An Etymological Dictionary of Astronomy and Astrophysics

The chemical reaction that gives rise to → carbon monoxide in the → interstellar medium. According to models, several processes may lead to CO formation. For example, HCO⁺ + e → CO + H. The molecule HCO⁺ is itself produced through several paths, for example: H₃⁺ + C → CH₂⁺ + H, CH₂⁺ + H₂ → CH₃⁺ + H, CH₃⁺ + O → HCO⁺ + H. Alternatively:
C⁺ + H₂O → HCO⁺ + H. Another possibility:
C⁺ + OH → CO⁺ + H, CO⁺ + H₂→ HCO⁺ + H.

See also: → carbon monoxide; → formation.

A spectral line energy distribution which plots the intensity of each → carbon monoxide (CO) transition as a function of the upper J number. This type of → diagram is a powerful diagnostic tool, where models show that these CO ladders have very different shapes depending on the type of excitation (i.e. photon dominated region, PDR or X-ray dominated region, XDR) as well as density and radiation environment.

See also: → carbon monoxide (CO); → ladder.

→ com-.

An image made up of several individual images of relatively short exposure times which are added together in order to produce a final image of higher quality.

Etymology (EN): Co-added, from → co- “together” + added p.p. of → add; → image.

Etymology (PE): Tasvir, → image; hamafzudé from ham- “together”, → com-, + afzudé p.p. of afzudan, → add.

A theory according to which
the Earth and Moon formed from the → protoplanetary disk at the same time.

See also → giant impact hypothesis, → capture theory, → fission theory.

See also: → co-; → formation; → theory.

Of or relating to two or more celestial bodies that share, or almost share, the same orbit.

See also: → co-; → orbital.

The motion of two or more bodies around the Sun on different orbits when it takes them the same amount of time to complete one revolution. There are three possible types of co-orbital motions of a small body associated with a planet: → tadpole orbits, → horseshoe orbits, and → quasi-satellite orbits.

See also: → co-orbital; → motion.

Any of satellites which either share the same orbit or which occupy immediately adjacent orbits that change periodically as the satellites approach one another (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → co-orbital; → satellite.

The action or quality of a → co-orbiting asteroid.

Etymology (EN): From co- “together,” → com- +
→ orbit + → -ing.

Etymology (PE): From ham- “together,” → com- + madâr→ orbit + -i noun suffix.

An asteroid having a → co-orbital motion.

See also: → co-orbiting; → asteroid.

For any rotating planetary body, a thermal limit beyond which the → rotational velocity at the equator intersects the → Keplerian orbital velocity. Beyond this corotation limit, a hot planetary body forms a structure, called a → synestia, with a corotating inner region connected to a disk-like outer region.

Beyond this limit a body cannot have a single → angular velocity. It can instead exhibit a range of morphologies with disk-like outer regions. The (CoRoL is a function that depends upon the composition, thermal state, → angular momentum and mass of a body

(Simon J. Lock nd Sarah T. Stewart, 2017, arXiv:1705.07858v1).

See also: → co-; → rotational; → limit.

1. (v.int.) Generally, of liquids, to change into a thickened mass, curdle; congeal.
   

2. Biology, Medicine: of blood, to form a clot.
   

3. Physical chemistry: of colloidal particles, to flocculate or cause to flocculate.
   

4. Astrophysics: of dust grains in the interstellar medium and protoplanetary disks, to grow into larger entities.
   → dust coagulation.

Etymology (PE): Mâsidan “to coagulate, clot,” originally “of milk, to turn into yogurt,” mâst “clotted milk, yogurt;” Gilaki mas, Lori mâs, Kurd. mâzd, mâst, Sangesari must, Baluchi madhagh, mastagh; Mid.Pers. mâs- “to coagulate, become hard;” cf. Skt. mástu- “milk cream,” Arm. macum “soar milk,” macanim “to clot, congeal.”
Rocidan from Lori roc “congealed,” rocesse “to congeal, clot.”
Laxté bastin lit. “coagulate into (solid) piece,” from laxté “piece, part, portion,” + bastan “to coagulate, congeal; to bind, shut” (Mid.Pers. bastan/vastan “to bind, shut,” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” PIE *bhendh- “to bind,” cf. Ger. binden, E. bind).

Verbal noun from → coagulate.
Physical chemistry: A separation or precipitation of particles from a dispersed state in a colloid solution.
Astrophysics: The mechanism by which dust grains grow into
larger entities in the interstellar medium and protoplanetary disks. → dust coagulation.

See also: Verbal noun from → coagulate.

A black, hard mineral consisting of carbon and various carbon compounds. Coal is formed from the decomposition of ancient plants buried deep in the Earth’s crust
for millions of years. It is currently the most widely used substance to generate electricity and heat. Its combustion products are used as raw material for a variety of products including cement, asphalt, and plastics. Due to the harmful gases that it releases, the use of coal is constantly being reduced as alternative fuels are found.

Etymology (EN): M.E. cole, from O.E. col “charcoal, live coal;” (cf. O.Fr. kole, M.Du. cole, Du. kool, O.H.G. chol, Ger. Kohle, from PIE root *g(e)u-lo- “live coal” (cf. Irish gual “coal”).

Etymology (PE): From zoqâl, → charcoal, + sang, → stone.

A prominent → dark nebula visible to the naked eye as a dark patch silhouetted against the starry band of → Milky Way in the Southern sky. It obscures an area of about 5 by 7 degrees on the sky and extends beyond the borders of → Crux into neighboring → constellations → Centaurus and → Musca. It lies at a distance of approximately 500 → light-years.

Etymology (EN): → coal; sack, from M.E., from O.E. sacc, from L. saccus, from Gk. sakkos, of Semitic origin (cf. Heb. saq “sack”).

Etymology (PE): Guni “sack;” zoqâl, → charcoal.

To grow together; to come together so as to form one whole, to fuse. → merge;
→ fusion.

Etymology (EN): From L. coalescere, from co- + al-, stem of alere “to nourish, make grow” + -esce, from -escere, a suffix conveying an inchoative meaning.

Etymology (PE): Âhamidan, from â- nuance prefix + ham “together” (Av. hama- “similar, the same;” Skt. samah “even, level, similar, identical;” Gk. hama “together with, at the same time,” homos “one and the same,” PIE *samos “same,” from base *sem- “one, together”) + -idan infinitive suffix.

1. General: The act or state of growing together, as similar parts; the act of uniting by natural affinity or attraction; the state of being united.
   

2. Merging of the stars composing a → binary system after having undergone → supernova explosion. General relativity predicts that binary systems of → compact objects will emit energy in the form of → gravitational radiation, and that this loss of energy eventually will lead to the coalescence of the system.

See also: Verbal noun from → coalesce.

A scenario for building up → massive stars through merging of → intermediate-mass protostars. It occurs in the cores of dense stellar clusters that have undergone core contraction due to rapid → accretion of gas with low → specific angular momentum. The required densities are, however, very high, 10⁸ stars pc^-3, which are extremely rare (Bonnell et al. 1998, MNRAS 298, 93).

See also: → coalescence; → model.

To move without further use of propelling power. → coasting flight, → coasting Universe.

Etymology (EN): M.E. coste, from O.Fr., from L. costa “rib, side,” cf. Mid.Pers. kust, kustag “side, direction; district,” Mod.Pers. xost, xwast “a beaten road; island;” PIE *kost- “leg, bone.”

Etymology (PE): Rahâraftan, from rahâ “free, set free”
(O.Pers. rad- “to leave,” Skt. rah-, rahati “separates, leaves,” Av. razah- “isolation;” PIE *redh-) + raftan “to go, walk” (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack”).

The unpowered flight of a spacecraft or missile after propulsion cutoff or between the burnout of one stage and the ignition of the next.

See also: Coasting, verbal adjective from → coast; → flight.

A Universe whose density is just less than or equal to the critical value and expands forever with no change in the expansion rate.

See also: Coasting, verbal adjective from → coast; → Universe.

To → cover with a → thin  → layer of a → substance, as → aluminum over the → surface of a → mirror, → aluminize.

Etymology (EN): Verb from noun coat, from M.E. cote, from O.Fr. cote “coat, robe,” from some Germanic source; cf. O.S. kot “woolen mantle,” O.H.G. chozza “cloak of coarse wool,” Ger. Kotze “a coarse coat,” of unknown origin.

Etymology (PE): Andudan, variant andâyidan, from Mid.Pers. handudan, from O.Iranian *ham-dâvaya-, from ham- “together” + *dâvaya-, from dav- “to rub, clear,” cf. Av. dav- “to clean, polish,” Skt. dhâv-, PIE *dheu- “to shine”.

An open cluster of about 40 stars at the border of → Vulpecula and → Sagitta. It has an apparent size of about 1° and
lies 420 → light-years away. Also called Collinder 399 and → Brocchi’s Cluster.
Six of its brighter stars, of sixth and seventh magnitude, are lined up in a nearly perfect row, from the center of which four stars form a hook to resemble the coathanger shape. To the naked eye, it appears as an unresolved patch first recorded by the Persian astronomer Sufi in A.D. 964. It was later rediscovered by Giovanni Battista Hodierna (1597-1660). The Coathanger shares roughly the same motion with several other clusters, including the → Pleiades.

Etymology (EN): → coat; hanger, from hang, M.E. han(i)gen, fusion of O.E. hon “suspend” and hangian “be suspended;” also probably influenced by O.N. hengja “suspend” and hanga “be suspended” (cf. O.Frisian hangia, Du. hangen, Germ. hängen).

Etymology (PE): Raxtâviz, from raxt “clothes, garment, wearing apparel” + âviz “hang,” → pendulum.

A → thin → layer of a → substance spread over a → surface.

See also: Noun from → coat; → -ing.

A silver gray, brittle, hard metallic → chemical element which is highly magnetic; symbol Co. → Atomic number 27; → atomic weight 58.9332; → melting point 1,495°C; → boiling point about 2,870°C; → specific gravity 8.9 at 20°C. It is used in many → alloys, and in particular its compounds have been used since ancient times (Egyptians, Persians, Greeks) to produce a blue color in glass and ceramics. Cobalt was discovered in 1735 by the Swedish chemist Georg Brandt (1694-1768). It has several radioactive isotopes, including Co-56, half-life about 77 days, Co-57, 272 days, Co-58, 71 days, Co-60, 5.27 years. The → light curve of → type I supernovae is explained by the radioactive decay of nickel-56 through cobalt-56 to iron-56.

See also: From Ger. kobold “evil spirits or goblins,” who were superstitiously thought to cause trouble for miners, since the mineral contained arsenic which injured their health and the metallic ores did not yield metals when treated with the normal methods.

Geology: A → sedimentary particle that is between 64 and 256 mm in size. Cobbles are larger than → pebbles but smaller than → boulders. Cobbles have typically been rounded by abrasion during sedimentary transport (geology.com/dictionary).

Etymology (EN): From M.E. cobill, kobill, probably a diminutive of M.E. *cob, *cobb, ultimately from Proto-Germanic *kubb- (“lump; round object”)

• -le.

Etymology (PE): Qolve, variant of gorde “kidney.”

A web spun by a spider to entrap its prey; a single thread spun by a spider; something resembling a cobweb; anything finespun, flimsy, or insubstantial (Dictionary.com).

Etymology (EN): M.E. coppeweb, derivative of O.E. -coppe “spider” in atorcoppe “poison spider;” + → web.

Etymology (PE): Karu “cobweb, web,” variants kari, kartané, kartiné, kârtanak, kârtané, kare tan (all in Dehxodâ), (Malâyeri, Hamadâni) kâtena, (Gilaki) kârtang, (Kermâni) kerâš, (Qêyeni) kalaš, (Qomi) kârye, (Tabari) kel, kuli, (Yazdi) kare, from *kar-, *kâr-, *kel- “to weave;” cf. (Ormuri, in Pakistan, Afghanistan) gal-/galôk- “to weave;” PIE base *ker- “to weave; rope.”

An emission nebula located about 3,000 light-years away toward the → constellation  → Cygnus. It is thought to be a region of active → star formation.

Etymology (EN): Cocoon, from Provençal Fr. coucoun,
from O.Fr. coque “egg shell, nut shell,” L. coccum “berry,” from Gk. kokkos “berry, seed;” → star; → nebula.

