<< < B r bac Bal Bar Bar Bay bed Bes bi- bif bin bin bio bis bla bli blu Boh Bol Bos bou bra bri bro buo > >>
Boltzmann's equation hamugeš-e Boltzmann Fr.: équation de Boltzmann 1) An equation that expresses the relative number (per unit volume) of → excited atoms in different states as a function of the temperature for a gas in → thermal equilibrium: N_{u}/N_{l} = (g_{u}/g_{l}) exp (-ΔE/kT_{ex}), where N_{u} and N_{l} are the upper level and lower level populations respectively, g_{u} and g_{l} the → statistical weights, ΔE = hν the energy difference between the states, k is → Boltzmann's constant, and h → Planck's constant. → Boltzmann's constant; → equation. |
Boltzmann's relation bâzâneš-e Boltzmann Fr.: relation de Boltzmann A relation between the → entropy of a given → state of a → thermodynamic system and the → probability of the state: S = k . ln Ω where S is the entropy of the system, k is → Boltzmann's constant, and Ω the thermodynamic probability of the state. Boltzmann's relation connects → statistical mechanics and → thermodynamics. Ω is the number of possible → microstates of the system, and it represents the → randomness of the system. The relation also describes the statistical meaning of the → second law of thermodynamics. This expression has been carved above Boltzmann's name on his tombstone in Zentralfreihof in Vienna. Same as → Boltzmann's entropy formula. → Boltzmann's constant; → relation. |
bond band (#) Fr.: lien The → attractive force that holds together neighboring → atoms in → molecules. Bond, variant of band, from M.E. bende, O.E. bend, from O.Fr. bande, bende, PIE *bendh- "to bind" (cf. Goth bandi "that which binds;" Av./O.Pers. band- "to bind, fetter," banda- "band, tie" (see below); Skt. bandh- "to bind, tie, fasten," bandhah "a tying, bandage"). Band "band, tie," from Mid.Pers., O.Pers./Av. band- "to bind," banda- "band, tie," also present stem of bastan "to bind, shut," → shutter. |
Bond albedo sepidâ-ye Bond Fr.: albedo de Bond The fraction of the total amount of electromagnetic radiation falling upon a non-luminous spherical body that is reflected in all directions by that body. The bond albedo takes into account all wavelengths at all → phase angles. Compare with → geometric albedo. Named after the American astronomer George Phillips Bond (1825-1865), who proposed it; → albedo. |
Bondi-Hoyle accretion farbâl-e Bondi-Hoyle Fr.: accrétion de Bondi-Hoyle The → accretion of mass by a star (assumed as point particle) moving at a steady speed through an infinite, uniform gas cloud. It is directly proportional to the star mass (M) and the medium density (ρ) and inversely proportional to the relative star/gas velocity (v). In its classical expression: 4πρ(G M)^{2} / v^{3}, where G is the → gravitational constant. See Bondi & Hoyle (1944, MNRAS 104, 273) and Bondi (1952, MNRAS 112, 195). For a recent treatment of accretion in a turbulent medium see Krumholtz et al. 2006 (ApJ 638, 369). Named after Hermann Bondi (1919-2005), an Anglo-Austrian mathematician and cosmologist and Fred Hoyle (1915-2001), British mathematician and astronomer best known as the foremost proponent and defender of the steady-state theory of the universe; → accretion. |
Bondi-Hoyle accretion radius šo'â'-e farbâl-e Bondi-Hoyle Fr.: rayon de l'accrétion de Bondi-Hoyle In the → Bondi-Hoyle accretion process, the radius where the gravitational energy owing to star is larger than the kinetic energy and, therefore, at which material is bound to star. The Bondi-Hoyle accretion radius is given by R_{BH} = 2 GM / (v^{2} + c_{s}^{2}) where G is the gravitational constant, M is the stellar mass, v the gas/star relative velocity, and c_{s} is the sound speed. → Bondi-Hoyle accretion; → radius. |
Bonner Durchmusterung (BD) Bonner Durchmusterung Fr.: Bonner Durchmusterung A catalog of 324,188 stars in the → declination zones +89 to -01 degrees. The goal of the survey was to obtain a → position and estimated → visual magnitude for every star visible with the 78 mm → refracting telescope at Bonn. Actual → magnitude estimates were made and reported to 0.1 mag for all stars down to 9.5 mag. Positions are given to the nearest 0.1 sec in → right ascension and 0.1 arcmin in declination. The survey was carried out by Friedrich W. Argelander (1799-1875) and his assistants in the years 1852-1861. The Ger. name means Bonn Survey. |
