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

A collapsing object whose radius is less than its Schwarzschild radius must collapse into a singularity.

→ Penrose process; → theorem.

The → moment of inertia of a plane object (→ lamina) about an axis perpendicular to the plane is equal to the sum of the moments of inertia about any two perpendicular axes in the plane. Thus if x and y axes are in the plane, I_z = I_x + I_y.

→ perpendicular; → axis; → theorem.

A specific type of → photoluminescence that continues for an appreciable time after the stimulating process has ceased. Phosphorescence is due to the existence of metastable → excited states of the atoms and molecules from which a change to the normal state is hindered for some reason or other. The change from the → metastable metastable state to the normal one becomes possible only as a result of some additional excitation, for example the application of heat.

→ phosphorus; → -escence.

In an → isolated system, any initial state will occur again in the course of the → evolution of the system over a sufficiently long but finite → time.

→ Poincaré sphere; → recurrence; → theorem.

1) A small hole such as a space especially in a rock, soil, etc. → porous dust grain, → porosity.
2) A minute orifice in an animal or plant, as in the skin or a leaf.
3) Sun: A small → sunspot that does not have a → penumbra. Pores are up to about 2,500 km across and are lighter than a sunspot's → umbra. They appear and disappear at the surface of the Sun where sunspots are rarely formed, e.g. at low latitudes. Their lifetimes can be as short as 1/2 hour and as long as several days.

From L. porus "a pore," from Gk. poros "a pore," literally "passage, way," from PIE root *per- "to lead, pass over."

Lik, from Gilaki lik "hole," variants luk, luke, liuk, luxa, Tabari luk, li, Sangesari, Semnâni lu, Aftari lo.

The space through which electromagnetic radiation passes is filled with electric and magnetic fields at right angles to each other and to the direction of propagation of the radiation. The rate of energy transfer is given by the Poynting vector.

In honor of John Henry Poynting (1852-1914), English physicist; → theorem.

A precursor of a small, loosely bound → star cluster (→ bound cluster) as well as an → OB association, with masses ranging from about 10 to 1000 → solar masses or more.

→ pre-; → cluster; → core.

A small, gravitationally unstable molecular → clump of typical size of less than 0.1 pc resulting from → gravitational collapse and → fragmentation of a larger → molecular cloud. It is a centrally concentrated structure which evolves into a → class 0 object, where eventually a single star or a stellar system is formed. Core masses range between 0.5 and 5 solar masses, with a mean number density of at least 10⁴-10⁵ cm^-3, and a temperature as low as about 10 K. A pre-stellar core evolves into a → Class 0 object. Also called dense core.

→ pre-stellar; → core.

The proposition that the → square of the → hypotenuse of a → right triangle is equal to the → sum of the squares of the other two sides: a² + b² = c².

After Pythagoras (c570 BC-c495BC), Greek philosopher and mathematician; → theorem.

Any group of three → integers that satisfy the relations specified by the → Pythagorean theorem. Some examples: 3, 4, and 5; 5, 12, and 13; 8, 15, 17.

→ Pythagorean theorem; → triple; → triplet.

The part of a → nuclear reactor in which → nuclear fission takes place and huge quantities of heat energy are generated.

→ reactor; → core.

1) General: Any theorem that expresses various reciprocal relations for the behavior of some physical systems, in which input and output can be interchanged without altering the response of the system to a given excitation.
2) In classical electromagnetism, the theorem stating that the current in a detector divided by the voltage at the source remains constant when source and detector are interchanged, as long as the frequency and all the impedances are left unchanged.

Reciprocity, from L. reciproc(us) "returning the same way, alternating" + → -ity; → theorem.

Farbin, → theorem; dosuyegi, quality noun of dosuyé nuanced term of dosu "two-sided," from do, → two, + su "direction, side," from Mid.Pers. sôk "direction, side."

The theorem stating that the value of the line integral of a complex function, taken along a simple closed curve encircling a finite number of isolated singularities, is given by 2πi times the sum of the residues of the function at each of the singularities.

→ residue; → theorem.

1) To bring back into existence, use, or the like; reestablish.
2) To bring back to a former, original, or normal condition, as a building, statue, or painting (Dictionary.com).

M.E., from O.Fr. restorer, from L. restaurare "repair, rebuild, renew," from → re- "back, again" + -staurare, from PIE root *sta- "to stand, set down, make or be firm," → state.

Bâzsâxtan, from bâz- "back, again," → re-, + sâxtan "to build, make, fashion; to adapt, adjust, be fit" (from Mid.Pers. sâxtan, sâz-, Manichean Parthian s'c'dn "to prepare, to form;" Av. sak- "to understand, to mark," sâcaya- (causative) "to teach").
Bâzgardândan, from bâz-+ gardândan "to turn (around)," accusative of gardidan, gaštan "to change, to turn, → revolve."

An image that has been upgraded by a process of → image restoration.

Restored, p;p. of restore, → restoration; → image.

Tasvir, → image; bâzsâxté, p.p. of bâzsâxtan, → restoration.

If a function f(x) is → continuous on an interval [a,b] and is → differentiable at all points within this interval, and vanishes at the end points x = a and x = b, that is f(a) = f(b) = 0, then inside [a,b] there exists at least one point x = c, a < c < b, at which the derivative f'(x) vanishes.

Named after Michel Rolle (1652-1719), a French mathematician; → theorem.

A uniqueness theorem involving the equations of state of stellar structure. → Vogt-Russell theorem.

Named after the German astronomer Heinrich Vogt (1890-1968) and the American astronomer Henry Norris Russell (1877-1957); → theorem.

Same as → Nyquist-Shannon sampling theorem.

→ sampling; → theorem.

A hydrostatic object predicted to result from the → second collapse of a → molecular cloud in an early stage of star formation.

→ second; → core.

Same as → sampling theorem.

→ Shannon entropy; → sampling; → theorem.