Etymology (PE): Miq, → nebula; pilé “the silkworm’s cocoon; a purse”, cf. Skt. patta- “woven silk.”

A star hidden in a dense envelope of gas and dust which is a strong source of infrared emission.

Etymology (EN): → Cocoon nebula; → star.

1. A system used for brevity or secrecy of communication, in which arbitrarily chosen words, letters, or symbols are assigned definite meanings.
   

2. Computers: The symbolic arrangement of statements or instructions in a computer program in which letters, digits, etc. are represented as binary numbers; the set of instructions in such a program (Dictionary.com).

Etymology (EN): M.E., from O.Fr. code, from L. codex “book, book of laws,” later form of caudex
“tree trunk,” hence “document made up of wooden tablets.”

Etymology (PE): Ramz “secret writing, enigma,” loan from Ar.

The complement of → declination; the angular distance along a great circle from the celestial pole,
i.e., 90&deg - declination.

Etymology (EN): Codeclination, from → co- + → declination.

Etymology (PE): Hamvâkil, from ham-, → co-, + vâkil,
→ declination.

A manuscript text in book form which was common before the invention of printing. The codex is the earliest known form of a bound book which replaced the scroll. It was a Roman invention. → Dresden codex.

Etymology (EN): From L. codex “book,” → code.

Etymology (PE): Nebigân, from nebi / nepi / nevi “book, scripture,” from Mid.Pers. nibêg “writing, scripture, book,” related to neveštan, → write, + -gân suffix denoting collective nature.

The set of values that a → function is allowed to take (i.e. may possibly come out of a function), as opposed to the → range.

See also: → co-; → domain.

1. Math.: A number or letter placed before an algebraic expression to indicate that the expression is to be multiplied by that factor, e.g. in the expression 2 x³, 2 is the coefficient of x³. In general, any factor of a product is called the coefficient of the product of the remaining factors.
   

2. Physics: Factor which measures some specified property of a given substance, and is constant for that substance under given conditions, such as the coefficient of friction. → factor.

Etymology (PE): Hamgar, from ham- “together,” → com- + -gar agent suffix, from kar-, kardan “to do, to make,” Mid.Pers. kardan, O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “makes,” cf. Skt. kr- “to do, to make,” krnoti “makes,” karma “act, deed;” PIE base k^wer- “to do, to make.”

A quantity that indicates a property of fluids and is defined by the ratio of shearing → stress to the rate of change of shearing → strain. It is also simply called viscosity. The coefficient of viscosity is expressed by: μ = (F/A) / (dv/dy), where F is the force required to maintain a steady velocity difference dv between any two parallel layers of the fluid, A is the area of the layers, and dv/dy is the → velocity gradient between two points separated by a small distance measured at right angles to the direction of flow. The unit of viscosity is that of force times distance divided by area times velocity. Thus, in the cgs system, the unit is 1 dyne.cm/cm².(cm/s), which reduces to 1 dyne.s/cm². This unit is called 1 → poise.

See also: → viscosity; → coefficient.

A flat mirror with a clock-drive mounted in such a way that it moves from east to west to compensate for the apparent
rotation of the Earth in order that the image of a particular area of sky remains fixed in the focal plane. See also → siderostat and → heliostat.

Etymology (EN): Coelostat, from L. coelo-, for caeli-, combination form of coelum “sky” + -stat prefix denoting something that stabilizes, keeps, fixes, from -stata, from Gk. -states “one that causes to stand,” or statos “standing,” from *sta- “to stand.”

Etymology (PE): Âsmândâštâr, from âsmân, → sky, + dâštâr “holder, maintainer,” from dâštan “to hold, maintain; to have; to possess,” Mid.Pers. dâštan, O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind,” Skt. dhr-, dharma- “law,”
Gk. thronos “elevated seat, throne,” L. firmus “firm, stable,” Lith. daryti “to make,” PIE *dher- “to hold, support.”

1. To compel by force, especially by law or authority.
   

   2. To obtain through the use of force, threat, or other
      forms of constraint.

Etymology (EN): M.E., from O.Fr. cohercier, from L. coercere “to restrain, surround,” form → com- “together” + arcere “to enclose, confine, keep off,” from PIE *ark- “to hold, contain, guard.”

Etymology (PE): Pazuridan, literally “to force against,” from pa- “contrary to; against; opposing,” → counter- + zur “power, force,” → strength, + infinitive suffix -idan.

The act, practice, or power of using physical or moral force to compel a person to do something.

See also: Verbal noun of → coerce.

Serving or tending to coerce.

See also: → coerce; → -ive.

Same as → coercive force.

See also: → coercive; → field.

The strength of an external → magnetic field that brings to zero the → magnetic flux density of a magnetic material when that field is caused to operate in the opposite direction from the orientation of the → magnetization of the material. Also called coercivity. Coercive force is a measure of the magnetization of a → ferromagnetic material. It is usually measured in the units of → oersted or ampere/m.

See also: → coercive; → force.

1. General: The quality of being coercive.
   

2. Physics: Same as → coercive force.

See also: → coercive; → -ity.

Of the same age.

Etymology (EN): From L. co-, → com-, + aevum “age,” → aeon.

Etymology (PE): Hamsenn “equally old,” from ham-, → com-, + senn→ age.

A number associated with an → element of a → determinant. If A is a square matrix [a_ij], the cofactor of the element a_ij is equal to (-1)^i+j times the determinant of the matrix obtained by deleting the i-th row and j-th column of A.

See also: → co-; → factor.

1. The mental process of knowing, including → awareness, → perception, → reasoning, and judgment.
   

2. The product of such a process; something thus known, perceived, etc. (Dictionary.com).

Etymology (EN): M.E. cognicioun; L. cognitionem (nominative cognitio, from cognitus p.p. of cognoscere, from → co-

• gnoscere, noscere “to learn;” cognate with Pers. šenâs, šenâxt, as below.

Etymology (PE): Šenâxtâr, verbal noun of šenâxtan “to know, recognize,” dânestan “to know;” O.Pers./Av. xšnā- “to know, learn, come to know, recognize;”
cf. Skt. jñā- “to recognize, know,” jānāti “he knows;” Gk. gignoskein “to know, think, judge;” L. gnoscere, noscere “to come to know” (Fr. connaître; Sp. conocer); O.E. cnawan; E. know; Rus. znat “to know;” PIE base *gno- “to know.”

1. Of or pertaining to the act or process of knowing, perceiving, remembering, etc.; of or relating to → cognition.
   

2. Of or pertaining to the mental processes of perception, memory, judgment, and reasoning, as contrasted with emotional and volitional processes (Dictionary.com).

See also: → cognition; → -ive.

1. To stick together; be united; hold fast, as parts of the same mass.
   

2. Physics (of two or more similar substances) to be united
   within a body by the action of molecular forces (→ cohesion).

Etymology (EN): From L. cohaerere “to cleave together,” from → com- “together,” + haerere “to stick.”

Etymology (PE): Hamdusidan, from ham- “together,” → com-,

• dusidan (Dehxodâ) “to stick, to adhere,” → adhere.

The property of two or more electromagnetic waves when they are in
fixed phase relationship over time. If the crests and troughs of the waves meet at the same time and place they are said to be in phase.

See also: → cohere + -ence, → -ance.

Of an → electromagnetic wave, the area of a surface perpendicular to the direction of → propagation, over which the wave maintains a specified → degree of coherence. According to the van Cittert-Zernike theorem, the coherence area is given by: A_c = D²λ²/(πd²),

where d is the diameter of the light source and D is the distance away.

The coherence area is an important parameter in photon correlation experiments. In the → Young’s experiment the → interference pattern is only seen if slits are inside one coherence area.

See also: → coherence; → area.

The distance over which an → electromagnetic wave train maintains a specified → degree of coherence.

The coherence length is related to the → coherence time multiplied by vacuum → velocity of light.

See also: → coherence; → length.

The time over which a propagating → electromagnetic wave may be considered → coherent. The coherence time of an interferometer is the interval during which the fringe phase remains stable.

See also: → coherence; → time.

Two or more wave sources are said to be coherent sources if the phase difference between a pair of points, one in each source, remains constant.

See also: Coherent, adj., → coherence.

Light waves that have the same wavelength and possess a fixed phase relationship, as in a laser.

See also: → coherent; → light.

A branch of optics that uses coherent radiation to produce holographic three-dimensional images of objects.

See also: → coherent; → optics.

A scattering process in which the scattered radiation bears the same frequency and phase as the incident radiation.

See also: → coherent; → scattering.

One of two light beams derived from the same source in → interference experiments. It is impossible to obtain interference from two separate sources because their → wavefronts do not have a constant → phase difference. In → Young’s experiment, → Fresnel’s biprism, → Fresnel’s mirrors, and → Lloyd’s mirror the two sources always have a point-to-point correspondence of phase, since they are both derived from the same source.

See also: → coherent; → source.

Holding together.
Physics: The attraction between the molecules of a solid or liquid that holds the parts of the substance together.

Etymology (EN): From L. cohæsus, p.p. of cohærere “to stick together,” → coherence.

Etymology (PE): Hamduseš, verbal noun from hamdusidan “to cohere,” → coherence.

1. Of or pertaining to the molecular force → cohesion.
   

2. Characterized by or causing → cohesion.

See also: Adjective from → cohere.

The energy associated with the chemical bonding of atoms in a solid.

See also: → cohesive; → energy.

The force of → attraction between the molecules of the same substance.

See also: → cohesive; → force.

A device consisting of a length of electrical wire wound in a spiral to provide magnetic field by → electromagnetic induction.

Etymology (EN): Maybe from M.E. cull, from M.Fr. culier, coillir “to gather,” from L. colligere “to bind together,” → collect.

Etymology (PE): Picé “a curled, a twisted figure or object,” from picidan “to twist, invove, enttwine, coil.”

1. (Of two more objects) to correspond in area and outline; to occupy the same place.
   
2. To happen at the same time.

Etymology (EN): Coincide, from Fr. coincider, from M.L. coincidere, from L. → co- “together” + incidere “to fall upon,” from in- “upon” + cadere “to fall,” PIE base *kad- “to fall”.

Etymology (PE): Barhamoftâdan, from bar- “to, upon, together” + oftâdan “to fall,” Mid.Pers. patet “falls,” opastan “to fall,” Av. pat- " to fly, fall, rush," cf. Skt. patati “he flies, falls,” L. petere “to fall, rush out,” Gk. piptein “to fall,” petomai “I fly,” PIE base *pet- “to fly, to rush.”

1. Fact, event, or condition of coinciding.
   
2. The occurrence of events simultaneously in a striking manner but without any causal connexion between them.

See also: → coincide.

Occupying the same area in space or happening at the same time. Of two geometric figures, matching point for point.

Etymology (EN): From Fr. coincident, from M.L. coincident-, coincidens, pr.p. of coincidere, → coincide.

See also: → coincide.

The polar angle on a sphere measured from the north pole instead of the equator; equal to “90° - latitude”.

See also: → co- + → latitude.

Having a relatively low temperature.

Etymology (EN): M.E., from O.E. cald, ceald “cold, cool” (cf. O.Fr. and O.Sax. kald, O.H.G. and Ger. kalt, Goth. kalds “cold”), from PIE root *gel-/*gol- “cold;” cf. L. gelare “to freeze,” gelu “frost,” glacies “ice;” Kurd. girsân, girsiân “to coagulate” (Cheung 2007).

Etymology (PE): Sard “cold, cool,” afsordan, afsârdan “to congeal;” Mid.Pers. sard/sart “cold;” Av. sarəta- “cold;” cf. Skt. śiśira- “cold;” L. calidus “warm;” Lith. šaltas “cold;” Welsh clyd “warm;” PIE *keltos- “cool.”

A broad → absorption feature observed in → X-ray spectra of → active galactic nuclei (AGN).
It is caused by material associated with the → interstellar medium in our → Galaxy and/or the host galaxy of the AGN or cold material near the AGN.

→ Quasars commonly have their X-ray spectrum absorbed by cold gas between us and the quasar X-ray source (along our → line of sight). This is particularly common in less luminous quasars. See also → warm absorber.

See also: → cold; → absorber.

1. A type of → accretion flow by a → compact object such as a → black hole that consists of cool → optically thick gas and has a relatively high mass → accretion rate, in contrast to → hot accretion flows.
   

