe. These particles interact wealky and they are usually massive, so they move slowly - they are cold. An example of such a particle is the lightest supersymmetric partner. Astronomers have evidence from the motion of galaxies, and from the large-scale structure of the universe, that cold dark matter exists. [K2000]
Cold Emission	Emission of electrons from a solid by a process other than thermionic emission. The term is usually used to describe either field emission or secondary emission. [DC99]
Cold-Gas Approximation	In MHD studies: An approximation in which the sound speed is much less than the Alfvén speed or the gas pressure is much less than the magnetic pressure. [H76]
Collapse of the Wavefunction	In the Copenhagen interpretation of quantum mechanics, the result of an act of measurement, in which the potentialities inherent in the quantum wavefunction take on a specific value, namely, that which is measured. [HH98]
Collapsed Star	see Black Hole. [H76]
Collider	A collider is made by accelerating beams of particles and causing them to collide. The energy of the colliding beams can provide much more energy (that can be used to make new particles) than if the beams hit stationary targets. Two challenges confront colliders: getting to larger energies and getting to higher intensities. [K2000]
Collimate	To make parallel, neither diverging nor converging. All rays from a given field point travel in the same direction. [McL97]
Collimator	An arrangement for producing a parallel beam of radiation for use in a spectrometer or other instrument. A system of lenses and slits is utilized. [DC99]
Collinear (Collinearity)	Three or more points lying in a straight line. [H76]
Collisionless Damping	The tendency of weakly interacting (collisionless) matter to smooth out gravitational perturbations by freely streaming from overdense to underdense regions. [HH98]
Colloid	A substance containing very small particles (sizes in the range 10-9-10-5 m). Sols, gels, and emulsions are examples of colloids. [DC99]
Color	(a) An attribute which distinguishes otherwise identical quarks of the same flavor. Three colors red, green and blue - are required to distinguish the three valence quarks of which baryons are composed. It must be stressed that these colors are just labels and have nothing to do with ordinary color. Color is the source of the strong force which binds quarks together inside baryons and mesons, and so the three colors (r, g, b) can be thought of as three different color charges analogous to electric charge. [CD99] (b) A property possessed by quarks and gluons. A `threefold type' of charge akin to electrical charge, believed to be the source of the strong force between quarks and described by the quantum chromodynamic theory of the strong interaction. [D89] (c) Each flavor of quark can exist in three variations, called colors, usually labeled as red, green, and blue. The color of a quark has no relation to its visual appearance, but the word color is used because there are three variations, in analogy with the three primary colors. Measurable properties of the quarks, such as electric charge and mass, depend on the flavor but not the color, but the color is responsible for the interactions that bind the quarks together (see Yang-Mills theories). Individual quarks cannot exist independently, but are forever confined within baryons or mesons, each of which is colorless. Baryons achieve colorlessness by being composed of three quarks, one of each color, while mesons achieve colorlessness by pairing each colored quark with its corresponding antiquark. [G97] (d) Property of quarks that expresses their behavior under the strong force. Analogous to the concept of charge in electromagnetism, except that, whereas there are two electrical charges (plus and minus), the strong force involves three color charges - red, green, and blue. The term is whimsical, and has nothing to do with color in the conventional sense, any more than quark "flavor", which determines the weak force behavior of quarks, has anything to do with taste. [F88]
Color Charge	Color charge and strong charge are the same thing. [K2000]
Color-Color Plot	Traditionally, a plot of B - V versus U - B. [H76]
Color Excess	Difference between the observed color index of a star and the intrinsic color index corresponding to its spectral type. It indicates the amount of reddening suffered by the light from the star when it passes through interstellar dust. [H76]
Color Field	Any particle carrying color charge (or strong charge) has an associated color field (or strong field) around it. Any other particle carrying color charge feels that field and interacts with the first particle. [K2000]
Color Force	The force between two particles carrying color charge. The color force (or strong force) binds quarks into protons and neutrons. The residual color force outside protons and neutrons is the nuclear force that binds protons and neutrons into nuclei. The color force is mediated by the exchange of gluons. [K2000]
