An Etymological Dictionary of Astronomy and Astrophysics
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A → photometric system defined by reference to → monochromatic magnitudes in such a way that, when monochromatic → flux f_ν is measured in ergs sec^-1 cm^-2 Hz^-1, the magnitude will be: AB = -2.5 logf_ν - 48.60. The constant is set so that AB is equal to the V magnitude for a source with a flat → spectral energy distribution. The → zero point is defined by the flux of the star → Vega at 5546 Å. In this system, an object with constant flux per unit frequency interval has zero color.

→ magnitude; → system.

A material system that is subject to a constant force in each and every one of its instantaneous points of trajectory.

→ accelerating; → system.

An → adaptive optics instrument used on the → European Southern Observatory (ESO) 3.6-m telescope at La Silla. It was an upgraded version of COME-ON-PLUS, the → Very Large Telescope (VLT) adaptive optics prototype. It had 52 → actuators and performed corrections of the mirror 200 times per second. The reference → wavefront was sensed in the → visible. The observation was done in the → near-infrared (1-5 μm).

→ adaptive; → optics; → near-infrared; → system.

An → optical system that uses → adaptive optics.

→ adaptive; → optics; → system.

An optical system with object and image points at infinity.

→ afocal; → system.

A system of three stars, the → close binary Alpha Centauri A (→ spectral type G2 V) and Alpha Centauri B (K1 V), and a small and faint → red dwarf, Alpha Centauri C (M6 Ve), better known as → Proxima Centauri. To the unaided eye, the two main components (AB) appear as a single object with an → apparent visual magnitude of -0.27, forming the brightest star in the southern constellation → Centaurus and the third brightest star in the night sky, after → Sirius and → Canopus. The individual visual magnitudes of the components A, B, and Proxima are +0.01, +1.33, and +11.05, respectively. The masses of A and B are 1.100 and 0.907 Msun, respectively. Their → effective temperatures are (A) 5,790 K and (B) 5,260 K; their luminosities (A) 1.519 Lsun and (B) 0.500 Lsun. The binary members are separated in average by only 23 → astronomical units. They revolve around a common center of mass with a period of about 80 years. Both have a distance of 4.37 → light-years. Proxima Centauri, lying about 15,000 AU apart from AB, is → gravitationally bound to them. It has a mass of 0.1 Msun, a radius of 0.1 Rsun, a luminosity of about 0.001 Lsun, and an → effective temperature of ~ 3,000 K.

→ alpha; → Centaurus; → system.

The coordinate system in which the position of a body on the → celestial sphere is described with respect to an observer's → celestial horizon and → zenith. The coordinates of a point in this system are its → altitude on the → vertical circle, and its → azimuth westward (clockwise) along the celestial horizon from the observer's south. Same as → horizon coordinate system.

→ altazimuth; → coordinate; → system.

An optical system whose optical power, and imaging scale, differs in the two principal directions. See also → anamorphosis.

→ anamorphic; → system.

An → optical system that is able to produce an image essentially free from → spherical aberration and → coma. See also the → Abbe sine condition.

→ aplanatism; → system.

An optical system that is → apochromatic.

→ apochromatic; → system.

Any system of → logic which explicitly states → axioms from which → theorems can be → deduced.

→ axiomatic; → system.

A → numeral system that has 2 as its base and uses only two digits, 0 and 1. The positional value of each digit in a binary number is twice the place value of the digit of its right side. Each binary digit is known as a bit. The decimal numbers from 0 to 10 are thus in binary 0, 1, 10, 11, 100, 101, 110, 111, 1000, 1001, and 1010. And, for example, the binary number 11101₂ represents the decimal number (1 × 2⁴) + (1 × 2³) + (1 × 2²) + (0 × 2¹) + (1 × 2⁰), or 29. In electronics, binary numbers are the flow of information in the form of zeros and ones used by computers. Computers use it to manipulate and store all of their data including numbers, words, videos, graphics, and music.

→ binary; → number; → system.

Two astronomical objects revolving around their common center of mass.

→ binary; → system.

Râžmân, → system; dorin→ binary.

A system composed of several material bodies the total energy of which (the sum of kinetic and potential energies) is negative, e.g. a → bound cluster.

Bound, p.p. of → bind; → system.

Aâžmân, → system; bandidé p.p. of bandidan, → bind.

An optical system in which the light is reflected only.

→ catoprtics; → system.

The system of → CGS units.

→ CGS unit; → system.

A system that is → deterministic through → description by mathematical rules but can evolve highly → nonlinearly depending on → initial conditions. See also → chaos.

→ chaotic; → system.

A → binary system in which the distance separating the stars is comparable to their size. Most close binaries are spectroscopic binaries (→ spectroscopic binary) and/or eclipsing binaries (→ eclipsing binary). In most of them → mass transfer occurs at some stage, an event which profoundly affects the → stellar evolution of the components. The evolution of close binaries depends on the → initial masses of the two stars and their → separation. When the more massive star evolves into a → red giant first, material will spill through the inner point onto its companion, thereby affecting its companion's evolution. Mass transfer can also alter the separation and → orbital period of the binary star.

→ close; → binary; → system.

Thermodynamics: A system which can exchange energy with the surroundings but not matter. → open system; → isolated system.

→ closed; → system.

A binary star system which is composed of a collapsed object (→ degenerate dwarf, → neutron star, or → black hole) in orbit with a low-mass (≤ 0.5 Msol) secondary star, wherein the collapsed star → accretes matter from its → companion. These two objects form a binary system of overall dimensions 10⁶ km with an orbital period of only hours or less. See also: → X-ray binary.

→ compact; → binary; → star; → system.