2. Gas accreting from the → intergalactic medium (IGM) onto → galactic haloes with sufficiently low velocities so that it will not be shocked to the → virial temperature of the halo, but will instead flow at a relatively low temperature
   (T ~ 10⁴ K).

Galaxies grow by accreting gas from → cosmic filaments. Feedback from star formation and → active galactic nuclei returns a significant fraction of the → interstellar medium (ISM) to the halo and may even blow it out of the halo into the IGM. This “cold accretion” will happen if the cooling time of → virialized gas is too short to maintain a hot, → hydrostatic halo. The existence of such a cold accretion mode has been confirmed by simulations, which have furthermore demonstrated that cold mode accretion can also be important for halos sufficiently massive to contain hot, hydrostatic gas. Because gas accretes preferentially along the filaments of the cosmic web, the streams of infalling gas have relatively high gas densities and correspondingly low cooling times. This allows the cold streams to penetrate the hot, hydrostatic halos surrounding massive galaxies, particularly at → high redshifts (F. van de Voort et al., 2012, MNRAS 421, 2809).

See also: → cold; → accretion; → flow.

Any → hypothetical → non-baryonic  → dark matter that is → non-relativistic at the point of → decoupling in the → early Universe. CDM plays a key role in → cosmic structure formation. See also → CDM model, → lambda cold dark matter, → Meszaros effect, → missing satellites problem.

See also: → cold; → dark; → matter.

An accretion process whereby material coming from an → accretion disk settles onto the → protostellar surface through a geometrically thin layer or thin accretion columns. Heat brought into the protostar in the accretion flow radiates freely into space until the temperature attains the photospheric value. Most of the stellar surface is unaffected by the accretion flow (see, e.g., Hosokawa et al. 2010, ApJ 721, 478).

See also: → cold; → disk; → accretion.

The act or process of working with another or others on a joint project.

Etymology (EN): From Fr. collaboration, noun of action from L. collaborare from → com- “with” + labore “to work.”

Etymology (PE): Hamkâri, from ham-, → com-, + kâr, → work, + -i verbal noun suffix.

The collapse of the core of a very massive star which results in a black hole accompanied by a very energetic → gamma-ray burst.

See also: From collaps(e) + (st)ar.

1. (v.) To fall inward abruptly under its own → gravity.
   

2. (n.) The act of falling inward abruptly under its own gravity. → gravitational collapse; → core collapse.

Etymology (EN): From L. collapsus, p.p. of collabi “fall together,” from → com- “together” + labi “to fall, slip.”

Etymology (PE): 1) Mod.Pers. rombidan “to fall apart, to crumble,” Hamadâni, Malâyeri: rommidan, Lori remese “get destroyed,” remane “to destroy a building,” possibly
cognate with E. crumble “to break into small fragments,” from O.E. cruma, akin to D. kruim, Ger. Krume “crumb,” L. grumus “heap of earth,” root of Fr. grumeau “lump.”

2. Rombeš from present stem romb “collapse” + noun suffix -eš.

The idea, central to the → Copenhagen Interpretation of quantum theory, whereby at the moment of observation the → wave function changes irreversibly from a description of all of the possibilities that could be observed to a description of only the event that is observed. More specifically, quantum entities such as electrons exist as waves until they are observed, then “collapse” into point-like particles. According to the Copenhagen Interpretation, observation causes the wave function to collapse. However it is not known what causes the wave function to collapse. Same as → wave collapse.

See also: → collapse; → wave function.

A star that has undergone → collapse.

Etymology (EN): Collapsed p.p. of → collapse; → star.

Etymology (PE): Setâré, → star; rombidé p.p. of rombidan, → collapse.

1a) To bring together into a one body or place.

1b) To gather and gradually accumulate in a place.

2. to gather together; assemble.

Etymology (EN): From O.Fr. collecter, from L. collectus, p.p. of colligere “gather together,” from → com- “together” + legere “to gather,” cf. L. lignum “wood, firewood,” lit. “that which is gathered”;
cognate with Gk. legein “to say, tell, declare,” from PIE *leg- “to pick together, gather, collect”

Etymology (PE): 1) Gerdâvardan “to collect,” from gerd “round; around,” → disk; alternatively, ultimately from PIE base *ger- “to gather,” cf. Iranian ger-, gor-, etc. “to gather,”
→ category, + âvardan “to bring,” → production.
Anbâštan, anbârdan “to fill, to replete;” Mid.Pers. hambāridan “to fill;” from Proto-Iranian *ham-par-, from prefix ham-, → com-, + par- “to fill;” cf. Av. par- “to fill,” parav-, pauru-, pouru- “full, much, many;” O.Pers. paru- “much, many;” Mid.Pers. purr “full;” Mod.Pers. por “full, much, very;” PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru- “much, abundant;” Gk. polus “many,” plethos “great number, multitude;” O.E. full.

2. Gerdâmadan “to come together,” with âmadan “to come,” → efficiency.

A → sequential star formation model involving → massive stars and → H II regions. The energetic ultraviolet photons from a massive star born in a → molecular cloud drive a spherical → ionization front radially outward from the star at a velocity much higher than the → sound speed in the cold neutral gas. The supersonic expansion of the H II region through the surrounding neutral gas creates a → shock front, sweeping up an increasingly massive and dense shell of cool neutral gas. This is the collect phase of the process in which the H II region simply acts like a snowplough. If the expansion of the H II region continues for long enough, the surface density of the shell increases to the point where the shell becomes self-gravitating. The shell is then expected to collapse and fragment.
Individual fragments may then enter a non-linear collapse phase, possibly forming massive stars. This model was first proposed by Elmegreen & Lada (1977, ApJ 214, 725), who used a one-dimensional analysis. Whitworth et al. (1994, MNRAS, 268, 291) developed an analytical model for the collect and collapse process which predicts the fragmentation time, the size, number, and mass of the fragments (see also Elmegreen 1998, in ASP Conf. Ser. 148, Origins, eds. Woodward et al., p. 150 and references therein). → stimulated star formation, → triggered star formation.

See also: → collect; → collapse; → model.

Of an interferometric telescope made up of several mirrors, the hypothetical mirror created by the combination of the individual mirrors.

See also: → collect; → area.

1. The act or process of collecting.
   

2. A group of objects or works to be seen, studied, or kept together.

See also: Verbal noun from → collect.

Forming a whole; of or characteristic of a group taken together.

See also: → collect + → -ive.

1. The whole behavior of a system of many interacting components,
   which differs from that of the individual components. Some classical examples are the → many-body problem, → phase transition, most of thermodynamic phenomena,
   collective oscillations occurring in solids, etc. Physics dealt with collective behavior much earlier than the term collective behavior was even coined.
   

2. Sociology: The more or less unpredictable act of people when being in group, which is totally different from the behavior they would have if acted individually.
   

3. Biology: A large number of group phenomena such as pattern formation, self organization, trail formation, and synchronization seen among many living creatures.

See also: → collective; → behavior.

Formation of stars, especially → massive stars, in group as opposed to individual formation.

See also: → collective; → star; → formation.

1. The principle of ownership of the means of production, by the state or the people.
   

2. A social system based on this principle (Dictionary.com).

See also: → collective; → -ism.

1. Collective character.
   

2. A collective whole.

See also: → collective; → -ity.

The process of forming collectives or collective communities where property and resources are owned by the community and not individuals (TheFreeDictionary.com).

See also: Verbal noun of → collectivize.

To organize (a people, industry, economy, etc.) according to the principles of → collectivism (Dictionary.com).

See also: → collective; → -ize.

A device that collects: a solar energy collector; a dust collector.

The semiconductor region in a bipolar junction transistor in which most of the current flows.

See also: From → collect + -or.

1. To strike together with forceful impact, such as two astronomical objects.
   
2. Of two or more atomic particles, to approach closely resulting in an abrupt change of momentum or exchange of energy.

Etymology (EN): From L. collidere “to strike together,” from col- variant of → com- “together” + laedere “to strike,” of unknown origin.

Etymology (PE): Ham- “together, with,” → com-

• kubidan, kuftan “to beat, strike, pound,” kus “drum,” Mid.Pers. kôftan and kôstan “to beat, strike.”

A particle accelerator in which two beams of particles are forced to collide head on.

See also: Agent noun from → collide + → -or.

Same as → collider.

See also: → collide; → beam; → accelerator.

To pack and align photons or atomic particles parallel to a particular direction.

Etymology (EN): L. collimatus, p.p. of collimare, alteration of collineare “to make straight,” from → com- + linea, → line.

Etymology (PE): From ham- “together, with,” → com- + râstâ, → direction, + -idan infinitive suffix, kardan “to do,” → work.

Made accurately parallel or brought into line.

See also: Past participle of → collimate.

A → beam of → photons or → subatomic particles with a narrow → cross section that has little or no spatial spread.

See also: → collimated; → beam.

A beam of particles in which every particle would have exactly the same direction of travel, thereby the jet moves without → dispersion. Collimated jets of → plasma are associated with → protostars.

See also: → collimated; → jet.

1. The process of restricting a beam of photons or particles to a given area.
   
2. The process of adjusting a telescope to place all optical elements in precise alignment. → align a telescope.

See also: Verbal noun of → collimate.

An instrument that creates a tight parallel beam of light or particles.

See also: Agent noun from → collimate.

A catalog of → open clusters published in 1931.

See also: Named after Per Collinder (1890-1975) the Swedish astronomer who created the catalog; → catalog.

Lying on the same straight line. Collinear points are a set of points on the same line.

See also: → com- + → linear.

A system of two or more forces that lie along the same → line of action.

See also: → collinear; → force.

A mathematical property where all points lying on a line initially still lie on a line after transformation.

See also: → collinear; → -ity.

The act or process of colliding, → collide.

See also: Verbal noun from → collide .

Of or relating to → collision.

See also: → collision; → -al.

Same as → cross section.

See also: → collisional; → cross; → section.

The process when the energy difference between the excited and non excited states of an atom is taken away by an electron during a collision.

See also: → collisional; → decay.

A physical process which is caused by the free electrons that are energized either by → photoionization or → collisional ionization. Collisional excitation puts ions, atoms, and molecules into excited states from which they may decay radiatively. Collisional excitation is important in the → interstellar medium.

See also: → collisional; → excitation.

A physical process whereby heat is imparted to (e.g. → interstellar dust grains or → molecular hydrogen) through collisions (with hot electrons, ions, etc.).

See also: → collisional; → heating.

An → ionization resulting from a → collision.

See also: → collisional; → ionization.

A plasma in which the → mean free path of the → charged particles between two → collisions is much smaller than the size of the system containing the plasma.

See also: → collisional; → plasma.

Same as the → Landau damping.

See also: → collision; → -less; → damping.

1. An informal meeting for the exchange of views.
   
2. A usually academic seminar on a broad field of study, usually led by a different lecturer at each meeting.

Etymology (EN): From L. colloquium “conference, conversation,” from → com- “together” + loqui “to speak”.

Etymology (PE): From Lori hacâyé “conversation among two or more people,” maybe from Av. ušyāi- “to speak, talk,” from vak- “to speak,” Skt. vacas “speech, word,” cognate with L. vox “voice,” vocare “to call,” Gk. ops “voice,” epos “song;” PIE root *wek- “to speak”.

1. A visual sensation produced in the brain when the eye views various wavelengths or frequencies of light.
   

2. Short for → color index.
   

3. A property of → quarks required to distinguish the three basic quarks of which → baryons are composed. In all → hadrons, the quarks (red, green, blue) are combined in such a way as to yield a colorless overall particle.

Etymology (EN): From M.E. colour, from O.Fr. colur, from L. color
“color, hue,” from Old L. colos “a covering,” from PIE *kel-os- “that which covers,” from *kel- “to cover, conceal.”

Etymology (PE): Rang, from Mid.Pers. rang “color” (abrang “splendor”), related to Mod.Pers. razidan “to color,” raxš “a mixture of red and white,” also the name of Rostam’s horse (loan from Sogd.?),
awrang “glory, beauty, throne;” Kurd. raš “black;” cf. Skt. raj- “to color, to become red;” Gk. rezein “to color;” PIE base *(s)reg- “to color, paint” (Cheung 2007).