Color Index	Difference between the photographic and photovisual magnitudes of a star; or more generally, the difference in magnitudes between any two spectral regions. Color index is always defined as the short-wavelength magnitude minus the long-wavelength magnitude. In the Johnson-Morgan UBV system, the color index for an A0 star is defined as B - V = U - B = 0; it is negative for hotter stars and positive for cooler ones. [H76]
Color-Magnitude Diagram	Plot of absolute or apparent visual magnitude against color index for a group of stars. (C-M Diagram, CMD ) [H76]
Color Temperature	A stellar temperature determined by comparison of the spectral distribution of the star's radiation with that of a blackbody. [H76]
Column Density (N)	The number of particles per square centimeter along a specified path with a length equal to the distance to the probing source. [H76]
Coma	(a) An aberration common in traditional reflecting telescopes, in which off-axis rays of light striking different parts of the objective do not focus in the same image plane. It produces elongated, comet-like images at the outer edge of the field. It is mainly because of coma that the Hale telescope is limited to on-axis work and has a usable field of only 10' without special corrector lenses. This problem has largely been solved by the Schmidt telescope and the Kitchey-Chrétien design. [H76] (b) Is rather similar in cause, effect, and correction to astigmatism. After refraction by a lens, a cone of rays from an off-axis object tends to have a tadpole-shaped section because of coma. (c) An off-axis aberration which produces images with flared tails like comets. [McL97]
Coma (of a comet)	The spherical region of diffuse gas, about 150,000 km in diameter, which surrounds the nucleus (q.v.) of a comet. Together, the coma and the nucleus form the comet's head. [H76]
Coma Cluster (Abell 1656)	A symmetric cluster of about 1000 galaxies (primarily E and S0) about 92 Mpc distant (z = 0.023). Luminous mass 4 × 1014 M = 8 × 1047 g; virial theorem mass about 5 × 1048 g; mass needed to bind the cluster about 4 × 1049 g. R 9 × 1024 cm. It is also an X-ray source (see Coma X-1). [H76]
Coma Cluster	(a) An open cluster of about 100 stars in our Galaxy (about 80 pc distant). Similar to the Hyades in overall binary frequency. [H76] (b) The nearest massive cluster of galaxies. The Coma cluster, about 300 million light years from us, contains about 1,000 galaxies in a region about 10 million light years across. (see Cluster; Galaxy.) [LB90]
Coma X-1 (3U 1257+28)	An extended X-ray source in the Coma cluster of galaxies. [H76]
Combination Variable	see Symbiotic Star. [H76]
Comet	A diffuse body of gas and solid particles (such as CN, C2, NH3, and OH), which orbits the Sun. The orbit is usually highly elliptical or even parabolic (average perihelion distance less than 1 AU; average aphelion distance, roughly 104 AU). Comets are unstable bodies with masses on the order of 1018 g whose average lifetime is about 100 perihelion passages. Periodic comets comprise only about 4% of all known comets. Comets are obviously related in some manner to meteors, but no meteorites from a comet have ever been recovered. Observations of comets Bennett and Kohoutek have established that a comet is surrounded by a vast hydrogen halo. [H76]
Comets, Nomenclature	When a newly discovered comet is confirmed, the IAU assigns an interim designation consisting of the year of discovery followed by a lowercase letter in order of discovery for that year. Frequently the discoverer's name precedes the designation - e.g., comet Bennett 1969i. If a reliable orbit is later established, the comet is given a permanent designation consisting of the year of perihelion passage followed by a roman numeral in order of perihelion passage - e.g., comet Bennett 1970 II. If the comet is periodic, the letter P followed by the discoverer's (or computer's) name is used - e.g., comet 1910 II P/Halley. [H76]
Comets, Family of	An aggregation of comets with similar aphelion distances (e.g., Jupiter's family). [H76]
Comets, Group of	An aggregation of comets with identical orbits except for phase. [H76]
Cometary Nebula	A reflection nebula with a fan shape that bears a superficial resemblance to a comet. Classical examples of the heads of cometary nebulae are R Mon, R CrA, and RY Tau. All have A0-G0 type spectra that resemble the spectrum of a T Tauri star, and their brightness varies from year to year. [H76]
Commensurate Orbits	A term applied to two bodies orbiting around a common barycenter when the period of one is an integral multiple of that of the other. [H76]
