An Etymological Dictionary of Astronomy and Astrophysics
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Any of a set of physical properties whose values determine the characteristics or behavior of a system; for example, → mass, → size, → temperature, → luminosity, etc.

→ physical; → parameter.

In plasma physics, a dimensionless quantity which is the ratio of the plasma thermal pressure to the magnetic pressure: β = nkT/(B²/2μ₀). When β is much smaller than 1, like in the → solar corona, the magnetic forces dominate over the plasma forces.

→ plasma; β, Gk. letter of alphabet.

A dimensionless quantity characterizing the → interstellar extinction, defined by the total-to-selective extinction ratio: R_V = A_V/E(B-V). The typical value found for the reddening parameter in the Milky Way is R_V ~ 3.1, but it is known to vary from one line of sight to another, from values as 2 to as large as 6. Very large → dust grains would produce extinction with R_V → ∞.

→ reddening; → parameter.

The northward variation of the Coriolis parameter, arising from the sphericity of the Earth.

→ Rossby number; → parameter.

A quantity that defines the type of → scattering.

→ size; → parameter.

In a → power-law distribution or → regression, the → exponent that represents the effect of the → independent variable, x, on the → dependent variable, y. X has no association with y if the slope parameter = 0 and x has strong association with y if the slope parameter is large.

→ slope; → parameter.

Four parameters which are needed to fully describe the → polarization state of → electromagnetic radiation. They involve the maximum and minimum intensity, the ellipticity, and the direction of polarization. The four Stokes parameters are traditionally defined as follows:
I ≡ total intensity.
Q ≡ I₀ - I₉₀ = difference in intensities between → horizontal and → vertical  → linearly polarized components.
U ≡ I₊₄₅ - I_-45 = difference in intensities between linearly polarized components oriented at +45° and -45° (or 135°).
V ≡ I_rcp - I_lcp = difference in intensities between right and left → circularly polarized polarized components.

→ Stokes; → parameter.

In celestial mechanics, a combination of orbital elements commonly used to distinguish between comets and asteroids. Objects whose Tisserand's parameter value is smaller than 3 are considered to be dynamically cometary, and those with a value larger than 3 asteroidal. Also called Tisserand's invariant.

Named after François Félix Tisserand (1845-1896), French astronomer, Director of the Paris Observatory (1892).

A quantity that measures the stability of a differentially rotating disk of matter against → gravitational collapse. It is expressed by the relation: Q = c_sκ / πGΣ, where c_s is the → sound speed, κ the → epicyclic frequency, G the → gravitational constant, and Σ the → surface density. The disk is linearly stable for Q > 1 and linearly unstable for Q < 1.

After Alar Toomre (1936-), an American astrophysicist of Estonian origin, professor of mathematics at the Massachusetts Institute of Technology; → parameter.

A dimensionless parameter that measures the ratio of thermal plus kinetic energies to gravitational energy of a physical system, such as a molecular cloud. The virial parameter is expressed as: α_vir = 5σ²R / GM, where R and M are the radius and mass of the cloud respectively, σ is the one-dimensional → velocity dispersion inside the cloud, and G the → gravitational constant. It indicates whether a cloud could be bound or not. For molecular clouds that are confined by their surface pressure and for which self-gravity is unimportant, α_vir is much larger than unity, whereas α_vir is ~ 1 when the gravitational energy of a clump becomes comparable to its kinetic energy. See, e.g., Bertoldi & McKee, 1992 (ApJ 395, 140). See also → virial theorem.

→ virial; → parameter.