In the → standard model of particle physics, a property possessed by → quarks and → gluons that determine rules for how these particles may interact in the context of → quantum chromodynamics.

Color charge is analogous to electromagnetic charge, but it comes in three types rather than two, which results in a different type of force, the → strong interaction.
There are three pairs of colors and anti-colors, named red (anti-red), green (anti-green), and blue (anti-blue). The terminology has nothing to do with visible color. It is only a word that is used to designate three independent types of the strong charge characteristic.

See also: → color; → charge.

The difference between the observed → color index of a star and the intrinsic color index corresponding to its → spectral type. It indicates the modification brought to a color index by the → interstellar absorption.

Etymology (EN): → color; excess, M.E., from O.Fr., from L. excessus “departure, going beyond,”
p.p. excedere “to depart, go beyond,” from → ex- “out” + cedere “to go, yield.”

Etymology (PE): Fozuni, from afzuni “excess,” afzuni kardan “to exceed bounds,” → add; → color.

The difference between the → apparent magnitude of a star measured at one standard wavelength and the apparent magnitude at another longer, standard wavelength, allowing the quantitative measure of a star’s color.

See also: → color; → index.

The temperature of that black-body which has the same spectral energy distribution in a limited spectral region, as the object under study has.

See also: → color; → temperature.

Empirical mathematical transformation applied to the observed magnitudes in order to convert them into a standard system, or into a different system.

See also: → color; → transformation.

A diagram based on two photometric colors usually representing the same class of astronomical objects.

See also: → color; → diagram.

Fr.: diagramme couleur-couleur

Same as → color-color diagram.

A form of → Hertzsprung-Russell diagram in which the luminosity is the vertical axis and the → color index the horizontal axis.

See also: → color; → luminosity,
→ diagram.

A form of → Hertzsprung-Russell diagram in which the visual absolute magnitude M_v is the vertical axis and the → color index the horizontal axis.

See also: → color; → magnitude,
→ diagram.

The measurement and definition of unknown colors in terms of standard colors.

See also: → color; → -metry.

The Dove. A small → constellation in the Southern Hemisphere just south to → Canis Major and → Lepus. Abbreviation: Col; genitive: Columbae.

Etymology (EN): L. columba “dove.”

Etymology (PE): Kabutar “pigeon,” Mid.Pers. kabôtar, from kabôd “grey-blue; pigeon,” cf. Skt. kapota- “a dove, pigeon; the grey color of a pigeon.”

CCD detector: Series of pixels arranged under one another.

Etymology (EN): Column, from O.Fr. columpne, from L. columna “pillar,” collateral form of columen “top, summit,” from PIE *kel- “to project.”

Etymology (PE): Sotun, from Mid.Pers. stun, from O.Pers. stênâ “column,” Av. stuna-, Skt. sthuna- “column.”

Density of the interstellar matter lying between an object and the Earth in a cylinder with a unity base.

See also: → column; → density.

Either of two great circles of the celestial sphere that passes through the poles and meets the ecliptic at either the solstice points (the solstitial colure) or the equinox points (the equinoctial colure).

Etymology (EN): From L. colurus, from Gk. kolouros “dock-tailed,” from kol(os) “docked” + -ouros “-tailed,” from oura “tail;” so called because the lower part is permanently hidden beneath the horizon.

Etymology (PE): Koldom, from Mod.Pers. kol “docked, short,” most probably cognate with the Gk. term, as above, + dom(b) “tail,” Av. duma- “tail.” Recorded in classical dictionaries, kol has several variants in a large number of dialects: kola, kalta, kel, kelma, koc, kall, kor, kul in Gilaki, Tâleši, Lori, Malâyeri, Hamedâni, Qâeni, and others, cf. Av. kaurva- “bald, docked,” kaurvôduma- “with a bald tail,” kaurvôgaoša- “with bald ears.”

Prefix denoting “together; with; joint; jointly”. It is sometimes used for intensification as in complete, complain, convince.

Etymology (EN): M.E., from O.L., classical L. form cum “together, together with,” Gk. koinos “common,” from PIE *kom- “beside, near, by, with.”

Etymology (PE): Ham- and ham “together, with; same, equally, even,” Mid.Pers. ham-, like L. com- and Gk. syn- with neither of which it is cognate. O.Pers./Av. ham-, Skt. sam-, sa-; also O.Pers./Av. hama- “one and the same,” Skt. sama-, Gk. homos-;
originally identical with PIE numeral *sam- “one,” from *som-. The Av. hąm- (nasal a) appears in various forms: ham-, han- (before gutturals, palatals, dentals) and also həm-, hən-, ha- (Bartholomae, 1772). Variants in Pers. ha- as in (Anâraki) ha-bend, → connect, and (Kurd.) hasûn “to whet, sharpen,”
and hâ- as in hâ-dâdan, hâ-gereftan, see Dehxodâ.

1. The glowing envelope of gas and dust that surrounds a comet’s nucleus.
   

2. An elongated, → comet-shaped → image of a → point source of → light
   caused by → aberration in the → optical system. In → telescopes it is often because → off-axis rays of light striking different parts of the → objective do not focus in the same → image plane.
   → Coma Berenices; → Coma cluster; → hydrogen coma.

See also: L. coma “hair,” from Gk. kome “hair;” → hair.

Berenice’s Hair. A → constellation made up of many faint stars and located near the north Galactic pole between → Canes Venatici to the north, → Virgo to the south, → Leo to the west, and → Boötes to the east. Abbreviation: Com; genitive: Comae Berenices.

See also: → coma; L. Berenices genitive of Berenice,
a queen of Egypt, wife of Ptolemy III, who sacrificed her hair to Aphrodite, begging her husband’s victory in the war with the Assyrians, who had killed his sister. While the story is an old one, the constellation is relatively new, being introduced by Tycho Brahe (1546-1601).

The nearest rich cluster of galaxies which contains more than a thousand known galaxies, is about 20 million light-years in diameter, and lies about 280 million light-years away in the → constellation  → Coma Berenices. Also known as Abell 1656.

See also: → coma; → cluster.

1. General: The act of combining or the state of being combined.
   

2. Math: The number of ways elements making up a set can be arranged into various groups without regard to their order in the group. → permutation

See also: Noun from → combine

A branch of mathematics dealing with the → combination and → permutation of
sets of elements and mathematical relations that characterize their properties.

Etymology (EN): From combinator(ial) (from combinatorial analysis),

• → -ics.

Etymology (PE): Miyâzešik, from miyâzeš, → combination,

• -k, → -ics.

To cause to join in a close union or whole; unite.

Etymology (EN): From M.Fr. combiner, from L.L. combinare “to unite, yoke together,” from L. → com- “together” + bini “two by two,” adv. from bi- “two, twice,” cf. Av. biš “twice,” bi-, dva- “two,” Skt. dvi- “two,” Gk. di-, O.E. twi-.

Etymology (PE): Miyâzidan, infinitive from miyâz-, variant of miz- in â-miz-, âmixtan “to mix,” âmizé, âmižé “mixture,” âmiq “mixture; copulation;”
Mid.Pers. âmêz-, âmêxtan (Proto-Iranian *āmis- ,*āmiz-;
PIE *meik- “to mix”);
cf. Av. mayas- “to mix;” Skt. miks- “to mix, mingle,” miśr- “to mix, blend, combine;” Gk. misgein “to mix, mingle;” L. miscere (p.p. mixtus) “to mix;” O.C.S. meso, mesiti “to mix,” Rus. meshat, Lith. maisau “to mix, mingle.”

1. Capable of catching fire and burning; inflammable.
   

2. A combustible substance.

See also: → combustion; → -ible.

1. Any chemical reaction in which a substance (fuel) combines with oxygen to produce heat and often light. Combustion reactions usually involve a complex sequence of free-radical chain reactions. The light is produced by excited atoms, molecules, or ions.
   

2. → nuclear combustion.

Etymology (EN): M.E., from O.Fr. combustion, from L. combustionem (nominative combustio) “a burning,” noun of action from p.p. stem of comburere “to burn,” from → com-, intensive prefix + urere “to burn.”

Etymology (PE): Suzeš, → burning

To approach or move toward a particular person or place.

Etymology (EN): M.E., from O.E. cuman “come, approach, arrive;” cf. Du. komen, Ger. kommen, Goth. qiman; cognate with Pers. âmadan, as below.

Etymology (PE): Âmadan “to come, to occur;” Mid.Pers. âmatan;
O.Pers. gam- “to come; to go;” Av. gam- “to come; to go,” jamaiti “goes;” Proto-Iranian *āgmatani; Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come.”

A small body of → gas and → dust which revolves around the → Sun in a usually very → elliptical or even → parabolic → orbit. It is seen to be composed of a → head, or → coma, and often with a spectacular gaseous → tail extending a great distance from the head. The rocky-icy head is called the → comet nucleus. As the comet nears the Sun, the increased temperature causes the → ice in the nucleus to → sublimate
and form a gaseous halo around the nucleus, called the coma. Comets often possess two tails, a → dust tail that lies in the orbit behind the comet generated by surface activity, and a brighter, ionized → gas tail, that points away from the Sun, driven by → solar wind. → Long-period comets
are thought to originate in the → Oort cloud, at distances exceeding 50,000 → astronomical units (AU). They are perturbed by the planets (especially → Jupiter) to fall in toward the Sun. Their orbits typically have random inclinations and a very large → eccentricity; some → hyperbolic orbits have been observed.
→ Short-period comets
apparently arise in the → Kuiper belt in the zone from 20 to 50 AU. Their orbits typically have small eccentricities. Both cometary reservoirs are thought to represent primordial solar system material. A comet with a dust coating on its surface that inhibits gas production might be classified as an → asteroid. Because of this ambiguity, objects such as → Chiron, a → Centaur asteroid, have been reclassified as comets.

Comets are primarily composed of amorphous → water ice, but also contain → carbon dioxide (CO₂), → carbon monoxide (CO), → formaldehyde (H₂CO), → methanol (CH₃OH), → methane (CH₄) at a few percent level (with respect to water), and many other molecules at a lower level. See also → comet designation.

Etymology (EN): From O.Fr. comète, from L. cometa, from Gk. (aster) kometes, “long-haired (star),” from kome “hair of the head,” so called from resemblance of the comet’s tail to streaming hair.

Etymology (PE): Dombâledâr, from
dombâlé “tail,” from domb, dom (Mid.Pers. dumb,
Av. duma- “tail”) + -âlé, -âl resemblance suffix, → -al

• dâr “having, possessor,” (from dâštan “to have, to possess,” O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind,” Skt. dhr-, dharma- “law,”
  Gk. thronos “elevated seat, throne,” L. firmus “firm, stable,” Lith. daryti “to make,” PIE *dher- “to hold, support”).

A → comet with an irregular → nucleus of roughly 3 × 5 km across orbiting the Sun between → Jupiter and → Earth with a period of 6.45 years. The comet has been observed from Earth on seven approaches to the Sun: in 1969, 1976, 1982, 1989, 1996, 2002, and 2009. It was also imaged by the → Hubble Space Telescope in 2003, which allowed estimates of its size and shape. It arrived at → perihelion on 13 August 2015. In 2014 the → European Space Agency probe → Rosetta, launched in 2004, was placed on an orbit around 67P/Churyumov-Gerasimenko. Over an entire year, as it approached the Sun, Rosetta mapped the comet’s surface and studied changes in its activity.

See also: → comet; Named after its discoverers, Klim Churyumov and Svetlana Gerasimenko, Ukrainian
astronomers, who first noticed the comet in 1969.