Common-User	A shared facility or common resource, such as a CCD spectrograph, a computer data reduction program or even a telescope, which has been carefully designed to meet the needs of many research programs. Common-user facilities are expected to be very reliable and well-supported. [McL97]
Commutation Relations	in quantum mechanics, if one has two operators A and B, then it is often the case that the action of the product operator AB is not the same as that of BA. The difference, AB-BA, is called the commutator of A and B. Specifying the value of the commutator is known as a commutation relation. [D89]
Co-Moving Coordinates	(a) A set of coordinates which do not change in an expanding (or otherwise moving) medium. i.e. the coordinates of a distant galaxy do not change just because of the expansion of space. [C97] (b) Coordinates fixed with respect to the overall Hubble flow of the Universe, so that they do not change as the Universe expands. [HH98]
Co-Moving Sphere	A hypothetical and arbitrary spherical surface (about any point) that is expanding along with the rest of the Universe. Relative to the comoving sphere, the particles on it are at rest. [Silk90]
Compact Galaxy	A galaxy similar to an N galaxy but with no disk or nebulous background. It is an object of high surface brightness which appears slightly nonstellar on photographs and which has a larger redshift than normal stars in our Galaxy. Nearest "compact" galaxy is M32. [H76]
Compact HII Region	A dense (ne 103 cm-3) H II region of small linear dimensions ( 1 pc). [H76]
Compact Infrared Sources	Strong compact infrared sources embedded in nebulosity. [JJ95]
Compact Radio Source	A radio source which has a small angular extent and is strongest at shorter wavelengths (cf. extended source). [H76]
Compactification	(a) The process in which a space of many dimensions effectively reduces its dimensions. Some new theories of particle physics the superstring theories claim that the Universe actually has 10 spatial dimensions but that 7 of these dimensions have become "compactified" down to subatomic size and thus are unobservable. (see Superstring Theory.) [LB90] (b) The process of "curling up" six of the ten dimensions of superstring theory. [P88]
Companion of Sirius (Sirius B)	A white dwarf of about 1 Solar mass but of only 0.03 Solar radii (R = 5400 km, Teff = 32,000 K). [H76]
Companion Star	Either one of a binary star system (although usually the less massive), sometimes only detectable by spectroscopy. [A84]
Comparison Band	The wavelength interval measured in the continuum outside a spectral feature - e.g., the 21-cm line. [H76]
Complex Analytic	A particular property of mathematical representations of physical or mathematical systems. To have complex analytic structure, a system must be able to be represented by complex numbers, among other things. The representation must also have the property that its value at one point suffices to define its value at all other points. (see Complex Number.) [LB90]
Complex Number	A type of number that is the sum of two parts, the first of which produces a positive number when multiplied by itself (like ordinary numbers), and the second of which produces a negative number when multiplied by itself (unlike ordinary numbers). Complex numbers were discovered mathematically in the nineteenth century and have been found to play a major role in physics. [LB90]
Complex Number Astrophysics	The basis of twistor theory. [A84]
Complexity	in information sciences, complexity measures the length of the shortest description of a given (finite) sequence of symbols. In the physical sciences, complexity is associated with the ability of a system to display long range coherence in space and time, and to undergo transitions between different states. These two alternative views might merge in chaotic dynamics. [D89]
Composite Object	Any object made of other objects is composite, as are atoms, nuclei, and protons. If quarks and leptons had followed the historical trend that matter at each level turned out to be composites of smaller constituents, experiments should already have shown evidence of their compositeness. This, combined with theoretical arguments, strongly suggests that quarks and leptons may be the ultimate constituents of matter-the indivisible "atoms" of the Greeks. [K2000]
Composite Particle Theory	A class of elementary particle theories, according to which there are increasing numbers of elementary particle states of higher and higher mass. [Silk90]
Composite Spectrum Stars	Objects with a spectrum due to superposition of the spectra of two different stars. [JJ95]
Compound lens	Two or more lenses used together as a unit. For instance, the eyepiece of a telescope and the lens of a camera are both normally compound lenses; each has a number of elements (the single lenses) along the same optical axis. Insect compound eyes are not compound lenses in this sense - the elements are not on a single axis. The elements of a compound lens may or may not touch. The function of compound (rather than single) lenses is usually to minimize aberrations. (see also Doublet) [DC99]
