An Etymological Dictionary of Astronomy and Astrophysics
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A basic rule in → group theory stating that if a and b are a group element then a * b is also a group element.

→ closure; → axiom.

In astronomical interferometry, a method using triplets of telescopes in an array to calculate the phase information and get over the effects of atmospheric turbulence. The method, used in high-resolution astronomical observations, both at radio and at optical wavelengths, allows imaging of complex objects in the presence of severe aberrations.

→ closure; → phase.

The central part of a cluster (globular, galaxies, etc.) where the spatial density of the objects making up the cluster is much higher than the average value.

→ cluster; → core.

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).

From L. cohaerere "to cleave together," from → com- "together," + haerere "to stick."

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.

→ 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.

→ 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.

→ 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.

→ 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.

Coherent, adj., → coherence.

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

→ coherent; → light.

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

→ coherent; → optics.

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

→ 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.

→ coherent; → source.

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).

→ cold; → accretion; → flow.

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).

→ cold; → disk; → accretion.

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

→ collect; → area.

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

→ color; → temperature.

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).

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.

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."

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.

→ 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 envelope of → gas and → dust around a → comet nucleus, also known as → coma. As the comet approaches the → Sun, the frozen materials → sublimate and give rise to an expanding atmosphere. The atmosphere is composed of dust, → molecules, → radicals, and molecular → ions released from the inner coma with velocities ~ 0.5 to 1 km s^-1, well above the → escape velocity for the nucleus. The → chemical species observed in cometary spectra can be divided into several categories: (i) atoms and molecules related to → water (H, O, OH, OH⁺, H2O, H2O⁺), (ii) carbon and related molecules (C, C⁺, CO, CO⁺, CO₂⁺, C₂, CH, CH⁺, HCO, H₂CO), (iii) → nitrogen and related molecules (CN, CN⁺, HCN, CH₃CN, NH, NH₂, N₂⁺, NH₃, NH₄), (iv) → sulphur and related molecules (S, CS, S₂, H₂S⁺), (v) → metals (Na, K, Ca, Co, Cr, Cu, V, Fe, Mn, Ni). For a typical average comet the neutral atmosphere is first seen when the heliocentric distance is d ≤ 3 → astronomical units.

→ cometary; → atmosphere.