Bonnor-Ebert mass jerm-e Bonnor-Ebert Fr.: masse de Bonnor-Ebert The largest gravitationally stable mass of the → Bonnor-Ebert sphere. After W.B. Bonnor (1956) and R. Ebert (1955); → mass. |
Bonnor-Ebert sphere epehr-e Bonnor-Ebert, kore-ye ~ Fr.: sphère de Bonnor-Ebert A sphere of interstellar gas at uniform temperature in equilibrium under its own gravitation and an external pressure. The pressure of a hotter surrounding medium causes the sphere to collapse. → Bonnor-Ebert mass. → Bonnor-Ebert mass; → sphere. |
book ketâb (#), nâmé (#), nask (#) Fr.: livre A bound set of printed or manuscript pages. M.E., from O.E. boc "book, written document;" cf. Ger. Buch "book;" Du. boek; O.N. bôk; Gothic boka. Ketâb, loanword from Ar. |
Boolean Booli (#) Fr.: de Boole A → variable or → function which takes the value → true or → false. → Boolean algebra. After the English mathematician George Boole (1815-1864), the founder of mathematical, or symbolic, logic. |
Boolean algebra jabr-e Booli (#) Fr.: algèbre de Boole Any of a number of possible systems of mathematics that deals with → binary digits instead of numbers. In Boolean algebra, a binary value of 1 is interpreted to mean → true and a binary value of 0 means → false. Boolean algebra can equivalently be thought of as a particular type of mathematics that deals with → truth values instead of numbers. → Boolean; → algebra. The term Boolean algebra was first suggested by Sheffer in 1913. |
Boomerang Nebula miq-e bumerâng Fr.: nébuleuse du Boomerang A → nebula displaying two nearly symmetric lobes of matter that are being ejected from a central star at a speed of about 600,000 km per hour (each lobe nearly one light-year in length). The Boomerang Nebula resides 5,000 → light-years from Earth in the direction of the Southern constellation → Centaurus. Boomerang, adapted from wo-mur-rang, boo-mer-rit, in the language of Australian aborigines; → nebula. |
Bootes Gâvrân, Gâyâr Fr.: Bouvier The Herdsman, the Ox Driver. A constellation in the northern hemisphere, at right ascension about 14h 30m, north declination about 30°. Its brightest star is → Arcturus. Abbreviation: Boo; genitive form: Boötis. L. Boötes, from Gk. bootes "plowman," literally "ox-driver," from bootein "to plow," from bous "ox," from PIE *gwou- "ox, bull, cow;" compare with Av. gao-, gâuš "bull, cow, ox," Mod.Pers. gâv, Skt. gaus, Armenian kov, O.E. cu. Gâvrân "ox-driver," from gâv "ox, cow" + rân
"driver," from rândan "to drive." |
Borda circle dâyere-ye Borda Fr.: cercle de Borda An instrument which was an improved form of the → reflecting circle, used for measuring angular distances. In Borda's version the arm carrying the telescope was extended right across the circle. The telescope and a clamp and tangent screw were at one end, and the half-silvered horizon glass at the far end from the eye. In practice, with the index arm clamped, the observer first aims directly at the right hand object and by reflection on the left, moving the telescope arm until this is achieved. He then frees the index arm, sights directly on the left hand object with the telescope arm clamped, and moves the index arm until the two coincide again. The difference in the readings of the index arm is twice the angle required, so that the final sum reading must be divided by twice the number of double operations. Borda's instrument greatly contributed to the French success in measuring the length of the meridional arc of the Earth's surface between Dunkirk and Barcelona (1792-1798). The operation carried out by Jean Baptiste Delambre (1749-1822) and Pierre Méchain (1744-1804) was essential for establishing the meter as the length unit. After the French physicist and naval officer Jean-Charles de Borda (1733-1799), who made several contributions to hydrodynamics and nautical astronomy. Borda was also one of the most important metrological pioneers; → circle. |