Compton Effect	(a) An increase in the wavelength of x-rays or gamma rays when scattered by loosely bound electrons in substances, the electrons being ejected. Theoretically the phenomenon is treated as a collision between a photon and an electron, the latter being regarded as a free particle initially at rest. The photon transfers energy and momentum to the electron, hence the wavelength increases. (see also Duality) [DC99] (b) Decrease in the frequency of high-energy radiation (such as X-rays) caused when a photon loses some of its energy to a free electron by collision. [H76]
Compton Scattering	(a) The scattering of photons by free electrons in an ionized medium. [C97] (b) Scattering of a photon due to the Compton effect (see also Non-coherent Scattering). [H76]
Compton Wavelength	(a) The wavelength of a photon containing the rest energy of a particular particle. [C97] (b) In relativistic quantum mechanics the quantity h/mc represents the wavelength of the quantum wave associated with a particle of mass m. This is called the Compton wavelength (after Arthur Compton) because it appears in the theory of Compton scattering - the scattering of photons by electrons. [D89] (c) The quantum wavelength of a particle with a highly relativistic velocity. [HH98] (d) The dual nature of particles and waves results in the association of a fundamental wavelength with each elementary particle. It is equal to / mc, where is Plancks constant, c is the speed of light, and m is the particle mass. [Silk90]
Concave	Curving inward, away from the viewpoint. A concave mirror is one with a concave reflecting surface. A concave lens is either a biconcave or a plano-concave lens. [DC99]
Concavo-Convex	Describes a lens with one concave surface and one convex surface. Most spectacle lenses have this shape. Concavo-convex lenses can be converging or diverging. They are sometimes called meniscus lenses. [DC99]
Condensed Matter Physics	primarily the physics of solids and liquids. Analogous behavior is expected in some stellar interiors, particularly in white dwarfs and neutron stars. [D89]
Conduction	1. A process of heat transfer through a substance without movement of the substance itself. The rate of transfer depends on the sample length and cross-sectional area, the temperature difference, and the nature of the material. Good conductors, such as copper and silver, are often metals that have free electrons. The energy is transmitted by movement of these. In poor solid conductors (insulators) there are no conduction electrons and the transfer is only by vibrations of atoms or molecules in the crystal structure. 2. Passage of charge through a sample under the influence of an electric field. The charge may be carried by electrons (e.g. in metals), by electrons and positive holes (in electrolytes and semiconductors), or by ions (in electrolytes and gases). (see also Energy Bands) [DC99]
Conduction Band	(a) In an insulator or semiconductor this is the lowest empty energy band, and electrons excited to the conduction band can carry current. In a metal the conduction band is the partially filled band in which the current-carrying electrons occur. [D89] (b) The combined unoccupied higher energy levels or orbits of all the atoms in a semiconductor crystal. [McL97]
Configuration	(a) The arrangement of electrons in shells around the nucleus of an atom. The electron configuration largely determines the chemical behavior of an element as well as some physical properties, such as electrical conductivity. [DC99] (b) All the data that refer to the location of each mass point in ordinary space. [H76]
Configuration Mixing	The superposition of a number of wave functions belonging to different configurations. [H76]
Confinement	The inability of quarks to escape the bonds that hold them in pairs and triplets at the energy levels found in the Universe today. (see Gluon Lattice; Asymptotic Freedom) [F88]
Confinement	The property of quarks which implies that they cannot exist as free particles, but are forever bound into protons, neutrons, etc. (see Color) [G97]
Conformal Geometry	Conformal geometry is related to the stretchings of space-time that preserve the light cone structure. A space containing only null lines, such as Penrose's basic twistor theory, is conformally invariant. Both mass and nonlinear interactions, such as gravity, break conformal invariance. [P88]
Conic Constant	A number used in optics to specify the shape of a surface which is a conic section, i.e. parabolic, hyperbolic, elliptical. Conic sections are obtained by slicing through a cone at the appropriate angle. [McL97]