A → nomenclature system for naming → comets. In early 1995, a new comet designation system was established by the → International Astronomical Union. The main rules are as follows:
a) If the comet is a newly discovered one, it first gets a provisional name,
which closely matches the → asteroid designation system. For example, the first comet discovered in the first half of 1998 January is designated 1998 A1, the second 1998 A2, etc.
b) The name of the person(s) who discovered the comet may be added to this designation (limited, however, to three names). For example, comet → Hale-Bopp (C/1995 O1) has its full name as Hale-Bopp C/1995 O1, whereas its designation is C/1998 O1. If several people are involved with a discovery at an observatory, the comet may be named after the observatory instead of the individuals.
c) → Long-period comets and one-apparition → periodic comets receive only a provisional designation.
d) A → short-period comet would get the P/designation until it is recovered in a second → apparition. At this point, the P/Year designation would be replaced with a number followed immediately by an upper case P, and a slash followed by the name of the discoverer(s). The number here is one more than the number of known periodic comets that have reappeared. For example, the comet Hug-Bell (P/1999 X1) was given the full name 178P/Hug-Bell after it reappeared in 2007. Previously, 177 periodic comets had got assigned numbers.
e) Long-period comets are indicated by the prefix C.
f) If the comet is destroyed, or if it fails to appear after several apparitions, it would be prefixed D/ (→ defunct comet) followed by the year of its discovery. For example, → Comet Shoemaker-Levy 9 has been assigned D/1993 F2 since it was discovered in the second half of March in 1993 and was destroyed when it crashed into Jupiter in 1994.
g) Comets that lack sufficient position measurements for an orbital determination are given the designation of X/ followed by the year of their discovery and the appropriate letter and number code.
h) When a → comet nucleus nucleus splits, each fragment is given the comet designation followed by A, B, C, etc (for fragments).

See also: → comet; → designation.

A group of comets with similar orbital characteristics. They result from perturbations by planets which change the diverse original orbits of the comets into those they now occupy.

See also: → comet; → family.

One of the brightest comets seen in the twentieth century, even though it came no closer to Earth than 1.32 AU (on 22 March 1997). It was visible to the naked eye for many months. The → nucleus of Hale-Bopp was estimated to be about 30 to 40 km across. Hale-Bopp has an orbital period of 2,380 years and is predicted to be seen again in AD 4377.

See also: Discovered independently by American amateur astronomers Alan Hale and Thomas Bopp on July 22, 1995; → comet.

A → long-period comet found in January 1996, which became the brightest comet since → Comet West in 1976. It was a bright naked-eye object and remained so in March, April, and May of 1996. At closest approach to Earth on March 25, is was only 0.10 AU away,
displaying a long tail of up to 100 degrees. Small fragments were observed to break off the main nucleus. Hyakutake was the first comet from which X-ray emission was detected.

See also: → comet; Named after the Japanese amateur astronomer Yuuji Hyakutake (1951-2002), who discovered this comet in the morning of January 30, 1996.

The solid, centrally located part of a → comet. The nucleus is a mass of dust and frozen gases. When heated by the → Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the → coma, which is later swept into an elongated tail.

Reliable measurements of cometary nuclei indicate sizes from a few km to 10 or 20 km. The nucleus of → Comet Hale-Bopp is one of the largest (perhaps 40 km).

The composition of the nucleus is determined by measuring the composition of the coma (except for 67P/Churyumov-Gerasimenko).
The dominant → volatile is → water, followed by → carbon dioxide (CO₂), → carbon monoxide (CO), → formaldehyde (H₂CO), → methanol (CH₃OH), → methane (CH₄) at a few percent level (with respect to water) and many other molecules at a lower level.

See also: → comet; → nucleus.

A comet, formally designated D/1993 F2, whose shattered nucleus crashed into the planet → Jupiter over the period of July 16-22, 1994, several months after its discovery. The collision produced scars that were visible from Earth even in small telescopes. The cataclysmic event was the first collision between two → solar system bodies ever observed. The comet had been discovered on March 24, 1993, from photographs taken using the 0.46 m → Schmidt telescope at → Palomar Observatory.

The appearance of the comet was reported as “most unusual”: the object appeared as a “dense linear bar’’ with a “fainter, wispy tail.’’ The comet’s brightness was reported as about magnitude 14, more than a thousand times too faint to be seen with the naked eye. Later observations revealed that the “bar’’ was made up of as many as 21 pieces “strung out like pearls on a string,’’ according to one researcher.

Orbit calculations show that on July 7, 1992, the comet had passed only 25,000 km
above Jupiter. The differential pull of the planet’s enormous → gravitational force on the near and far sides of the comet fragmented it into the 21 or more pieces with sizes estimated at up to 2 km and an enormous amount of smaller debris. The comet had been in a rapidly changing orbit around Jupiter for some time before this, probably for at least several decades.

See also: → comet; Named after the husband and wife scientific team of American Carolyn S. (1929-) and Eugene M. Shoemaker (1928-1997) and Canadian amateur astronomer David H. Levy (1948-)

A spectacular comet that at its closest approach to Earth reached a brightness of -1 magnitude. It was so bright that could be seen even at sunrise. The comet reached → perihelion on 1976 Feb. 25 at 0.20 A.U. and had a fan-shaped tail of dimensions 25° x 25° x 15° on the sky. A few days after perihelion, the nucleus split in four fragments. The → carbon monoxide (CO) molecule in comets was first detected in West. The comet’s orbit has a period of about 500,000 years. Formerly designated 1976 VI.

See also: After the Danish astronomer Richard M. West (1941-), who worked at the → European Southern Observatory (ESO); → comet.

Of or relating to or resembling a → comet.

See also: → comet; → -ary.

The appearance of → gas and → dust features from the rocky-icy nucleus of a comet when approaching the Sun (→ cometary atmosphere, → cometary tail). The → sublimation of → water can explain cometary activity at distances from the Sun up to about 4 → astronomical units. At larger distances, the average temperature of the → comet nucleus’ surface is less than 140 K, too low for efficient sublimation of water → ice. However, there are many examples of cometary activity at larger distances. This can probably be due to the sublimation of more → volatile → chemical species. Indeed, radio spectroscopic observations of comets at large distances have revealed an important → outgassing of → carbon monoxide (CO), which can sublimate at temperatures as low as 25 K.

See also: → cometary; → activity.

The envelope of → gas and → dust around a → comet nucleus, also known as → coma. As the comet approaches the → Sun, the frozen materials → sublimate and give rise to an expanding atmosphere. The atmosphere is composed of dust, → molecules,
→ radicals, and molecular → ions
released from the inner coma with velocities ~ 0.5 to 1 km s^-1, well above the → escape velocity for the nucleus. The
→ chemical species observed in cometary spectra can be divided into several categories: (i) atoms and molecules related to → water (H, O, OH, OH⁺, H2O, H2O⁺),

(ii) carbon and related molecules (C, C⁺, CO, CO⁺, CO₂⁺, C₂, CH, CH⁺, HCO, H₂CO),

(iii) → nitrogen and related molecules (CN, CN⁺, HCN, CH₃CN, NH, NH₂, N₂⁺, NH₃, NH₄),

(iv) → sulphur and related molecules (S, CS, S₂, H₂S⁺),

(v) → metals (Na, K, Ca, Co, Cr, Cu, V, Fe, Mn, Ni).

For a typical average comet the neutral atmosphere is first seen when the heliocentric distance is d ≤ 3 → astronomical units.

See also: → cometary; → atmosphere.

A relatively small cloud of → dust and → gas in the → interstellar medium shaped like a comet with a bright-rimmed head. Cometary globules are situated near
young → massive stars with a strong → stellar wind. The wind ionizes gases
on the side facing the → O stars and sweep away the low-density gas toward the tail. Cometary globules are believed to be → molecular cloud condensations, which are so dense that they are not disrupted when an → H II region expands into the molecular cloud surrounding it.
The → Rosette nebula is a good example of an H II region which shows an abundance of cometary globules.

See also: → comet; → globule.

→ comet nucleus.

See also: → cometary; → nucleus.

The → path followed by a → comet in the → solar system around the → Sun.

Most cometary orbits appear to be → elliptical, or in some cases → parabolic. The orbits of → short-period comets are elliptical, carrying them out to a region lying from → Jupiter to beyond the orbit of → Neptune. Those of → long-period comets are very elliptical. The orbits may be strongly influenced if they pass near the Jovian planets, particularly Jupiter itself. The cometary orbits are also influenced to some degree by gases shooting out of comets, so their orbits are primarily but not completely determined by gravity.
Newton (1644-1727) was the first to compute a cometary orbit. He found that the comet of 1680 was following a parabolic orbit around the Sun. Edmond Halley (1656-1742), following the methods of Newton, computed the → orbital elements of 24 comets. He realized that the comets of 1531, 1607 and 1682 had very similar elements and postulated that they were in fact the same object, orbiting an elongated ellipse. He predicted the next return to occur in 1758 or early 1759. The return of what is now called Halley’s comet was observed after his death, This first observation of a “predicted” comet is manifestly one of the major successes of → celestial mechanics.

See also: → cometary; → orbit.

A formation of → gas and/or → dust that streams away from the → coma of many comets under the influence of the Sun’s → radiation pressure and the → solar wind. See also:

→ dust tail, → gas tail, → ion tail, → plasma tail, → sodium tail, → Type I tail, → Type II tail, → antitail.

See also: → cometary; → tail.

1. Eating together at the same table.
   

   2. Ecology: Of a form of → symbiosis in which two different species of plant or animal live in close association without injury to either.

Etymology (EN): From M.L. commensalis, from → com- “with, together” + mensa (genitive mensalis) “table,” → Mensa.

Etymology (PE): Ham-miz “(eating together) at the same table,” from ham-, → com-, + miz “table,” → Mensa.

A mode of → survey particularly in → radio astronomy such that two different observing goals are achieved simultaneously. Commensal survey necessitates compromises whenever necessary.

See also: → commensal; → survey.

(adj.) Of the same size, extent, or duration as another; proportionate.

Etymology (EN): L.L. commensuratus, from → com- “together, with” + mensuratus, p.p. of mensurare “to measure,” from menusra “measure.”

Etymology (PE): Hammasâ, from ham- “together,” → com- + masâ “size, greatness,” from Mid.Pers. masây, masâk “size,” Av. masah- “size, greatness, length,” maz-, masan-, mazant- “great, important,” mazan- “greatness, majesty,” mazišta- “greatest,” cf. Skt. mah-, mahant-, Gk. megas, L. magnus; PIE *meg- “great.”

Of two bodies orbiting around a common barycenter, when the orbital period of one is an exact fraction, for example one-half or two-thirds, of the other.

See also: → commensurate; → orbit.

1a) A remark, observation, or criticism (Dictionary.com).

1b) A note in explanation, expansion, or criticism of a passage in a book, article, or the like; annotation (Dictionary.com).

2. To make remarks, observations, or criticisms (Dictionary.com).

Etymology (EN): M.E. coment, from O.Fr. coment “commentary;” L.L. commentum “comment, interpretation,” in classical L. “invention, fiction,” from comminisci “to contrive, devise,” from → com-, intensive prefix + base of meminisse “to remember,” related to mens, → mind.

Etymology (PE): Hâmat, from hâ-, intensive prefix,
variant of ham-, → com-,

• mat- “think; thought,” from Av. mat- matay- “to think,” humat- “good thought;” cf. Skt. matāy-.

1. A series of comments, explanations, or annotations (Dictionary.com).
   

2. An explanatory essay or treatise (Dictionary.com).

Etymology (EN): From M.E. commentaries (plural), from L. commentarium “notebook,” noun use of neuter of commentarius, from comment(um), → comment, + -arium, → -ary.

Etymology (PE): Hâmatgân, from hâmat, → comment, + gân suffix forming plural entities, from Mid.Pers. -gânag, -gâna.

To deliver, to write a → commentary on.

See also: Verb from → comment.

1. A person who makes commentaries.
   

2. A person who discusses news, sports events, weather, or the like, as on television or radio.

See also: Agent noun from → commentate.

Putting a telescope or an observing instrument into active service after their final construction.

Etymology (EN): From L. commissionem “delegation of business,” from commissus, p.p. of committere “to bring together,” from → com- “together” + mittere “to put, send.”

Etymology (PE): Râhandâzeš, verbal noun from compound verb râhandâxtan “to set in work; to prepare,” from râh “way, path” + andâxtan “to cast; to make, do; to throw.”

A period during which a newly constructed observing instrument is used for test.

See also: → commissioning; → period.

Belonging to or shared by two or more or all in question. → common denominator, → least common multiplier.

Etymology (EN): From M.E. comun, from O.Fr. comun, from L. communis “in common, public, general, shared by all or many,” originally “sharing common duties,” akin to munia “duties, public duties, functions,” from
munus “task, duty, gift.”

Etymology (PE): Hamdâr, literally “possessing together, sharing together,” from ham-,
→ com-, + dâr present stem of dâštan “to have, to possess,” from Mid.Pers. dâštan, O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind,” Skt. dhr-, dharma- “law,”
Gk. thronos “elevated seat, throne,” L. firmus “firm, stable,” Lith. daryti “to make,” PIE *dher- “to hold, support.”