born zâdé (#) Fr.: né Brought forth by → birth. Past participle of bear. → born-again AGB star. M.E., from O.E. boren, p.p. of beran "to bear, bring, wear", from P.Gmc. *beranan (O.H.G. beran, Goth. bairan "to carry"), from PIE root *bher- "to bear; to carry" (cf. Av./O.Pers. bar- "to bear, carry," bareθre "to bear (infinitive)," bareθri "a female that bears (children), a mother," Mod.Pers. bordan "to carry," bâr "charge, load", bârdâr "pregnant," Skt. bharati "he carries," Gk. pherein). Zâdé "born," p.p. of zâdan "give birth" (Av. zan- "to bear, give birth to a child, be born," infinitive zizâite, zâta- "born," cf. Skt. janati "begets, bears," Gk. gignesthai "to become, happen," L. gignere "to beget," gnasci "to be born," PIE base *gen- "to give birth, beget"). |
born-again AGB star setâre-ye AGB-ye bâzzâdé Fr.: étoile AGB recyclée A → post-AGB star that undergoes a last → thermal pulse when it is already on the → white dwarf → cooling track. The thermal pulse will expand the hot central star, whereby hydrogen will be ingested into the → helium burning shell. This will temporarily return the star to the → AGB phase it has previously left. → born; → again; → asymptotic giant branch; → star. |
born-again planetary nebula miq-e sayâreyi-ye bâzzâd Fr.: nébuleuse planétaire recyclée A → planetary nebula which is thought to have experienced a → very late thermal pulse (VLTP) when the central star (→ CSPN) was on the → white dwarf cooling track. The VLTP event occurs when the thermonuclear → hydrogen shell burning has built up a → shell of helium with the critical mass to ignite its → fusion into carbon and oxygen (→ helium shell burning). Since the → white dwarf envelope is shallow, the increase of pressure from this last helium shell flash leads to the ejection of newly processed material inside the old planetary nebula, leaving the stellar core intact. As the stellar envelope expands, its → effective temperature decreases and the star goes back to the → asymptotic giant branch (AGB) region in the → H-R diagram. The subsequent stellar evolution is fast and will return the star back to the → Post-AGB track in the H-R diagram: the envelope of the star contracts, its effective temperature and ionizing photon flux increase, and a new fast stellar wind develops (see, e.g. J. A. Toalá et al. 2015, ApJ 799, 67). |
boron bor (#) Fr.: bore A soft, brown, nonmetallic chemical element; symbol B. → Atomic number 5; → atomic weight 10.81; → melting point about 2,300°C; → specific gravity 2.3 at 25°C; → valence +3. Boron occurs as borax and boric acid. It is used for hardening steel and for producing enamels and glasses. Since it absorbs slow neutrons, it is used in steel alloys for making control rods in nuclear reactors. Boron was separated in 1808 by Joseph Louis Gay Lussac (1778-1850) and Louis Jacques Thénard (1777-1857) and independently by Sir Humphry Davy (1778-1829). From bor(ax), from M.Fr. boras, from M.L. borax, from Ar. buraq, from Pers. burah "borax, nitre, used in soldering gold" + (car)bon. Bor, loan from Fr., as above. |
Bose-Einstein condensate (BEC) cagâlâk-e Bose-Einstein Fr.: condensat de Bose-Einstein A state of matter in which a group of atoms or subatomic particles,
cooled to within → absolute zero,
coalesce into a single quantum mechanical entity
that can be described by a → wave function.
When a group of atoms are cooled down to very near
absolute zero, the atoms hardly move relative to each other, because
they have almost no free energy
to do so. Hence the atoms clump together and enter
the same → ground energy states.
They become identical and the whole group starts behaving as though it were a
single atom. A Bose-Einstein condensate results from a
→ quantum transition phase
called the → Bose-Einstein condensation.
This form of matter was predicted in 1924 by Albert Einstein on
the basis of the quantum formulations of the Indian physicist
Satyendra Nath Bose. → boson; → Einstein; → condensate. |
<< < B r bac Bal Bar Bar Bay bed Bes bi- bif bin bin bio bis bla bli blu Boh Bol Bos bou bra bri bro buo > >>