An Etymological Dictionary of Astronomy and Astrophysics
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Statistics: Two random variables X and Y if and only if the value of X has no influence on the value of Y and vice versa.

→ dependent; → random; → variable.

Math.: A variable whose value determines the value of other variables. For example, x, in the expression y = f(x), is the independent variable. See also → dependent variable.

→ independent; → variable.

In → predicate logic, a variable which represents an unspecified → subject. Individual variables work just as do variables in algebra, they stand in for a value, but have no fixed value of their own. They are represented by lower case letters taken from the end of the alphabet x, y, z; and if additional variables are needed by u, v, w, as well.

→ individual; → variable.

A variable star whose fluctuations in brightness are due to natural changes in the luminosity of the star itself, not by external causes, such as in → extrinsic variable stars.

→ intrinsic; → variable.

Mechanics: For a rotating rigid body not subject to external torque, a plane which is perpendicular to the angular momentum vector of the body, and which is always tangent to its → inertia ellipsoid.

Invariable negation of → variable; → plane.

A type of variable star in which variations in brightness show no regular periodicity. There are two main types, irregular eruptive variables and irregular pulsating variables.

→ irregular; → variable.

One of several → input or → output variables of a → fuzzy logic system whose values are → words or → sentences from a natural language, instead of numerical values.

→ linguistic; → variable.

A type of → variable star in which variations in brightness occur over long time-scales of months or years. The term generally refers to → Mira variable types.

→ long; → period; → variable.

A high-luminosity variable star, which represents a transition phase in the life of a massive star when it evolves off the main sequence to become a supernova. Only about a dozen confirmed LBVs are presently known in our Galaxy. → Hubble-Sandage variable.

→ luminous; → blue; → variable.

A periodic → variable star with cycles 100-500 days, and of → spectral types K, M, S, and C; also called → long-period variables.

→ Mira; → variable.

A type of eruptive variable star, mainly young FU Orionis and T Tauri types, associated with nebulosity.

→ nebular; → variable.

A member of a small subset of quasars consisting of bright radio galaxies whose flux of visible light output can vary by as much as 50% in a single day.

→ optically; → violent; → variable; → quasar.

A variable → supergiant star with typical periods of the order of 10 to 100 days and amplitudes less than a few tenths of a magnitude. PVSGs are thought to be pulsating → g modes, caused by a density inversion, arising from an → opacity bump, most likely from Fe, H, and/or He.

→ periodical; → -ly; → variable; → supergiant.

Logic: A → propositional symbol that represents any → proposition whatsoever.

→ propositional; → variable.

A quantity that takes different real values as a result of the → outcomes of a → random event or experiment involving specified probabilities.

→ random; → variable.

A → dynamical variable which does not have an → imaginary number part.

→ real; → dynamical; → variable.

A type of giant or supergiant pulsating variable star, with intermediate or late spectra, showing noticeable periodicity in its light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light curve may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes (usually 1-2 magnitudes in the V filter). Examples are Betelgeuse, Antares, and Rasalgethi. Semiregular variables are classified in several subtypes: SRA, SRB, SRC, and SRD.

→ semi-; → regular; → variable.

A variable star that has a relatively short period with respect to stars of similar types.

→ short; → period; → variable.

A → variable star that displays changes in its → spectrum. In such stars line intensities may vary and new lines may appear. Examples include → AG Carinae, HD 108, HD 191612, and HD 148937.

→ spectroscopic; → variable.

Something that may or does vary. → variable star.
Math.: A symbol, usually a letter, which in a mathematical or logic expression represents any member of a set of possible values or possible objects.

M.E., from O.Fr. variable, from L. variabilis "changeable," from variare "to change," → vary.

Vartandé agent noun from vartidan "to change," → vary.