A quantity into which all the denominators of a set of fractions may be divided without a remainder.

See also: → common; → denominator.

A fraction written as a/b where a and b are → positive  → integers, as opposed to a → decimal fraction; for example, 5/7. Common fractions are sometimes also called → vulgar fractions.

See also: → common; → fraction.

The logarithm with → base 10. It is known also as
the → decimal logarithm, decadic logarithm, or Briggsian logarithm, after Henry Briggs, an English mathematician who pioneered its use.

See also: → common; → logarithm; → decimal.

A calendar year which is not a → leap year. In the → Gregorian calendar, a year with 365 days divided into 12 → months, and only 28 days in February. In the → Iranian calendar, a year consisting of 365 days with 12 months, and only 29 days in the month of Esfand.

See also: → common; → year.

The ordinary people, as distinguished from those with authority, rank, station, etc.; the common people. Also, commonality (Dictionary.com).

Etymology (EN): M.E., from M.Fr. comunalte, from comunal- “communal,” → common, + -te “-ty,” a suffix denoting state, quality, etc.

Etymology (PE): Mid.Pers. pâtram “common people, commonalty” (MacKenzie), variant pâyram, prefixed from ram “herd, flock,” Mod.Pers. ramé “herd, flock.”

The fact or state of being → common or happening often.

See also: → common; → -ness.

1. A small group of persons living together, sharing possessions, work, income, etc., and often pursuing unconventional lifestyles.
   

2. Any community organized for the protection and promotion of local interests, and subordinate to the state (Dictionary.com).

Etymology (EN): From M.Fr. commune “free city, group of citizens,” from M.L. communia, noun use of neuter plural of L. adj. communis, literally “that which is common,” from communis, → common.

Etymology (PE): Hamdâré, from hamdâr, → common.

1a) To impart knowledge of; make known.

1b) To give to another; impart; transmit (as a disease).

2a) To give or interchange thoughts, feelings, information, or the like, by writing, speaking, etc.

2b) To express thoughts, feelings, or information easily or effectively (Dictionary.com).

Etymology (EN): From L. communicatus, p.p. of communicare
“to share, divide out; impart; inform; join, participate in,” literally “to make common,” from communis, → common.

Etymology (PE): Hamdârgandan, literally “to diffuse to the public,” from hamdâr, → common, + gan, variant kan “to throw, diffuse” (as in parâgan-, parâgandan “to diffuse, disperse, scatter;” afgandan “to throw; to lay, place;” kandan “to dig; to extract”), from Proto-Ir. *kan- “to throw, place, put, disperse, → scatter” + -dan infinitive suffix.

1. The act or process of communicating; fact of being communicated.
   

2. The imparting or interchange of thoughts, opinions, or information by speech, writing, or signs (Dictionary.com).

See also: Verbal noun of → communicate.

A theory or system of social organization based on the holding of all property in common, actual ownership being ascribed to the community as a whole or to the state (Dictionary.com).

See also: → commune; → -ism.

A member of a communistic community; an advocate of such a community.

See also: → community; → -ary; → -an.

A doctrine or system of social organization that upholds the importance of communities. It tends to lessen the focus on individual rights and increase the focus on communal responsibilities.

See also: → communitarian; → -ism.

Of or relating to a community; communal.

See also: → community; → -ary.

1a) A social group whose members share common characteristics or interests, such as values, identity, and often a common location (e.g. a village, town, or neighborhood).

1b) Ecology: A group of populations of different species (plants and animals) within a specified location in space and time.

2. Common ownership or participation; joint possession.

Etymology (EN): Ultimately from L. communitas “partnership, society, fellowship,” from communis, → common “common, public, general,” + -itas, → -ity

Etymology (PE): Hamdârgân, from hamdâr, → common, + -gân suffix forming plural entities, from Mid.Pers. -gânag, -gâna, on the model of hamegân, → public.

General: A substitution, exchange, or interchange.
Electronics: The transfer of current from one channel to another in a gas tube.
Electricity: The reversal of direct current into alternating current.
Math: A commutative operation, where a . b = b . a.

See also: Noun from → commute.

The specification of commutators of operators that in quantum physics correspond to the coordinates and momenta of a system.

See also: → commutation; → rule.

(adj.) Of or pertaining to → commutation.

See also: Adj. from → commute.

A principle holding for the operations of addition and multiplication (in some number domains) that asserts that the consequence of the given operation is not affected by the order in which the terms are considered. Thus x + y = y + x; x . y = y . x.

See also: → commutative; → law.
The first use of the word commutative in this sense is ascribed to
the French mathematician François Joseph Servois (1768-1847).

The state or quality of being commutative.

See also: Noun from → commutative.

Math.: The commutator of a and b is the element c of a group such that bac = ab.

See also: Agent noun from → commute.

To substitute (one thing for another); exchange. → mutation.
Math., logic: To satisfy or engage in a commutative operation.

Etymology (EN): From L. commutare “to change altogether,” from →
com- + mutare “to change,” from PIE base *mei-
“to change, go, move;” cf. Av. miθô “inverted, false,” miθaoxta-
“wrong spoken;” Skt. methati “changes, alternates,
joins, meets,” mithah “mutual, reciprocal;”
L. meare “to go, pass,” mutuus “done in exchange;” Goth.
maidjan “to change;” E. prefix mis- (in mistake).

Etymology (PE): Âmutidan, from intensive prefix â- + mut,
from L. mutare cognate with Av. miθô, as above,

• -idan infinitive suffix.

Moving together or in a correlated way.

Etymology (EN): From → com- “together,” + pr.p. of → move.

Etymology (PE): Hamjonb, ham-miyâv “comoving,” from ham- “together,” → com-, + jonb or miyav
“moving, mover,” present stems of jondidan and miyâvidan “→ move.”

A system of coordinates used in cosmology which is
fixed with respect to the overall → Hubble flow of the universe. A given galaxy’s location in comoving coordinates does not change as the Universe expands.

See also: → comoving; → frame.

1. A distance in → comoving coordinates
   between two points in space at a given cosmological time. In other words, the distance between two nearby objects in the Universe which remains constant with epoch if the two objects are moving with the → Hubble flow.

More specifically, it is the → proper distance divided by the ratio of the → scale factor of the Universe between then, a(t)_em, and now, a(t)_obs:

D_C = D_proper . [a(t)_obs/a(t)_em].

In terms of → redshift (z), it is the proper distance multiplied by (1 + z).

At the present epoch, i.e. a = a(t_obs) = 1,
D_C = D_proper.

If the objects have no peculiar velocity their comoving distance at any time is the same as their distance today.

The comoving distance of the → cosmic horizon is about 48 × 10⁹→ light-years.

2. Transverse comoving distance: In a non-flat Universe, the comoving distance between two events at the same → redshift but separated on the sky by some angle. It is expressed by trigonometric functions of → curvature, → comoving distance, and the → Hubble distance accounting for the curvature of space. In a flat universe (Ω_k) it is the same as the → comoving distance.
   
   

3. Line-of-sight comoving distance: The total line-of-sight comoving distance from us to a distant object computed by integrating the infinitesimal comoving distance
   contributions between nearby events along the radial ray from the time t_emit, when the light from the object was emitted, to the time t_obs, when the object is observed.

See also: → comoving; → distance.

A → reference frame that is attached to a moving object. The object in this frame is therefore at rest.

See also: → comoving; → frame.

The volume that a structure at → redshift  z would have if it was seen at the → current cosmological epoch (defined by z = 0).

See also: → comoving; → volume.

1. Closely and firmly joined or packed together;
   Occupying little space compared with others of its type.
   

2. To reduce in size or volume.

Etymology (EN): M.E., from L. compactus “concentrated,” p.p. of compingere
“to fasten together,” from → com- “with, together” +
pangere “to fix, fasten,” cf. Gk. pegnunai “to fasten,
coagulate;” PIE *pag-/*pak- “to fasten.”

Etymology (PE): 1) Hampak, from ham-, → com-, +
pak, from pakidé [Mo’in, Dehxodâ]
“thick, dense, compact,” in Hamadâni pukida
“much, full, abundant,” Kordi pêk “together, joint,”
pêk hatin “to be made up of,” pêk hênan
“to collect, constitute,” from PIE *pag-/*pak- as above?

2. Hampakidan, from hampak + -idan
   infinitive suffix; hampak kardan, from hampak +
   kardan “to do, make, perform,” → -ize.

A binary star system which is composed of a collapsed object (→ degenerate dwarf, → neutron star, or → black hole) in orbit with a low-mass (≤ 0.5 Msol) secondary star, wherein the collapsed star → accretes matter from its → companion. These two objects form a binary system of overall dimensions 10⁶ km with an orbital period of only hours or less. See also: → X-ray binary.

See also: → compact; → binary; → star; → system.

An → X-ray source detected close to the center of young → supernova remnant (SNR)s
that has no apparent emission in other wave-bands and no binary companions. Although these sources have been known and studied for several decades without much understanding of their nature, exciting results over the past few years have brought them into the forefront of → neutron star studies. They have soft, exclusively thermal spectra in the few hundred eV range and X-ray luminosities around 10³³ - 10³⁴ erg s^-1. About ten CCOs are presently known, including the central sources of CasA,
Puppis A and Kes 79 supernova remnants. Several, J1852+0040 in Kes79, J0822.0-4300 in Puppis A and 1E 1207.4-5209 in PKS 1209-51/52,
have detected pulsations in the hundreds of milliseconds range. J1852+0040 has a detected → period derivative, indicating that it is spinning down like a → rotation-powered pulsar (RPP). The measured period and either measurements or constraints on period derivative indicate that these sources have very low → magnetic fields in the range 10¹⁰ - 10¹¹ G assuming magnetic dipole braking. Since their SNRs are all young, ~ 10³ - 10⁴ yr, they were probably born with unusually low magnetic fields, which makes them “anti-magnetars” (A. K. Harding, 2013, Front. Phys. 8, 679 and references therein).

See also: → compact; → central; → object.

A galaxy belonging to a comparatively rare class of galaxies possessing very small radii
and high central → surface brightnesses. The prototype is the → Local Group  → dwarf galaxy M32. At the low mass end of the → early-type galaxy population, the well-known → mass-size relation splits into diffuse and compact branches. The compact branch is composed of compact elliptical galaxies (cEs) and may even extend to the regime of → ultracompact dwarfs.

Compact ellipticals have → effective radii (R_e) generally less than 0.6 kpc, while their diffuse counterparts, the → dwarf elliptical galaxies (dEs) or → dwarf spheroidals (dSphs), have R_e ~ 0.6-3 kpc at similar mass. One formation scenario for cEs proposes that they are low-mass classical → elliptical galaxies, in accordance with the fact that they follow the same trend on the fundamental plane as the giant ellipticals. This implies formation through hierarchical mergers, as in “normal” ellipticals. Most cEs are notably more → metal-rich than dEs and are outliers from the → mass-metallicity relation of massive early type galaxies and low-mass galaxies in the Local Group.

An alternative formation scenario addresses the problem of high metallicity by proposing that cEs are the remnants of larger, more massive galaxies.

In this scenario, their disks are stripped by strong tidal interactions (→ tidal stripping) with an even more massive host galaxy, leaving only the compact, metal-rich bulges (Du et al., 2018, arxiv/1811.06778 and references therein).

See also: → compact; → elliptical; → galaxy.

A galaxy with no disk or nebulous background and a high surface brightness that appears only barely larger than a star-like point on a sky survey photograph.

See also: → compact; → galaxy.

A Galactic H II region with an electron density ≥ 10³ cm^-3 and of a linear dimension ≤ 1 pc.

See also: → compact; → H II region.

A population of relatively small (typically < 2°) → high-velocity clouds, which are spatially and kinematically isolated from the gas distribution in their environment. They are thought to be located in the → intergalactic medium of the
→ Local Group.

See also: → compact; → high-velocity cloud.

