An Etymological Dictionary of Astronomy and Astrophysics
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A → dimensionless parameter indicating the degree to which a star is close to the → Eddington limit. It is expressed as Γ = L / L_Edd = κ L / (4πGMc), where L and M are the star luminosity and mass respectively, κ is the opacity, c the speed of light, and G the → gravitational constant. At the Eddington limit, Γ = 1, the star would become unbound. Because stellar luminosity generally scales with a high power of the stellar mass (L∝ M^3-4), → massive stars with M larger than 10 Msun generally have electron Eddington parameters of order Γ ≅ 0.1-1.

After Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → parameter.

The effective value of the → Eddington parameter in a non-homogeneous system (porous opacity).

→ effective; → Eddington limit; → parameter.

In cosmology, a → dimensionless parameter introduced by the → equation of state representing the ratio of the pressure to the energy density of a fluid, such as the → dark energy: w = p/ρ. The → deceleration or → acceleration of an → expanding Universe depends on this parameter (→ accelerating Universe). A number of numerical values of this parameter are as follows: for the → cosmological constant: w = -1, for → non-relativistic matter (present-day → baryons): w = 0, and for → relativistic matter (photons, neutrinos): w = +1/3. Together with Ω(dark energy) and Ω(matter), w provides a three-parameter description of the dark energy. The simplest parametrization of the dark energy is w = constant, although w might depend on → redshift.

→ equation; → state; → parameter.

A space project, initially → LISA, consisting of a configuration of three satellites, aimed to detect low frequency → gravitational waves that cannot be measured by ground-based detectors. The detection range will be from about 0.1 milliHz to 1 Hz. One "mother" and two "daughter" spacecrafts will be brought into an orbit around the Sun, which is similar to the Earth's orbit. The satellites will fly in a near-equilateral triangle formation, with a constant distance of one million km between, following the Earth along its orbit at a distance of around 50 million km. The mother spacecrafts carries two and each of the daughter spacecraft carry one free-flying → test masses that will be kept as far as possible free of external disturbances. The mutual distances of the test masses from satellite to satellite will be measured by means of high-precision, → Michelson-like laser → interferometry. In this way, the extremely small distance variations between the test masses of two satellites can be detected which are caused by the passages of a gravitational waves. The required measurement accuracy of the distances amounts to typically 1/100 of the diameter of a hydrogen atom (10^-12 m) at a distance of two million km.

→ evolve; → laser; → interferometer; → space; → antenna.

A → scale factor that relates the size of the Universe R = R(t) at time t to the size of the Universe R₀ = R(t₀) at time t₀ by R = aR₀. The expansion parameter represents the history of expansion of the Universe.

→ expansion; → parameter.

In photography, an instrument used to measure the intensity of light from a scene to be photographed and to indicate the camera lens and shutter settings required to expose the film correctly.

→ exposure; → meter.

A type of interferometer wherein the beam of light undergoes multiple reflections between two closely spaced partially silvered surfaces. Part of the light is transmitted each time the light reaches the second surface, resulting in multiple offset beams which can interfere with each other. The large number of interfering rays produces an interferometer with extremely high resolution, somewhat like the multiple slits of a diffraction grating increase its resolution.

The design was conceived by French physicists Charles Fabry (1867-1945) and Alfred Pérot (1863-1925) in the late nineteenth century; → interferometer.

An instrument used with a telescope for accurately measuring small angular separations between two celestial bodies (as between binary stars). The instrument consists of two parallel fine wires with one wire being fixed and the other movable by means of a finely threaded screw.

Filar, from L. fil(um) "a thread" (see the paragraph below) + -ar variant of the adjective-forming suffix → -al; → micrometer.

Kehsanj, → micrometer; zehi adj. of zeh "string, bow-string;" Mid.Pers. zih "bow-string," zig "string; astronomical table" (loaned into Ar. as zij); Av. jiiā- "bow-string;" cf. Skt. jiyā- "bow-string;" Gk. bios "bow;" L. filum "thread;" Arm. jil "string, line;" Lith. gijà "thread;" Russ. žica "thread;" PIE base *g^whi- "thread, tendon."

