An Etymological Dictionary of Astronomy and Astrophysics
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1) Electronics: The extent to which an → analogue → amplifier boosts the strength of a → signal. Also called → gain.
2) In → gravitational lensing, the ratio of the lensed brightness to unlensed brightness. This factor depends on the mass of the → lensing object and the closeness of the alignment between observer, lens, and source (→ impact parameter).

→ amplification; → factor.

The ratio of the radiation intensity after traversing a layer of matter to its intensity before.

→ attenuation; → factor.

The factor e^-E/kT involved in the probability for atoms having an excitation energy E and temperature T, where k is Boltzmann's constant.

→ Boltzmann's constant; → factor.

The ratio f_cl = <ρ²> / <ρ >², where ρ represents the → stellar wind density and the brackets mean values. Unclumped wind has f_cl = 1 and → clumping becomes significant for f_cl≅ 4.

→ clumping; → 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.

→ co-; → factor.

In thermodynamics, the quantity Z = pV_m/RT, in which P is the gas pressure, V_m the molar volume, R the gas constant, and T the temperature. The compression factor is a measure of the deviation of a real gas from an ideal gas. For an ideal gas the compression factor is equal to 1.

→ compression; → facteur.

1) A numerical factor that, by multiplication or division, translates one unit or value into another.
2) In → molecular cloud studies, a factor used to convert the → carbon monoxide (CO) line intensity to → molecular hydrogen (H₂) → column density; usually denoted X_CO = I(CO) / N(H₂). This useful factor relates the observed CO intensity to the cloud mass. A general method to derive X_CO is to compare the → virial mass and the ¹²CO (J = 1-0) luminosity of a cloud. The basic assumptions are that the CO and H₂ clouds are co-extensive, and molecular clouds obey the → virial theorem. However, if the molecular cloud is subject to ultraviolet radiation, selective → photodissociation may take place, which will change the situation. Moreover, molecular clouds may not be in → virial equilibrium. To be in virial equilibrium molecular clouds must have enough mass, greater than about 10⁵ solar masses. The way → metallicity affects X_CO is a matter of debate, and there is no clear correlation between X_CO and metallicity. Although lower metallicity brings about higher ultraviolet fields than in the solar vicinity, other factors appear to be as important as metallicity for the determination of X_CO. In the case of the → Magellanic Clouds, X_CO(SMC) = 14 ± 3 × 10²⁰ cm^-2 (K km s^-1)^-1, which is larger than X_CO (LMC) = 7 ± 2 × 10²⁰ cm^-2 (K km s^-1)^-1. An independent method to derive X_CO is to make use of the gamma ray emission from a cloud. The flow of → cosmic ray protons interacts with interstellar low-energy hydrogen nuclei in clouds creating neutral → pions. These pions quickly decay into two gamma rays. It is therefore possible to estimate the number of hydrogen nuclei and hence the cloud mass from the gamma ray counts. Such a gamma-ray based conversion factor is estimated to be 2.0 × 10²⁰ cm^-2 (K km s^-1)^-1 for Galactic clouds, in good agreement with the result obtained from the virial method. However, the gamma ray flux is not well known in general, so this method is uncertain as well. See, e.g., Fukui & Kawamura, 2010 (ARAA 48, 547).

→ conversion; → factor.

A quantity, denoted a(t), which describes how the distances between any two galaxies change with time. The physical distance d(t) between two points in the Universe can be expressed as d(t) = R(t).x, where R(t) is the → scale factor and x the → comoving distance between the points. The cosmic scale factor is related to the → redshift, z, by: 1 + z = R(t₀)/R(t₁), where t₀ is the present time and t₁ is the time at emission of the radiation. The quantity (1 + z) gives the factor by which the → Universe has expanded in size between t₁ and t₀. It is also related to the → Hubble parameter by H(t) = R^.(t)/R(t), where R^.(t) is the time → derivative of the scale factor. In an → expanding Universe the scale factor increases with time. See also the → Friedmann equation.