A galaxy with a stellar mass of M ≥ 10¹¹Msun and an → effective radius of R_e ≤ 1.5 kpc. Many studies have shown that massive galaxies with low → star formation rates were remarkably compact at a → redshift of z≥ 2. At fixed stellar mass of Mstars ≅ 10¹¹Msun,
quiescent galaxies are a factor of ~ 4 smaller at z = 2 than at z = 0. As the stellar mass of the galaxies also evolves, the inferred size growth of individual galaxies is even larger. It is unlikely that all massive galaxies in the present-day Universe had a compact progenitor. However, the vast majority of CMGs that are observed at z = 2 ended up in the center of a much larger galaxy today. Their size growth after z = 2 is probably dominated by minor → mergers.
Such mergers are expected because other mechanisms cannot easily produce the observed scaling between size growth and mass growth (P. G. van Dokkum1 et al., 2015, ApJ 813, 23).

See also: → compact; → massive; → galaxy.

An astronomical object that is substantially denser or more compact
than most objects of its class. More specifically, a → neutron star or a → stellar black hole.

See also: → compact; → object.

A star whose spectrum shows striking similarities to → B[e] stars and is evolving into a → planetary nebula (→ preplanetary nebula).

See also: → compact; → planetary; → nebula; → B[e] star.

An object emitting intense energy in radio wavelength from a small, unresolved central region.

See also: → compact; → radio source.

A topological space for which every collection of open sets that covers the space has a finite subset that also covers the space.

See also: → compact; → space.

1. Math.: A process applied to topological spaces having many dimensions to make them compact spaces.
   

2. Physics: In string theory, the process of “curling up” the extra dimensions (six in the superstring theory). According to these theories, the Universe actually has 10 spatial dimensions but that 7 of them have become “compactified” down to subatomic size and thus are unobservable.

See also: Compactification, n. from → compactify.

Verbal form of → compactification.

See also: Compactify, from → compact + -ify
“cause to become,” M.E. -fien, from O.Fr. -fier,
from L. -ficare, root of facere “to make, do;” PIE base *dhe- “to put, to do” (cf. Skt. dadhati
“puts, places;” Av. dadaiti “he puts,” O.Pers. ada “he made,” Gk. tithenai “to put, set, place.”

1. The act of compacting or the state of being compacted.
   

   2. Geology: The physical process whereby the volume of a fixed mass of sediment reduces either by the weight of overlying deposits or by the pressure of earth movements.

See also: → compact; → -tion.

1. General: The quality of being compact.
   

2. For a → compact object, a rough measure of the importance of gravity, defined by a dimension-less quantity which is the ratio between its gravitational radius (→ Schwarzschild radius) and its physical radius. It is expressed as φ = 2 GM / c² R, where M and R are the mass and radius of the object respectively, and G and c are the → gravitational constant and
   speed of light respectively. For a → black hole  φ is of order unity (by definition), for a → white dwarf it is of order 10^-4, and for a → neutron star  φ is around 0.5.

See also: → compact + → -ness.

The fainter of the two components in a → binary system.

Etymology (EN): Companion from O.Fr. compaignon “fellow, mate,” from
L.L. companionem, lit. “bread fellow, messmate,” from
L. → com- “with” + panis “bread.”

Etymology (PE): Hamdam “bearthing together, i.e. an intimate companion, friend,” from
ham- “with”, → com- + dam “breath,” Mod.Pers./Mid.Pers. damidan “to breathe, blow,” Av. dāδmainya- “blowing up,”
cp. Skt. dham- “to blow,” dhamati “he bows,” Proto-Indo-Iranian
*dhamH-, Proto-Iranian *damH- “to blow, breathe; to swell,” PIE
*dhemH- “to blow.”

A faint star of 8th magnitude in a binary system with → Sirius.
Called also Sirius B, it is a → white dwarf with a mass comparable to
that of the Sun, but with a diameter smaller than that of the Earth. The mean distance
between the stars is about 20 A.U. (angular separation 4.6 seconds of arc), and their period of
revolution about 50 years. This star was discovered in 1844 by Friedrich Bessel, who noticed that Sirius had a slight wobbling motion, as if it was orbiting an unseen object. In 1863, the optician and telescope maker Alvan Clark spotted the companion. The star was later determined to be a → white dwarf.

See also: → companion; → Sirius.

1. Of or relating to comparison.
   

2. Proceeding by, founded on, or using comparison as a method of study.
   

3. Grammar: Being, noting, or pertaining to the intermediate degree of the comparison of adjectives, as better and more beautiful, the comparative forms of good and beautiful, and of adverbs, as nearer and more carefully, the comparative forms of near and carefully (Dictionary.com).

See also: → comparison; → -ive.

A device that compares the measured property of an object
(position, shape, color, brightness) with a standard.
→ blink comparator.

Etymology (EN): L.L. comparator “comparer,” from compara, → compare,

• -tor suffix forming personal agent nouns, cf. Gk. -tor, Skt.
  -tar.

Etymology (PE): Hamsanjgar, agent noun from hamsanjidan→ comparison.

To examine in order to note similarities and differences.

Etymology (EN): From O.Fr. comparer, from L. comparare “to match, make equal with, liken,” from → com- “with” + parare “to make or esteem equal,” from par “equal.”

Etymology (PE): Sanjidan “to compare; to measure,” from Mid.Pers. sanjidan “to weigh,”
from present tense stem sanj-, Av. θanj- “to draw, pull;” Proto-Iranian
*θanj-. Other terms from this base in Pers.: lanjidan “to pull up,”
hanjidan, âhanjidan “to draw out,” farhang “education, culture.”

The act of comparing or the process of being compared.

Etymology (EN): From O.Fr. comparaison from L. comparationem, from p.p. stem of comparare, → compare.

Etymology (PE): Hamsanješ, from hamsanj present tense stem of hamsanjidan, → compare, + -š verbal noun suffix.

A spectrum of known substances used as a standard of comparison for investigating spectra of celestial objects.

See also: → comparison; → spectrum.

A non-variable star used to monitor the variations of another nearby star.

See also: → comparison; → star.

A device for determining directions, as by means of a freely rotating → magnetic needle that indicates → magnetic north.

Etymology (EN): M.E. compas, from O.Fr. compas “circle, pair of compasses,” from compasser “to go around, measure,” from V.L. *compassare “to pace out,” from L. → com- “together” + passus “a step.”

Etymology (PE): Qotbnemâ, literally “pole indicator,” from qotb, → pole,

• nemâ “indicator, showing,” from nemudan “to show,” → display.

The needle in a → magnetic compass.

See also: → compass; → needle.

An instrument that consists of two arms, joined at the top, one arm of which serves as a stationary reference point or pivot, while the other describes a circle or is extended. It is used for measuring angles, transferring lengths, and drawing circumferences. Also pair of compasses.

Etymology (EN): → compass.

Etymology (PE): Pargâr “a pair of compasses,” from Proto-Ir. *pari-kar- “to draw around, to plough around,” from *pari- “around, round about” (Mod.Pers. par-, pirâ-, cognate with Gk. peri-), → circum-, + *kar-, karš- “to draw (a furrow), till, plant” (Mod.Pers. kašidan “to draw,” kâštan, kâridan “to plant”); Lori, Hamadâni kerr “line, scratch;” Laki keronen, Hamadâni kerândan “to pull along, drag;” Tâleši kernye; Tabari kərəš “to drag;” cf. Av. pairikara- “a furrow round about,” Skt. parikara- “girdle, waist-band, zone.”

General: The fact or conditions of existing or working together.
Informatics: The ability of a software or peripheral device to function with another computer or computer
system without modification.

See also: Noun from → compatible.

<i<General: Capable of existing harmoniously with another or others.
Informatics: of a software or peripheral device, capable of working with another computer or computer
system without modification.

Etymology (EN): M.Fr. compatible, from M.L. compatibilis, literally “sympathetic,” from L.L. compati, from → com- “together”

• pati “to suffer, undergo;” PIE base *pe(i)- “to hurt” (cf. Skt. pijati “reviles, scorns,” Gk. pema “ruin, affliction,” Av. pâman- “scab; skin disease”) + -ible, → -able.

Etymology (PE): Sâzgâr variant sâzkâr “consonant, in accordance, agreeing,” from sâz- present tense stem of sâzidan, sâxtan “to adapt, adjust, be fit; to build, make, fashion,” Mid.Pers. sâxtan, sâz-, Manichean Parthian s’c’dn “to prepare, to form,” Av. sak- “to understand, to mark,” sâcaya- (causative) “to teach;” + agent suffix -gâr, variant -kâr, O.Pers. kara-, O.Pers./Av. kar- “to do, make, build,” Av. kerenaoiti “makes,” cf. Skt. kr- “to do, to make,” krnoti “makes,”
karma “act, deed;” PIE base k^wer- “to do, to make.”

To force or oblige (someone) to do something.

Etymology (EN): M.E. compellen, from O.Fr. compellir and directly from L. compellere “to drive together, drive to one place” (of cattle), “to force or compel” (of persons), from → com- “together” + pellere “to drive.”

Etymology (PE): Vadâridan, vâdâr kardan, from vâdâr present stem of vâdâštan “to keep back, restrain, oblige; to provoke, impel, instigate; to hide, conceal,” from vâ-, → de-, + dâr-, dâštan “to hold, have, possess,” → possession.

1. Tending to compel, as to force or push toward a course of action; overpowering.
   

   2. Having a powerful and irresistible effect; requiring acute admiration, attention, or respect (Dictionary.com).

See also: → compel; → -ing.

To reduce or balance the effect of something unwanted by exerting an opposite force or effect.

Etymology (EN): From L. compensatus, p.p. of compensare “to weigh one thing (against another),” thus, “to counterbalance,” from → com- “with”+ pensare, frequentative of pendere “to weigh, to hang.”

Etymology (PE): Pâhangidan, from pâhang (Dehxodâ) “that which is put in a balance scale to equalize the weights,” from pâ- variant of pâd-, → counter-, + hangidan variant of sanjidan “to measure; compare, put in balance,” → object.

One who, or that which, compensates.

See also: → compensate; → -or.

A transparent plate inserted in one of the arms of an interferometer, in particular the → Michelson interferometer, to compensate for a longer → optical path in the other arm. Its function is to ensure that the beams transverse the same total thickness of glass in both arms of the interferometer. This is not essential for producing → interference fringes in monochromatic light. The compensator plate is of the same glass and the same thickness as the → beam splitter plate.

See also: → compensator; → plate.

To strive to outdo another for acknowledgment, a prize, supremacy, profit, etc.; engage in a contest (Dictionary.com).

Etymology (EN): M.E., from M.Fr. compéter, from L.L. competere “to strive in common,” from → com- “together” + petere “to strive, go forward, seek,” from PIE base *pet- “to fly, rush” (cf. Av. pat- “to fly, fall, rush,” patarəta- “winged;” Mid.Pers. patet “falls,” opastan “to fall;” Mod.Pers. oftâdan “to fall;” Skt. patati “he flies, falls,” pátra- “wing, feather, leaf;” Gk. piptein “to fall,” pterux “wing;” O.E. feðer “feather;” L. penna “feather, wing”).

Etymology (PE): Hâjuyidan, from hâ-, variant of ham-, → com-, + juyidan “to strive for, seek,” Parthian Mid.Pers. ywdy- “to strive for,” Khotanese juv- “to fight,” Av. yūd- (yaoda-) “to fight, struggle,” yūdiieiti “fights,” cf. Skt. yodh- “to fight,” yúdh- “fight,” Gk. hysmine “battle, fight,” Lith. judus “belligerent.”

1. The act of competing; rivalry for supremacy, a prize, etc.

2. A contest for some prize, honor, or advantage (Dictionary.com).

See also: Verbal noun of → compete + → -tion.

1. Of, pertaining to, involving, or decided by competition.
   

2. Well suited for competition; having a feature that makes for successful competition.
   

3. Having a strong desire to compete or to succeed (Dictionary.com).

See also: → compete + → -ive.

A scenario for → massive star formation whereby developing → protostars in their natal → molecular clouds compete with each other to gather mass. The protostars → accrete mass with a rate which depends on their location within the protocluster. They use the same reservoir of gas to grow. Therefore those protostars nearest the center, where the potential well is deep, and gas densities are higher, have the highest → accretion rates. The competitive accretion model explains the observational fact that the most massive stars are generally found in cluster cores. It accounts also for the distribution of stellar masses. In this model the accretion process depends on the content of the cluster. In clusters where gas dominates the potential (e.g. at initial stages of cluster formation),
the accretion process is better modeled by using the → tidal radius as the accretion radius. In contrast, when the stars dominate the cluster potential and are virialized, the accretion is better modeled by → Bondi-Hoyle accretion (Bonnell et al. 1997, MNRAS 285, 201; 2001, MNRAS 323, 785).

