An Etymological Dictionary of Astronomy and Astrophysics
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The time integral associated with the evolution of a physical system in the phase space.

→ action; → variable.

The same as eclipsing binary stars, the prototype of which is Algol.

→ Algol; → variable.

A variable star, of early B or late O types, undergoing radial pulsations with short periods (< 1 day). Beta Cephei stars are confined within a narrow band of the → H-R diagram above the upper → main sequence. They are believed to be near the end of core hydrogen-burning stars of approximately 10 to 20 solar masses. The famous bright stars → Spica and → Mirzam belong to this family.

→ Beta Cephei; → variable.

A generalized coordinate and its → conjugate momentum.

Canonically, adverb from → canonical; → conjugate; → variable.

A → variable star that shows a sudden and dramatic change in brightness, including → flare stars, → novae, and some types of → symbiotic stars. They are believed to be very → close binary systems consisting of an → accreting → white dwarf  → primary and an evolved → late-type secondary star that has filled its → Roche lobe. For systems with an → accretion disk, it is believed that a thermal instability is the cause of repetitive outbursts observed in cataclysmic variables called → dwarf novae.

→ cataclysmic; → variable.

A → variable star belonging to the class of → Cepheids.

→ Cepheid; → variable.

Circular band-pass interference filter whose thickness and central wavelength vary along the perimeter. They are used in low-resolution spectrophotometers mainly in the infra-red.

→ circular; → variable; → filter.

A → dynamical variable which has an → imaginary number part.

→ complex; → dynamical; → variable.

A variable which has changes continuously, in contrast to → discrete variables.

→ continuous; → variable.

A member of a class of → pulsating stars with periods less than 0.3 days, → spectral types A or F, and visual light amplitudes in the range from a few thousands of a magnitude to about 0.8 mag. On the → H-R diagram, δ Scuti stars form a group which lies in an → instability strip which includes the classical → Cepheids at its brightest end and the pulsating → white dwarfs at its faintest limit. These stars can show very complex light variations since, while some of them are pulsating in one radial mode only, others may be pulsating simultaneously in several radial and non-radial modes.

Named after the prototype star δ of constellation → Scutum; → variable.

Math.: A variable whose value depends on the value assigned to another value. For example, in the equation y = 2x, the value of y depends on that of x. See also → independent variable.

→ dependent; → variable.

A → differential equation of the form: M₁(x) N₁(y) dx + M₂(x) N₂(y) dy = 0, which can be reduced to a → differential equation with separated variables.

→ differential; → equation; → separate; → variable.

A → differentail equation that can be transformed into the form: M(x)dx + N(x)dy = 0.

→ differential; → equation; → separate; → variable.

A variable which has only → discrete values and has no in-between values.

→ discrete; → variable.

Mechanics: One of the variables used to describe a system in classical mechanics, such as coordinates (of a particle), components of velocity, momentum, angular momentum, and functions of these quantities.

→ dynamical; → variable.

Same as → eclipsing binary.

→ eclipse; → variable.

same as → cataclysmic variable.

→ eruptive; → variable.

A star whose variation in apparent brightness is not due to changes in the star itself but to some external cause, such as eclipsing by a companion.

→ extrinsic; → variable; → star.

A theory based on the hypothesis that the discrepancies with respect to classical reality found in → quantum mechanics stem from our lack of knowledge about the observed system (→ EPR paradox). According to this hypothesis, the system should be described by additional quantum parameters, of still unknown nature, but different from position, velocity, spin, etc. The hidden variable theory has been ruled out by the violation of → Bell's inequality for all theories with local property, as suggested by the → Aspect experiment.

→ hidden; → variable.

A type of highly luminous → blue supergiant star with variable light, first discovered in the M31 and M33 galaxies; also called → S Doradus stars. They are now believed to be part of the class of → Luminous Blue Variable stars.

→ Hubble; Allan Rex Sandage (1926-2010), American cosmologist.