An Etymological Dictionary of Astronomy and Astrophysics
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The fraction of normally incident light that is absorbed per unit path length or by a unit mass of absorbing medium.

→ absorption; → coefficient.

The fraction of a beam of → X-rays or → gamma rays that is absorbed or scattered per unit thickness of the → absorber. The linear attenuation coefficient, denoted by the symbol μ, appears in the equation I(x) = I₀e^-μx, where I(x) is the intensity at depth of x cm and I₀ is the original intensity.

→ attenuation; → coefficient.

The factor multiplying the variable in a term of a → binomial expansion. For example, in (x + y)⁴ = x⁴ + 4x³y + 6x²y² + 4xy³ + y⁴ the binomial coefficients are 1, 4, 6, 4, and 1. In general, the r-th binomial coefficient in the expression (x + y)ⁿ is: (n,r) = n!/[r!(n - r)!].

→ binomial; → coefficient.

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.

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.

→ viscosity; → coefficient.

A number between -1 and 1 which measures the degree to which two variables are linearly related.

→ correlation; → coefficient.

A parameter that indicates the strength of the interaction between two systems.

→ coupling; → coefficient.

A factor of proportionality involved in the → diffusion equation. It may be defined as the amount of the quantity diffusing across a unit area through a unit concentration gradient in unit time. → magnetic diffusivity.

→ diffusion; → coefficient.

Of a → non-zero polynomial, the coefficient of the → monomial which has the highest → degree.

Dominant, adj. from → dominant; → coefficient.

A measure of the probability that a particular atomic transition leading to the formation of an atomic spectral line occurs. The coefficient of spontaneous emission is denoted by A_ij, and the coefficient of stimulated emission by B_ij, i representing the lower level and j is the upper level.

Named after Albert Einstein (1879-1955) who introduced the coefficients in 1916; → coefficient.

Electromagnetic flux emitted by a source per unit volume per unit time.

→ emission; → coefficient.

Gradient of apparent magnitude with air mass.

→ extinction; → coefficient.

One of the coefficients a_n or b_n of cos (nx) and sin (nx) respectively in the → Fourier series representation of a function. They are expressed by:
a_n = (1/π) ∫f(x) cos nx dx, for n≥ 0, summed over 0 to 2π
b_n = (1/π) ∫f(x) sin nx dx, for n≥ 1, summed over 0 to 2π.

→ Fourier analysis; → series.

According to the → von Zeipel theorem, the emergent flux, F, of total radiation at any point over the surface of a rotationally or tidally distorted star in → hydrostatic equilibrium varies proportionally to the local gravity acceleration: F ∝ g_eff^α, where g_eff is the → effective gravity and α is the gravity darkening coefficient. See also the → gravity darkening exponent.

→ gravity; → darkening; → coefficient.

A measure of the rate of absorption of radiation, expressed as the linear absorption coefficient divided by the density of the medium through which radiation is passing.

→ mass; → absorption; → coefficient.

A measure of the specific rate at which oppositely charged ions join to form neutral particles. It is given by the rate at which those ions recombine, divided by the product of the densities of the two species involved.

→ recombination; → coefficient.

A dimensionless quantity determined from the comparison of the observed → Balmer decrements with respect to the theoretical values for given physical conditions of electron temperature and density. The reddening coefficient at Hβ is defined as c(Hβ) = log (I(Hβ)/F(Hβ)), where I(Hβ) and F(Hβ) are → de-reddened and reddened Hβ fluxes respectively. Also called logarithmic extinction.

→ reddening; → coefficient.

The ratio given by the → amplitude (or energy) of a reflected wave divided by the amplitude (or energy) of the incident wave.

→ reflection; → coefficient.

The slope of the straight line that most closely relates two correlated variables.

→ regression, → coefficient.

The fraction of light scattered per unit distance in a medium.

→ scattering; → coefficient.