Conifold Transition	Evolution of the Calabi-Yau portion of space in which its fabric rips and repairs itself, yet with mild and acceptable physical consequences in the context of string theory. The tears involved are more severe than those in a flop transition. [G99]
Conjugate Points	Any pair of points relative to a lens, reflector, or other optical system such that either is imaged at the other. Compare the symmetry in object distance u and image distance v in the equation: 1 / v + 1 / u = 1 / f    [DC99]
Conjunction	The phenomenon in which two bodies have the same apparent celestial longitude (see Longitude, Celestial) or right ascension as viewed from a third body. Conjunctions are usually tabulated as geocentric phenomena. For Mercury and Venus, geocentric inferior conjunction occurs when the planet is between the Earth and Sun, and superior conjunction occurs when the Sun is between the planet and Earth. [S92]
Conservation Laws	Laws that identify a quantity, such as energy, that remains unchanged throughout a transformation. All conservation laws are thought to involve symmetries. [F88]
Conservation of Angular Momentum	(a) The principle that the angular momentum of a system (the momentum of rotation about a point) remains the same as long as no external torque acts. [HH98] (b) The total angular momentum of an isolated dynamical system does not change during the course of its evolution. [Silk90]
Conservation of Energy	(a) The principle that the total energy of a closed system never changes, that energy is only converted from one form to another. This principle must be enlarged under special relativity to include mass-energy. [HH98] (b) The total energy of a system (including kinetic energy and gravitational energy) is conserved and does not vary. Thus, kinetic energy can only increase at the expense of gravitational potential energy. Modern physics has modified the law of conservation of energy, since matter can be created or annihilated; a more general law is the conservation of mass and energy. [Silk90]
Conservation of Mass and Energy	Important physical principle and one of the basic laws of physics stating that matter is neither created nor destroyed (although mass may become energy, the energy quantitatively represents the mass). One exception to this principle is a singularity; another follows from the theory of virtual particles. [A84]
Conservation of Matter	The principle that matter is neither created nor destroyed. This principle is only approximately true, since special relativity shows that matter and energy are equivalent and interconvertible. [HH98]
Conservation of Momentum	The principle that the linear momentum of a system (in Newtonian mechanics, mass times velocity) remains the same as long as no external force acts. [HH98]
Conservative Field	A field of force such that the work done on or by a body that is displaced in the field is independent of the path. Hence if a body is moved in a closed path (back to the starting point) the net work done is zero. Gravity and an electrostatic field in a vacuum are conservative. Any form of friction prevents the field from being conservative, although the total energy of any isolated system is conserved in all cases. [DC99]
Conservative Scattering	Scattering that occurs in the absence of absorption. [H76]
Conservative System	(a) A system in which total energy is conserved in time, and the evolution of the observable properties is indifferent with respect to the direction of time. [D89] (b) A system in which energy is conserved; that is one in which there is no dissipation of energy. [H76]
Conserved Quantity	A quantity that remains unchanged in the course of the evolution of a dynamical system. There are seven known quantities that are conserved: energy (including mass), momentum, angular momentum (including spin), charge, electron-family number, muon-family number, and baryon-family number. [H76]
Consistent	The property possessed by a scientific theory when it contains and extends an earlier well-supported theory; for example, general relativity is consistent with Newtonian gravity. [HH98]
Constant of Precession	see Precession of the Equinoxes. [A84]
Constants	Avogadro's number 6.02 × 1023; 1 amu = 1.66 × 10-24 g; me = 9.1 × 10-28 g; mp = 1.00728 amu; mH = 1.67 × 10-24 g; c = 299,792.46 km s-1; 1 AU = 1.49598 × 1013 cm; 1 lt-yr = 9.4605 × 1017 cm = 6.324 × 104 AU; 1 lt-min 0.13 AU; 1 pc = 3.084 × 1018 cm = 206,265 AU = 3.26 lt-yr. G = 6.668 × 10-8 dyn cm2 g-2. [H76]
Constellation	(a) A grouping of stars, usually with pictorial or mythical associations, that serves to identify an area of the celestial sphere. Also, one of the precisely defined areas of the celestial sphere, associated with a grouping of stars, that the International Astronomical Union has designated as a constellation. [S92] (b) 1. As used by astronomers, constellation refers to a particular region of the sky. There are 88 official constellations that blanket the entire sky, so every star, known or unknown, is