→ cosmic; → scale; → factor.

The ratio between the D/H value in → water and in → molecular hydrogen, as expressed by:
f = [(1/2)HDO/H₂O]/[(1/2)HD/H₂] = (D/H)_H2O/(D/H)_H2.
When f> 1, there is → deuterium enrichment.

→ deuterium; → enrichment; → factor.

The energy density of a radiation field divided by the equilibrium value for the same color temperature.

→ dilution; → factor.

Same as → Eddington parameter.

→ Eddington limit; → factor.

1) One that actively contributes to the production of a result.
2) Math.: Any of the numbers or symbols that when multiplied together form a → product.

M.Fr. facteur "agent, representative," from L. factor "doer or maker," from facere "to do" (cf. Fr. faire, Sp. hacer); from PIE base *dhe- "to put, to do;" cf. Skt. dadhati "puts, places;" Av. dadaiti "he puts;" Hitt. dai- "to place;" Gk. tithenai "to put, set, place;" Lith. deti "to put;" Rus. det' "to hide," delat' "to do;" O.H.G. tuon; Ger. tun; O.S., O.E. don "to do."

Karvand, from kar- root of Mod.Pers. verb kardan "to do, to make" (Mid.Pers. kardan; O.Pers./Av. kar- "to do, make, build;" Av. kərənaoiti "he makes;" cf. Skt. kr- "to do, to make," krnoti "he makes, he does," karoti "he makes, he does," karma "act, deed;" PIE base k^wer- "to do, to make") + -vand a suffix forming adjectives and agent nouns.

A diagram representing a systematic way of determining all the prime factors of a number.

→ factor; → tree.

1) (n.) The product of all the positive integers from 1 to n, denoted by symbol n!
2) (adj.) of or pertaining to factors or factorials.

→ factor + -ial, from L. -alis, → -al.

The operation of resolving a quantity into factors.

→ factor + → -ize.

Of a molecular cloud or a nebula, the ratio of the volumes filled with matter to the total volume of the cloud.

Filling, from fill, from O.E. fyllan, from P.Gmc. *fullijan (cf. Du. vullen, Ger. füllen "to fill"), a derivative of adj. *fullaz→ full; → factor.

Karvand, → factor; pori, from por, → full.

In the atomic theory of spectral line formation, a quantum mechanical correction factor applied to the absorption coefficient in the transition of an electron from a bound or free state to a free state.

Gaunt, after John Arthur Gaunt (1904-1944), English physicist born in China, who significantly contributed to the calculation of continuous absorption using quantum mechanics; → factor

A function that converts a → differential equation, which is not exact, into an → exact differential equation. This is done by multiplying all terms of the original equation by the integrating factor.

→ integrate; → factor.

A quantity used in studies of → emission nebulae to convert the → ionic abundance of a given chemical element to its total → elemental abundance. The elemental abundance of an element relative to hydrogen is given by the sum of abundances of all its ions. In practice, not all the ionization stages are observed. One must therefore correct for unobserved stages using ICFs. A common way to do this was to rely on → ionization potential considerations. However, → photoionization models show that such simple relations do not necessarily hold. Hence, ICFs based on grids of photoionization models are more reliable. Nevertheless here also care should be taken for several reasons: the atomic physics is not well known yet, the ionization structure of a nebula depends on the spectral energy distribution of the stellar radiation field, which differs from one model to another, and the density structure of real nebulae is more complicated than that of idealized models (see, e.g., Stasińska, 2002, astro-ph/0207500, and references therein).

→ ionization; → correction; → factor.

The constant of proportionality relating the separations of lines of successive pairs of adjacent components of the levels of a spectral multiplet to the larger of the two J-values for the respective pairs. The interval between two successive components J and J + 1 is proportional to J + 1.

After Alfred Landé (1888-1976), a German-American physicist, known for his contributions to quantum theory; → facteur.