See also: → competitive; → accretion; → model.

The state of being competitive.

See also: → competitive; → -ness.

A person, team, company, etc., that competes; rival (Dictionary.com).

See also: → compete + → -or.

The act of compiling; something compiled.

See also: → compile; → -tion.

1. To put together (documents, selections, or other materials) in one book or work.
   

2. Computers: To translate (a computer program) from a high-level language into another language, usually machine language, using a → compiler (Dictionary.com).

Etymology (EN): M.E., from O.Fr. compiler “compile, collect,” from L. compilare “to plunder, rob,” probably originally “bundle together, heap up;” from → com- “together” + pilare “to fix firmly, accumulate.”

Etymology (PE): Hâtalidan, from hâ- variant of ham-, → com-,

• tal, → pile, + -idan suffix of infinitives.

A → software program that compiles program source code files into an executable program.

See also: → compile + → -er

To express dissatisfaction, pain, uneasiness, censure, resentment, or grief; find fault (Dictionary.com).

Etymology (EN): M.E. compleinen, from O.Fr. complaindre “to lament” from V.L. *complangere, originally “to beat the breast,” from L. → com- an intensive prefix

• plangere “to strike, beat the breast.”

Etymology (PE): Mid.Pers. garz- “to weep, lament,” garzišn “complaint;” Mod.Pers. geristan, geryidan “to weep,” gelé “gripe, complaint, lamentation;” Kurd (Kurm.) girižin/giriž- “to be grumpy, growl;” Av. gərəz- “to lament, weep;” cf. Skt. grah “to complain;” Proto-Ir. *garz- “to lament, weep.”

An expression of discontent, regret, pain, censure, resentment, or grief; lament; faultfinding (Dictionary.com).

Etymology (EN): M.E. compleynte, from O.Fr. complainte “complaint, lament,” noun from complaindre “to lament,” → complain.

Etymology (PE): Verbal noun of garzidan, → complain.

1. Math., logic: With reference to any set A, conceived as a subset of some larger set U, all the elements of U that are not contained in A.
   

2. Optics: A complementary color that, when combined to another, creates neutral gray.

Etymology (EN): From O.Fr. complement, from L. complementum “that which fills up or completes,” from complere “fill up,” → complete.

Etymology (PE): Osporân, from ospor present tense stem of osporidan “to → complete.”

The state or quality of being → complementary.

Etymology (EN): From → complementary + → -ity.

Etymology (PE): From osporandé, → complementary, + -gi, same as -i noun suffix.

Physical principle, put forward by Niels Bohr in 1928,
that a complete knowledge of phenomena on atomic dimensions requires a description of both wave and particle properties.

See also: → complementarity; → principle.

Forming or serving as a complement; completing.

See also: From → complement.

Any angle that when added to another one creates a 90° angle.

See also: → complementary; → angle.

Same as → complementary screens.

See also: → complementary; → aperture.

Two apertures where opaque and transparent areas are inverted. If A is an aperture that has some opaque areas and some transparent ones, the complementary pattern A’ is the pattern in which exactly the opposite areas are opaque and transparent. Thus, the complimentary screen of a single slit is a wire of the same size, and the complementary screen of a circular opening is a dot. A better term would be → complementary apertures.

See also: → complementary; → screen.

1a) General: Having all necessary parts, elements, or steps.

1b) Math: Of a deductive theory or system, the property that every statement formulated in terms of the theory can be either proved or disproved.

2. To make whole or entire; to make perfect.

Etymology (EN): From O.Fr. complet “full,” from L. completus, p.p. of complere “to fill up,” from → com- + plere “to fill,” PIE *pelu- “full,” from *pel- “to be full;” cf. O.Pers. paru- “much, many,” Av. parav-, pauru-, pouru-, par- “to fill,”
Mod.Pers. por “full,” Skt. puru-, Gk. polus.

Etymology (PE): 1) Ospor, from Mid.Pers. uspurr “complete, entire,” from prefix us-, os-, → ex-, + por “full,” O.Pers. paru- “much, many,” Av. parav-, pauru-, pouru-, par- “to fill,” PIE *pelu- “full,” from *pel- “to be full;” cf. Skt. puru-, Gk. polus,
O.E. full “completely, full,” from P.Gmc. *fullaz, O.H.G. fol, Ger. voll, Goth. full.

2. From ospor + -idan infinitive suffix.

In → graph theory, a simple graph with an → edge between every pair of → vertices.

See also: → complete; → graph.

Quantum mechanics: A set of states such that any state can be represented as a superposition of them.

See also: → complete; → set.

1. The state of being complete and entire.
   

2. Logic: The condition of a → formal system if every → valid  → inference is → provable by means of the rules of the system.

See also: From → complete + → -ness.

In photometric studies of a → population of astronomical objects (usually stars or galaxies), the magnitude that represents the faintest members of the population.

See also: → completeness; → magnitude.

1. General: (n. & adj.) A whole comprised of many elements. → complex wave.
   
2. (adj.) Complicated and hard to solve.
   
3. Math: Pertaining to or using → complex numbers.

Etymology (EN): From Fr. complexe, from L. complexus “surrounding, encompassing,” p.p. of complecti “to embrace, encompass, include,” from → com- “together” + plectere “to weave, twine.”

Etymology (PE): Hamtâft, from ham- “together,” → com- + tâft past stem of tâftan, tâbidan “to twist, to spin, to bend, to crook,” p.p. tâftah “spun, silk or linen cloth,” loaned into E. taffeta (from O.Fr. taffetas, from It. taffeta); similarly Gk. tapetion “little carpet” is probably from this Iranian origin (from which tapestry, tapis); Proto-Ir. *tap- “to spin,” related to tan-, tanidan “to spin; to stretch,” → tension.

A → complex number in which the → real part of the number remains the same, but i is replaced by -i. For example, the complex conjugate of x + iy is x - iy. Same as → conjugate complex number.

See also: → complex; → conjugate.

A → dynamical variable which has an → imaginary number part.

See also: → complex; → dynamical; → variable.

The complex notation for the → Fourier series of a function f(x). Using → Euler’s formulae, the function can be written in cimplex form as

f(x) = Σ c_n e^inx (summed from -∞ to ∞), where the → Fourier coefficients are

c_n = (1/2π)∫ f(x) e^-inx dx (integral from -π to +π).

See also: → complex; → Fourier series.

A fraction in which the → numerator or → denominator, or both, contain fractions. For example (3/5)/(6/7). Also called → compound fraction.

See also: → complex; → fraction.

A function whose → variables are → complex numbers.

See also: → complex; → function.

A molecule in the → interstellar medium comprising at least six atoms (e.g. Herbst & van Dishoeck, 2009, ARA&A).

See also: → complex; → molecule.

Any number of the form u = a + bi, where a and b are → real numbers and i imaginary, i.e. i² = -1.

See also: → complex; → number.

A → complex molecule containing C, H, O, and N atoms and involved in life developing processes. See also → organic molecule.

See also: → complex; → organic; → molecule.

Same as → composite population.

See also: → complex; → population.

A wave that is composed of several frequencies.

See also: → complex; → wave.

A constituent part; an individual part of an organized whole.
One of two or more vectors having a sum equal to a given vector.

Etymology (EN): L. componentem, pr.p. of componere “to put together,” → compose.

Etymology (PE): Hamneh from ham- “together,” → com- + neh present tense stem of nehâdan “to put, place,” → compose.

To create or form by putting together things, parts, or elements.

Etymology (EN): From O.Fr. composer “put together, arrange,” from → com- “together” + poser “to place,” from L. ponere “to put, place.”

Etymology (PE): Hamnehâdan from ham-, → com-, + nehâdan “to put, place,” Mid.Pers. nihâtan, Av. ni- “down; into,” → ni- (PIE),

• dâ- “to put; to establish; to give,” dadâiti “he gives,” cf. Skt. dadâti “he gives,” Gk. didomi “I give,” L. do “I give;” PIE base *do- “to give.”

Made up of distinct components. The components may retain part of their identities. → compound, → combination.

Etymology (EN): From O.Fr. composite, from L. compositus, p.p. of componere “to put together,” → compose.

Etymology (PE): Hamnehâdé, p.p. of hamnehâdan→ compose.

A whole number which is the product of whole numbers other than itself and 1. The opposite of prime number. → compound number.

See also: → composite; → number.

A stellar population consisting of more than one → simple population. Also called complex population.

See also: → composite; → population.

A stellar spectrum displaying lines characterising two stars of different types.

See also: → composite; → spectrum.

1. General: The act or manner of composing; the result or product of composing.
   

2. Chemistry: The proportion of the elements of which a substance is made up.
   

3. Mechanics: The determination of a force whose effect is the same as that of two or more given forces acting simultaneously.

Etymology (PE): Hamnehesh, from ham-, → com-,

• neheš verbal noun from nehâdan “to put, place,” → compose.

1a) General: Consisting of two or more separate substances, ingredients, elements, or parts. A compound usually constitutes a new and independent entity with regard to its components. → composite, → combination.

1b) Chemistry: A substance whose molecules are made up of at least two different elements.

1c) Linguistics: A word consisting of two or more independent elements such as sunflower, eyeball, football.

2a) To combine so as to form a whole; mix.

2b) To produce or create by combining two or more ingredients or parts; compose or make up.

Etymology (EN): M.E. compouned, p.p. of compounen, M.Fr. compon-, stem of compondre, from L. componere, → compose.

Etymology (PE): Hamnât, literally “placed together,” from ham- “together,” → com-, + nât “placed, put,” variant of nâd, nâde, from Lori nâdan “to place, put” (Tabari da-nâyan “to place, put”), variant of nehâdan “to place, put,” → compose.

An angle that results from the combination of two other angles. These angles
are expressed by → compound angle formulae.

See also: → compound; → angle.

One of eight equations that give the → trigonometric functions of → compound angles.

sin(A± B) = sinA.cosnd angleB± cosA.sinB

cos(A + B) = cosA.cosB - sinA.sinB

cos(A - B) = cosA.cosB + sinA.sinB

tan(A + B) = (tanA + tanB)/(1 - tanA.tanB)

tan(A - B) = (tanA - tanB)/(1 + tanA.tanB).

See also: → compound; → angle; → formula.

A curve that is made up of a series of successive tangent circular arcs.

See also: → compound; → curve.

In statistics, an event whose probability of occurrence depends upon the probability of occurrence of two or more independent events.

See also: → compound; → event.

Same as → complex fraction.

See also: → compound; → fraction.

A lens system consisting of two or more lenses with the same radius used together in order to remove aberrations found in a single lens.

See also: → compound; → lens.

A temporary excited state of the atomic nucleus which results from the collision of an incident energetic particle with a target nucleus. Subsequently the nucleus
decays by emitting particle(s).

See also: → compound; → nucleus.

A quantity expressed as the sum of two or more quantities of differing units. For example 5 hours and 15 minutes or 4 meters and 20 centimeters. → composite number.

See also: → compound; → number.

A statement formed from simple statements by the use of words such as “and,” “or,” “not,” “implies,” or their corresponding symbols.

See also: → compound; → proposition.

A verb that is made up of several words, such as tear up, take away, double-click.

See also: → compound; → verb.

1. Grasp mentally; understand.
   

   2. Include, comprise, or encompass.
      

   Comprehend and understand are very often interchangeable. In fact, both words mean “grasp the meaning of,” but in some cases understand stresses the final result, while comprehend stresses the process of getting there.

See also: From L. comprehendere “to take together, to unite; include; seize,” from → com- “together, with,” + prehendere “to seize,” → prehend.

1. The ability to → comprehend something.
   

   2. In logic, the sum of all the essential attributes which analysis can find in a concept or term. For example, the comprehension of the term man is rational animal; the → extension of the term man, is all men and women. Comprehension and extension are inversely proportional. The greater is the comprehension of a term, the less is its extension.

See also: Verbal noun of → comprehend.

1. Completely or broadly covering; including a large proportion of something.
   

   2. Having or showing extensive understanding.