An Etymological Dictionary of Astronomy and Astrophysics

6 terms — F › F

F FA FE FH FI FL FO FR FU

F corona   تاج ِ F   tâj-e F Fr.: couronne F The exterior part of the → solar corona, illuminated by solar light scattered or reflected by dust particles. The same phenomenon also produces the → zodiacal light, much farther away from the Sun. The dust particles are at most several microns in size and make up a disk stretching over almost 1 solar radius (700,000 km) from the Sun’s surface. Unlike electrons, which are responsible for the → K corona, the dust particles move relatively slowly. Thus, the light scattered by them has the same spectrum as the → photosphere and shows the → Fraunhofer lines. The F corona is the most luminous part of the corona at 1.5 solar radii from the Sun’s surface (M.S.: SDE). See also: F referring to the Fraunhofer lines; → corona.

f mode   ترز ِ f، مُد ِ ~   tarz-e f, mod-e ~ Fr.: mode f Waves that propagate only on the stellar surface. See also: → oscillation mode; → g mode; → p mode. See also: f referring to → fundamental; → mode.

F ring   حلقه‌ی ِ F   halqe-ye F Fr.: anneau F The → Saturn’s ring, with a width of 30-500 km, lying just outside of the → A ring, at 140,210 km from the center of Saturn. See also: → ring.

f-number   عدد ِ کانونی   adad-e kânuni (#) Fr.: nombre d'ouverture Same as → focal ratio. See also: f, from → focal; → number.

F-type star   ستاره‌ی ِ گونه‌ی ِ F   setâre-ye gune-ye F Fr.: étoile de type F A star whose spectrum is characterized by strong → absorption lines of ionized → calcium, Ca II (→ H and K lines), which become much stronger than the hydrogen lines of the → Balmer series. A multitude of fainter metallic lines are also present. Ca II lines strengthen to later types. → Main sequence F stars, of which → Procyon is an example, have a → surface temperature of 6,000 to 7,400°C and a mass of 1.1 to 1.4 → solar masses (Habets & Heintze, 1981, AASS 46, 193). See also: B, letter of alphabet used in the → → Harvard classification; → type; → star.

f(R) gravity   گرانی ِ (R)f   gerâni-ye f(R) Fr.: gravité f(r) An extension of Einstein’s → general relativity derived from relaxing the hypothesis that the → Hilbert-Einstein action for the → gravitational field is strictly linear. This was done by replacing the → Ricci scalar, R, with a non-linear function of R: S = (1/2κ)∫d4x (-g)1/2f(R) + Sm, where κ ≡ 8πG and Sm is the matter part of the action. The case of f(R) = R represents the simplest type of f(R) gravity theories. The discovery of → dark energy in 1998 stimulated the idea that → cosmic acceleration today may originate from some modification of gravity to general relativity. Dark energy models based on f(R) theories have been extensively studied as the simplest modified gravity scenario to realize the late-time acceleration. There are three versions of f(R) modified gravity: metric (or second order) formalism, Palatini (or first order) formalism, and metric-affine gravity. See also: f(R), function of the → Ricci scalar; → gravity.