An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A star's → intrinsic brightness, i.e. the total amount of energy radiated by the star per second. → Luminosity is often expressed in units of watts or erg/sec. The Sun's absolute luminosity is 3.86 × 10³³ erg/sec.

→ absolute; → luminosity.

Same as → viscosity and → dynamic viscosity.

→ absolute; → viscosity.

A → transit circle that defines the position of the → Greenwich Meridian since the first observation was taken with it in 1851. Airy's transit circle lies at longitude 0°, by definition, and latitude 51° 28' 38'' N.

Named after Sir George Biddell Airy (1801-1892), Astronomer Royal, at the Royal Observatory in Greenwich from 1835 to 1881. Airy transformed the observatory, installing some of the most advanced astronomical apparatus of his day and expanded both staff numbers and their workload; → transit; → circle.

Discrepant luminosity classes derived for the same → Am star when different criteria are used. Lore specifically, a luminosity criterion may indicate a → giant star, wheras another criterion indicates a → supergiant.

→ anomalous; → luminosity; → effect.

1) The position on the celestial sphere at which a heavenly body would be seen from the Earth at a particular time.
2) A position given by the coordinates calculated for a star, if it were seen from the Earth's centre, relative to the → real equator and the → real equinox, at a certain date. It includes the displacements from one heliocentric direction, given in a stellar catalogue, due to → precession, → nutation, → aberration, → proper motions, → annual parallax, and light gravitational deviation (M.S.: SDE).

→ apparent; → position.

A certain place whose characteristics, as to location, altitude, atmospheric conditions, etc., make it appropriate for astronomical observations.

→ astronomical; → site.

In → propositional logic, a → sentence without any → connectives. See also → molecular proposition.

→ atomic; → proposition.

A change in the → energy level or → state of an → atom in which a → quantum of energy is either gained or lost. See also → forbidden transition; → permitted transition; → semiforbidden transition.

→ atomic; → transition.

The → variety of → plant and → animal → species in a particular → environment.

→ bio-; → diversity.

The total rate of energy output of an object integrated over all wavelengths.

→ bolometric; → luminosity.

A transition between two energy levels of an electron bound to a nucleus. The electron remains tied to the nucleus before and after the transition. → bound-free transition; → free-free emission.

Bound, p.p. of → bind; → transition.

A transition in which a bound electron is liberated. → free-bound emission; → free-free emission.

Bound, p.p. of → bind; → free.

A characteristic luminosity around which the → luminosity function of a sample of galaxies changes to a steeper slope or exponentially declines.

→ break; → luminosity.

A photon generation mechanism in an atom in which a transition initiates a series of secondary transitions from lower electronic levels.

→ cascade; → transition.

Same as catalog place and → mean catalog place.

→ catalog; → position.

In a → syllogism, a → proposition or statement that deals with inclusion or exclusion of members of → subject classes in → predicate classes. Categorical propositions are of four basic forms, see → Aristotelian form.

→ categorical; → proposition.

The → electric charge per unit volume in space, or per unit area on a surface, or per unit length of a line. They are respectively called volume- (ρ), surface- (σ), or line (λ) charge density.

→ charge; → density.

The identities, and relative abundances of the → chemical elements or → compounds that make up a substance.

→ chemical; → composition.

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 form of → Hertzsprung-Russell diagram in which the luminosity is the vertical axis and the → color index the horizontal axis.

→ color; → luminosity, → diagram.