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

A typical or most likely mass for the formation of an astronomical object. In current star formation models, it is of order of a few tenths of a → solar mass.

→ characteristic; → mass.

The quantity kT in the → Maxwell-Boltzmann distribution law, where k is → Boltzmann's constant and T the gas temperature. See also → thermal energy.

→ characteristic; → thermal; → energy

The way a number of → electric charges are arranged in space with respect to the point of observation.

→ charge; → distribution.

The science of the composition, structure, properties, and reactions of chemical elements and compounds and their interactions with matter and energy.

Chemistry, from chemist, from Gk. chemia "alchemy" + -ry, from M.E. -rie, from O.Fr.

Šimi, from Fr. as above.

A probability density function, denoted χ², that gives the distribution of the sum of squares of k independent random variables, each being drawn from the normal distribution with zero mean and unit variance. The integer k is the number of degrees of freedom. The distribution has a positive skew; the skew is less with more degrees of freedom. As degrees of freedom increase, the chi-square distribution approaches a normal distribution. The most common application is chi-square tests for goodness of fit of an observed distribution to a theoretical one. If χ² = 0 the agreement is perfect.

Chi Gk. letter of alphabet; → square; → distribution.

Vâbâžeš, → distribution; do, → two.

A abbreviated notation for various functions associated with quadratic differential forms. Each Christoffel symbol is essentially a triplet of three indices, i, j and k, where each index can assume values from 1 to 2 for the case of two variables, or from 1 to n in the case of a quadratic form in n variables. Christoffel symbols appear in many calculations in geometry where non-Cartesian coordinates are used. These symbols are fundamental in the study of tensor analysis.

Named after Elwin Bruno Christoffel (1829-1900), a German mathematician; → symbol.

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.

1) (followed by of) To be composed (of); be formed (of).
2) (followed by: in or of) To have its existence (in); lie (in); be expressed (by).
3) To be compatible or consistent; accord (TheFreeDictionary.com).

M.E., from M.Fr. consister, from L. consistere "to stand firm, take a standing position, halt," from → com- "with, together" + sistere "to place," causative of stare "to stand, be standing," cognate with Pers. ist-/istâdan.

Hânesidan, literally "to put together," from prefix hâ-, → com-, + nesidan "to put, place," variant of nehidan, nehâdan "to put, place," → position.

1) Agreement or accordance with facts, form, or characteristics previously shown or stated.
2) Agreement or harmony between parts of something complex; compatibility.
3) Physics: Degree of → viscosity or firmness.
4) The state or quality of holding or sticking together and retaining shape.
5) Conformity with previous attitudes, behaviour, practice, etc. (TheFreeDictionary.com).

→ consist; → -ency.

1) Showing consistency; not self-contradictory.
2) In agreement or harmony (with); accordant.
3a) Maths.: The quality of an equation if it has a solution.
3b) Maths.: (of a set of equations) Satisfied by at least one solution.
4) Logic: (of a formal system) Not permitting the deduction of a contradiction from the axioms (TheFreeDictionary.com).

→ consist; → -ent.

Hânesgâr, from hânes- present stem of hânesidan, → consist, + -gâr agect noun suffix (on the model of sâzgâr), → -or.

Measurement of the distances to the farthest objects in the Universe based on a bootstrapping series of methods, each applicable to more distant objects, and each dependent on the previous methods.

→ cosmic; → distance; → scale.

The study of the chemical composition of the universe and the processes that produced those compositions. Cosmochemistry is an interdisciplinary science that overlaps with geochemistry, geology, astronomy, astrophysics, and geophysics.

→ cosmo-; → chemistry.

The distance to a remote galaxy based on its redshift assuming that the redshift is caused by the → Doppler effect and reflects the general expansion of the Universe.

→ cosmological; → distance .

A function that gives the probability that a → random variable X is less than or equal to x, at each possible outcome: F(x) = P(X ≤ x), for -∞ < x < ∞. Same as → distribution function.

→ cumulative; → distribution; → function.

A person who believes in → deism.

→ deism; → -ist

Of, pertaining to, or dealing with → determinism.

From determinist + → -ic.

The classical representation of the laws of nature according to which a particular future state (B) will arise from a particular past one (A). In contrast to → quantum physics which deals with the probability for the transition from A to B.

Deterministic, adj. of determinism; → physics.

A theory in which specification of the initial value of all relevant variables of the system is sufficient to calculate the past values and to predict the future values of such variables for any arbitrary time. Moreover, it is possible, for any arbitrary time, to assign a value to all the variables characterizing the system. In quantum mechanics, the time evolution of the → wave function, governed by the → Schrodinger equation, is deterministic. Quantum mechanics, however, is a non deterministic theory because of the probabilistic nature of the predictions for the values of the → observables of a quantum system.

→ deterministic; → theory.

Geology: Impact ejecta found at distances more than 5 crater radii from the rim of the source crater.

Distal, from dist(ant), → distance + → -al; → ejecta.

Ešânâk, → ejecta; dur, → distance.

1) The separation/length in space/time between two things/events.
2) The state of being apart in space or time.
3) In cosmology four main distance definitions are used: → luminosity distance, → angular diameter distance, → comoving distance, and → light-travel distance. In a → flat Universe these four approaches give the same result for the present epoch for distances below 100 Mpc. In a non-flat Universe with the → Robertson-Walker metric they give different but related values.
See also:
→ apparent distance, → cosmic distance scale, → cosmological distance, → distance function, → distance modulus, → distance to the horizon, → focal distance, → Hubble distance, → perihelion distance, → polar distance, → proper distance, → redshift-distance relation, → velocity-distance relation, → zenith distance.

M.E., from O.Fr., from L. distantia "a standing apart," from distantem (nominative distans) "standing apart, separate, distant," pr.p. of distare "to stand apart," from → dis- "apart, off" + stare "to stand," (cf. Mod.Pers. istâdan "to stand," O.Pers./Av. sta- "to stand, stand still; set," Skt. sthâ- "to stand," Gk. histemi "put, place, weigh," stasis "a standing still").

Apest, literally "standing apart," from apa- prefix denoting "separation, away, off," → dis-, + est variant of ist, present stem of istâdan, to stand," as above;" cf. Choresmian bst "to stand apart," from *apa- + st- "to stand," → stand.
Durâ, duri, noun from dur "far, distant, remote," Mid.Pers. dur, O.Pers. dūra- "far (in time or space)," Av. dūra-, from dav- "to move away," cf. Skt. dūrá- "distant, far."