The 500 m diameter → radio telescope which is the largest → single-dish antenna in the world. It is an Arecibo type telescope nestled within a natural basin in China's remote and mountainous Dawodang, Kedu Town, in southeastern China's Guizhou Province. The → reflector consists of 4,450 triangular panels, each with a side length of 11 m. More than 2,000 → actuators are used, according to the feedback from the measuring system, to deform the whole reflector surface and directly correct for → spherical aberration. Several detectors are used to cover a frequency range of 70 MHz to 3 GHz.

→ five; → hundred; → meter; → aperture; → spherical; → radio; → telescope.

One of the three quantities (k, α, and δ) which are used in the → radiation-driven wind theory to express the radiation pressure due to spectral lines. These coefficients parametrize the radiation acceleration as: g_rad≅ k(dv/dr)^αg_e, where g_e = σ_eL/4πcR_*² is the radiative acceleration by electron scattering. The parameter k is dependent on the number of lines that produce the radiation pressure. The parameter α depends on the optical depth of the driving lines and varies between 0 (optically thin) and 1 (optically thick). The parameter δ describes the dependence of k on the density with k ≅ ρ^δ. The velocity law of radiation-driven winds depends on α and δ, but not on k. The → mass loss rate depends on k, α, and δ (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269 and references therein).

→ force; → multiplier; → parameter.

One of the parameters that characterize atmospheric → seeing. It is the diameter of the largest aperture that can be used before → turbulence starts to degrade the image quality. As the turbulence gets stronger, the Fried parameter, denoted r₀, becomes smaller. The Fried parameter is wavelength dependent: r₀ ∝ λ^6/5. On best astronomical mountain tops it ranges between 20 and 30 cm for λ = 5000 A.

Named after David L. Fried, who defined the parameter 10 1966; → parameter.

An instrument for measuring or detecting small → direct currents, usually by the mechanical reaction between the magnetic field of the current and that of a magnet.

→ galvano- + → -meter.

An instrument used to measure the angular separation of two stars that are too far apart to be included in the field of view of an ordinary telescope.  The instrument was originally designed for measuring the variation of the Sun's diameter at different seasons of the year.

→ helio- + → -meter.

→ frequency to wavelength conversion.

→ hertz; → meter; → conversion.

An → interferometer using a technique that involves introducing a small → frequency shift between the optical frequencies of the two interfering light beams. This results in an intensity modulation at the → beat frequency of the two beams for any given point of the → interference pattern. A convenient way of introducing such a frequency shift is by means of an acousto-optic modulator.

→ heterodyne; → interferometer.

→ Hubble-Lemaitre parameter.

→ Hubble; → parameter.

The rate pf change of the → cosmic scale factor: H(t) = (dR/dt)/R. The Hubble parameter is a time-dependent quantity and therefore is not constant. The → Hubble-Lemaitre constant is the Hubble-Lemaître parameter measured today.

→ Hubble-Lemaitre law; → parameter.

An instrument that measures the relative humidity of the air.

→ hygro- + → -meter.

1) A measure of the distance by which a collision fails being frontal.
2) In → gravitational lensing, the distance of closest approach of the light path to the → lensing object.
3) In → rainbows, the displacement of the → incident from an axis that passes through the center of the water droplet.

→ impact; → parameter

1) Physics: A device that divides a beam of light into a number of beams and re-unites them to produce → interference. → Fabry-Perot interferometer; → Michelson interferometer.
2) Astro.: An arrangement of two or more separate telescopes placed at some distance from one another, each one receiving electromagnetic radiation (optical, infrared, or radio) from a celestial object and brought together to form an interference pattern.

Interferometer, from interfer, → interfer, + -o- a connective/euphonic infix + → -meter.

Andarzanešsanj, from andarzaneš→ interference + -sanj, → -meter.