An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1a) A social group whose members share common characteristics or interests, such as values, identity, and often a common location (e.g. a village, town, or neighborhood).
1b) Ecology: A group of populations of different species (plants and animals) within a specified location in space and time. 2) Common ownership or participation; joint possession.

Ultimately from L. communitas "partnership, society, fellowship," from communis, → common "common, public, general," + -itas, → -ity

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

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

Noun from → commute.

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

→ commutation; → rule.

(adj.) Of or pertaining to → commutation.

Adj. from → commute.

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

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

The state or quality of being commutative.

Noun from → commutative.

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

Agent noun from → commute.

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

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

Âmutidan, from intensive prefix â- + mut, from L. mutare cognate with Av. miθô, as above, + -idan infinitive suffix.

Moving together or in a correlated way.

From → com- "together," + pr.p. of → move.

Hamjonb, ham-miyâv "comoving," from ham- "together," → com-, + jonb or miyav "moving, mover," present stems of jondidan and miyâvidan "→ move."

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

→ comoving; → frame.

1) A distance in → comoving coordinates between two points in space at a given cosmological time. In other words, the distance between two nearby objects in the Universe which remains constant with epoch if the two objects are moving with the → Hubble flow. More specifically, it is the → proper distance divided by the ratio of the → scale factor of the Universe between then, a(t)_em, and now, a(t)_obs: D_C = D_proper . [a(t)_obs/a(t)_em]. In terms of → redshift (z), it is the proper distance multiplied by (1 + z). At the present epoch, i.e. a = a(t_obs) = 1, D_C = D_proper. If the objects have no peculiar velocity their comoving distance at any time is the same as their distance today.
The comoving distance of the → cosmic horizon is about 48 × 10⁹→ light-years.

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

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

→ comoving; → distance.

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

→ comoving; → frame.

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

→ comoving; → volume.

1) Closely and firmly joined or packed together; Occupying little space compared with others of its type.
2) To reduce in size or volume.

M.E., from L. compactus "concentrated," p.p. of compingere "to fasten together," from → com- "with, together" + pangere "to fix, fasten," cf. Gk. pegnunai "to fasten, coagulate;" PIE *pag-/*pak- "to fasten."

1) Hampak, from ham-, → com-, + pak, from pakidé [Mo'in, Dehxodâ] "thick, dense, compact," in Hamadâni pukida "much, full, abundant," Kordi pêk "together, joint," pêk hatin "to be made up of," pêk hênan "to collect, constitute," from PIE *pag-/*pak- as above?
2) Hampakidan, from hampak + -idan infinitive suffix; hampak kardan, from hampak + kardan "to do, make, perform," → -ize.

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

→ compact; → binary; → star; → system.

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

→ compact; → central; → object.

A galaxy belonging to a comparatively rare class of galaxies possessing very small radii and high central → surface brightnesses. The prototype is the → Local Group  → dwarf galaxy M32. At the low mass end of the → early-type galaxy population, the well-known → mass-size relation splits into diffuse and compact branches. The compact branch is composed of compact elliptical galaxies (cEs) and may even extend to the regime of → ultracompact dwarfs. Compact ellipticals have → effective radii (R_e) generally less than 0.6 kpc, while their diffuse counterparts, the → dwarf elliptical galaxies (dEs) or → dwarf spheroidals (dSphs), have R_e ~ 0.6-3 kpc at similar mass. One formation scenario for cEs proposes that they are low-mass classical → elliptical galaxies, in accordance with the fact that they follow the same trend on the fundamental plane as the giant ellipticals. This implies formation through hierarchical mergers, as in "normal" ellipticals. Most cEs are notably more → metal-rich than dEs and are outliers from the → mass-metallicity relation of massive early type galaxies and low-mass galaxies in the Local Group. An alternative formation scenario addresses the problem of high metallicity by proposing that cEs are the remnants of larger, more massive galaxies. In this scenario, their disks are stripped by strong tidal interactions (→ tidal stripping) with an even more massive host galaxy, leaving only the compact, metal-rich bulges (Du et al., 2018, arxiv/1811.06778 and references therein).

→ compact; → elliptical; → galaxy.

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

→ compact; → galaxy.

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

→ compact; → H II region.

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

→ compact; → high-velocity cloud.