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

For a specified → bandwidth of radiation consisting of a continuous → frequency spectrum, the total → power in the bandwidth divided by the bandwidth. Spectral density describes how the power (or variance) of a time series is distributed with frequency. Also called power spectral density.

→ spectral; → density.

The state of a spectrum from an astronomical object in which the lines change with time as far as their intensity, profile, and wavelength are concerned.

→ spectral; → variability.

The quality or fact of being → spiritual.

→ spiritual; → -ity..

A condition in which a dynamical system slightly displaced from its equilibrium configuration always tends to return to this configuration. → instability, → instability strip.

Noun from adj. → stable.

The total amount of energy emitted by a star per unit time. According to the → Stefan-Boltzmann law, the stellar luminosity is given by: L_* = 4πR_*²σT_eff⁴, where R_* is radius, σ is the → Stefan-Boltzmann constant, and T_eff is → effective temperature. A star's luminosity depends, therefore, on two factors, its size and its surface temperature. Stellar luminosity is measured either in ergs per second or in units of → solar luminosity or in → absolute magnitude. See also → luminosity class.

→ stellar; → luminosity.

The metallicity derived from observations of stars in galaxies. It is mainly based on spectral → absorption lines in → ultraviolet (UV) and optical ranges. Stellar metallicity is a direct measure of the amount of metals in a galaxy, since large part of heavy elements lies in its stars.

→ stellar; → metallicity.

1) The state or quality of being → subjective. Contrasted with → objectivity.
2) The interpretation based on individual personal feelings and opinions rather than on external → facts.

→ subjective + → -ity.

The fact or condition, for an object, of not being capable to sustain the → hydrogen fusion because of its low mass (less than 0.08 → solar masses).

→ substellar + → -ity.

The phenomenon in which certain materials, when cooled to a sufficiently low temperature, lose all resistance to the flow of electricity.

→ super-; → conductivity

The phenomenon occurring in → liquid helium (→ helium I) cooled below the → lambda point temperature of 2.17 K, whereby it flows freely with no measurable → friction and → viscosity.

→ super-; → fluidity.

The amount of a quantity distributed over a surface area divided by the area, such as a surface-charge density.

→ surface; → density.

1) The rate at which a small object in free fall near the surface of a body is accelerated by the gravitational force of the body: g = GM / R², where G is the gravitational constant, and M and R are the mass and radius of the object. The surface gravity of Earth is equal to 980 cm s^-2.
2) → black hole surface gravity.

→ surface; → gravity.

State or character of being susceptible. → magnetic susceptibility

M.L. susceptibilitas, from susceptibilis "capable, sustainable, susceptible," from susceptus, p.p. of suscipere "sustain, support, acknowledge," from sub "up from under" + capere "to take" ......

Barxodgiri, from bar- "up; upon; on; in; into; at; forth; with; near; before; according to" (Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over") + xod "self, own" (Mid.Pers. xwad "self; indeed;" Av. hva- "self, own") + giri vebal noun of gereftan "to take, seize, hold" (Mid.Pers. griftan, gir- "to take, hold, restrain;" O.Pers./Av. grab- "to take, seize," cf. Skt. grah-, grabh- "to seize, take," graha- "seizing, holding, perceiving," M.L.G. grabben "to grab," from P.Gmc. *grab, E. grab "to take or grasp suddenly;" PIE *ghrebh- "to seize").

An ecological concept, the property or condition of being → sustainable.

Quality, state noun from → sustainable.

1) The instantaneous linear velocity of a body moving in a circular path. It is equal to the → angular velocity multiplied by the radius: v_t = ωr.
3) The component of the velocity of an object, such as a star, that is at right-angles to the observer's → line of sight; also known as transverse velocity. See also → radial velocity.

→ tangential; → velocity.

A hydrodynamic instability in the → Taylor-Couette flow that arises when the rotation velocity of the fluid exceeds a critical value. The instability arises for → Taylor numbers larger than about 1700. At the beginning the fluid elements will move in simple rolls, but turbulence in the form of complex spirals will appear with increasing rotation velocity.

→ Taylor-Couette flow; → instability.

Temporal character or nature.

→ temporal; → -ity.

A generalization of the tensor concept that like a tensor transforms, except for the appearance of an extra factor, which is the → Jacobian matrix of the transformation of the coordinates, raised to some power, in transformation law. The exponent, which is a positive or negative integer, is called the weight of the tensor density. → weight of a tensor density. Ordinary tensors are tensor densities of weight 0. Tensor density is also called → relative tensor.

→ tensor; → density.

1) The constant maximum velocity reached by a body falling under gravity through a liquid or gas, especially the atmosphere. The body ceases to accelerate downward because the force of gravity is equal to the opposing force of resistance by the medium.
2) The velocity acquired at the end of a body's motion.
3) The velocity attained by → stellar wind at very large distance from the star. The material that escapes from the outer layers of the stars is accelerated outward from a small radial velocity at the → photosphere of the star, to some high velocity at large distance from the star. The theory of → radiation-driven winds predicts that the terminal velocity scales with the → escape velocity as: v_∞  ≅ 2-3 v_esc. The winds of hot → O stars experience fast acceleration and reach 80% of their terminal velocity near the star (some 3 R_*). The terminal velocity ranges from about 10 km s^-1 for a cool → supergiant star to 3000 km s^-1 for a luminous hot star. Terminal velocities are quite accurately measured from the violet trough of saturated → P Cygni line profiles in the ultraviolet (mainly N V λ1239, C IV λ1548, Si IV λ1394).

→ terminal; → velocity.

The state or fact of being → testable. See also → falsifiability.

→ testable; → -ity.