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.

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.

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-.

→ 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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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 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.

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.

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.

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.

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”).

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.

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.

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”.

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.

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 → 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.

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.”

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.”

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.”

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.

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.

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.

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.

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.

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.”

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 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.”

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.

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.

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.

Serving or tending to coerce.

See also: → coerce; → -ive.

Same as → coercive force.

See also: → coercive; → field.

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.

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.

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.

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.

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. 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. 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.

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.

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.

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 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.

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.

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.

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 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.

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.

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.

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.

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 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.

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.”

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. (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.

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.

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.

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.”

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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. 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.

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.

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.

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.

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.”

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.

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.

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.

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.

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 → 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.

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.

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.

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.

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.

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 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.

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 .

The act or process of colliding, → collide.

See also: Verbal noun from → collide .

Of or relating to → collision.

See also: → collision; → -al.

Of or relating to → collision.

See also: → collision; → -al.

Same as → cross section.

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

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.

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 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.

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.

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.

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.

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. 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).

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.

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 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 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.

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.

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.

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.

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 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.

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 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.”

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.”

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.

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.”

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.

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.

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).

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.

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

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.

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.”

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. 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

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.”

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 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 → 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 → 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).